Effect of Column Orientation on Response Modification Factor (R-Factor) of Reinforced Concrete Frames

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Zaid, Ahmad Mahmoud
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An-Najah National University
Response Modification factor (R-Factor) is an essential seismic design parameter, which is typically used to describe the level of inelasticity expected in structural systems during an earthquake and is used to reduce the anticipated earthquake load due to the inherent inelasticity of the structure. International building codes provide fixed values for this factor for each category of building system despite the fact that its value depends on the details of the structural system and thus should differ for each building. One of the aspects of buildings in Palestine is their irregularity and this includes the disorientation of columns strong axes in the building plan to suit architectural needs. In Palestine, the international codes are generally applied with little to no guidelines on the validity of these codes to the buildings being designed. To-date, there are no guidelines as to how this R-Factor would change due to the disorientation of main axes of the load-bearing columns in the building. This study comes as a step towards investigating the validity of the code-specified values of the R-factor for framed buildings with disoriented columns. To achieve the above-mentioned goal, pushover analysis is considered as a nonlinear procedure to predict the inelastic behavior of framed buildings, by exposing the structure to increasing lateral loads, until failure occurs. The finite element software SAP2000 is used to generate the nonlinear behavior curve through incremental elastic-plastic analysis with concentrated plasticity in the plastic hinges within the structural members. Two building layouts were used in the study, one square and the other is rectangular, with variable number of storey’s and variable column orientation. The results show that the R-Factor increases as the number of storey’s increase, and it attains a maximum value when the loading direction coincides with the strong axes of the columns. The R-factor is minimum when the main quake load coincides with the weak axes of the columns. These results were invariable for both building layouts. Also, it is found that the R-Factor recommended by the seismic design provisions (IBC 2012 for example) may not be conservative for use in buildings with disoriented columns. In fact, it is found that for buildings of 4 floors, the value of R-factor from IBC 2012 is higher than that obtained from the push over analysis. This means that using IBC2012 value of R-Factor would give lower induced seismic forces for design, which may lead to detailing level that does not warrant the realistic R-Factor for the building being designed. The study is only a first step towards scrutinizing the validity of the international building codes for use in Palestine and further research is needed to advance this study. As a future research topic, it is recommended to conduct nonlinear time-history analysis using actual earthquake records in order to compare the inelastic behavior of these buildings to the actual earthquake loads in these buildings.
Column Orientation , Response Modification Factor