Proposed Changes to Steel Column Evaluation Criteria for Existing Buildings

An increasing number of high rise steel buildings, particularly of the “preNorthridge moment frame” era, are being evaluated and retrofitted in the San Francisco Bay Area. In addition to addressing the well understood vulnerabilities associated with the moment frame connections, the evaluations are also frequently identifying apparent column deficiencies when applying ASCE 41 criteria. ASCE 41 currently classifies any steel column with an axial compression ratio greater than 0.5 as force-controlled, which does not permit any flexural yielding. Nonlinear dynamic analyses of three case-study high rise steel moment frame buildings typical of the “pre-Northridge” era demonstrate that these forcecontrolled criteria cannot be satisfied for design level events, and frequently govern the performance measure prior to expected onset of t h e p r e N o r t h r i d g e moment frame connection failure. However, examination of column results typically indicates that plastic rotation demands are extremely low (<0.01 radian) with axial compression ratios less than unity. Other standards, such as the New Zealand Steel Structures Standard (NZS 3404), permit limited flexural hinging in columns with axial load ratios of up to 0.8. Recent testing at the University of California San Diego indicates that the forcecontrolled limitations of ASCE 41 are generally overly conservative for seismically compact steel columns, with little evidence of significant strength or stiffness degradation. The testing also suggests that the ASCE 41 plastic rotation limits may be non-conservative for columns with low levels of axial compression. Based on a review of other national standards, testing and the research used to derive the force-controlled limits in ASCE41, the authors propose a set of alternative modeling and acceptance criteria for steel columns. ATC & SEI 2015 255