Incremental Dynamic Analysis of Concentrically Braced Frames Subjected to Near Field Ground Motions

In last decades, most of the famous seismic design codes, had concentrated mainly on far field earthquakes. Generally no special requirements were introduced to minimize near fault earthquake effects. Near field ground motions have caused several structural damages in recent decades, causing seismic codes to be updated with related requirements. As a result, it seems necessary to evaluate domestic seismic design codes and their requirements to see their effectiveness in designing safe structures against near fault earthquakes. In this paper seismic behavior of concentrically braced frames (CBFs) designed based on different seismic design codes is comparatively studied. Various Frames of different heights, (5, 8 and 12 stories), and bracing types (X-bracing and inverted V bracing) are designed based on the Iranian seismic code (standard no.2800) and also taking into account the near fault requirements of AISC-ASD 89 and UBC-97. These frames are analyzed using a nonlinear time history analysis method, namely “Incremental Dynamic Analysis” (IDA) under near field ground motions. IDA is an emerging method in analysis of structures which allows estimating seismic capacity, limit states and demand via series of nonlinear dynamic analyses using multiple scaled ground motion records. A total of 15 near filed time histories are selected according to special characteristics namely: fault distance to site (less than 10 km), fault direction, earthquake moment magnitude, time history frequency content, time history velocity content, shear wave velocity in the site soil and near fault wave’s pulse nature.
The 15 selected earthquake time histories were scaled in several steps. To achieve a more accurate point for yielding point of the frames, 0.1g steps were used in the initial elastic region.
On other important step was to select proper intensity measure and damage measure. According to FEMA-350 requirements, the maximum inter story drift was selected as damage measure. For selecting a proper intensity measure, four intensity measures were selected and compared together, namely: first mode spectral acceleration , first mode spectral velocity , peak ground acceleration PGA and the equivalent first n modes spectral acceleration. Among them, first mode spectral acceleration showed the least dispersion in the analysis results.
Finally, all the 20 frames designed were analyzed using the 15 time histories selected in multiple steps and IDA curves were extracted. Using FEMA requirements the points corresponding to 2 performance levels namely IO and CP were defined on summarized IDA curves using which it was possible to compare the performance of the frames.
The results of this paper imply that frames designed based on the near field seismic design criteria of UBC-97 have better performance under near-field earthquakes comparing to others. It can be concluded that the requirements of the Iranian seismic code should be updated according to the effects of near fault earthquakes. Besides, frames with X-bracing system showed better performance in comparison with chevron ones. Also the deficiencies in the seismic requirements of the domestic codes are more obvious while designing taller frames and moving from 5 story frames to 12 story ones, their seismic performance obviously deteriorate.