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Showing 3 results for Higher Modes Effect

F.r. Rofooei, M. R. Mirjalili, N. K. A. Attari,
Volume 10, Issue 4 (12-2012)

The nonlinear static procedures (NSPs) proposed by design codes do not lead to reliable results especially for tall buildings.

They generally provide inconsistent estimates of inelastic seismic demands, especially for the top floors due to their inabilities in

considering the higher modes effects. In this paper, a new enhanced pushover procedure is proposed which is based on the

envelope of the structural responses resulting from two separate pushover analyses as a combination rule. Also, the suggested

pushover analyses are performed using a newly proposed modal load pattern, i.e., the Modal Spectra Combination (MSC), and

the ASCE41-06 required first mode load pattern. The MSC load pattern is consisted of a number of mode shapes combined with

appropriate weighting factors that depend on their modal participation factors, modal frequencies and design spectral values. A

number of 2-D steel moment resisting frame models with different number of stories are used to investigate the efficiency of the

proposed method. The inter-story drifts and the maximum plastic beam moment and curvature responses are used as a measure

to compare the results obtained from the nonlinear time-history analyses (NL-THA) and some other NSPs. The results obtained

through rigorous nonlinear dynamic analyses show that the application of the proposed method leads to acceptable results for

steel MRF systems in comparison to other available enhanced NSPs. The OpenSees program is used for numerical analysis.

F. Khoshnoudian, O. Nozadi,
Volume 11, Issue 2 (6-2013)

It has been pointed out the static lateral response procedure for a base-isolated structure proposed in International Building Code (IBC) somewhat overestimates the seismic story force. That is why in the current paper, vertical distribution of base shear over the height of isolated structures considering higher mode effects under near field earthquakes is investigated. Nonlinear behavior of isolation systems cause variation of frequencies transmitted to the superstructure and consequently higher modes effects should be considered. In this study base shear distribution obtained from nonlinear dynamic analysis is compared with that achieved from IBC for assessment of the international building code. This investigation has been conducted in two parts, in order to have an appropriate base shear distribution formula for isolated structures under near field earthquakes. In the first part using three first mode shapes of isolated structure and introducing coefficient corresponding to each mode, extracted from nonlinear dynamic analysis under near field earthquakes, a new formula has been derived. In the second part, the mode shape coefficients have been obtained theoretically and consequently a new base shear distribution over the height of isolated structures including the isolation system properties under near field ground motions was proposed.
M. Fazlavi, E. Haghshenas,
Volume 13, Issue 1 (3-2015)

In this paper we are going to show the importance of mode identification in microtremor array analysis. The idea come from four concentric ambient noise array recordings with aperture 100 to 1000 meters, performed in southern urban area of Tehran near the shrine of Imam Khomeini. These measurements were part of a comprehensive research project with the aim of determination of deep shear wave velocity model of Tehran alluvial deposits. Using appropriate signal processing techniques, including array processing methods as well as classical and time-frequency horizontal/vertical spectral ratio, the dispersion curves of surface waves, fundamental resonance frequency and Ellipticity of Rayleigh waves, were extracted. In the final step, the shear wave velocity profile of the site was determined by joint inversion of all of these attributes. The results show 2 different energetic trends in dispersion curves, for arrays of aperture 200 and 400 meters that one of them is coincide with 100m aperture array. For array with aperture 1000m any clear trend of energy could be observed because of deficiency of energy in low frequency. The inversion of data obtained by 100m aperture array alone, assuming the dispersion curve as fundamental mode (a common procedure in urban area) result in shear wave velocity that is not match with existing geological information. Performing the inversion, assuming 2 energetic trends, observed for larger arrays one as fundamental mode and another as mode 1 of Rayleigh waves, can modify significantly the shear wave velocity profile in accordance with existing geological and geotechnical information. This study show the importance of extracting of correct dispersion curves with detecting fundamental and higher modes, using array measurement with various aperture at one place to obtain more realistic shear wave velocity profile.

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