LIQUID STORAGE TANK Storage tanks are containers that hold liquids, compressed gases (gas tank; or in U.S.A "pressure vessel", which is not typically labeled or regulated as a storage tank) or mediums used for the short- or long-term storage of heat or cold.[1] The term can be used for reservoirs (artificial lakes and ponds), and for manufactured containers. The usage of the word tank for reservoirs is uncommon in American English but is moderately common in British English. In other countries, the term tends to refer only to artificial containers.
Review of Code Provisions on Design Seismic Forces for Liquid Storage Tanks by Dr O R Jaiswal Department of Applied Mechanics Visvesvaraya National Institute of Technology Nagpur Dr Durgesh C Rai Dr Sudhir K Jain Department of Civil Engineering Indian Institute of Technology Kanpur Kanpur Review of Design Seismic Forces for Liquid Storage Tanks Review of Code Provisions on Design Seismic Forces for Liquid Storage Tanks Abstract It is well recognized that liquid storage tanks possess low ductility and energy absorbing capacity as compared to the conventional buildings Accordingly, various design codes provide higher level of design seismic forces for tanks In this article, provisions of IBC 2000, ACI, AWWA, API, Eurocode and NZSEE guidelines are reviewed, to assess the severity of design seismic forces for tanks vis-à-vis those for buildings It is seen that, depending on the type of tank, design seismic force for tanks can be to times higher than that for buildings Based on the comparison of provisions in these documents, various similarities, discrepancies and limitations in their provisions are brought out At the end a brief description of Indian code is given along with a few suggestions to remove the inadequacies in Indian code INTRODUCTION Seismic safety of liquid storage tanks is of considerable importance Water storage tanks should remain functional in the post earthquake period to ensure potable water supply to earthquake-affected regions and to cater the need for fire fighting Industrial liquid containing tanks may contain highly toxic and inflammable liquids and these tanks should not loose their contents during the earthquake Liquid storage tanks are mainly of two types: ground supported tanks and elevated tanks Elevated tanks are mainly used for water supply schemes and they could be supported on RCC shaft, RCC or steel frame, or masonry pedestal Failure of tanks during Chilean earthquake of 1960 and Alaska earthquake of 1964 led to beginning of many investigations on seismic analysis of liquid storage tanks Following two aspects came to forefront: (a) Due consideration should be given to sloshing effects of liquid and flexibility of container wall while evaluating the seismic forces on tanks (b) It is recognized that tanks are less ductile and have low energy absorbing capacity and redundancy compared to the conventional building systems Studies focused on the first aspect resulted in the development of mechanical models of tank by Housner (1963) and Veletsos (1974), which represented tank-fluid system in a more realistic fashion Many investigations IITK-GSDMA-EQ01-V1.0 Review of Design Seismic Forces for Liquid Storage Tanks followed along this line to further refine these mechanical models to include effects of flexibility of soil (Hori (1990), Veletsos et al (1992)) and base uplifting of unanchored tanks (Malhotra (1997)) Further studies have provided more simplifications to these mechanical models (Malhotra (2000)) Most of the design codes use these mechanical models to represent dynamics of tank-fluid system, which are applicable to ground supported as well as elevated tanks The second aspect which is related to low ductility and redundancy in tanks as compared to the conventional buildings, has been dealt with in a rather empirical manner Lateral seismic coefficient for tanks is generally taken higher than for the buildings Wozniak and Mitchell (1978) state “… the high value of lateral seismic coefficient for tanks in comparison with buildings is appropriate because of the low damping inherent for storage tanks, the lack of nonstructural load bearing elements, and lack of ductility of the tank shell in longitudinal compression” Most of the design codes follow this approach and assign higher