COMPARATIVE SEISMIC PERFORMANCE OF RC FRAME BUILDINGS DESIGNED FOR ASCE 7 AND IS 1893

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1 ISET GOLDEN JUBILEE SYMPOSIUM Indian Society of Earthquake Technology Department of Earthquake Engineering Building IIT Roorkee, Roorkee October 20-21, 2012 Paper No. D019 COMPARATIVE SEISMIC PERFORMANCE OF RC FRAME BUILDINGS DESIGNED FOR ASCE 7 AND IS 1893 Vijay Namdev Khose 1, Yogendra Singh 2 and Dominik Lang 3 1 Senior Engineer, Thornton Tomasetti, Mumbai, vijaynkhose@gmail.com 2 Associate Professor, Department of Earthquake Engineering, IIT Roorkee, yogenfeq@iitr.ernet.in 3 Research Engineer, NORSAR, P.O. Box 53, 2027 Kjeller, Norway, Dominik.Lang@norsar.no ABSTRACT During the development process of the earthquake engineering discipline, seismic design codes evolved simultaneously and were updated from time to time in order to match the current state-ofthe-art at that time. Current international seismic design codes tend to converge on the issues of design methodology and the state-of-the-art. The existing codes differ significantly in specifying the limits on various control parameters and, if a building is designed for a given seismic hazard, using different seismic design codes, it is expected that seismic performance of the very same building will vary significantly. This article presents a comparative study of the seismic performance of an eightstorey RC frame building designed according to the U.S. American (ASCE ) and Indian (IS 1893-Part1 2002) seismic design codes for a given value of PGA corresponding to the Maximum Credible Earthquake (MCE). Keywords: Seismic design codes, seismic performance, seismic design, design criteria, RC frame buildings INTRODUCTION During the development process of the earthquake engineering discipline, seismic design codes evolved simultaneously and were updated from time to time in order to match the current state-of-theart at that time. Current international seismic design codes tend to converge on the issues of design methodology and the state-of-the-art. Most of the current design codes are based on a force-based design methodology, indirectly considering the effect of inelastic energy dissipation, through a Response Reduction Factor (also known as Behaviour Factor), with a check on interstorey drift to ensure stability of the structure and to control damage to drift-sensitive (mostly non-structural) components. Further, the controls applied on different design parameters such as, design base shear, ductility capacity, ductility demand, and drift, are prescriptive and the role of individual control parameters in ensuring the desired performance is not explicitly estimated. The existing codes differ significantly in specifying the limits on various control parameters; and if a building is designed for a given seismic hazard, using different seismic design codes, it is expected that seismic performance of building will vary significantly. Due to this reason, there is a need to conduct comparative studies that may lead to the harmonization of different international seismic design codes. This establishes also a

2 crucial step in the process of evolution of the next generation of design codes. This paper presents a comparative seismic performance of an eight storey RC frame building designed according to the U.S. American (ASCE ) and Indian (IS 1893-Part1 2002) seismic design codes for a given value of PGA corresponding to the Maximum Considered Earthquake (MCE). BUILDING DESIGN The seismic performance of different codes can be evaluated at best by comparing the estimated seismic performance of a building designed by the different codes for a given seismic hazard level. In the present study, an eight storey RC frame building is designed according to theasce 7-10 (2010) and IS 1893-Part1 (2002), for a given value of PGA corresponding to MCE. The building is considered to have residential occupancy (Importance Factor =1.0) and is located on ASCE 7-10 site class D (stiff soil, V s30 = m/s). In case of IS 1893, it is considered to be located on soil type II which is equivalent to ASCE 7 site class D. More detailed discussion on equivalent site classes can be found in Khose et al. (2012). A PGA value of 0.35g corresponds to the MCE (with a 2% probability of exceedance in 50 years) on ASCE 7-10 site class B (rock, V s30 = 760-1,500 m/s) is considered. ASCE 7 anchors design spectrum using short-period spectral acceleration at 0.2 sec period (S s ), and spectral acceleration at 1.0 sec period (S 1 ) rather than PGA value. Considering the similarity of spectral shape of the IS 1893 and ASCE 7 for site class B, S s and S 1 are assumed as 2.5 times and equal to the considered PGA, respectively. Figure 1 show the design spectra obtained from the two codes for the given PGA. It can be observed that there are significant differences in the design spectra for the two codes, mainly due to use of different site amplification models. There are further differences in the design seismic coefficients due to the different relationships between the Design Basis Earthquake (DBE) and MCE, used in the two codes. The IS 1893 defines DBE PGA as half of the MCE PGA, whereas the ASCE 7 considers it one third of the MCE PGA. Figure 1. MCE response spectra of ASCE 7 and IS 1893 for 0.35 g PGA on ASCE 7 site class B Table 1 shows the ductility classes and corresponding response reduction factors specified by ASCE 7 and IS ASCE 7 classifies RC frame buildings into three ductility classes, namely Ordinary Moment Resisting Frame (OMRF), Intermediate Moment Resisting Frames (IMRF) and Special Moment Resisting Frame (SMRF). However, IS 1893 classifies RC frame buildings into only two classes, namely Ordinary Moment Resisting Frame (OMRF) and Special Moment Resisting Frame (SMRF). Ductile detailing and response reduction factors of OMRF of both codes are similar but the SMRF of IS 1893 is comparable with IMRF of ASCE 7. Therefore, in the present study, seismic performance of IMRF of ASCE 7 and SMRF of IS 1893 are compared. Table 1. Ductility classes and corresponding response reduction factors specified by codes considered for study ASCE 7 IS 1893 Ductility class Response reduction Ductility class Response reduction

