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BUREAU OF INDIAN STANDARDS Preliminary Draft CRITERIA FOR DESIGN OF RCC STAGING FOR OVERHEAD WATER TANKS (First Revision of IS 11682) Special Structures Sectional Committee, CED 38 FOREWORD (Formal clauses of the standard will be added later.) This standard was first published in 1985. This first revision was taken up to keep abreast with the rapid developments in design and construction fields, and to bring further modifications in the light of experience gained. Liquid tanks are important public utility and industrial structures. Specifications, the design and construction method in reinforced concrete are influenced by the prevailing construction practices, the physical properties of the material and the environmental conditions. Based on the experience in design and construction of staging of elevated tanks, necessity of revising the standard was felt (see Commentary, E-1). While the common methods of design have been covered in this standard code, design of structures of special forms or in unusual circumstances should be left to the judgment of the Design Engineer and in such cases special systems of design & construction may be permitted on production of satisfactory evidence regarding their adequacy and safety by analysis or test or by both. If applicable at a particular location dust load should be accounted on roofs. In this standard it is assumed that the design of liquid tank and staging is entrusted to the qualified engineer knowledgeable with the current engineering practice related to RCC deign, and the execution of work is carried out under the direction of an experienced supervisor. The design and construction of container for storage of liquid have been covered by IS 3370 (Parts 1 to 4), and this standard lays down the principles of design of staging for elevated liquid tanks All requirements of IS 456, IS 3370 (Part 1), IS 3370 (Part 2) and IS 1893 Part 2 in so far as they apply, shall be deemed to form part of this standard except where otherwise laid down in this standard. It is proposed that as and when IS 1893 part 2 is published, the clause 10.5.3.1 & 10.5.3.2 & Annex D shall be withdrawn.
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The inner part staging in many cases is used for material and equipment storage, office space, and other applications. Provisions in design are required for such requirements. This standard is drafted for common types of staging. Enough details may not be available for all other types of staging and possible configurations, for which designer is responsible for additional criterion for design. “Liquid tank” & “Liquid container’ are treated as synonymous terms. In place of “liquid”, “water” may be used wherever appropriate by the user. Elevated water tanks in reinforced concrete are normally constructed under a lumpsum contract as deign & built contracts. The designs are checked by owner organizations or proof consultants. Hence all objective data should be clearly defined and for subjective decisions if required solutions should be defined, along with the data in contract document. For the purpose of deciding whether a particular requirement of this standard is complied with, the final value, observed or calculated, expressing the result of a test or analysis shall be rounded off in accordance with IS 2:1960. The number of significant places retained in the rounded off value should be the same as that of the specified value in this standard.
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Contents Scope References Terminology Symbols Specifications, Design Report & Drawings Exposure Condition Concrete Structural Configuration of Members Stability of Structure Loads Load Combinations Analysis Design Framed Staging Modeling P-δ Effect Columns Braces Foundations Stair Shaft Type Staging Detailing Construction Requirements Miscellaneous items & Appurtenances ANNEX A – Referred Indian Standards ANNEX B – Types of Stagings ANNEX C – Structural Configuration of Members ANNEX D – Response reduction factors (R) ANNEX E – Commentary Figures (To be included)
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BUREAU OF INDIAN STANDARDS Preliminary Draft CRITERIA FOR DESIGN OF RCC STAGING FOR OVERHEAD WATER TANKS (First Revision of IS 11682) Special Structures Sectional Committee, CED 38 1 SCOPE This draft standard lays down criteria for analysis, design and construction of reinforced cement concrete staging of framed type with columns or shaft type, for achieving a desirable level safety and durability of the supported liquid storage structure (container). Container may consist of any material like RCC, fiber concrete, ferrocement, steel, PVC, etc. While the provisions of this standard refer to stagings for the storage of liquids, the recommendations are applicable mainly to water storage or containment. Additional requirements necessary for containment of liquids other than ordinary or plain water are beyond the scope. The requirements given in this standard are not applicable for staging in reinforced masonry or un-reinforced masonry, may it be in concrete block, stone or bricks. 2
REFERENCES
The standards listed in Annex A, contain provision which through reference in this standard, constitute provisions of this standard. At the time of approval of this standard, the editions indicated are valid. All standards are subject to amendments and revision, and parties to agreements based on this standard are encouraged to investigate the possibility of applying the most recent editions of the standards being referred with their amendments. 3
TERMINOLOGY
For the purpose of this standard, the following definitions shall apply. 3.1 Capacity – Capacity of the tank shall be the volume of liquid it can store between designed full supply level (FSL) and lowest supply level (LSL that is, the level of the lip of the outlet pipe). Due allowance shall be made for applying lining, coating or plastering to the tank from inside if any, when calculating the capacity of tank. The designated capacity of tank excludes dead storage which is the quantity of liquid below lowest supply level (LSL). 3.2 Staging – It consists of components of structure supporting a liquid tank (container), to locate it significantly above general ground level. Pedestals or blocks
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of short heights supporting a tank will not be called as staging. In general the term staging includes the structural components for foundations also. 3.3 Height of Staging – Height of staging is the difference between the lowest supply level of tank and the average ground level at the tank site. 3.4 Liquid Depth – Liquid depth in tank shall be the difference of level between lowest supply level (LSL) and full supply level (FSL) or working top liquid level (WTL) of the tank. In case of liquid being water, the term ‘water depth’ can be used. The ‘design liquid depth’ for tank can be more than the ‘liquid depth’ due to dead storage and due to rise of liquid in freeboard zone to be accounted in design. 3.5 Framed Staging – Staging consisting of columns and braces. 3.6 Shaft Staging – Staging consisting of shell like a circular or polygonal cylinder or hollow prism. 3.7 Liquid Tower – The structure consisting of tank (i.e. container) together with the staging and foundation is termed as liquid tower. 3.8 Force actions – Include bending moments, torsion, shear forces, direct tension or compression. 4
SYMBOLS/ NOTATIONS
DL - dead load DL p - provisional dead load IL - imposed load IL s - imposed load due to storage IL p - imposed load due to an operation or equipment WL - wind load or seismic load FL – liquid (fluid) load WTL - normal working top liquid level FSL - full supply level MTL - maximum top liquid level LSL – lowest supply level P-δ effect – effect of vertical load with horizontal deflection resulting in increased bending moments. R – response reduction factor RCC - reinforced cement concrete k 1 , k 2 , k 3 - wind speed factors Ht - total height of tower (including container) h - depth of liquid in tank Cf - force coefficient (for wind load) SMRF – special moment resisting frame – ductile frame (ref IS 1893 & IS 13920) OMRF – ordinary moment resisting frame not confirming to IS 13920 f y - characteristic strength of reinforcement bars, yield or proof stress. R c – radius of the centerline of shaft t – thickness of shaft f ck - characteristic compressive strength of concrete
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5 SPECIFICATIONS, DESIGN REPORT & DRAWINGS 5.1 Documentation shall be prepared which should contain all salient features of the work and engineering data and maintenance scheme of the work. It should cover the following. Brief data and features like description of liquid to be contained, capacity of tank (in m³), height of free board (in m), staging height (in m). 5.2 Foundation investigation report and soil data, type of foundation, probable depth of foundation and net allowable bearing capacity of founding strata. The position of ground water table highest & lowest. Soil classification for seismic design. 5.3 Location of tower (e.g. polluted industrial area, sea front area, coastal area, urban area etc.) and purpose of storage of liquid (i.e. public water supply, fire fighting. Industrial etc.), pollutants, salts, soleplates if any in air, soil and ground water. 5.4 Specifications of concrete and its grade, type of cement to be used, limits of maximum and minimum cement content, grade of reinforcement bars. 5.5 Salient features of structure and construction, method of construction, guidance on release of form work. Clear cover of concrete on reinforcement bars for various members at different locations. Codes, standards, references for construction. 5.6 Design loads – Density of concrete, liquid, soil, masonry etc.; provisional loads of finishing, flooring, rendering, coating, lining etc. as applicable, railing, parapets, masonry wall etc.; imposed loads on roof, balcony, walkways, platform etc.; Seismic zone, zone factor, response reduction factor, importance factor, critical damping factor, soil factor; Basic wind speed, k1, k2, k3, terrain category, class A/B/C (see IS 875 part 3) ; Load of equipment if any etc.; Construction loads; any other loads. 5.7 Indian standards referred for design. 5.8 Design report containing basis of design, method of structural analysis, detailed computation of loads, structural analysis, design calculations with sizes of members and reinforcement. 5.9 Drawing with reinforcement detailing, instructions, brief specifications and notes. Locations of construction joints and its treatment should be specified on the drawing. 5.10 Guide for completion drawing, and completion report for record. Record of quality of construction. 5.11 Proposed scheme of condition survey and maintenance of structure. 6
EXPOSURE CONDITION
6.1 At the site of tower actual exposure condition should be assessed. Due to possible exposure, the probable mechanism which may cause loss of durability of RCC should also be assessed. Specifications of concrete, the structural design and Page 6 of 44
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construction of structure shall take in to considerations, imparting enough resistance to the structure against possible mechanisms of deterioration & loss of durability over the service life. 6.1.1 The design and construction should also take in to account the probable maintenance requirements expected during service life of structure. With the design report, maintenance aspects which can adversely affect the service life of structure within 30 years must be mentioned with its frequency. Structure shall be designed such that maintenance activities will be minimum possible. 6.2 Components of staging shall be treated as exposed to not less than moderate environment as defined in clause 8.2.2 of IS 456 & table 3, except for the components protected from external environment by permanent cladding similar to building work. Owner or designer may decide for higher exposure condition based on the location of the tank. For staging in coastal area and in area of heavy air pollution, higher environmental exposure condition like severe should apply. For foundations and components (like piles, footing, column, ground brace, etc.) in contact with ground / soil, based on actual ground or sub-soil conditions, higher exposure condition may be assumed for design. 6.3 For severe or higher exposure conditions, possible mechanism which could bring about durability loss shall be assessed and accordingly design actions, specification drafting, applications of coating or lining and precautions in construction shall be taken to achieve the designed service life of structure. 6.4 While deciding on the exposure condition for design, the possibility of small leakage through container due to construction error may be considered, which would make the components of staging occasionally wet and thus may need higher exposure condition. 7
CONCRETE
7.1 The requirements for concrete materials shall be governed by IS 456 for reinforced concrete, with the following additional requirement. Use of aggregate having high porosity (>5%) shall be permitted only after establishing its parameters, long term influence on concrete and specifically effect on durability. Prestressed members will be governed by IS 1343. Structural steel members will be governed by IS 800. 7.2 Concrete shall conform to provisions of IS 456. The grade of concrete for staging shall be maximum of the requirements in 7.2.1, 7.2.2, 7.2.3 & 7.2.4. 7.2.1 The grade of concrete shall not be less than that required by IS 456, table 5, depending upon the exposure condition.
