A New Efficient Multiple-Node Constraint Approach for FE Analysis of Radius-cut RBS Moment Frames in Highly Seismic Areas

 Dawit Alemayehu, Adil Zekaria, Samuel Kinde Kassegne⁺

Civil Engineering Department, Addis Ababa University, Addis Ababa, Ethiopia

⁺ Henri Samueli School of Engineering, University of California - Irvine, Irvine, CA

Abstract

The 1994 Northridge, California earthquake opened a new chapter in the design of steel moment frames. It was previously believed that steel moment frames are the ideal solution to resist earthquake induced ground motion due to their high ductility. However, observations after the Northridge earthquake proved otherwise. It was found that a number of brittle fractures were formed in weld at the beam-column connection. No actual collapse of moment frame buildings occurred, but had the earthquake continued for a longer time, greater damage and even total collapse could have resulted. Exactly one year after the Northridge earthquake, another earthquake hit Kobe, Japan, but with much more adverse consequences, killing more than 5000 people. A number of steel moment frame buildings suffered partial or total collapse, again confirming the fact that steel moment connections were not as safe as they were previously believed to be.

Following the Northridge earthquake, a series of studies were conducted to investigate the cause for the poor performance of the moment connections, and to come up with improved seismic design and retrofitting techniques for steel moment frames. The SAC Steel Project, over a 6-year period, published a series of documents on improved design of steel moment frames. One of the suggested solutions is the use of pre-qualified connection types for the design of new steel moment frames. The Reduced Beam Section (RBS) Connection is a pre-qualified connection in which a portion of the top and bottom flanges is cut out at some distance from the face of columns. The RBS is intended to pull the plastic hinge away from the face of the column where the stress and strain demands may cause the weld to fracture.

Currently there is no an efficient and accurate method for the analysis of moment frames with RBS. Analysis that does not account for the reduction in the beam stiffness at the RBS is too approximate. On the other hand, full finite element modeling of moment frames using programs like ANSYS is not efficient and practical.

This study intends to devise and implement an efficient and accurate method for the linear elastic analysis of moment frames with RBS. This is to be accomplished using the finite element method. The technique to be employed involves coupling elements of different dimension using multi-point constraints. The central portion of the beam will be modeled using computationally cheap beam elements while the RBS region will be modeled using either shell or solid elements to capture the local response. Ultimately a computer program implementing the multi-point constraints will be developed.