BUREAU OF INDIAN STANDARDS. Preliminary Draft. CRITERIA FOR DESIGN OF RCC STAGING FOR OVERHEAD WATER TANKS (First Revision of IS 11682)

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1 Doc: CED38 (7811)P 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 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 It is proposed that as and when IS 1893 part 2 is published, the clause & & Annex D shall be withdrawn. 1

2 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. Page 2 of 44

3 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) Page 3 of 44

4 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 Page 4 of 44

5 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 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 Page 5 of 44

6 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 Guide for completion drawing, and completion report for record. Record of quality of construction 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

7 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 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 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 Structural steel members will be governed by IS 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, & The grade of concrete shall not be less than that required by IS 456, table 5, depending upon the exposure condition. Page 7 of 44

8 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 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.) 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 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 Ready Mixed Concrete conforming to IS 4926 may also be preferred. 7.3 Cement 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 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 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. Page 8 of 44

9 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 Construction joints in columns, braces and shaft should be as less as possible. 7.7 Formwork should comply with IS 456 and IS 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 ). Page 9 of 44

10 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 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 WL]. See also clause 17.2 of IS 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 of IS 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

11 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 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 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 N/m³ 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 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 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

12 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) 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) 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) 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 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 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, 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)] 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

13 10.5 Seismic Forces (WL) For seismic load both tank empty and tank full conditions shall be considered as per IS 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 or 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 The two mass model is technically more appropriate, and in most cases also gives an economical design of staging 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 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) 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 to 3.0. (Note: Guidance given in Annex D for types and shapes of staging for range of R). b) Framed staging OMRF to 2.5 ; c) RCC shaft with reinforcement on each face (and conform to ductile detailing as per clause 9 of IS 13920) ; d) RCC shaft with reinforcement in middle Alternately refer 1893 (Part 1):2002 in conjunction with IS 1893:1984 may be used. However R shall be as given above Seismic base shear shall be estimated for a load combination of (1DL + 1 FL + 0 IL + 1 ILs 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 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 Page 13 of 44

14 Seismic base shear shall not be less than 1% of the gravity loads 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 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 of IS 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 The design for blast or explosion shall be done, as mutually agreed between the relevant parties Construction loads Temporary loads resulting from construction activity should be considered in design of structural components required to support construction loads 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) 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. Page 14 of 44

15 11.2 Table 1 Load Combination & Load Factors Load Limit State of Collapse Limit State of Serviceability Combination DL FL IL WL DL FL IL WL DL +FL + IL DL + WL a DL + WL DL+FL+IL+WL [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 FL IL + 1 ILs ILp) and this base shear be multiplied by the load factor specified for seismic load 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 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

16 vertical and horizontal loads acting in any direction when the tank is full as well as empty For the analysis of frame, including P-δ effect, modulus of elasticity of concrete will be taken as per IS 456 clause For P-δ effect, refer 39.7 of IS As an option for not taking in to account the effect of deflection (P-δ effect), the provisions of clause of IS 456 shall apply if conditions in are fulfilled Simplified analysis as given in IS 456 clause and 22.5 shall not be applicable 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 Staging and other reinforced concrete members including foundation shall be designed by limit state method in accordance with the requirements of IS Alternately staging and foundations can be designed by working stress method, with the check as required in Columns shall be checked by limit state method also, even if design of staging is done by working stress design method 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

17 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% 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 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 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 ) does not constitute a detailed P-δ analysis. Page 17 of 44

18 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 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 c) Brace size is larger than a requirement given in (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 of IS 456. No correction for creep is necessary Columns 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 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 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 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 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

19 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 Design of columns shall be governed by following guidelines 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 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 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 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 P- δ analysis is done with δ enhanced by 1.3 times 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 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 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. Page 19 of 44

20 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 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 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 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. Page 20 of 44

21 Moments and shears arising from local vertical loading, if any, shall be accounted in the design 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 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 Foundations Foundations shall comply with the requirements of IS 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 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 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 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 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) In case of load combination with wind or seismic forces, enhancement of allowable bearing capacity shall be permitted as per the relevant standard. Page 21 of 44