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7.2.2 For conformance to the requirement of maximum water cement ratio, concrete grade required may be higher than the minimum specified in the table 5 of IS 456. 7.2.3 The minimum grade of concrete should not be less than the following: (a) M25 - For all staging except as below ; (b) M30 - For towers with any one of following conditions, (i) Tanks of capacity more than 1000 m³, (ii) Tanks of capacity >500 m³ in seismic zone IV, (iii) Tanks of capacity >500 m³ & staging height > 20 m, (iv) Tanks of capacity >200 m³ in seismic zone V or more, (v) Tanks of capacity >200 m³ & staging height > 30 m; (c) M20 may be permitted for staging of tanks less than 120 m³ in rural non industrial area (not subject to air pollution), with staging height less than 13 m and not located in seismic zone IV or above, provided the tank is neither located in coastal area nor the area having basic wind speed above 45 m/sec. (This permission is to provide continuity to present practice of constructing staging in M20, it is hoped that in near future this clause will be deleted.) 7.2.4 For tank staging in area where basic wind speed specified in IS 875(Part 3) is 50 m/sec or more, concrete grade shall not be less than M30. Grade of concrete for staging in coastal area shall not be less than M30. Where staging is located near sea face such that the structure can be subject to salt laden wind, higher than M 30 grade of concrete will be required for durability. 7.2.5 Concrete of grades higher than that recommended in this standard are preferable and acceptable. It may be suggested that the grade of concrete for staging may not be less than that for container for convenience in construction. 7.2.6 Ready Mixed Concrete conforming to IS 4926 may also be preferred. 7.3
Cement
7.3.1 The cement content should normally not be in excess of 400 kg/m³ in concrete. If mineral admixtures are added while mixing concrete, the limit applies to ordinary Portland cement content only. Cement shall be as per 5.1 of IS 456. 7.3.2 Use of blended cements (Portland pozzolana cement confirming to IS 1489 Part 1 and Portland slag cement conforming to IS 455) is preferable, unless 7 days strength of more than 20 N/mm² is targeted. Brand, grade and type of cement shall not be changed during construction unless mix proportioning is again verified by trial mix. 7.3.3 Site mixing of mineral admixture requires very efficient and thorough mixing. Unless a batch mixing plant or highly efficient mixer is used to deliver concrete, site mixing of mineral admixture may not be done.
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7.3.4 Use of sulphate resisting cement shall be used only if exposure condition requires its use. Its use may be required for members below ground level, if would be subjected to sulphate attack. 7.4
Fibers
For enhancing the performance of concrete, addition of fibers is permitted in concrete. In general steel or polypropylene fibers can be added. For any other fiber, its long term chemical stability shall be established by the designer. Use of fibers is very useful in controlling plastic shrinkage cracks, as well temperature shrinkage cracks in young age of concrete. Structural fibers like steel can improve the dispersion of cracks due to loads in service life. 7.5 Nominal cover to reinforcement shall be governed by the exposure condition assumed for design. Refer the recommendations in 26.4 of IS 456. 7.6 Construction joints in columns, braces and shaft should be as less as possible. 7.7 Formwork should comply with IS 456 and IS 14687. 8
STRUCTURAL CONFIGURATION OF MEMBERS
For general information on types of staging, reference may be made to Annex B. The types indicated therein are not exhaustive, and other variations may be possible. Annex C gives guidelines on the layout & configuration of staging. The configuration for economy does depend upon method of construction, number of tanks in a contract, number of repetition of formwork and experience of construction, and hence can not be governed by general rules. Most optimization studies do not consider the parameters influenced by construction and hence results have limited applications. 8.1 Before taking up designs, the designer should decide the most suitable configuration of the tank and staging. 8.2 At top of staging, container shall be connected to it so as to prevent relative horizontal & vertical movements between member at top of staging and the container. The connection must be designed to withstand the design forces to which it may be subjected, and more specifically for tension and bending. For container in reinforced concrete, monolithic connection between members of container and staging are preferred. In case container is not of concrete, there should be arrangement for safe and efficient load transfer from container to staging including occasional uplift (due to horizontal loads). 8.3 In case of framed staging, all members carrying vertical loads shall be tied together at top as well as at bottom of staging. Staging top connected monolithically to container will not require additional tie members. Bottoms of columns will be considered as connected if connected by (a) foundation beam or strip foundation, (b) connected by braces such that the clear distance between top of structural foundation and bottom of brace shall not be more than three times the size of column or pedestal in this height (also see 14.5.2). Page 9 of 44
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9
STABILITY OF STRUCTURE
Stability of the structure shall be checked as per following provisions. Also reference may be made to the relevant provisions in IS 1904 such as clause 17. 9.1 The stable equilibrium of a structure as a whole against overturning shall be ensured so that the restoring moment shall be not less than the sum of 1.2 times the maximum overturning moment due to the characteristic dead load, and 1.4 times the maximum overturning moment due to the characteristic imposed loads, wind or seismic loads. In cases where dead load provides the restoring moment, only 0.9 times the characteristic dead load shall be considered. Restoring moment due to imposed loads shall be ignored. 9.2 During construction and service, foundation area, anchorages or counterweights (if required) shall be such that static equilibrium should be maintained, even if overturning moment is one and half times. This also amounts to a load combination [(1.2 or 0.9) DL + 1.5 WL]. See also clause 17.2 of IS 1904. Normally over turning check will be critical with (a) DL (no IL & FL) & wind load, and (b) DL + FL (no IL) & seismic load. Under the load combination for stability check, the maximum bearing pressure on soil shall not exceed the ultimate bearing capacity of foundation strata. 9.3 Sliding The structure shall have a safety factor against sliding of not less than 1.4 under the most adverse combinations of the applied characteristic forces. In these cases only 0.9 times the characteristic dead load shall be taken into account. See also clause 17.1.1 of IS 1904. 9.4 Probable Variation in Dead Load To ensure stability at all times (& as in 9.1 & 9.2), account shall be taken of probable variations in dead load and liquid load during construction, repair or other temporary measures. Provisional dead load may be neglected, if DL helps in stabilizing. Wind and seismic loading can be treated as overturning or de-stabilizing loads. 9.5 Moment Connection In designing the framework of staging, provisions shall be made by designing adequate moment connections or by a system of bracings to effectively transmit all the horizontal forces to the foundations. All junctions of columns and braces shall be designed and detailed so as to avoid failures within junctions. 9.6 Lateral Sway Under design wind load or designed seismic load the lateral sway at the top should not exceed Ht/500, where Ht is the total height of the tower (including container) Page 10 of 44
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measured from top of structural foundation. For seismic loading, provisions in IS 1893 (Part 2) shall also be applicable. For unusual configuration of staging, the loss of elastic stability should adequately be studied. 10 LOADS In structural design, account shall be taken of the dead, imposed and wind loads and forces such as those caused by earthquake, and effects due to shrinkage, creep, temperature, etc, where applicable. Liquid (FL or water load/pressure) do not fall in the classification either as DL or IL. 10.1 Dead Loads (DL) Dead loads can be calculated on the basis of unit weights taken in accordance with IS 875 (Part 1). Unless more accurate calculations are warranted, the unit weight of reinforced concrete made with sand and gravel or crushed natural stone aggregate may be taken as 25000 N/m³. Loads due to finishes, lining in tank, plaster, piping, parapet, railing, staircases etc. should also be considered. For concrete in contact with aqueous liquid, its wet density shall be considered. Wet density of concrete for members retaining aqueous liquids, should be determined, and in absence of an appropriate value wet density of reinforced concrete can be taken as 25600 N/m³. 10.1.1 Part of dead loads may be provisional dead load (DLp), which may or may not be considered for the design of a particular member of the structure under different load combinations. [Example – provision of a wall load on a beam]. Some design forces at sections of the beam may be more critical if the provisional wall load is not considered along with the WL combination. For design of particular member, in the load combinations both with and without provisional dead load should be considered. 10.2 Liquid Load (FL) The effect or weight or pressure of the liquid/ fluid/ water shall be considered for the design of staging. FL should account for the actual density of the contained liquid. Density of water can be taken as 9810 N/m³. Aqueous solutions or suspensions can have higher densities. In some cases deposited silt, accumulated sludge, lime, etc will add to the load. Liquid load includes dead storage wherever applicable. In any combination, FL may be accounted at zero or partial liquid load or full liquid load as may make the combination more critical. The arrangement of FL should be such as to cause the most critical effects. The term liquid load also includes the effect of liquid pressure. 10.2.1 Occasionally liquid may rise above WTL (or FSL). A small rise will result, while liquid is overflowing. For over flow to match the rate of incoming liquid, the heading of liquid above WTL is usually of the order of 20 to 50 mm. Such a heading Page 11 of 44
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of liquid can be neglected for load combination. However for the rare event of overflow blocked, the liquid level can rise can rise above WTL, to a level controlled by alternate path of overflow, and such a rise of level can be substantial. While accounting FL, it should be the total quantity of liquid assumed up to the following levels: i) Working top liquid level (WTL or FSL) including dead storage. ii) Level under maximum overflow rate or to maximum top level (MTL) to which liquid can rise assuming the normal liquid outlet or overflow provision are blocked. In limit state of collapse for load combination without wind or seismic, FL will be considered up to MTL. For all other load combinations (in limit state of collapse and in limit state of serviceability), the liquid load (FSL) shall be accounted up to working top liquid level (i.e. FSL). 10.3 Imposed Loads (IL) Imposed loads like live loads shall be in accordance with IS 875 (Part 2). Snow loads shall be in accordance with IS 875 (Part 4). 10.3.1 Storage or piling of material or sustained load over long periods, and which may not be permanent, and are called as storage imposed load (ILs). Imposed load may also be due to processing, or provisional/ operating equipment and its impact allowance (ILp). 10.4 Wind load (WL) Wind load shall be estimated in accordance with IS 875 (Part 3). Load combinations shall take in to account both the tank empty and tank full conditions. The worst combination of the load on account of above shall be considered while working out the force action and the stresses. Wind and seismic loads shall not be assumed to act together. 10.4.1 Wind load shall be accounted as pseudo-static wind force as per section 5 & 6 of IS 875 (Part 3). The tower can be divided into different height zones and the wind pressure and resultant force are calculated for each of these zones. 10.4.1.1 While force coefficients (Cf) are estimated as per IS 875 (Part 3), for the members the effective values of Cf shall not be less than the following: Cylindrical wall – 0.50, Circular column – 0.80, Braces – 1.20 , Rib of beams attached to slab 1.2, 10.4.2 If specially required or mutually agreed between the parties, the wind load can be estimated by gust factor method [as per 8 of IS 875 (Part 3)]. 10.4.3 For very flexible and slender staging, if specially required or mutually agreed between the parties, the wind load can be estimated by gust factor method [as per 8 of IS 875 (Part 3)/and or specialists/it may be required]. Page 12 of 44
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10.5
Seismic Forces (WL)
For seismic load both tank empty and tank full conditions shall be considered as per IS 1893. Wherever critical, the effect of surge due to wave formation of liquid should be considered. Effect of sloshing or convective mass of liquid should be considered for design of staging. Both impulsive and convective effects shall be considered simultaneously as per the treatment referred in 10.5.1 or 10.5.3. 10.5.1 In dynamic analysis the mass of liquid should be considered separately as convective mass and impulsive mass. For earthquake analysis, the liquid tower shall be idealized by two-mass model. The impulsive mass of liquid, with the mass of container and the equivalent mass of the staging together shall be accounted as a mass. The convective mass of liquid shall be separately accounted as second mass. Refer IS 1893 (Part 2) for details. 10.5.1.1 The two mass model is technically more appropriate, and in most cases also gives an economical design of staging. 10.5.2 For design of staging of small tanks having maximum horizontal spread of liquid less than 15m, at the option of designer, simplification by considering one mass model wherein total liquid is treated as impulsive mass only is acceptable. 10.5.3 The seismic load on the staging and its analysis shall be in accordance with IS 1893 (Part 1) and IS 1893 (Part 2) (being published). 10.5.3.1 Till IS 1893 (Part 2) is published, “Codal provisions on seismic analysis of liquid storage tanks : a review”, Report no. IITK-GSDMA-EQ-04-V1.0, Indian Institute of Technology. Kanpur may be referred. However the response reduction factor (R) should be taken as below: a) Framed staging conforming as SMRF - 2.0 to 3.0 . (Note: Guidance given in Annex D for types and shapes of staging for range of R). b) Framed staging OMRF - 1.5 to 2.5 ; c) RCC shaft with reinforcement on each face (and conform to ductile detailing as per clause 9 of IS 13920) - 2.0 ; d) RCC shaft with reinforcement in middle - 1.5 10.5.3.2 Alternately refer 1893 (Part 1):2002 in conjunction with IS 1893:1984 may be used. However R shall be as given above. 10.5.4 Seismic base shear shall be estimated for a load combination of (1DL + 1 FL + 0 IL + 1 ILs + 0.7 ILp) for load combination 3 in table 1. This base shear shall be multiplied by an appropriate partial load factor for a load combination. For load combination 2 in table 1, no FL & no IL will be accounted. 10.5.4.1 If imposed loads are other than live loads on roof, and of nature like a process or operations or equipment (ILp), an appropriate part of such ILp excluding impact allowance shall be accounted for estimating base shear in 10.5.4.