design seismic action for tanks as compared to buildings How high this design action should be, is perhaps decided on ad-hoc basis or based on past experiences, however, it is influenced by type of tank, supporting subgrade, type of anchorage to tank etc Basically it depends on how good ductility and energy absorbing capacity a particular type of tank can provide For elevated tanks, ductility, redundancy and energy absorbing capacity is mainly governed by the supporting structure, which could be in the form of a RCC shaft, RCC frame, Steel frame or even masonry pedestal This article presents an assessment of design seismic force for tanks visà-vis design seismic force for buildings as mentioned in the following documents: (a) IBC 2000 (b) ACI Standards ACI 371 (1998) and ACI 350.3 (2001) (c) AWWA D-100 (1996), AWWA D-103 (1997), AWWA D-110 (1995) and AWWA D-115 (1995) (d) API 650 (1998) (e) Eurocode (1998) IITK-GSDMA-EQ01-V1.0 Review of Design Seismic Forces for Liquid Storage Tanks (f) NZSEE guidelines and NZS 4203:1992 It may be noted here that IBC 2000, ACI, AWWA and API standards are from USA The quantification of design seismic action in ACI, AWWA and API standards is in a different fashion than IBC 2000 However, FEMA 368 (NEHRP 2000) has provided modifications to these quantifications to bring them in conformity with provisions of IBC 2000 In the present article, provisions of ACI, AWWA and API standards will be discussed along with the modifications of FEMA368 Similarly, in New Zealand, the NZSEE recommendations (Priestly et al., 1986) on seismic design of tanks, is being presently revised by a study group to bring it in line with New Zealand loading code NZS 4203:1992 The outline of the procedure proposed by this study group is given by Whittaker and Jury (2000) In the present article, procedure described by Whittaker and Jury is considered along with NZS 4203:1992 The assessment of design seismic force for tanks is presented in terms of design response spectra This assessment is done with respect to corresponding design seismic force for buildings Such a comparative assessment helps in knowing how severe design seismic action for tank is, as compared to that for a building under similar seismic exposures First, provisions on design seismic action for tanks described in the above-mentioned documents are discussed, followed by a comparison of design seismic actions from various codes At the end a brief description of Indian Standard, IS 1893:1984 is given Inadequacies of IS 1893:1984 in quantifying suitable seismic design forces for tanks are brought out and a few modifications are proposed to remove these limitations IBC 2000 International Building Code (IBC) 2000 does provide provisions for certain types of non-building structures which include tanks For buildings, the seismic base shear is given by V = Cs W, where, W is the effective seismic weight Seismic response coefficient or base shear coefficient, Cs should be minimum of the following two values Cs = SDS SD1 or Cs = , where SDS and R/I ( R / I )T SD1 are the design spectral response accelerations at short periods and second IITK-GSDMA-EQ01-V1.0 Review of Design Seismic Forces for Liquid Storage Tanks period, respectively; I is importance factor; R is response modification factor and T is the fundamental time period of building The minimum value of Cs should not be less than 0.044 SDS I IBC suggests a value of R = 8.0 for buildings with ductile frames For most of the buildings, importance factor, I = 1.0 Figure shows the variation of base shear coefficient, Cs = (V/W) with time period The values of SDS and SD1 are taken as 1.0 and 0.6 respectively, which correspond to SD = 1.5, Fa = 1.0, S1 = 0.6 and Fv = 1.5 with site class D For tanks, due to low ductility and redundancy, low values of R are specified Table gives details of various types of tanks mentioned in IBC along with their R values Four values of R are specified, i.e R = 1.5, 2.0, 2.5 and 3.0 For most of the tanks the value of importance factor I will be I =1.25 However, for tanks containing highly toxic materials, importance factor could be I =1.5 The expression for base shear of tank is same as that for building with suitable values of R and I For tanks, the minimum value of Cs should not be less than 0.14 SDSI as against 0.044 SDSI