3 factor factor OMRF 3 OMRF 3 IMRF 5 SMRF 5 SMRF 8 - Figure 2 shows the plan and elevation of the under investigation. In the transverse direction, beams are not provided across the corridor and the two blocks separated by the corridor are connected by rigid diaphragms of floor and roof slabs. The 3D model of the building (Error! Reference source not found.), is developed in SAP2000 (2010). The beams and columns have been modeled as frame elements and in-plane rigidity of the slab is simulated using rigid diaphragm. The columns are assumed to be fixed at the base. The effective stiffness of RC members is considered as per the provisions of the respective design codes. Since both design codes do not provide guidelines for the modeling of joints, the recommendations of Elwood et al. (2007) regarding the modeling of RC beam-column joints have been used, whereas columns are considered as rigid and beams are considered as flexible, within the joint. Figure 2. Plan (left) and transverse elevation (right) of the model building. The building is designed as per ASCE 7-10 (2010) and IS 1893-Part1 (2002) and the corresponding RC design codes ACI 318M-08 (2008) and IS 456 (2000), respectively. ACI 318 uses cylinder ' strength, f c as the measure of concrete compressive strength, while IS 456 uses cube strength, f ck. In the present study, the cube compressive strength f ck is considered as 30 MPa, which corresponds to a ' cylinder strength, f c (=0.8 f ck ) of 24 MPa. Values of modulus of elasticity of the concrete have been estimated using the relationships provided in the corresponding codes. Specified yield strength, f y and modulus of elasticity, E s of reinforcing steel are considered as 500 MPa and 2x10 5 MPa, respectively. The seismic load has been estimated according to the considered codes and the building is designed for the combined effect of gravity and seismic loads, considering all the design load combinations specified in each code. The inelastic drift limit as per ASCE 7 is 2.5%, whereas for IS 1893 it is 0.4%, at the elastic design load.

4 Figure 3. Space frame model of the model building in SAP2000 It is to be noted that ASCE 7 and IS 1893 are the two codes among the current major national codes, which provide capping on the design period. However, the ceiling on the design period in the two codes is different. The IS 1893 restricts the maximum design period to T, where T is given as T h (1) where, T is approximate fundamental period of vibration, in sec, and h is height of the building, in m, excluding the basement storeys, where basement walls are connected with the ground floor deck or fitted between the building columns; but including basement storeys in case that they are not connected. ASCE 7 allows the maximum design period up to C u times T a, where C u and T a are given as T x a C h n t (2) where, T a is approximate fundamental period in sec, C t is for moment-resisting concrete frames, h n is height of building, in m, above the base to the highest level of the structure, x = 0.9 for moment-resisting concrete frames and C u is coefficient for upper limit on calculated period. Table 2. Comparison of fundamental periods and design base shear in longitudinal and transverse directions of buildings designed using ASCE 7 and IS 1893 Parameter Direction Without Capping With Capping ASCE 7 IS1893 ASCE 7 IS1893 Period (sec) Longitudinal Transverse Design Base Shear (kn) Longitudinal Transverse Table 2 shows the design periods and design base shears of the model building, with and without capping. It can be observed that significant difference exists in the design period of buildings designed for ASCE 7 and IS ASCE 7 results in higher periods in both directions as compared to Indian code. This is because ASCE 7 considers cracked section for effective stiffness of RC members,

