Successful NEES Grand Challenge Tests on Non-Ductile Beam-Column Joints Experiencing Axial Collapse under Simulated Seismic Loading

The axial collapse potential of existing gravity load designed RC buildings is a great concern during intense seismic events.

Wael Hassan, UC Berkeley PhD Candidate, under supervision of Jack Moehle, UC Berkeley Professor and former PEER director is experimentally and analytically investigating the axial collapse likelihood following shear failure of unreinforced older-type construction corner beam-column joints.

Testing of four full scale corner beam-column joint subassemblies, including floor slabs, is under way. The goal is to evaluate unreinforced corner joints shear strength and axial residual capacity under high axial load reversals varying with lateral loads; representing intense ground motion overturning moment effects. Gravity axial load is 0.20f ’c Ag , while the overturning axial loads vary with displacement reversals to range the joint axial load from tension to high compression (0.45f ’c Ag). A sophisticated test setup was constructed to simulate realistic boundary conditions of actual buildings. A drift based history is used to simulate lateral loading. The main test parameters are axial load level, joint aspect ratio, beam reinforcement ratio, and loading history (unidirectional vs. bidirectional displacement reversals).

The results of this investigation will provide essential input to update strength and ductility provisions of existing buildings assessment documents (ASCE/SEI 41-06). Test results also will help quantify and prioritize the axial collapse vulnerability of shear damaged unreinforced beam-column joints. Throughout the analytical stage of the current research, a simplified shear strength model was developed and verified using test results. In addition, test and analytical model outcomes will be implemented into nonlinear dynamic analysis simulations of existing RC buildings aiming to assess collapse risk during seismic events.

Two specimens have been tested so far, while the remaining two specimens will be tested by the end of September 2010. Stay tuned to watch the interesting upcoming bi-directionally loaded joint test!