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10.5.4.2 Seismic base shear shall not be less than 1% of the gravity loads. 10.5.4.3 Horizontal seismic force and vertical seismic effect shall be assumed to act simultaneously. If the tank or staging do not have over hanging or cantilever members, the effect of vertical seismic force can be neglected for tanks in zone II & III. 10.6 Blast Load or Vibration effect Forces– Design shall be checked for the forces induced due to excitation causing vibration and impact, by blast action (see IS 6922) as experienced in mines, collaries and in the close proximity of railway tracks, etc. or explosion (IS 4991). This load shall be assumed not to act simultaneously with wind or seismic, which gives critical actions in a member of structure. Note 1 – In most cases the effect of vibration or blast due to the charge normally permitted per delay, may be less significant than the seismic consideration. Note 2 – The structure will be designed for the explosion only if required under a contract as specification of owner by specifying the probable charge and its distance. Note 3 – For tanks located near mines, in addition to vibration forces, effect of mining subsidence could also be given due consideration, if the necessary data from experts is given to the designer. Refer clause 5.3.1 of IS 1904. 10.6.1 For design against explosion the survival of staging shall be checked for condition of loss of one column or a significant portion of the shaft staging. This design condition will require substantial increase in the cost of staging. 10.6.2 The design for blast or explosion shall be done, as mutually agreed between the relevant parties. 10.7 Construction loads Temporary loads resulting from construction activity should be considered in design of structural components required to support construction loads. 10.8 The structural effects of temperature variation, temperature gradient, shrinkage of concrete together with creep, with their restraining effects are usually not significant, and permitted to be neglected. In situation where designer feels that these effects may induce significant stresses and affect the safety, the same may be evaluated. 11 LOAD COMBINATIONS Load Combinations will be as below. (see Commentary, E-2). 11.1 For limit state design the partial load factors for load combinations shall normally be as given in Table 1. Any additional load combination may be mutually decided between the parties concerned.
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11.2 Table 1 Load Combination & Load Factors Load
1 2 2a 3
Combination 1 DL +FL + IL DL + WL DL + WL DL+FL+IL+WL
Limit State of Collapse DL 2 1.5 1.5 0.9 1.2
FL 3 1.5 0 0 1.2
IL 4 1.5 0 0 1.2
WL 5 0 1.5 1.5 1.2
DL 6 1.0 1.0
Limit State of Serviceability FL IL WL 7 8 9 1.0 1.0 0 0 0 0.8
1.0
0.7
0
0.3
[Under serviceability limit state, combination 2 & 3 can be deleted as well.] [Under ultimate limit state, combination 2 can be deleted as well.] Note 1. For any combination, the load factor for liquid load (or partially filled FL) may also reduce if the reduced value is expected to give more critical design action at a section of a member. Liquid load can be present in part i.e. may vary from zero (tank empty) to any specified value (say 1 or 1.2 or 1.5) in a combination. Similar is the situation of earth load (/pressure) in load combinations. Note 2. Base shear (for seismic) shall be worked out for a combination (1.0 DL + 1.0 FL + 0.0 IL + 1 ILs + 0.7 ILp) and this base shear be multiplied by the load factor specified for seismic load.
11.1.1 For load combination with wind or seismic, the columns and braces shall also be checked by limit state design method with P-δ effect. In working stress design method, structure should be designed for liquid up to MTL (above FSL) for combination without WL. For resistance to crack, check liquid up to WTL (/FSL) may only be considered. For combination with WL, structure should be designed for liquid up to WTL (/FSL). For load combination with wind or seismic, the allowable stress can be exceeded by 33% in concrete & steel. (see Annex E-2 for Commentary) 12
ANALYSIS
12.1 General Force actions (i.e. bending moments, torsion, shear forces, direct forces) in the components of structure shall be adequately analyzed in accordance with principles of mechanics, recognized methods of design and sound engineering practice. In particular, adequate consideration shall be given to the effects of monolithic construction in assessment of member forces. All the provisions on analysis in IS 456 shall be applicable, unless modified or overruled by provisions in this standard. For analysis of staging, some guidelines on structural modeling are given in 14.1. 12.1.1 The designer should correctly estimate the loads and statical equilibrium of structure particularly in regard to overturning of overhanging members. The design should be based on the worst possible combination of force actions, arising from Page 15 of 44
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vertical and horizontal loads acting in any direction when the tank is full as well as empty. 12.2 For the analysis of frame, including P-δ effect, modulus of elasticity of concrete will be taken as per IS 456 clause 6.2.3.1. 12.3.1 For P-δ effect, refer 39.7 of IS 456. 12.3.2 As an option for not taking in to account the effect of deflection (P-δ effect), the provisions of clause 39.7.1 of IS 456 shall apply if conditions in 14.2.1 are fulfilled. 12.3 Simplified analysis as given in IS 456 clause 22.4.2 and 22.5 shall not be applicable. 12.4 For seismic design, eccentricity is the distance between center of mass and center of rigidity measured in a horizontal plane. For tank and staging symmetrical about two axis in plan, the eccentricity will be assumed as negligible. In case the structure has an eccentricity, same shall be accounted without magnification, in the dynamic analysis of staging. The effect of vertical pipe assemblies on eccentricity can be neglected. 13
DESIGN BASIS OF DESIGN FOR REINFORCED CONCRETE MEMBERS
Design is the process in which appropriate size of member is arrived at and adequate reinforcement is estimated and detailed, such that all the checks of serviceability, safety and durability over service life satisfy an appropriate level of probability and acceptability. Analysis is the process in which by appropriate method, force actions at various parts of members are calculated under the action of loads, environmental effects and material characteristics. 13.1 Staging and other reinforced concrete members including foundation shall be designed by limit state method in accordance with the requirements of IS 456. 13.1.1 Alternately staging and foundations can be designed by working stress method, with the check as required in 13.1.2. 13.1.2 Columns shall be checked by limit state method also, even if design of staging is done by working stress design method. 13.1.3 For members of foundation, under service load condition, the stress in steel shall not be more than fy/2. This is applicable both for limit state design & working stress design. 14 FRAMED STAGING Framed staging shall consist of column and braces. Frame coupled with shear wall can also be provided. In case of dual system, horizontal shear shared by columns will be determined by relative stiffness of columns and shear wall. However, columns shall be designed for a minimum horizontal shear not less than or equal to 1% of the vertical/gravity load on columns, both for framed staging and dual system. Page 16 of 44
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14.1
Structural Modeling
It should be noted that simplifications in modeling may affect the design force actions in members near and also away from location of simplification. Some examples are as below. a) Columns at top of staging (at junction with container member) may be assumed as rotationally fixed. Such assumption reduces the design moments in the braces at a level just below container (i.e., top most brace level). Alternatively, if stiffness of container member are underestimated (say container floor beams are considered only rectangular, neglecting stiffness contribution of slab or dome) the column moments at the top junction will be under estimated and the moment in brace below will be over estimated. b) Normally it is permissible to assume the base of column fixed at the level of top of structural foundation, for the analysis of staging. Such simplification underestimates the moments in the brace just above foundations. Hence design moment in first level brace (near to GL or nearly plinth level), should be suitably enhanced. In the absence of an analysis for the possible increased moment in brace, moment enhancement may be taken as 30%. 14.1.1 For analysis and design, the frame along the center line of members should be considered and length of member shall be the length between two ends as points at junctions with other members. The junctions of column and brace (/beam) have finite size. The junction can be assumed rigid or rigidity factor for junction can be reduced to a value of 0.5 to 1.0. In most cases the width of brace shall be smaller than width of column, and in such cases brace can be designed for section at face of junction. Design column moments shall be at, top face of column pedestal above foundation, bottom face of container member (like floor beam or wall), and middle of junction at brace junctions. 14.1.2 Provision of stair (or staircase) or some other feature may provide eccentricity between center of mass and center of rigidity (or stiffness) for a dynamic analysis. Configuration of staging should be symmetrical along two mutually perpendicular directions to avoid eccentricity behaviour. Such effect will be significant if staircase is provided as a tower with more than one column, which is connected to staging of tank. In such cases eccentricity of mass and stiffness shall be accounted in the analysis of staging. Where staircase is on a single column, its effect to cause eccentricity will be small and may be neglected. 14.2
P-δ Effect
Staging consisting of columns & braces must be designed for P-δ effect. In this standard wherever “detailed P-δ analysis” is specified, it means a second order analysis accounting the effects of deflection. The simplified calculation of additional moments (as in 14.2.1) does not constitute a “detailed P-δ analysis”. Page 17 of 44
