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Received: 23 September 2009 / Accepted: 24 October 2010 / Published online: 17 November 2010© Springer Science+Business Media B.V. 2010Abstract The current Indian Standard (IS) code for seismic design of structures(IS 1893:2002) specifies the use of time history analysis for structures with height greaterthan 40m. However, for structures less than 40m it recommends the concept of equivalentstatic analysis. This study attempts to investigate the adequacy of the current design codewhen it comes to the actual evaluation of structures shorter than 40m subjected to seismicloading using dynamic analysis as opposed to the code specified static analysis. Incrementaldynamic analysis, which subjects a structure to a progressively increasing series of intensitymeasures, has been adopted here for the purpose. 52583
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Three 2D moment resisting steel struc-tures under the 1991 Uttarkashi and the 2001 Bhuj earthquakes (both of which predate thecurrent IS1893) have been studied—a single storeyed portal frame, a 2 storey 3 bay frameand a 3 storey 2 bay frame. While it can be argued that two records are never enough forany generalization, and that only a full probabilistic analysis can determine if the limitingcollapse prevention probability has been exceeded for these structures, the IS code in bothcases does significantly under predict the seismic demands on the structures. At the sametime, and perhaps why the codal provisions usually work, the structural capacities are inmostcases underestimated as well. These suggest that a thorough study is in order and that thereis scope for rationalization in the IS codal provisions.Keywords Base shear · Steel · Deflection · Collapse ·
Earthquake · Incremental dynamicanalysis · Indian Standard code · Pushover analysis · Capacity demand ratio 1 IntroductionThe engineering challenges from major earthquakes in the Indian context cannot be overem-phasized. More than about 60% of the land area is considered prone to shaking of intensityVII and above (MMI scale). In fact, the entire Himalayan belt is considered prone to greatearthquakes of magnitude exceeding 8.0, and in a short span of about 50 years, four suchearthquakes (in Assam (1897), in Kangra (1905), in Bihar-Nepal (1934) and in Assam Tibet(1950)) have occurred (Jain andNigam2000).Although earthquake engineering applicationsstarted quite early in our country, extensive damage during several moderate earthquakes inrecent years (Shekhar et al. 2004) indicate that earthquake risk in the country has beenincreasing alarmingly and that there is significant scope of improvement in our design codes.In India, IS1893 (BIS 2002) is the main code outlining seismic design provisions. Forlow-rise structures (<40m high) the code focuses the design approach on base shear and itsdistribution to various floors of a building in an equivalent static sense (either taking the firstmode of vibration of a cantilever column, or through a modal analysis taking into accountthe first few modes). An effort has been made in this paper to study the actual response ofthree low rise 2-D steel moment-resisting frame structures (designed to IS1893 and IS800(BIS 2007) provisions) to two recent earthquake records (1991 Uttarkashi as captured fromstation Bhatwari and 2001 Bhuj as captured from station Ahmedabad), to find their yield andultimate capacities through full dynamic analyses, and to compare the results with the codalprovisions. 2 Structural design and evaluation methodologiesThe current Indian Standard code for designing/evaluating a structure for seismic loading isIS 1893:2002. IS 1893 recommends the use of dynamic analysis for certain structures (regu-lar structures with height greater than 40m and irregular structures with height greater than12min zone V). In this study however, dynamic analysis is not used for computing the designbase shear. This is because the structures used in this study, in addition to being shorter than40m, have almost their entire modal masses concentrated in the first mode of vibration, asdetailed below. IS 1893 (Clause 7.8.4.2) suggests incorporating only those modes for whichthe sum of modal masses is at least 90% of the total seismic mass to compute the design baseshear for each floor in each mode. The shear force on the i th floor in kth mode isQik = Akφik PkWi (1)where, Ak = design horizontal seismic coefficient for mode k, Pk = modal participationfactor,