5 whereas IS 1893 permits use of gross section for this purpose. Further, the capped design periods are much lower than the uncapped (analytically estimated) periods and have drastic effect on the design base shear, which is inversely proportional to the design periods, in the velocity-controlled range of the design response spectrum. Even though the analytically estimated fundamental period of the building designed for ASCE 7 is longer, the design base shear is higher, due to cumulative effect of differences in design spectra (Figure 1), and different factors used in the two codes for conversion of reference hazard (MCE) to design hazard (DBE). This difference is further increased for capped periods. However, it is to be noted that IS 1893 and ASCE 7 use different load factors on the estimated base shear for strength design. IS 1893 applies a load factor of 1.5 on the design base shear shown in Table 2, whereas ASCE 7 recommends a unity load factor for seismic design classes B and C. It is interesting to note that this difference in the applied load factors compensates for the difference in the design base shears, resulting in almost equal factored design base shears for the two codes. SEISMIC PERFORMANCE EVALUATION The expected performance of the designed-buildings is estimated using nonlinear static (pushover) analysis. Lumped plasticity models for beams and columns have been used to simulate the inelastic response of the building. Moment hinges at the ends of all the beams and interacting P-M-M hinges at the ends of all the columns have been assigned. Expected strength of materials to estimate members yield capacities and generalized force deformation relations as recommended by ASCE (2007) have been considered. Figure 4 shows the generalized force-deformation behaviour of RC members for modeling and acceptance criteria. IO LS CP C B Force A Deformation or deformation ratio Figure 4. Generalized force-deformation behavior of RC members for modeling and acceptance criteria (ASCE ). In Figure 4 linear elastic behavior is represented by line AB. The slope of line AB represents the effective elastic stiffness of the member. Point B represents the effective yield strength of the member. Line BC represents strain hardening. The slope of line BC is generally considered as 0 10% of the elastic stiffness, and taken as 10% in the present study. Point C represents the ultimate strength of the member. It is the point beyond which significant stiffness degradation begins. The range from point D to E represents the residual strength of the member. In the original model as per ASCE 41 the point D is just below point C. However, since this may lead to computational difficulties and inability to converge, slope (10 vertical to 1 horizontal units) is provided from point C to D. Point E is considered as failure of the member. Immediate Occupancy (IO), Life Safety (LS) and Collapse Prevention (CP) performance levels are also shown in Figure 4 and the corresponding acceptance criteria are considered as per ASCE 41 Update (Elwood et al. (2007). Effective member stiffness values as recommended by Elwood et al. (2007) and modulus of elasticity of concrete, E c as recommended by ACI 318M-08 (2008) have been considered for the seismic performance evaluation. D E

6 Pushover analysis has been performed with a load pattern proportional to the fundamental mode in the considered direction. In order to obtain the performance point, the displacement modification method of ASCE (2007) has been used. According to this method, the target displacement, δ t can be obtained using the following expression: 2 Te t C0C1C 2Sa g (3) 2 4 where, C 0 is the modification factor to relate spectral displacement of an equivalent single-degree of freedom (SDOF) system to the roof displacement of a multi-degree of freedom (MDOF) building. C 0 is obtained as the product of the first mode mass participation factor and the ordinate of the first mode shape at the roof. C 1 is the modification factor to relate the expected maximum inelastic displacements to the displacements calculated for equivalent linear elastic system, given as R 1 C1 1 (4) 2 at e For effective periods greater than 1 sec, C 1 = 1 and for effective periods less than 0.2 sec, C 1 is considered the same as at period equal to 0.2 sec. a is a site class factor which is taken as 130 for ASCE 7-10 site classes A, and B; 90 for site class C, and 60 for ASCE 7-10 site class D, E and F. R is the ratio of elastic strength demand to the yield strength and can be obtained using the following expression: R S a 1 Cm (5) Vy W where, V y is the yield strength estimated from the idealized bilinear capacity curve; W is the effective seismic weight; and C m is the effective mass factor. Values of C m for different heights of buildings are provided in ASCE (2007). C 2 is the modification factor to represent the effect of pinched hysteresis shape, cyclic stiffness degradation, and strength deterioration on maximum displacement response. 2 1 R 1 C (6) T e where, T e is the effective/equivalent period of the building representing secant stiffness at first yield in the direction under consideration; S a is the spectral acceleration at effective fundamental period; and g is the acceleration due to gravity. For effective periods greater than 0.7 sec, C 2 = 1. RESULTS AND DISCUSSIONS Figure 5 shows the capacity curves and performance points of the building designed for ASCE 7 and IS Although significant difference was noted in the design base shear, the capacity curves are quite close. In case of Design Basis Earthquake (DBE), buildings designed for both the codes have achieved better than the intended performance objective of Life Safety (LS), whereas, in case of MCE, the performance is Collapse Prevention, except for of the building designed for IS 1893, which resulted in LS performance level in transverse direction.. The similarity in estimated performance for the two buildings, despite use of different design spectra, is due to the almost identical design base shears resulting from the combination of different capping on design period and use of different load factors in the two codes.