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14.2.1 For staging design, the requirement of “P-δ analysis” can be fulfilled by estimation of additional moments in columns for design, as per IS 456 clause 39.7.1. Such additional moments are permitted in lieu of detailed P-δ analysis, if the following conditions are satisfied, or else detailed P-δ analysis shall be carried out. a) Staging height is less than 20m. b) Storey height of column is within limit specified in 14.3.6.1. c) Brace size is larger than a requirement given in 14.4.3 (iii). d) The elastically computed first order lateral defection of any storey is not more than 625th (i.e. 0.16%) of the storey height. For calculating horizontal deflections (with P-δ effect), modulus of elasticity of concrete shall be as per 6.2.3.1 of IS 456. No correction for creep is necessary. 14.3 Columns 14.3.1 Forces and Moments on Columns The entire load on tanks shall be considered to be transferred to the columns in the manner in which the floor of the tank contributes to each column. The effects of continuity of the beams and wall at the top of the columns, if any shall be accounted for in calculating the reactions on columns. For continuity effect, proper stiffness of members meeting at junctions shall be accounted. In addition to tank load, force actions (axial forces, bending moments, etc) due to wind, earthquake or vibration shall be considered. 14.3.2 All columns shall be designed for minimum eccentricity, equal to the unsupported (i.e.) length of column/500 plus lateral dimensions/30, subject to a minimum of 20 mm. It is sufficient to ensure that eccentricity exceeds the minimum about one axis at a time. For deign, bending moment shall not be less than the product of most critical (maximum) load and the minimum eccentricity specified here. In limit state design, the load will be the maximum factored load. 14.3.3 Horizontal Loads Forces and moments resulting from horizontal loads may be calculated for the critical direction and used in the design of the structure. Analysis may be done by any of the accepted methods (like moment distribution, stiffness matrix, etc.) considering the staging as space frame. 14.3.3.1 Horizontal loads shall act on all parts of the tank as well as the staging. Axial forces in columns, due to horizontal loads can be calculated by equating the moments due to all horizontal forces above the level of considerations to the restraining moment offered by axial forces in columns, unless frame is analyzed as space frame. 14.3.3.2 Due to horizontal load, bending moment in a column shall be critical (maximum), if in plan the column lies on the bending axis of staging as a whole, or the column is nearest to bending axis. This criterion will govern the direction of horizontal force with respect to column position for analysis. Page 18 of 44
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14.3.3.3 Due to horizontal load, additional axial load in a column shall be maximum, if in plan the column is at maximum distance from the bending axis of staging as a whole. 14.3.4 Design of columns shall be governed by following guidelines. 14.3.4.1 For column size less than 500 mm, the strength capacity of column shall be reduced by multiplying by the ratio of column size (diameter or smaller size of section) in mm to 500mm. In no case column shall be less than 300 mm size. 14.3.4.2 The columns inside the container and connected to the container such that all the horizontal forces (>99.5%) are resisted by the walls of container or the column is a non sway column, the size of such columns shall not be less than 200 mm. For such columns reduction in strength capacity shall be the ratio of column size (diameter or smaller size of section in mm) to 300mm. 14.3.5 At any junction of column with braces, the moment of resistance of column sections above and below, considered in any vertical plane shall not be less than smaller of the following: a) Moment of resistance of braces resolved in the vertical plane. b) 1.4 times the design moments in braces, resolved in the plane. Check can be carried out in the plane of the brace considered. The above test is applied on designed section by limit state design, and is to avoid possible plastic hinge mechanism in columns. 14.3.6 Storey Drift Under maximum design horizontal wind or seismic load, for any storey of column the sway shall not be more than 0.20% of storey height (i.e. height/500). This permissible sway will also include P-δ effect. This limit of sway can be exceeded if Pδ analysis is done with δ enhanced by 1.3 times. 14.3.6.1 In lieu of detailed P-δ analysis, additional moments may be estimated if following is satisfied. The storey height of column shall be not more than 10 times the size of column (diameter or smaller size of section). If in a staging, columns of different sizes are present, the storey height shall not be more than 12 times the smallest size of column. Note: This may be avoided if detailed P-δ analysis is carried out. 14.3.6.2 To reduce storey drift, the stiffness of column and/or brace can be increased, by increasing the grade of concrete or by increase in sizes of members. 14.3.7 For economy in material cost it is advisable to have smaller spacing of columns (say 3 to 4.5 m c/c). However for over all construction economy (due to less number of members) higher spacing may be about 5 to 8 m can be selected.
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14.4
Braces
Each column shall be connected by minimum two braces, each of which shall be in two separate vertical planes. As far as possible these braces shall make an angle 60 to 120° between them. In case all columns are on a circle (say for Intz tank), the angle between the braces if exceeds 135° the response reduction factor shall be reduced; and if exceeds 150° detailed P-δ analysis will be necessary (and also for seismic design modal analysis shall be done, accounting. A column need not be connected to all the columns in its vicinity. 14.4.1 For staging height above foundation to container bottom, greater than 16 times the column size, the column shall be rigidly connected by horizontal bracings suitably spaced at intermediate levels. 14.4.2 Bending moments in horizontal brace due to horizontal loads shall be calculated when horizontal force on staging is acting in a critical direction which is parallel to the brace. Moment in a brace will be critical while horizontal load is acting along the vertical plane contained by longitudinal axis of the brace or a plane parallel to it. The moments in braces shall be the sum of moments in the upper and lower columns at the joint resolved in the direction of horizontal braces. 14.4.3 Analysis and design of braces shall be governed by following guidelines. i) Width of braces shall be not less than 1/25th of the clear distance between column or other crossing brace. For brace with a flange, the width of braces shall be not less than 1/36th of the clear distance. For brace having flange on both faces (top & bottom) width restriction (as a ratio of length) shall not apply. ii) For rectangular section of brace the width to depth ratio shall not be less than 0.3 However for economy, this ratio should not be much higher. iii) If section of brace is not conforming to the any one requirement given below, the detailed P-δ analysis should be done. a) The percentage of concrete in braces should not be less than 40% of total concrete of staging b) Depth of any braces shall not be less than half the size of column. Middle braces are other than those just above foundation (i.e. GL brace), and the top brace (just below container). Depth of middle brace shall be not less than ¾th of column size. c) Alternately cross-sectional area of middle brace should not be less than 44% of average column section and for top & bottom brace 30% of column section. (iv) Brace width should be minimum 200 mm or more if required by constructability. For better constructability it is advisable to have one 80 mm gap between longitudinal bars to facilitate concrete pouring and vibration by immersion vibrator. For convenience in construction, for all braces in a staging a standardized width of brace may be adopted.
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14.4.4 Moments and shears arising from local vertical loading, if any, shall be accounted in the design. 14.4.5 All ground braces or braces just above foundation shall be designed for a minimum direct tension equal to the one fifth of base shear in the column to which it connects. Such tension will be in addition to the design force actions (including moments) on the brace. 14.4.6 For staging in seismic zones IV & V (or where design seismic coefficient exceeds 0.05) or where basic wind speed is 50m/sec or more, twin diagonal vertical bracing of steel or RCC in addition to the horizontal bracing may be provided. The typical sketch of diagonal vertical bracing is shown in Fig. 6. 14.5
Foundations
Foundations shall comply with the requirements of IS 1904. For staging with columns on a circle, requirements of towers & silos shall be complied with. For framed staging, requirements of RCC framed structure shall also be complied with. 14.5.1 Individual footings may be provided for columns designed as per requirements of IS 456. Combined footing with or without tie beam, or strip foundation may be provided where required. Mat foundation or raft foundation in accordance with IS 2950 may be provided. Ring foundations may comply with IS 11089, however design forces should be verified by equations given in other documents. Alternately other established equation or the method of finite element analysis can also be used. 14.5.2 All columns shall be tied together above foundation level and near ground by a structural member such as braces. As far as possible such brace shall be partly or fully with in ground level except if brace is just at top of foundation. Such situation may occur if foundation depth is small. Clear height between foundation top and such a tie shall not be more than three times the size of pedestal or column as applicable. Alternate to such a ground brace above foundation, continuous strip (or annular strip) foundation, mat or raft foundations should be provided. 14.5.3 The foundation shall be so proportioned that under vertical loads of tower (with tank full as well as empty) and effects of horizontal forces, the pressure on the soil is within the net allowable bearing capacity. 14.5.3.1 From tests, gross bearing capacity can be arrived at. Safe bearing capacity will be obtained by applying a factor of safety between 2 to 3. Factor of safety may be higher for individual footings and will also depend upon method of testing and uniformity of strata. Allowable bearing capacity shall be arrived at from permissible settlement considerations, but it shall not be more than the safe bearing capacity. Net capacity indicates the capacity at a founding depth in addition to the existing burden of soil (i.e. weight of existing soil at founding level due to height from founding level to GL). 14.5.3.2 In case of load combination with wind or seismic forces, enhancement of allowable bearing capacity shall be permitted as per the relevant standard.
