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The peakbase shear for this amplified record is identified and is found to be Ah = 0.85. The peak IDRfor the same record is 0.79%. Collapse occurs not via mechanism formation but when thestructure violates the 0.3∗ Se law as mentioned in previous section, at a PGA of 1.05 g whenSa = 2.4 g. Just prior to collapse, the peak values of base shear reached is Vb = 112 kN(Ah = 0.95) and the peak IDR is 2.7%.Moving to the Uttarkashi record, the first yielding record anywhere in the structure occursfor Sa = 0.8 g, corresponding to which Ah = 0.835. The peak IDR for the same record is0.725%. Collapse occurs (because it violates the rule described in Sect. 2.4) when the slopeof the graph becomes less than 0.3 which occurs at a PGA of 1.58 g when Sa = 2.2g. Justprior to the collapse, the peak values reached are: Vb = 112.8kN(Ah = 0.96),IDR=3.8%.3.1.4 Analysis of the portal frame resultsThe results from static pushover analysis, actual time history analysis and incrementaldynamic analysis, for both Bhuj and Uttarkashi records, are summarized in Table 2.Fivestructural response categories are selected—deflection, base shear, moments in critical sec-tions and spectral acceleration; the PGA is also listed where relevant. It should be noted thatall design demands (column 3) include the load factor of 1.3, and except deflection, all capac-ities (column 2) incorporate the capacity factor of 1.2. The base shear capacity of 23.4 kN hasbeen found to be that value which causes the combined axial and bending capacity-demandratio of the beam to be 1.0.Let us first look at the time history analyses of the portal frame under the actual Bhujand Uttarkashi records. Clearly, there is significant difference in each category of responseunder the two records—a difference that would perhaps be missed in a code-specified staticanalysis. Further, while the ratios of the structural responses for these two records are around1.5, the Uttarkashi PGA is 2.4 times that of the Bhuj PGA, underscoring the unsuitability ofPGA as an intensity measure.When each of these two records is scaled up, we first reach yield, and if the amplificationis continued, we encounter collapse. In other words, we get the yield and collapse structuralcapacities in each category of response under the given excitation. As may be expected forthis SDOF structure, although the responses under the actual records vary by a factor of about1.5, the yield capacities under the two records are very close (within 5—10% of each other),and the collapse capacities are even closer (with the exception of the roof deflection). We now compare the efficacy of the static analysis for this problem. The design demandcolumn (column 3) is compared with the time history analysis columns; the pushover yieldcolumn is compared with the IDA yield columns; the pushover collapse column is comparedwith the IDA collapse columns. Finally the capacity (as per IS code) column is comparedwith the static and the IDA capacity columns (in yield as well as in collapse).The design demand (per IS code) severely underestimates the actual (i.e., peak dynamic)deflection and base shear demands on the structure; the other demands are comparable. Itshould be remembered that the design demand column is the result of factored loads whereasthe time history analyses make use of unfactored loads.The pushover yield capacities are comparable to the IDA yield capacities; although push-over analysis seems to be conservative in estimating deflection and base shear capacities.The collapse capacities tell a different story: the pushover analysis overestimates all collapsecapacities compared to the IDA counterparts.The code-specified capacity (column 2) grossly underestimates the capacity in every cat-egory of response, by a factor of at least 1.5 at yield, and at least 2.0 at collapse.We now analyze two more structures in the same manner as above and pull the inferencestogether in Sect. 4. 3.2 Two storeyed three bay frameThe second structure analysed in this paper is a 2 storey 3bay steel moment resisting frame(Fig. 9) designed according to the current IS codes (BIS 2002, 2007). Damping coefficient istaken as 5% as before. The first mode period of the structure is 0.70 s. The member sectionsare found to be ISMB 225 and ISMB 250 for beams and columns respectively. The yieldstrength is taken to be 250MPa.The governing criterion is strength. Combined axial and flexural check is performed andbeam (1) is found to be critical with the ratio of 0.98 followed by column (2) with the ratioof 0.92.When subjected to pushover loading (Fig. 10), It is seen that the structure collapses at abase shear coefficient of 0.45. A point worth noting is that the slope of the curve (Fig. 10) just prior to collapse is not too small as compared to the initial slope suggesting that prior tothe structural collapse, hinge formation has not occurred in all the members.The two-storey structure is nowsubjected to Bhuj andUttarkashi records, and the responsetime histories are shown in Fig. 11.When subjected to incremental dynamic analysis, the structure exhibits an interestingphenomenon called resurrection for both records (Fig. 12). There is an intermediate zoneof collapse between Sa = 0.7 g and 1.3 g for Bhuj loading and Sa = 0.7 g and 0.8 g forUttarkashi loading. The structure becomes unstable in these ranges. However, it resurrectsback from failure and reaches collapse at Sa = 1.4 g and 0.9 g respectively.Table 3 summarizes the results of the pushover, time history and IDA analysis of thestructure subjected to Bhuj and Uttarkashi ground motion records. The results are discussedin detail in Sect. 4.3.3 Three storeyed 2 bay frameThe third structure analysed in this paper is a 3 storey 2 bay steel moment resisting frame(Fig. 13) designed according to the current IS codes (BIS 2002, 2007). Beam length andcolumn height are 5 and 4m respectively. Damping coefficient is taken as 5% as before. Thefundamental time period of structure is 0.945 s. The member sections are found to be ISMB350 and ISMB 250 for beams and columns respectively. The yield strength is taken to be300MPa.TheUDL shown here is the factored load obtained bymultiplying actual load by a factor of1.3. 源-自/751+文,论`文'网]www.751com.cn