7 Figure 5.Comparison of capacity curves and performance points in longitudinal (left) and transverse (right) directions of buildings. Figures 6 and 7 show the interstory drift ratios under DBE and MCE, respectively, for the buildings designed for the two codes. The interstory drift ratios are quite close at bottom and top stories. However, peak interstory drift ratio in transverse direction of the building designed for IS 1893 exceeds 2.5% (the limit on design drift as per ASCE 7) for DBE. In case of MCE, the peak interstory drift ratio exceeds 4% in case of the building designed for IS The higher interstorey drifts in case of IS 1893 are resulting due to use of gross section stiffness in design of RC buildings. Figure 6. Comparison of drift ratios under DBE in longitudinal (left) and transverse (right) directions of buildings.

8 Figure 7. Comparison of drift ratios under MCE in longitudinal (left) and transverse (right) directions of buildings. CONCLUSIONS A comparison of seismic performance of an eight-story RC frame building designed for ASCE 7 and IS 1893 has been presented. It is noted that there are significant differences in the site amplification models of the two codes resulting in different design spectra. Further the OMRF and SMRF of IS 1893 have detailing and response reduction factors similar to OMRF and IMRF, respectively of ASCE7. Capping on the design period has a significant effect on design base shear of the building. It is very interesting to note that despite the differences in design spectra, factors used for conversion of reference hazard (MCE) to design hazard (DBE), capping on design period, and load factors in the two codes, the final design base shear is identical in the two codes. The capacity curves of the buildings designed for ASCE 7 and IS 1893 are quite close.. Both the buildings have achieved better than the intended performance objective of LS in DBE and Collapse Prevention in MCE.. However, the peak interstory drift ratio in case of the building designed for IS 1893 exceeds the intended limits, due to use of gross-section stiffness in design. ACKNOWLEDGMENTS The work described in the present manuscript was carried out under the Indo-Norwegian Program of Institutional Cooperation (INPIC) funded by the Royal Norwegian Embassy to India (New Delhi) and the Ministry of Human Resource Development, Government of India. REFERENCES 1. ACI 318M-08. (2008). Building Code Requirements for Structural Concrete (ACI 318M-08) and Commentary, American Concrete Institute, Farmington Hills, MI 48331, U.S.A. 2. ASCE (2010). Minimum design loads for buildings and other structures, American Society of Civil Engineers, Virginia, USA. 3. ASCE (2007). Seismic rehabilitation of existing buildings, American Society of Civil Engineers, Virginia, USA. 4. Elwood, Kenneth J., Adolfo B. Matamoros, John W. Wallace, Dawn E. Lehman, Jon A. Heintz, Andrew D. Mitchell, Mark A. Moore, Michael T. Valley, Laura N. Lowes, Craig D. Comartin, and Jack P. Moehle. (2007). Update to ASCE/SEI 41 Concrete Provisions. Earthquake Spectra 23:(3), IS 456. (2000). Plain reinforced concrete-code of practice, Bureau of Indian Standards, New Delhi, India.

9 6. IS 1893-Part1. (2002). Part I: Criteria for earthquake resistant design of structures -General provisions and buildings, Bureau of Indian Standards, New Delhi, India. 7. Khose, V. N., Y. Singh, and D. Lang. (2012). A Comparative Study of Selected Seismic Design Codes for RC Frame Buildings. Earthquake Spectra 28:(3). 8. SAP2000 Advanced : Structural Analysis program. Computers and Structures, Inc., Berkeley, CA USA.