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14.5.3.3 At the contact plane of founding stratum and structural foundation, tension (as no contact) if developed shall be small. Tension will be considered as small if it is not more than one fourth of the maximum compression for load combinations with maximum wind or maximum seismic. Higher tension can be permitted for foundations on rock. 14.5.3.4 The tension check (as in 14.5.3.1) shall be applied both for tank full and tank empty cases. For load combinations without wind or seismic such tensions can not be permitted, except as in 14.5.4. 14.5.3.5 In all cases neglecting tension in contact, redistribution of contact stress shall be worked out, and thus computed maximum compression will be governed by net allowable bearing capacity of founding stratum, and such condition shall be taken for strength design of foundation. 14.5.4 More tension (than dealt in 14.5.3.1) causing loss of contact with the founding stratum under foundation can be allowed if the net allowable bearing capacity of strata is very high (i.e. founding strata soft rock or rock), and the foundation is specifically checked for stability against overturning as per IS 456 clause 20.1 & 20.1.1. 14.5.5 Columns can be provided with pile cap and pile foundations. Group of pile shall be designed such that a tolerance for error in placing any pile up to 10 cm should be catered. Piles shall be designed for the moments and shear due to horizontal loads. In case of single pile proposed under a column more rigorous analysis of staging together with piles for horizontal loads and eccentricity of pile due to construction tolerance shall be carried out. All pile caps shall be connected by braces which are within ground level. 14.6 Stair-Case Access to the tank shall be provided by means of ladder (in steel / aluminum / RCC or any other suitable material), stairs with landings adequately tied to the staging. In case of vertical ladders safety cages shall be provided for storey height of stair exceeding 6 m. In such cases, cages shall start at the level of 3 m. Spiral staircase carried by a single column may be provided. Stair if carried by columns other than the columns of staging shall be braced with minimum two columns of the main tower (or at two places on staging minimum 2m apart). 14.6.1 For small tanks ladder may be preferable rather RCC stair or spiral stair, because these RCC members can impart significant eccentricity (center of mass to center of stiffness) to be considered in dynamic analysis of tank. For staging having total cross-sectional area of columns more than 8 times the cross-sectional area of columns of stair, the eccentricity due to spiral staircase can be assumed to be negligible. In case of single spiral staircase column the eccentricity effect can be assumed to be negligible. 14.6.2 For staging having total cross-sectional area of columns more than 0.8m², the eccentricity due to spiral staircase can be assumed to be negligible.
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14.7 In some cases provision of a room for office or storage may be made under the staging. In such case the load of masonry wall is to be carried by ground level braces, and the roof (RCC slab or sheeting) is to be carried by the brace level above GL. For seismic analysis, as a simplification the equivalent weight of such masonry & roof will be added to the DL of container. Equivalent load could be as below : W emr = ( W m × h m ³ + W r × h r ³ ) / h c ³ Where, W emr = Load equivalent to the load of masonry & roof of ground floor room, W m = dead load of masonry, W r = dead load of roof, h m = height of CG of masonry from base of staging (i.e. foundation top), h r = height of CG of roof from base of staging (i.e. foundation top), h c = height of CG of container from base of staging (i.e. foundation top). 15
SHAFT TYPE STAGING
15.1 The tower may be in the form of single shaft circular or polygonal in plan and may be tapering. The area enclosed within the shafts may be used for providing the pipes, stairs, electrical control panels, valves, etc. Recommendations about cylindrical shell of revolution only is given in this standard. For other type of shafts, designer has to take appropriate decision. 15.2 Circular Shafts Staging (Circular Cylindrical Shell) 15.2.1 Minimum thickness of shaft shall be decided by the considerations of constructability which also depends on the height of formwork for one lift wall concrete. For shaft having horizontal and vertical steel near to both (inside & outside) faces of shaft, the minimum thickness will be the summation of cover on each face, two layers of steel bars on each face (total 4 layers) and the gap in middle governed by the method of compaction of concrete and the type & size of vibrator used. Smaller thickness can be provided for the shaft having vertical and horizontal steel at the middle of the thickness of shaft (i.e. having only one mesh of steel), however strength capacity of such shaft shall be lower. In no case shaft shall be less than 150 mm thick. Thickness of the shaft shall not be less than that from the guideline given below: a) For shafts with center line radius less than 4 m, t min = 150 + (R c - 2000)/40 mm b) For shafts with center line radius equal to or greater than 4 m diameter, t min = 200 + (R c – 4000) / 60 mm Where, R c is center line radius of shaft, in mm.
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Additional thickening of shaft shall be provided at top and bottom ends of shaft (i.e. at junctions with foundation and with container). This is required to account for secondary moments and eccentricities. Additional vertical and circumferential reinforcement at the ends of shaft on each face shall not be less than 0.2% of the thickness. 15.2.2
Reinforcement in Shell
The percentage of steel given in clause 15.2.2.1 & 15.2.2.2 is for deformed bars 415 grade. For any other grade of steel bars the minimum percentage of steel shall be inversely proportional to the grade of steel being provided. 15.2.2.1 Vertical reinforcement – The minimum vertical reinforcement shall be 0.25% of the horizontal section under consideration. Reinforcement on the exterior face shall be 50% or more of the minimum requirement. Reinforcement on the interior face shall not be less than 40% of the minimum requirement. Total on the faces to make up the specified reinforcement. Thin shaft may have one mesh of reinforcement at middle, meeting the requirement minimum steel. Strength capacity of shaft having only one mesh of reinforcement shall be suitably reduced. The diameter of longitudinal (vertical) bars shall be minimum 10 mm or higher size, and the maximum centre-to-centre distance of reinforcement in each layer shall neither exceed twice the thickness of shell nor exceed 450 mm c/c. 15.2.2.2 Circumferential reinforcement – The circumferential reinforcement shall not be less than 0.20% of the concrete area in vertical section under consideration subject to a minimum of 400 mm² per meter height. If the vertical reinforcement is provided in two layers, the circumferential reinforcement shall also be provided in two layers and minimum reinforcement specified above shall be divided nearly equally in each layer. The spacing of bars on a face shall not be more than 300 mm or the shell thickness whichever is less. For mesh of bars in two layers, circumferential reinforcement shall be placed near the faces of the shell with specified cover. Vertical bars will have concrete cover more by size of horizontal bars. Smaller size of horizontal bars are preferred, however spacing be not less than 80 mm c/c. 15.2.2.3 The detailing of shaft at the opening shall take into consideration the stress concentration at corner of opening, and provision of effective continuity in the reinforcement above, at the sides and below the opening. The requirement of extra reinforcement shall be designed based on the detailed analysis of shaft in the region of opening. Such analysis can be by finite element or any other suitable method. In absence of such design extra reinforcement shall be provided as given below in (a), (b) & (c). a) At top and bottom of each opening, additional reinforcement shall be placed having an area at least equal to 70% of the area of the calculated design circumferential reinforcement interrupted by the opening, and shall Page 24 of 44
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extend beyond the opening to a sufficient distance as development length of bar plus 300 mm. This steel shall be placed within a height not exceeding the thickness of concrete at opening. b) At both sides of each opening, additional vertical reinforcement shall be placed having an area at least equal to 60% of the area of the established design vertical reinforcement interrupted by the opening, and shall extend beyond the opening to development length. Size of extra vertical bars shall be not less than 12 mm. If the vertical height of opening is more than 8 m vertical bars at the edge of opening shall have lateral ties as per 26.5.3.2 of IS 456. c) At the corner of rectangular opening, extra diagonal reinforcement with cross-sectional area not less than 30% of the total extra horizontal & vertical required, shall be placed at each corner of the opening. The diagonal bars shall extend from corner by development length +300 mm. For opening less than 0.6 m width (horizontally) the diagonal reinforcement may be half the value recommended above. d) For openings smaller than 2 times the thickness of shaft, only nominal extra vertical and horizontal steel can be provided. 15.2.3 The minimum clear concrete cover over the horizontal reinforcement shall be 35 mm for the outer face and 25 mm for the inner face of the shaft. More clear cover could be specified for the exposure condition being considered, as per requirement of IS 456. 15.3 Analysis for Circular Shaft Staging 15.3.1 BM due to Ovalling : Under wind load due to ovalling of horizontal slice of shaft as ring, bending moment will develop causing horizontal tension at one face and compression at other face of shaft. In absence of analysis, the ring moment can be assumed as below : M oe = M oi = 0.33 W p × R c 2
in Nm/m height of shaft ,
Where M oe = BM due to ovalling causing tension at external face, M oi = BM due to ovalling causing tension at internal face, W p = Design wind pressure at the level under consideration in N/m2, and R c = Mean radius of ring of the concrete shell, in m. [Note : The coefficient 0.33 specified here may change from 0.27 to 0.375 and will depend upon the shape of wind pressure diagram on the horizontal slice of shaft. The coefficient may not be equal for M oe & M oi by linear elastic analysis. However in view of elasto-plastic behaviour of concrete a safe value is specified. ] Bending moment due to ovalling may be considered negligible at a diaphragm if provided at the ends of shaft (top at container and bottom at foundation). This Page 25 of 44
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moment may be considered to be varying linearly to the design value at a distance 3√(t R c ). Shaft may be deemed to be safe for the effect of hoop force and shear stress due to ovalling. 15.3.1.1 As long as flexural tension due to the ring moment at service load is within 0.25√fck , horizontal reinforcement need not be designed for the BM, and only minimum reinforcement can be provided either near the faces or at the middle of shaft thickness. If the flexural stress exceeds the limit, reinforcement shall be provided in two layers, ignoring the flexural tensile strength of concrete. Minimum horizontal steel as provided can be accounted to resist the ovalling BM. 15.3.2
Vertical Stress due to Horizontal (Wind/Seismic) load on Tower
For shaft as cylindrical shell, membrane stresses in vertical direction (meridional) shall be estimated. Membrane stresses in vertical direction (meridional) shall be the combination of stress due to gravity load and that due to cantilever bending of shaft as a whole. Vertical stress as calculated (in 15.3.3.1, 15.3.3.2, 15.3.3.3) shall be considered as direct compressive (membrane) stress in the shell. These equations are based on linear elastic behaviour. For limit state design method, results of these equations can also be used with appropriate load factors; alternately equations or design-aids derived for limit state design with assumptions as in 15.3.2.1, 15.3.2.2 & 15.3.2.3 below shall be used. For vertical stresses in shaft (as a wall), a section of shaft shall be considered which will have unit length (say 1m) along the centerline of shaft, and depth of cross section shall be the thickness of shaft. 15.3.2.1 For the purpose of design, total membrane stress should be considered as direct compression and not as bending compression. Bending stress in the shell may occur due to bending moment developed as a result of construction defects (such as out of alignment) & errors, differential wind pressure (gust) on the surface of shaft, local ovalling or distortion of shaft, etc. (See 15.3.2.5). For limit state design, the following assumption shall apply: a) For meridional stress, the maximum compressive strain in concrete as axial compression shall be limited to 0.002. For combined meridional compression and bending, the maximum compressive strain at the highly compressed extreme fiber in concrete when there is no tension on the section shall be 0.0035 minus 0.75 times the strain at the least compressed extreme fiber. 15.3.2.3 While calculating the resistance to vertical stress, the contribution of steel in compression shall be effective only if vertical steel bars are laterally tied conforming to all requirements (including transverse reinforcement) of IS 456 clause 26.5.3; or else the contribution of steel shall be neglected.