for buildings For ground-supported tanks (i.e., at-grade tanks), IBC suggests to include the effects of sloshing Similarly, for elevated tanks (i.e., above-grade tanks), IBC states that when sloshing mode period of the stored liquid is within 70 percent to 150 percent of the fundamental period of the supporting structure, the effects of sloshing shall be included in the design of tank and supporting structure However, IBC 2000 does not provide any particular details on evaluation of sloshing or convective mode forces Thus, the values of R specified for tanks can be considered only for impulsive modes The variation of base shear coefficient (BSC) for tanks, with time period is also shown in Figure It is seen from this figure that depending on the type of tank, base shear coefficient is to times higher than that of a ductile building The ratio of base shear coefficient of tank and building, (BSCtank/BSCbldg), plotted in Figure 2, directly indicates how severe design base shear for tank is with respect to a ductile building (R = 8.0) The effect of response reduction factor of tank is seen up to sec For time period greater than sec, all types of tank have same base shear coefficient, which is about four times that for a ductile building IITK-GSDMA-EQ01-V1.0 Review of Design Seismic Forces for Liquid Storage Tanks ACI STANDARDS ACI 371 and ACI 350.3 describe provisions for seismic design of liquid storage concrete tanks ACI 371 deals with pedestal supported elevated RCC tanks only; on the other hand, ACI 350.3 deals with ground supported as well as elevated tanks Further, ACI 371 describes consideration of impulsive mode only, whereas, ACI 350.3 has provisions for impulsive as well as convective modes The quantification of design seismic action in these ACI standards is in a manner different from IBC 2000 In order to bring these quantifications in conformity with IBC 2000, FEMA 368 has suggested modifications to the base shear coefficient expressions of ACI standards Prior to study of various provisions of ACI standards, it will be appropriate to review the modifications suggested by FEMA 368 Table gives the details of base shear coefficient expressions of ACI 371 and ACI 350.3 along with the modified expressions of FEMA 368 From Table it is seen that as per ACI standards, in velocity-critical range of spectra, the impulsive mode base shear coefficient, Cs decreases as a function of 1/T2/3 However, in FEMA 368, impulsive base shear coefficient, Cs decreases as 1/T For convective mode base shear also similar difference can be noted To have a better understanding of modifications proposed by FEMA, a comparison of base shear coefficient obtained from ACI 371 expression and one obtained from the modified expression of FEMA368 is shown in Figure The base shear coefficient values shown in Figure 3, correspond to the most sever zone of ACI 371 and equivalent seismic conditions of FEMA 368 It is seen that in short period range ( T < 0.6 s), base shear coefficient values of FEMA368 are about 12% higher than one obtained from ACI 371 In the long period range, values obtained from both the expressions match well It may be noted that in the ACI 371, the importance factor does not appear in the expression for base shear coefficient, whereas, FEMA368 modification has introduced an importance factor I =1.25 Similarly, comparison of base shear coefficient of impulsive mode obtained from the expression of ACI 350.3 and the one modified by FEMA 368 IITK-GSDMA-EQ01-V1.0 Review of Design Seismic Forces for Liquid Storage Tanks is shown in Figure 4a These results are for the most severe zone of ACI 350.3 and FEMA 368 It may be noted that in ACI 350.3 value of response modification factor, Rw is in the range of 2.0 to 4.75, whereas in FEMA 368, R varies in the range of 1.5 to 3.0 In Figure 4a, base shear coefficients are shown for the lowest and the highest values of response modification factor It is seen that for the highest value of response modification factor (i.e Rw = 4.75 and R = 3.0), base shear coefficient from both the expressions match well For the lowest value of response modification factor (i.e Rw = 2.0 and R = 1.5), results of FEMA 368 are on lower side by 15% In Figure 4b, base shear coefficient corresponding to convective mode is compared For T > 2.4 s, ACI 350.3 and