جستجوی مقالات مرتبط با کلیدواژه « Higher mode effects » در نشریات گروه « عمران »

Nonlinear Time History Analysis (NTHA) contains a complex and rigorous process for structural seismic evaluation. Nonlinear static (Pushover) analysis can simplify this process. This paper aims to develop an energy-based seismic assessment methodology using pushover analysis. This methodology can estimate the response of mid-rise buildings with much fewer computational operations than NTHA and consider higher mode effects. Other advantages of the proposed procedure include using the capacity curve of Multiple Degrees Of Freedom (MDOF) systems directly instead of the Equivalent Single Degree Of Freedom (ESDOF) and computing the energy demand of the structure based on the mean spectrum corresponding to the desired hazard level instead of the various earthquake record spectrums. The proposed methodology converts the pushover capacity curve to the energy capacity curve for each mode, and the energy demand curve is superimposed on it. The intersection of these two curves is considered the target response. NTHA and Modal Pushover Analysis (MPA) are employed to validate and compare the proposed methodology with other ones. Also, 4, 8, and 9-story steel moment frame buildings are selected, modeled, and analyzed using OpenSEES software. The results show that the proposed methodology can estimate the responses of the building with reasonable accuracy compared to the mean results of the NTHA. Also, the proposed method significantly reduces the error of the responses compared to the MPA. Nevertheless, it can be concluded that the proposed energy-based methodology can be a simple, efficient, and rapid alternative for NTHA.

In this research, the placement of energy dissipaters at the base of self-centering base-rocking walls has been investigated. For this purpose, structures with 4-, 8-, 12-, 16- and 20-floors were investigated. The nonlinear dynamic behavior of these structures was investigated subjected to 22 Far-Field (FF), 14 Near-Field without Pulse (NF-non Pulselike), and 14 Near-Field with Pulse (NF-Pulselike) seismic ground-motions. The models have been analyzed in two dimensions via OpenSees software. The considered seismic ground-motions are scaled and applied to the structure at DBE and MCE levels. The results showed that changing the location of energy absorbers can be effective in the value of the moment and shear demands of walls as well as the response of residual drift and roof acceleration. By increasing the distance of the energy dissipaters from the middle of the section wall, the moment and shear demands could be reduced about 14% and 13% at the DBE level, and about 19% and 17% at the MCE level, respectively. Furthermore, the reduction values of the moment and shear demands of the structures under NF-Pulselike and DBE or MCE levels have been observed more than other seismic ground-motions. Finally, increasing the distance of the energy absorbers from the middle of the wall, the accelerations of the roof decrease.

In the present study, the seismic behavior of self-centering rocking wall systems in both types of base-rocking and double-rocking was investigated. To conduct seismic analyses, three sets of seismic records were considered including 22 Far-Field (FF) ground motions and 28 Near-Field (NF) ground motions that half of which are Pulse-like (Pulse). These ground motions were used for nonlinear time-history analysis of structures with 8, 12, 16 and 20 floors. Based on the area of prestressing cables, three types of double-rocking walls are considered and compared with the base-rocking and the fixed base walls. Numerical modelling was conducted via OpenSEES software in two-dimensional space. For the sake of validation, the available experimental data of base rocking and fixed base walls was used. To compare the seismic performance of the structures, some desirability coefficients have been defined. These coefficients were based on the reduction of the higher mode effects and the reduction of the inter-story residual drifts. The results showed that generally, the double-rocking walls provides higher desirability coefficients than the other considered systems. Furthermore, the double-rocking walls by reducing the cable area in the bottom block (R2-H1) are more

In the equivalent static analysis (ESA) procedure the response of a structure is determined based on the first vibrational mode and elastic spectrum acceleration. In this procedure, the assumption of not contributing all vibrational modes in a structure behavior and using a constant behavior factor for a specific structural system may lead to an incorrect estimation of story ductility demands in intermediate- and long-period structures. In this thesis, in order to take into account the effects of higher modes in the linear and nonlinear response of steel moment resisting frames (MRFs), concentrically braced frames (CBFs) and eccentrically braced frames (EBFs), the equivalent static base shear correction factor was estimated. In linear regime, the base shear correction factor is determined as the ratio of spectral base shear to the ESA base shear. These factors were determined for the studied systems located on soil types I, II, III and IV. The obtained results for elastic regime showed that the correction factor depends on the spectral shape and the ratio of modal periods. The effect of higher modes increases as the ratio of the periods of second mode to first mode decreases and also increases as the spectral acceleration drops more rapidly with an increase in period. For inelastic regime, the base shear correction factor was determined using nonlinear time-history dynamic analysis and adjusting story ductility demands to a target value based on an iteration procedure. To this end, some 3-. 10- and 20-story steel MRFs, 7-, 20- and 30-story CBFs and 4-, 15- and 25-story EBFs on soil types I and II were designed. The inelastic regime results showed that applying the base shear correction factor takes into account the effects of higher modes as well as limits story ductility demands to the target ductility.

Information regarding the nonlinear dynamic response of steel moment resistant structures aected by earthquake should be as complete and integrative as possible to insure the ful llment of pre-determined limitations in design. Meanwhile it should be simple enough for implementation by professional engineers. To this end, the main objective of this study is to increase awareness of the nonlinear dynamic response of steel moment resistant frames (SMRF) and to provide methods for measuring the maximum load seismic demands of these structures using maximum global (roof) and interstory (intermediate) demands (which can be estimated by SDOF system demands). A further objective of this study is to provide a relation between interstory drift and the maximum local demand of beams. To this end, a considerable number of SMRF, short, mid and high rise structures, with a dierent number of stories and bays, were modelled in DRAIN2DX and analysed using nonlinear time history (NTHA) and conventional pushover analyses (CPO). The results of studies showed that beam ductility values are signi cantly higher than interstory and global ductility. Furthermore, higher mode eects on the increase of rotational ductility in the upper stories of high-rise buildings are completely sensible. This was seen on the distribution of force and deformation demands in CPO and NTHA analysis states. It is possible to calculate maximum beam ductility, in respect to target ductility, the number of bays and the foundamental period of frame, with acceptable precision, by the proposed equation of the models in this study.