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15.3.3 Equations for Vertical Stress 15.3.3.1 The whole section is under compression, if : (from existing code, these equations are for working stress method. a) for annular sections e 1 Rc 2
. . . . . . . . . (1) In such cases the maximum vertical compressive stress in concrete is given by: W 2e 1 cv 2 Rc t Rc . . . . . . . . . (2) b) for annular section with one opening: 2 1 sin2 e 3 sin Rc 2 cos sin
. . . . . . . . . (3) than in such cases, the maximum vertical compressive stress is given by: e sin cos sin 2 W 1 Rc cv 1 2 sin 2 2 Rc t sin 2 2 . . . . . . . . . (4) where Moment in vertical plane at the section M under consideration in Nmm e W Total vertical load above section under consideration in N Rc = Mean radius of circular section under consideration in mm; t = thickness of shaft shell at section under consideration in mm; β = Half the angle subtended by the neutral axis as a chord on the circle of radius r, in degrees radians unless otherwise specified; and cv = vertical stress in concrete at center of shaft (point farthest from bending axis) in N/mm2. 15.3.3.2 If e/R c is greater than the corresponding right hand side of expressions (1) or (3) of 15.3.3.1, then α defining the position of neutral axis may be calculated from the general expression (5) by trial (see Fig. 7). Put β = 0, for annular section without opening, where α = one half the central angle subtended by neutral axis as a chord on the circle of radius R c , in degrees radians unless otherwise specified. e A Rc B . . . . . . . . . (5) where A= 1 2
1 2
(1- ρ) (α - sin α - cos α)
1 -2
(1- ρ + mp) ( β + sin β + cos β – 2 cos α sinβ) +
m π ρ ; and Page 27 of 44
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B = (1- ρ) ( sin α - α cos α) - (1- ρ + mρ) ( sin β - β cos α ) - m π ρ cos α where, m = modular ratio; and ρ = ratio of total area of effective vertical reinforcement to gross total area of concrete of shaft shell at section under consideration. Refer 15.3.2 (iii). 15.3.3.3 The maximum vertical compressive stress in concrete due to combined effect of vertical loads and lateral wind loads, cv may be calculated by: t cv cv' 1 2 Rc cos cos cos . . . . . . . . (6) where cos cos W 'cv 2 Rc t 1 sin cos 1 mp sin cos m cos . . . . . . . . (7) 15.3.3.4 Maximum vertical tension in tension (at center line of thickness) σ` sv = m σ` cv (1 + cos α) / (1 - cos α) and at extreme fiber σ tv = σ` cv (1 + cos α + 0.5 t/R c ) / (1 - cos α) 15.3.3.5 In addition to vertical membrane stress estimated, vertical bending stress will also occur in the shell. Vertical bending moment will develop due to local ovalling and bulging effect and also due to eccentricity (out of geometry for middle surface of shaft) achieved and variation in stiffness of shell in construction and few other irregularities. (see Annex E-3 for Commentary) 15.3.3.6 In addition to vertical membrane force shaft shall also be designed for a bending moment. In absence of a detailed analysis to asses vertical bending moment, it shall be assumed as due to an eccentricity ecc m . ecc m = Rc/250 mm & not less than 20 mm. Mv = ecc m σ cv t (in Nm/m) Where σ cv is maximum axial compression in N/mm², & t is thickness in mm. Note : For Limit State Design, eccentricity moment shall be multiplied by partial load factor 1.5 . 15.4
Reduction of Strength Capacity
In both methods of design (i.e. working stress & limit state) strength capacity of concrete will be reduced by multiplication factors as below for vertical stresses.
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15.4.1 If vertical steel bars conform to all requirements (including transverse reinforcement) of IS 456 clause 26.5.3, the capacity reduction factor C rd (for detailing) shall be 1.0, or else it will be 0.85. 15.4.2
Based on constructability capacity reduction factor C rt will be as below : t = thickness of shaft in mm, C rt shall not be more than 1.0 ;
t/400 ;
(a) Shaft having steel on both faces : C rt = t / 200 , for C rt ≤ 1.0 (b) Shaft having steel in one layer in middle , for 0.6, ≥ C rt ≥ 0.4, C rt = 0.15 +
15.4.3
Slenderness of wall expressed as C rs = 40 t / R c , not greater than 1.0 .
15.4.4
Capacity reduction factor C r = C rd × C rt × C rs
15.5 Working Stress Design Method (see Annex E-4 Commentary) 15.5.1 The compressive stress due to membrane action in concrete shall not exceed the following permissible stresses for various combination of loads. For combination of membrane and bending stress also the allowable stresses are given: Load combination a
DL + FL + IL
Allowable compressive stress Direct Compression+ben compression ding 0.25 f ck 0.33 (f ck - σ cm )
b
DL + WL (or EQ) {no FL}
0.25 f ck
0.33 (f ck - σ cm )
c
DL + FL + IL + WL (or EQ)
0.33 f ck
0.44 (f ck - σ cm )
Where σ cm = minimum extreme fiber stress (direct - bending) in compression (N/mm²). f ck = characteristic compressive cube strength of concrete in N/mm2. (see Annex E-5 Commentary) Reinforcement - The stresses in steel shall not exceed the following permissible stress for various combinations of loads.
a) b) c) d)
Load combination DL + FL + IL DL + WL (or EQ) {no FL} DL + FL + IL + WL (or EQ) Circumferential stress in steel due to ring moment only by WL/EQ
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Allowable tension 0.50 f y 0.50 f y 0.625 f y 0.50 f y
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Where f y = Characteristic strength (i.e. yield or proof stress) of steel in N/mm2. Note : 1. Circumferential stress in steel due to wind induced ring moment only shall be 0.50 f y 2. If local bending effect due to secondary deformations, errors and defects are estimated by carrying out detailed analysis, under worst combination of the loads the stress limit may taken as 0.67 f y . 15.6 Limit State Design Method For limit state design proper value of W (membrane forces) & BM (bending moments, etc.) after multiplying with the appropriate load factor shall be accounted. Load factors are specified in 11.1; for limit state of collapse these shall be multiplied by 1.1 times. Formulae give in 15.3.2.1, 15.3.2.2, 15.3.2.3 can be used to asses the vertical meridian force as axial load. Section of shaft is to be designed for axial load and bending moment. Capacity reduction factors as in 15.4 shall also be applicable. 15.7 Eccentricity of Container For shaft and foundation, eccentricity may occur if: (a) the tank is not concentric with the support shaft, (b) the support wall is out-of-plumb, or (c) the foundation tilts because of differential settlement. Shaft shall be designed for an eccentricity of container not less than the value given below. eg = 25 mm + 0.0025 * staging height Above eccentricity is minimum and can be assumed to include the allowance for angular distortion of foundation and eccentricity due to error in construction, etc. Additional bending moment at base of shaft shall be equal to that given below: =
(Vertical loads from container in a wind or seismic combination + half the DL of staging) * eg
This BM will be in addition to the BM due to horizontal load (wind/seismic). While this eccentricity effect is considered, classical P-δ can be neglected. As the above eccentricity is accounting many factors, it should not be taken as maximum tolerance in construction. Higher construction tolerance will induce significant local bending stresses in shaft, which will very much affect the critical buckling load. 15.8 Eccentricity due to Settlement The section of the shaft shall also be checked for stresses resulting from the possible differential settlement of foundation as per IS 1904. 15.9 Polygonal Shaft
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Polygonal shaft may be designed as a circular shaft considering equivalent radius equal to radius of corner for the middle surface of the shaft, if it is a regular polygon having length of each side not more than 12 times the thickness of shaft. For thinner and larger shafts detailed analysis for bending moments and stability shall be carried out. 15.10 Shaft should be provided with opening for door entry, and for ventilation and natural light. Vertical stiffeners may be provided on the sides of opening. 15.10 Foundation Provisions in 14.5 as relevant shall be applicable. 15.10.1 Type of foundation will be governed by the considerations of economy. Shaft can be provided with strip or annular mat (ring mat) or raft foundation. Full circular mat or raft foundations may also be designed. Slab with opening is viewed as ring or annular foundation. Provisions in 14.5.3 & 14.5.4 will also be applicable with additional provision in 15.9.3. Raft foundation can be designed taking account of modulus of sub grade reaction (i.e. stiffness of soil and relative stiffness of raft. Foundation slab can be designed assuming it as a mat. Reference can be made to IS 11089. However other authentic formulae are available in literature and one can use the same for design. Most equations available are for uniform thickness, however as per practice higher thickness may be provided near the faces of shaft. Such a deviation is normally permissible. 15.10.2 Annular foundation of shaft can be designed as radial strip with following assumption. If the additional pressure on foundation due to bending of tower is with in 30% of the pressure due to vertical loads only, this simplified method is applicable. Each radial strip will be treated as rigid element having center of gravity of its area at the radius of center of shaft. Following equations can be used. All units in N & m. This approach is applicable till ro/rc < 1.45 . In this simplification the circumferential moments are assumed negligible. Area of foundation = p (r o ² - r i ²) CG of radial strip at radius = (r o ² + r o r i + r i ²) / (1.5 (r o + r i ) = R c Outer projection from face of shaft will be at radius r c + t f /2 t o r o At outer face of shaft BM due to outer projection (in Nm/m) = p (r o - R c + t f /2) × [(r o + R c + t f /2)/6 - r o ²/{3(R c + t f /2)}] At outer face of shaft BM due to outer projection (in Nm/m) =
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p (R c – t f /2 – r i ) × [(r i + R c – t f /2)/6 – r i ²/{3(r c – t f /2)}] Where, p = uniform design pressure causing bending of foundation (N/m²), r o = outer radius of strip foundation (m), r i = inner radius of strip foundation (m), t f = thickness of shaft at top of foundation (m). 15.10.3 For a circular mat foundation maximum eccentricity up to 0.20 times the outer diameter of foundation can be permitted. However neglecting tensile stress in contact redistribution of stress shall be worked out, and thus the maximum compression will be governed by net allowable bearing capacity of founding stratum, and such condition shall be taken for strength design. 16
DETAILING
Detailing of reinforcement shall conform to the requirements of IS 456. For staging designed as special moment resisting frame (ductile frame) the detailing shall also conform to the requirements of IS 13920. 16.1.1 If the staging is designed as ordinary moment resisting frame, The column beam junction shall be provided with lateral ties such that steel is not less than 6φ at 75 mm c/c or 8φ at 120 mm c/c, for rectangular column 8φ at 85 mm c/c. 16.1.2 The column beam junction shall be enclosed in the rings at a spacing not exceeding 85 mm c/c irrespective of the frame being ordinary moment resisting frame or special moment resisting frame which is ductile. Where column size (width) is more than brace width, the column rings will continue in the junction; and if width of brace is more than lateral size of column, the rings of the brace shall continue through the junction. 16.2 The tensile steel shall be well anchored. At ‘T’ or ‘L’ junction the tension bars changes its direction by right angle within the junction. Middle of the curved portion of the bar should be assumed to develop critical stress, hence the anchor length shall be counted from the mid point of the curved portion of bar. 16.3 Minimum reinforcement ratio in brace at the support ends shall be not less than 0.25(√f ck )/f y times the product of width and effective depth, This minimum steel shall be on top face, and at bottom face minimum can be 20% lower. 16.4 Typical reinforcement details for column and braces are given in Figures. 16.5 Galvanic Corrosion – Dissimilar metals should be electrically isolated to prevent galvanic corrosion. 17 CONSTRUCTION REQUIREMENTS Construction shall be proper to achieve quality in-line with behaviour assumed in design. This is necessary, in view of collapses of elevated tanks noticed during testing. Suggestions are given below, which are not exhaustive. The designers and
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construction engineers are expected to have competence to take adequate measures to ensure required structural performance. Some requirements of minimum size, constructability and tolerance have been provided in other clauses of the standard. Additional requirements are given below. 17.1.1 In columns, construction joints shall be avoided in special confining zone (i.e., near brace-column junction). Otherwise higher confinement reinforcement shall be provided near construction joints to compensate for lower strength at construction joint. In the absence of any estimate and calculation for the same, one set of 12mm diameter stirrup shall be provided on either side of joint within 40 mm. For circular column, 10mm diameter stirrup can be used. Location of construction joint shall be marked specified on the drawings. The interface between old and new concrete at construction joint should be rough and should not have loose material, mortar layer, laitance, etc. 17.1.2 If all bars of a column are spliced simultaneously within confining zone without staggering, the lap length shall be enhanced by 40% over the lap length in tension, and lateral ties in form of spiral shall be spaced at not more than 85 mm centre to centre over the lap length and 100 mm beyond the lap on both ends of lap. 17.1.3 Overlap can be provided for all the bars at location outside the special confining zone in a column, and the lap length shall be increased by 40% over compression lap and lateral ties shall be not less than 8 mm in diameter provided at maximum 150 centre to centre over the lap length and 100 mm beyond the lap on both ends of lap. 17.1.4 For small height of column lifts, laps may be avoided . 17.1.5 Formwork at brace-column junction should be designed and fabricated. It shall be rigid enough to avoid bulging as well as prevent leakage of slurry from plastic concrete. 17.1.6 All brace column junctions shall be designed and detailed, so as to avoid failure within junction and allow the ductility in the whole frame. Tension within the junction shall be resisted by adequate reinforcement. The drawing shall show detailing at each typical junction. 17.2 Shaft Staging 17.2.1 The wall of shaft is subjected to large compressive forces and generally requires a high degree of accuracy with regard to shell tolerance. Properly designed climbing forms or jump forms with through ties with proper workmanship and checks can achieve the required tolerances. Vertical alignment should be controlled with precise optical or other instruments. Wall forms should be designed for the full concrete head equal to form height, to avoid overloading and excessive deflection that can occur when forms designed for less than the full head are accidentally overfilled.