FEMA 368 expressions give same values of convective base shear coefficient 3.1 ACI 371 (1998) This ACI standard provides recommendations for evaluating design seismic forces on concrete pedestal supported elevated tanks With FEMA modifications, the base shear is given by V = Cs W, where W is the summation of weight of water, container and support structure above the base The base shear coefficient Cs is given by Cs = SD1 ( R / I)T Cs = 4S D1 ( R / I)T Cs ≤ S DS ( R / I) for Ts < T < s for T ≥ s and C s ≥ 0.2S DS (1) (2) (3) The quantities, SDS, SD1, R, I and T are same as defined in IBC 2000 and Ts = SD1/SDS FEMA 368 states that except for the above stated modifications, concrete pedestal supported elevated tanks should be designed as per provisions of ACI 371 In this ACI standard, a load factor of 1.1 is given for strength design method, as opposed to unity in IBC 2000 A closer look at the base shear formula mentioned above reveals that, this is same as one given in IBC 2000 except for the minimum values of Cs = 0.2 SDS (which is 0.14 SDS I in IBC 2000) and a load factor of 1.1 for strength design FEMA 368 specifies R = IITK-GSDMA-EQ01-V1.0 Review of Design Seismic Forces for Liquid Storage Tanks for pedestal supported elevated tanks The ratio of base shear coefficient of tank obtained from ACI 371 (i.e from Eqs to 3) and base shear coefficient of building (obtained as per IBC 2000 with R = 8) i.e BSCtank / BSCbldg is plotted in Figure The ratio of tank to building base shear coefficient as obtained from IBC 2000 is also shown in Figure It is seen that values of base shear coefficient obtained from ACI 371 are higher than the one obtained from IBC 2000 The higher value of base shear coefficient as per ACI 371 is due to a load factor of 1.1 and higher value of lower bound limit ACI 371 does not give details about convective mass component, and recommends its consideration if water weight is less than 80 percent of the total gravity load of tank 3.2 ACI 350.3 (2001) ACI 350.3 gives procedure for seismic design of liquid containing concrete tanks, with detailed description of impulsive and convective components It mainly deals with ground supported concrete tanks and limited information on pedestal supported elevated tanks is also provided Considering the modifications suggested by FEMA 368, the base shear coefficient for impulsive mode, is given by (0.6 (Cs ) i = SDs Ti + 0.4SDS ) T0 1.4(R / I) for < Ti s), severity of tank shear as compared to that of a building reduces This is due to the fact that for tanks no lower bound on design seismic force has been defined in ACI 350.3, whereas IBC 2000 specifies a lower bound on design seismic force for buildings For convective mode, ACI 350.3 and FEMA 368, specify 0.5% damping and for impulsive mode damping is 5% Convective mode period is usually grater than 2.0 sec and hence from eq (6) and (7) it is seen that convective mode spectrum (0.5% damping) is 8.4R/T times higher than impulsive mode spectrum (5% damping) AWWA STANDARDS American Water Works Association (AWWA) standards provide guidelines for design and manufacturing of different types of water storage tanks AWWA D-100 (1996) deals with welded steel tanks, AWWA D-103 (1997) is for factory-coated bolted steel tanks Similarly, AWWA D-110 (1995) deals with wire- and strand- wound, prestressed concrete water tanks and IITK-GSDMA-EQ01-V1.0 Review of Design Seismic Forces for Liquid Storage Tanks AWWA D-115 (1995) is for prestressed concrete water tanks with circumferential tendons All these AWWA standards deal with circular tanks only and quantification of seismic loads provided in them is in a fashion different from IBC 2000 To bring these quantifications in line with IBC 2000, FEMA 368 and its commentary (FEMA 369), has provided modifications to the expressions for base shear coefficients of these AWWA standards The provisions of these AWWA standards will be discussed along with the modifications of FEMA 368 4.1 AWWA D-100 (1996) and D-103 (1997) AWWA D-100 and AWWA D-103 deal respectively with welded steel tanks and factory-coated bolted steel tanks AWWA D-100 has provisions for ground supported as well as elevated water tanks, whereas AWWA D-103 has provisions for ground-supported tanks only Provisions of AWWA D-100 and D-103 for seismic design of