Discussion: Seismic response of concentrically braced steel frames

Abstract

The article discusses the nonlinear response of a number of braced frame structures designed in accordance with CAN/CSA-S16.1-M89 (CSA 1989) and the National Building Code of Canada (NRC 1990), using a time history analysis. The article considers designs based on the rules for ductile braced frames, nominally ductile braced frames, and frames with no provisions for ductility. Firstly, the writer found some aspects of the paper somewhat confusing, particularly in the authors' use of the terms and when referring to the braces acting in compression. From the information supplied it appears that the failure mechanism in is buckling, not yielding. The use of in reference to a buckling-type failure is unusual. Further, the writer does not see how the discussion of compression ductility is relevant to the performance of an X-braced system; the key element is the tension brace, not the brace. Secondly, from the article, it appears that most of the braces either did not yield or only yielded once in tension. While not conclusive, this may indicate that the approach taken in CSA (1989) and other codes for the design of braced frames is too conservative. This hypothesis is supported by the generally good behaviour of braced frame structures in prior earthquakes, despite the bracing being both substantially smaller in size and more slender than required by current standards. One possible explanation for this apparent discrepancy is the recent focus on the energy dissipation capabilities of ductile elements. Due to their pinched hysteresis loop, steel braces are not able to dissipate energy as well as plastic hinges in either steel or confined concrete. What appears to have been neglected, however, is the isolating effect once the braces have yielded in tension, which reduces the energy braces. The reduced energy input into the structure partially offsets the poorer dissipation capacity of braced frames when compared to structures with plastic hinges. If this explanation is correct, then there are only two ways to rationally design a braced frame structure: either elastically or as a ductile braced frame. For elastic design, it would be prudent to use an R value of less than 1 to account for potential peak responses, as essentially no provisions would be made for ductility. For a ductile braced frame, it is imperative that general yielding can occur in the braces, while the connections and other members must be capable of resisting the forces resulting from the yielding of the braces and the impact that occurs as the braces take up load. As many braces are field-bolted, there is a tendency for the braces to fail at the net section through the bolts before general yielding can occur (Ellingwood 1980). To preclude this type of failure, the writer recommends the area around bolt holes be reinforced, so that the net section at the bolts is greater than the gross area of the brace. This should permit yielding across the gross section of the brace, greatly increasing ductility. This line of reasoning, while different from the authors', results in many of the same conclusions expressed by them.