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17.2.2 Vertical Alignment – The center point of shaft shall not vary from its vertical axis by more than 0.2% of shaft height. 17.2.3 Over any height of 1.6 m, wall of shaft shall not be out of plumb by more than 10 mm and over any 10m height not more than 25 mm. 17.2.4 Shaft diameter – The measured centerline radius of shaft at any location shall not vary from the specified radius by more than 10 mm plus 0.1 % of the specified radius. 17.2.5 Shaft thickness – The measured wall thickness shall not vary from the specified wall thickness by more than - 5 mm or +10 mm. 17.2.6 At construction joints in shaft additional circumferential bar on either side of joint within 50 mm distance shall be provided near the outer face of shaft. 17.2.7 Laps shall be staggered such that not more than one-third of vertical bars shall be spliced at any section. For circumferential bars, lap length shall be 1.4 times development length in tension. 17.2.8 If the height of opening in shaft is more than 12 times the thickness of shaft, the vertical edge of the shaft at opening will require a detailed check for the compressive stresses. At the edge of opening, stiffener may be provided, which may project on any one face of shaft. 17.2.9 Additional vertical dowel bars may be designed at the construction joint, if required. 17.2.10 At the construction joint, the interface surface of old and new concrete should be rough and should not have loose material, mortar later, laitance etc. Segregation and honey-comb should be avoided at and above the joint. All the joints should be grouted to compensate for the loss of strength and stiffness at the joint. 17.3 Selected excavated material shall be backfilled by placing and compacting in uniform horizontal layers. Method of compaction shall be such that any damage to the ground brace, columns or shaft is avoided, as well as the fill near these members surface is properly compacted. Similar precautions are required for plinth filling. 17.3.1 Site grading around the tank should provide positive drainage away from the tank to minimize percolation of water in to and prevent ponding of water in the foundation area. 18 MISCELLANEOUS ITEMS & APPURTENANCES 18.1 Elevated liquid tower shall have following facilities & appurtenances. 18.2 Access to top of tower and inside of tank shall be provided. Normally access is provided in three parts. First part is from ground level to the floor level of container or the bottom of wall of container. Second part of access is to roof of tank, and third part is from roof to floor of tank inside.
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18.3 At or near the ground level, arrangement shall be such that un-authorized persons should not have access for the safety of people as well as safety of tower and threat of pollution etc. 18.4 For first two parts steel ladder is commonly provided with landings at intermediate brace levels. Ladder shall be minimum 450 mm wide, the stringer may consist of 75mm ×10mm MS flat (or 65×65mm×6mm MS angle), steps of 2×16φ deformed bars at 300mm c/c. ladder may consist of other materials and have different design. 18.5 For medium and large tanks (say >250m³) usually spiral stair case in RCC is provided, utilizing precast RCC steps. Such spiral stair is supported on a single core column, and braced to the staging. For large tanks, an independent staircase tower of four columns, having dog-legged stair may be provided. The minimum radius of step should be 1.0m. Stair should have railing or parapet. 18.6 For going inside the tank, commonly ladder is provided. Steel ladder with anticorrosive coating is the common material. Alternately RCC ladder or stair may be provided. 18.7 The tower shall be provided with lightning arrestor at its top. Height of arrestor above the roof shall such that the whole area of tower in plan is covered within conical protection zone. To reduce height of arrestor multiple arrestors can be provided. The arrestors shall be connected by conductor to the earthing arrangement. The whole arrangement shall confirm to IS --- and the electrical rules. It shall be tested for the resistance as per rules. 18.8 The vertical assembly of pipelines should be adequately be supported by the members of staging. The vertical assembly of pipe shall have a proper foundation, which in most cases may consists of a concrete or masonry pedestal. 18.9 For shaft type staging, a door should be installed with lock and key arrangement. 18.10 Adequate number of openings for ventilation and day light should be provided. 18.11 A suitable mechanical or electrical arrangement for indicating the level of liquid in tank should be installed. This arrangement should such that it is convenient to read the level from a position at ground. 19 QUALITY MANAGEMENT IN CONSTRUCTION 19.1 Quality Assurance Manual: A quality assurance plan should be prepared, to verify that the construction conforms to the design requirements. It should include the following: (a) Construction stages at which inspection and testing are required, forms for recording inspections and testing, and the qualification of personnel performing such work; (b) Procedures for exercising control of the construction work, and the personnel exercising such control;
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(c) Methods and frequency of reporting, and the audit of quality reports.
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ANNEX A LIST OF REFERRED INDIAN STANDADS & Other documents IS 456:2000 IS 3370 Part 1 : 2009 IS 3370 Part 2 : 2009 IS 1893 : 1984 IS 1893 Part 1 : 2002 (under revision) IS 1893 Part 2 : 20** (under print) IS 875 Part 1 : 1987 IS 875 Part 2 : 1987 IS 875 Part 3 : 1987 IS 875 Part 4 : 1987 IS 875 Part 5 : 1987 IS 4926 : 2003 IS 1343 : 20** (under print) IS 800 : 2007 IS 1489 Part 1 IS 455 : IS 6922 IS 4991 IS 13920 (under revision), IS 1904 IS 2950 IS 11089
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ANNEX B TYPES OF STAGINGS B.1 Elevated tanks can be classified based on the liquid capacities in m³. Very small 10 to 80 m³, Small 80 to 250 m³, Medium 250 to 800 m³, Large 800 to 2300 m³, Very large above 2300m³. B.2 Elevated tanks as in plan, can be (a) square or rectangular, or (b) circular. B.2.1 Square and rectangular tanks are planed on square grid, may be some times on rectangular grid of columns. These shapes are adopted when owner’s requirement or availability of land space restricts the shape. Except for very small tank (<30m³), walls of rectangular tanks are costlier than the circular wall. For a given area of tank, circular wall requires least form work. B.2.2 Circular tanks can be planed on different column configurations. B.2.2.1 Columns on square grid : This is a most common configuration preferred due to over all economy. These tanks have cylindrical wall. The floor and roof of tank consist of slab beam system. Slab-beams of roof may be replaced by flat-slab system. The possible number of columns can be 4, 9, 12, 16, 21, 24, 32, 37, 44, 52 etc. for a tank. Of these more popular and convenient for construction are the configurations with 4, 12 & 24 columns. Spacing of columns is governed by span of slab and thus largely influences the economy. The spacing ranges from 3 to 4.5 m and increases with BC of foundation & also with height of staging. By using secondary beams bearing on primary spanning over columns, the span of slab can be reduced to half, and thus spacing of column may range from 4.5m to 7.5m. Small tanks have been constructed on 4 column staging and large tanks on 24 columns. B.2.2.1.1 In configurations of 12, 16, 24, 32, 44, 52 etc. It is possible to have number of braces at a level to be equal to number of columns (i.e. for 24 column staging brace at a level can also be 24). The braces are not continuous over all the spans, but on almost on alternate spans. This configuration makes the construction very economical and fast. However the dynamic behaviour of the staging changes in modal analysis. Hence a lower value response reduction factor (R say 3.5 rather 4) is recommended for such configurations. B.2.2.2 Circular tanks on columns placed on grid on radial and circumferential pattern. In this pattern the columns are placed on circles (2 or more) at different radius. Columns are connected by braces which are along radial and circumferential direction. The floor of the tank can consist of slab, and beam layout similar to braces. In middle (over the smallest circle) floor dome may be provided. The layout of roof is usually same as that of floor. These types of tanks may be provided for elevated units of treatment plants. These shapes are un-common now a days.