ground-supported tanks are identical First the provisions for ground-supported tanks will be discussed 4.1.1 Ground supported tanks In Table 3, expressions for base shear coefficients for ground-supported tanks as given in AWWA D-100 and D-103 are given along with the modified expressions of FEMA 368 It is seen that for ground supported steel tanks, both impulsive and convective modes are considered For impulsive base shear a constant value independent of time period is given Since ground supported steel tanks will mostly have time period in the constant-acceleration range of spectra, hence it suffices to mention the constant value It may be noted here that AWWA D-100 specifies the base shear coefficient for working stress design and hence the modified expression given in FEMA 368 contains a factor of 1.4 in the denominator In AWWA standards, importance factor I is taken as I = 1.25 Further in AWWA D-100, the response reduction factor, Rw varies in the range from 3.5 to 4.5, whereas in FEMA 368, it varies from 2.5 to 3.0 To have better understanding of modifications suggested by FEMA 368, it will be appropriate to compare the numerical values of base shear coefficients IITK-GSDMA-EQ01-V1.0 10 Review of Design Seismic Forces for Liquid Storage Tanks Table 8: Different types of tanks with their ductility factor, μ (Whittaker and Jury (2000)) Type of Tank Steel Tanks on Grade Elastically supported Unanchored tank designed for uplift (elephant foot shell buckling may occur under seismic overload) Unanchored tank designed for uplift and elastic (diamond shaped) shell buckling mode Anchored with non-ductile holding down bolts Anchored with ductile tension yielding holding down bolts Ductile skirt pedestal On concrete base pad designed for rocking Concrete Tanks on Grade Reinforced Concrete Prestressed Concrete Tanks of other materials on Grade Timber Non-ductile materials (eg Fiberglass) Ductile materials and failure mechanisms Elevated Tanks μ 1.25 2.001 1.25 1.25 3.002 3.002 2.002 1.25 1.00 1.00 1.00 3.00 As appropriate for support structure Notes Check that elastic buckling does not occur before elephant foot Capacity design check required to protect against other forms of failure Capacity design approach shall be used to protect elevated tanks against failure while yielding occurs in the chosen support system Table 9: Correction factor, Cf (Whittaker and Jury (2000)) Ductility factor, μ 1.0 1.25 1.5 2.0 2.5 3.0 4.0 IITK-GSDMA-EQ01-V1.0 0.5 1.75 0.92 0.75 0.58 0.49 0.43 0.36 Damping level, ξ (%) 1.0 2.0 5.0 10.0 15.0 1.57 1.33 1.00 0.80 0.71 0.88 0.83 0.72 0.62 0.58 0.72 0.68 0.61 0.54 0.51 0.56 0.54 0.48 0.44 0.42 0.48 0.46 0.42 0.38 0.36 0.43 0.41 0.38 0.35 0.33 0.36 0.35 0.33 0.30 0.29 20.0 0.67 0.55 0.48 0.40 0.35 0.32 0.28 39 Review of Design Seismic Forces for Liquid Storage Tanks Table 10: Parameters for a low ductility tank Code IBC 2000 and ACI 350.3 Eurocode NZSEE guidelines Parameters R = 1.5, I =1.25 q = 1.0, γI = 1.2 Sp = 1.0, μ = 1.25, ξ = 5%, Cf = 0.72 Table 11: Parameters for a high ductility tank Code IBC 2000 and ACI 350.3 Eurocode NZSEE guidelines Parameters R = 3.0, I =1.25 q = 2.0, γI = 1.2 Sp = 1.0, μ = 3.0, ξ = 5%, Cf = 0.38 Table 12: Proposed values of Response reduction factor, R for IS 1893:2002 Tank Ground supported tanks Unanchored steel tank Anchored steel tank Concrete tank with unconstrained flexible base Concrete tank with non-sliding base Elevated tanks Supported on RCC shaft Supported on RCC frame staging Supported on steel frame staging Supported on masonry shaft IITK-GSDMA-EQ01-V1.0 Proposed value of R 1.80 2.25 1.10 1.50 1.50 2.25 2.25 1.10 40 Review of Design Seismic Forces for Liquid Storage Tanks R = 8.0 For building R = 1.5 Base shear coefficient (BSC) 0.8 R = 2.0 For tanks R = 2.5 0.6 R = 3.0 0.4 0.2 0 Time Period (S) Figure 1: Variation of base shear coefficient with natural period; IBC 2000 (SD =1.5, S1 = 0.6, Fa =1.0, Fv = 1.5, Class D site) R = 1.5 7.5 R =2.0 For tanks R = 2.5 BSCtank / BSCbldg R = 3.0 4.5 (SS = 1.5, S1 = 0.6, Fa = 1.0, Fv = 1.5, site class D) (I = and R = for building; I = 1.25 for tanks) 1.5 0 Time Period (S) Figure 2: Ratio of tank and building base shear coefficient (IBC 2000) IITK-GSDMA-EQ01-V1.0 41 Base shear coefficient (BSC) Review of Design Seismic Forces for Liquid Storage Tanks 0.8 0.6 0.4 0.2 