An envelope-based pushover analysis procedure is presented which assumes that the seismic demand for each response parameter is controlled by a predominant system failure mode that may vary according to the ground motion. To be able to simulate the most important system failure modes, several pushover analysesneed to be performed, as in a modal pushover analysis procedure, whereas the total seismic demand is determined by enveloping the results associated with each pushover analysis. The demand for the most common system failure mode resulting from the "first-mode" pushover analysis is obtained by response history analysis for the equivalent "modal-based" SDOF model, whereasdemand for other failure modes is based on the "failure-based" SDOF models. This makes the envelope-based pushover analysis procedure equivalent to the N2 method provided that it involves only "first-mode" pushover analysisand response history analysis of the corresponding "modal-based" SDOF model. It is shown that the accuracy of the approximate 16th, 50th and 84th percentile response expressed in terms of IDA curves does not decrease with the height of the building or with the intensity of ground motion. This is because the estimates of the roof displacement and the maximum storey drift due to individual ground motions were predicted with a sufficient degree of accuracy for almost all the ground motions from the analysed sets.
COBISS.SI-ID: 6305121
The closed-form solution for assessing the proportion of the mean annual frequency of limit-state exceedance as a function of integration limits is introduced, in order to study whether or not the mean annual frequency of limit-state exceedance is overestimated if the lower and(or) upper integration limit of the risk equation are(is) not selected in a physically consistent manner. Simple formulas for assessing the threshold value of the lower and upper integration limits are also derived. These formulas can be used to quickly assess the significant range of ground motion intensity that affects the mean annual frequency of limit-state exceedance. It is shown that the threshold values of the integration limits depend on the median intensity causing a limit-state, the corresponding dispersion and the slope of the hazard curve in the log domain. For several reinforced concrete buildings located in a region with moderate seismicity, it is demonstrated that the mean annual frequency of collapse can be significantly overestimated when assessed by integrating the risk equation over the entire range of ground motion intensity.
COBISS.SI-ID: 6420833
Usually seismic design procedures are based on elastic analysis by using design acceleration spectrum, which implicitly takes into account the ability of inelastic energy ab-sorption of the structural system, and capacity design principles. Thus, current standards for earthquake-resistant design of buildings do not control seismic risk to such an extent that would be acceptable for all types of structures and for all investors. Development is therefore oriented towards advanced design methods, which can be used to achieve well informed deci-sion-making based on target reliability. In this paper, the risk-based seismic design procedure is applied in order to design areinforced concrete building for a tolerable seismic risk. The procedure is based on the use of nonlinear methods of analysis (pushover-based method and nonlinear time history analysis). Herein the procedure for risk assessment is briefly described. It involves envelope-based pushover analysis procedure and the new closed-form solution for estimating the annual probability of exceeding selected limit states, which provide less con-servative results as that proposed by Cornell. The procedure is demonstrated by designing an eight-storey RC frame building for tolerable risk. It is shown that several iterations were needed in order to fulfill the requirement of tolerated seismic risk. However, the final configu-ration of the structure was checked using nonlinear response history analysis, where it is shown that the envelope-based pushover analysis procedure provided a slightly conservative estimate of collapse risk. The proposed design procedure enables explicit estimation of the seismic risk and verification of the collapse mechanism, which is an advantage in comparison to the design procedure prescribed by Eurocode 8.
COBISS.SI-ID: 6303585