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B.2.2.2.1 Circular staging may have all columns on a circle only. The tank shape may be Intz or cylindrical with domed bottom. Very small (or small) tank may have slab bottom. These tanks can be constructed on shaft type (cylindrical shell) staging. (i) Intz tank requires sizeable depth of liquid in tank, and is usually costly due to high cost of formwork. Tanks with 12 or more columns on a circle are very sensitive to construction error (column out of plumb or radial distance varying), and also critical buckling load, because the angle between braces at column is very high (say >150°). Intz tanks are more costlier if bearing capacity of foundation is low. (ii) Domed bottom cylindrical tanks can be constructed for small tanks. But this configuration is usually costlier than the slab bottom cylindrical tank. The form work for the dome consumes lot of cost and time of construction. For small tanks shaft type staging is costlier compared to staging of columns. B.3 Recommendations for tank Capacity Range B.3.1 Tanks of capacity up to 50 m3 may be square in plan and supported on minimum four columns. B.3.2 For small tank (say up to 250 m³ capacity) staging with four columns is suitable. Intz tanks up to 500 m³ have been constructed on four columns. Cylindrical tanks with floor slab and beams having cantilever, have been constructed up to 400 m³ capacity, method is suitable for tanks up to 300 m³. B.3.3 For medium capacity (250 m3 to 800 m3) the tank may be square, rectangular, circular or Intz type tank. The number of columns adopted shall be decided based on the column spacing which normally lies between 3.0 and 4.5 m for economy. For circular, Intz or conical tanks, a shaft supporting structure may be provided. Conical tank on shaft staging is quite a costlier form, but usually chosen for its aesthetic appeal. B.3.4 Large tanks can be cylindrical or Intz shape. Cylindrical tank can be designed for a depth of liquid of 3m to 5m usually. Whereas, Intz tank will require 5m to 8m depth of liquid. Either if depth of liquid is low or if BC of founding soil is low, cylindrical tank gives a reasonable solution. In all cases cylindrical tank with floor slab and beam gives an economical solution compared to Intz tank. B.3.5 Besides the general shapes discussed in B2 and B3, unusual shapes, such as spherical or multicell may also be adopted depending upon the discretion of the designer. B.3.6 Different shapes of water towers with certain arrangements of bottom construction will be shown later in typical figures.
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ANNEX C STRUCTURAL CONFIGURATION OF MEMBERS C.1 Generally the shape of tank, configuration of members of tank and staging, span of main members, and layout of staging for economical design, will be governed by the functional requirements some of which are enumerated below. a) Capacity of tank (Q in m³); b) Height of staging; c) Maximum liquid depth (h) allowable by functional / hydraulic design; d) Net allowable bearing capacity (BC) of foundation strata and type of foundation suitable; e) Other site conditions. C.1.1 Average capacity (in m³) supported by each column [i.e., ratio of tank capacity (Q) to number (Nc) of column = Q/Nc] increases with capacity of tank, and also with height of staging. C.1.2 As the net allowable bearing capacity of foundation strata reduces (below 200 kN/m²), Q/Nc will also reduce for economy. C.1.3 The cost of liquid tower (container & staging) reduces as liquid depth in the tank can increase, if BC permits the increase in depth (see C.1.5). C.1.4 If inflow to the tank is by pumping, the operational cost reduces with liquid depth due to saving in energy. In such cases recurring energy cost will govern the depth of liquid, rather than the cost of structure. C.1.5 For low bearing capacity (BC i.e. net allowable bearing capacity) of founding soil, the depth of liquid (h) in tank has to be limited. For relating depth of liquid with bearing capacity, a ratio as below be calculated. ( h × liquid density) = BC / Kbh or h = BC / (Kbh × liquid density) The ratio Kbh should not be less than 1.8 to 2.2 , and full mat or raft foundation is required. The individual footings are possible only if Kbh is substantially higher than the range (i.e. 1.8 to 2.2). For economical design (i.e. ratio of concrete to tank capacity in m³ low) it could be 1.8 and increases with staging height and also with increases with the thickness of members. As the ratio of DL (tank +staging) to liquid load (FL) in tank increases Kbh shall increase. For a low BC a mat foundation is required covering the whole area of tank. Thus bearing capacity governs the design depth of liquid in tank. Strip foundation or Individual footings can be provided, if the ratio Kbh is significantly larger than the recommended value (i.e. 1.8 to 2.2). C.2 Construction aspects influence the cost of tower significantly. Less number of columns or larger spans, though increases the quantity of concrete slightly, however may reduce the cost of construction due total reduction in formwork item (in m²) and total length of members together. Significant part of the working cost (other than materials) depends on the total length of members in a staging.
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C.3 Shells of revolution like dome or the shapes like Intz, can give smaller concrete quantity, but the formwork for these shells are very costly. Hence slab beam systems with cylindrical wall can give significant cost reduction. Further tanks with shells of revolution or Intz shape required much higher depth of liquid in tank. For shells (e.g. cone & dome) have sloping surfaces, and concrete in these members could not be compacted properly in absence of formwork on the top surface. Such concrete, not compacted properly, can not be acceptable, or the provision of back formwork at top surface makes such shapes extremely costly. Bottom forms for these curved shapes are already very costly. C.4 Reduction of total surface area of elements of staging, reduces the wind load on staging, which otherwise for higher staging heights (say >20m) increases the cost of tower. Reduction of surface area of staging also amounts to reduction of formwork area for staging construction, which is beneficial in reducing cost. Total length of column & braces to gather represents the working and influences the cost of construction significantly. Reduction of total length of column & braces can be brought about by reducing the number of columns (by increasing Q/Nc) and also number of braces to derive economy in total construction cost.
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ANNEX D Response reduction factors (R) D.1 All staging with proper and adequate configurations, and which are ductile frames confirming to IS 13920, the response reduction factor can be taken R = 3.0. For staging in class B or C as given below, R can be smaller. D.2 The response reduction factor shall be 2.5 for following types of staging for elevated tanks, which also qualifies as ductile frame. (i) Flat slab as floor slab of tank conforming to high ductility requirements. (ii) Staging of 4 columns with no spiral stair case (only ladder). (iii) Staging of more than 8 columns, all of which are on a circle in plan, having circumferential brace and additional internal braces or crossing braces. Such staging may be for Intz tank or dome bottom cylindrical tanks. (iv) Staging such that each columns has braces in nearly two perpendicular direction, but braces are not continuous in circumferential direction. Examples – tank on 24 column with 24 braces at a level. (v) Any staging of class A above, having brick wall in some bays in ground storey (say for enclosure of room). D.3. The response reduction factor shall be 2.0 for following types of staging for elevated tanks, which also qualifies as ductile frame. (i) Flat slab as floor slab of tank not conforming to ductility requirements (ii) Staging of 4 columns with one more column outside for spiral stair case. (iii) Staging of more than 8 columns, all of which are on a circle in plan, having circumferential brace only (i.e. no internal braces or crossing braces). Such staging may be for Intz tank or dome bottom cylindrical tanks. (iv) Tank is on few independent staging. These independent staging have adequate bracing within itself, but staging are not connected to each other by braces. Examples – tank on 4 tower of 4 column each, total 16 columns & 16 braces at a level. (v) Any staging of class B above, having brick wall in some bays in ground storey (say for enclosure of room). D.4. For tank with dual system (column frame & shear wall ductile), R = 2.0 Figures will be added in the document little later.
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ANNEX E COMMENTARY E-1 Other IS standards (e.g. IS 3370, IS 1893 part 2) are using “liquid” rather “water”, hence in this revision of standard also “liquid” is used in place of “water”. E-2 Liquid (FL or water load/pressure) do not fall in the classification either as DL or IL. It is not proper to classify the water load either as dead load or as live load. IS 456 deals with load combinations (refer table 18 of IS 456-2000), wherein no mention of water load is made. IS 456 does not deal with either water load/pressure or earth load or earth pressure. It should be noted that IS 456 is written as a code for building work, but also being treated as mother code for all types of concrete structures. Other documents dealing with special structures (i.e. other than buildings) have to deals cover up all the deficiencies of IS 456 as a mother code. Hence it is necessary that water load should be clarified in the relevant codes. Table 18 of IS 456, giving load factors & load combination Sl No 1 2 2a 3
Load Combination 1 DL + IL DL + WL DL + WL DL + IL + WL
Limit State of Collapse DL IL WL 2 3 4 1.5 1.5 0 1.5 0 1.5 0.9 1.5 1.2 1.2 1.2
Limit State of Serviceability DL IL WL 5 6 7 1.0 1.0 0 1.0 1.0 1.0
0.8
0.8
If water load (FL) is treated as DL, the above table modifies as below. Sl No 1 2 2a 3
Load Combination 1 Dl + IL DL + WL DL + WL Dl + IL + WL
DL 2 1.5 1.5 0.9 1.2
Limit State of Collapse FL IL WL 3 4 5 1.5 1.5 0 1.5 0 1.5 0.9 0 1.5 1.2 1.2 1.2
DL 6 1.0 1.0 1.0
Limit State of Serviceability FL IL 7 8 1.0 1.0 1.0 0 1.0
0.8
WL 9 0 1.0 0.8
In case of water tanks, IL (other than water load) is very small. For both limit states (collapse & serviceability) combination no.2 means wind/seismic added in combination no.1 with out changing load factors, and this combination will always be most critical. Above combination 2 is against the philosophy of limit state design. The load factor of 1.5 for wind/seismic combination with full DL & full water load is unacceptable. Similarly under limit state of serviceability inclusion of full wind/seismic is unacceptable. Under limit state of serviceability above combination 3 is also unacceptable. A quick look in to BS 8110 table 2.1 shows us that water & earth pressures are dealt in a separate column. Similarly see ACI 318 (& also ACI 350), wherein water & earth loads are separately dealt and neither as DL nor IL. It is necessary to include a table for load factors & load combinations in IS 11682. Below is a suggested table 1.
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E-3 Vertical stress due to bending of tower as a whole, is a membrane (meridional) stress and not bending. The bending (flexural) will give stress as compression on one face of shaft and tension on other face i.e. within the thickness of shaft stress will vary from compression to tension. Neglecting such vertical bending stresses requires higher factor of safety with the vertical membrane stress. E-4 Stress level need not be as per chimney, as DL effect is predominant and temperature effect is negligible compared to chimney. E-5 Note that stresses shall not be higher because – (i) Compared to chimney the shaft for water tower carries heavy load at top, (ii) Shaft of water tower is more susceptible to buckling as usually R/t ratio is quite high. Hence comparison can be made with cooling towers. Shaft of water tower is found to be very sensitive to errors in construction and also strength reduction at construction joints. [In WSM stresses are kept little lower than that as per IS 456, because of uncalculated stress which remains unaccounted. How similar reduction be applied in LSD, hence load factors are multiplied by 1.1 to the specified factors]
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