Distribution of internal forces in steel moment frames with knee braces in the near-fault region

Moment resistant steel frame is one of the structural systems used to resist earthquake loads in buildings. Many existing steel structures with this system need retrofitting to overcome deficiencies and to resist seismic loads. In the present study, the seismic behavior of rigid frames with knee-type braces were investigated and the distribution of internal forces in frame elements such as beams and columns were discussed. Two types of structures, including moment frames and braced frames with knee-type braces in different positions of joints, for 10, 20 and 30 stories were designed per the Iranian Seismic Code, the Iranian Steel Structures Design Code and the Seismic Provisions. Dynamic analysis using 5 earthquake records for moment frames and braced frames were carried out to obtain dynamic responses. According to the analysis results, the distribution of internal forces such as bending moments, shear forces and axial loads for braced frames in beams and columns turned out to be different from moment frames. Comparing shear and moment diagrams in moment frames and braced frames showed that utilizing knee-type braces cause the moments and shear forces at fixed ends to decrease. It means that the joint forces in braced frames are dissipated by a group of elements that are restricted by knee elements which change the joint from a point joint to a zonal joint.

Keywords:

steel moment frames , knee braces , near-fault region , seismic design