As per equation of ACI 371 (Ca = 0.44, Cv = 0.64, R =2.0, soil type D) 0 Time period (S) Figure 3: Comparison of base shear coefficient from ACI 371 and FEMA 368 IITK-GSDMA-EQ01-V1.0 42 Review of Design Seismic Forces for Liquid Storage Tanks Tank Cf = 0.72 Tank; Cf = 0.54 BSCtank / BSCbldg Tank; Cf = 0.38 0 Time Period (S) Figure 18: Ratio of base shear coefficient of tank and building (NZSEE Guidelines) Eurocode ( α = 0.3,S = 1, β = 2.5, γ = 1, Kd1 = 2/3, Kd2 = 5/3, TA = 0.15, TB = 0.6, q = 5, sub soil class B) Base shear coefficient (BSCbldg) 0.3 IBC 2000 ( SS = 1.5, S1 = 0.6, Fa = 1.0, Fv = 1.5, R = 8, I = 1, site class D) 0.2 NZSEE ( Z = 1.2, Sp = 0.67, R = 1, Lu = 1, Site category C) 0.1 0 Time (S) Figure 19: Comparison of base shear coefficient for ductile building obtained from various codes Most severe zone in each code is considered IITK-GSDMA-EQ01-V1.0 50 Review of Design Seismic Forces for Liquid Storage Tanks Eurocode (α = 0.3, S = 1, β = 2.5, η = 1, γ = 1.2, K1 = 1, K2 = 2, TA = 0.15, TB = 0.6, q = 1, sub soil class B) Base shear coefficient (BSCtank ) 1.2 0.8 NZSEE ( Z = 1.2, Sp = 1.0, R = 1.3, Cf = 0.72, Lu = 1, site category C) ACI 350.3 and AWWA D-110 0.4 IBC 2000 ( SS = 1.5, S1 = 0.6, Fa = 1.0, Fv = 1.5, R = 1.5, I = 1.25, site class D) 0 Time (S) Figure 20: Comparison of base shear coefficient for ground supported unanchored concrete water tank obtained from various codes Most severe zone in each code is considered NZSEE ( Z = 1.2, Sp = 1.0, R = 1.3, Cf = 0.72, Lu = 1, site category C) 10 IBC 2000 ( SS = 1.5, S1 = 0.6, Fa = 1.0, Fv = 1.5, R = 1.5, I = 1.25, site class D) BSCtank / BSCbldg ACI 350.3 and AWWA D-110 Eurocode (α = 0.3,S = 1, β = 2.5, η = 1, γ = 1.2, K1 = 1, K2 = 2,TA = 0.15, TB = 0.6, q = 1, sub soil class B) 0 Time (S) Figure 21: Comparison of ratio of base shear coefficients of tank and building from various codes (Low ductility tank) IITK-GSDMA-EQ01-V1.0 51 Review of Design Seismic Forces for Liquid Storage Tanks BSCtank / BSCbldg IBC 2000 ( SS = 1.5, S1 = 0.6, Fa = 1.0, Fv = 1.5, R = 1.5, I = 1.25, site class D) NZSEE ( Z = 1.2, Sp = 1.0, R = 1.3, Cf = 0.38, Lu = 1, site category C) ACI 350.3 and AWWA D-110 Eurocode (α = 0.3,S = 1, β = 2.5, η = 1, γ = 1.2, K1 = 1, K2 = 2, TA = 0.15, TB = 0.6, q = 2, Sub soil class B) 0 Time (S) Figure 22: Comparison of ratio of base shear coefficient of tank and building from various codes (High ductility tank) ACI 350.3 and AWWA D-110 ( SS = 1.5, S1 = 0.6, Fa = 1.0, Fv = 1.5, I = 1.25, site class D) Base shear coefficient Eurocode (S = 1, β = 2.5, η = 1.673, 1.5 γ = 1.2, K1 = 1, K2 = 2, TA = 0.15, TB = 0.6, q = 1, sub soil class B) API 650 (SS = 1.5, S1 = 0.6, Fa = 1.0, Fv = 1.5, I = 1.25, site class D) 0.5 1.5 2.5 3.5 4.5 5.5 Time (S) Figure 23: Comparison of base shear coefficient for convective mode IITK-GSDMA-EQ01-V1.0 52 Review of Design Seismic Forces for Liquid Storage Tanks IBC 2000 ( SS = 1.5, S1 = 0.6, Fa = 1.0, Fv = 1.5, R = 8, I = 1, site class D) Base shear coefficient 0.1 0.075 IS 1893 (Part I):2002 (Z = 0.36, R = 5, I = 1, soft soil) 0.05 0.025 IS 1893-1984 (K=1, β = 1, α0 = 0.08, soft soil) 0 0.5 1.5 2.5 Time Period (S) Figure 24: Comparison of base shear coefficient for building from IBC 2000, IS 1893:1984, IS 1893(Part 1):2002 IBC values are divided by 1.4 to bring them to working stress level 0.8 IBC 2000 ( Lowest value of R = 1.5) Base shear coefficient IS1893: 2002 (R = 1.1) 0.6 IBC 2000 ( Highest value of R = 3) IS1893: 2002 (R = 2.25) 0.4 IS1893: 1984 0.2 0 0.5 1.5 2.5 Time period (S) Figure 25: Base shear coefficients for tanks from IBC 2000, IS 1893:1984 and IS 1893(Part 1):2002 IBC values are divided by 1.4 to bring them to working stress level IITK-GSDMA-EQ01-V1.0 53 ... Design Seismic Forces for Liquid Storage Tanks Review of Code Provisions on Design Seismic Forces for Liquid Storage Tanks Abstract It is well recognized that liquid storage tanks possess low ductility... fighting Industrial liquid containing tanks may contain highly toxic and inflammable liquids and these tanks should not loose their contents during the earthquake Liquid storage tanks are mainly... Forces for Liquid Storage Tanks ACI STANDARDS ACI 371 and ACI 350.3 describe provisions for seismic design of liquid storage concrete tanks ACI 371 deals with pedestal supported elevated RCC tanks