Document Type : Research Article
Authors
1
MSc student in structural engineering, Department of civil engineering, Babol Noshirvani University of Technology
2
Faculty of structural and earthquake engineering, Department of civil engineering, Babol Noshirvani university of technology, Babol, Iran
3
Faculty of Structural and Earthquake Engineering, Department of Civil Engineering, Babol Noshirvani University of Technology, babol, Iran
Abstract
The basic idea of seismic design in today's codes is the capacity design approach. In moment frame systems, the desired mechanism is formed by the formation of plastic hinges at the two ends of the beams and the base of the first story column. To ensure preventing the formation of undesired mechanisms, the capacity design is used. For this purpose, the beam plastic moments are considered Displacement-Controlled (DC) actions, and the bending moments and shear forces in the columns (except the bending moment in the base of the first story column) are considered as Force-Controlled (FC) actions and must be designed to remain linear during earthquakes. In order to prevent the formation of plastic hinges and shear failure in columns, two issues should be considered in the design: (1) design of columns on the basis of the bending moment capacity of the beams plastic hinges, and (2) taking dynamic amplification factor into account which is mainly due to the higher mode effects. Although the first issue has been considered in most of the design codes, but the amplification of the column internal forces due to the higher mode effects has only been considered in some of the design codes. In this study, the higher mode effects on three reinforced concrete moment frames were investigated. For this purpose, three 8-, 12- and 20-story buildings, which were designed based on U.S. seismic codes, were subjected to 11 earthquake records. The nonlinear time history analyzes were performed for both Design-Based Earthquake (DBE) and Maximum Credible Earthquake (MCE) levels. The results of the analyzes based on the average plus the standard deviation index for DBE earthquake show that the column moments increase more than 50%, which is significantly influence the design considerations. The increase of shear forces in columns also reaches about 20%, which cannot be ignored due to the type of shear failure which is brittle and sudden. So, the shear force amplification should also be considered in the capacity design of columns. In addition, increasing the earthquake intensity from the Design-Based Earthquake (DBE) to the Maximum Credible Earthquake (MCE) levels increase the bending moment amplification factor about 20%. However, this issue shows less effect on the shear force amplification factors and the increase is usually less than 10%.
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