Experimental Investigation into the Cyclic Response of A1085 HSS Braces

Abstract

Concentrically braced frames (CBFs) are commonly used as the seismic force resisting system in buildings, where the ordinary (OCBFs) and special concentrically braced frames (SCBFs) provide moderate and high ductility, respectively. In both OCBFs and SCBFs, the braces are the primary yielding component and square HSS sections are commonly used for braces. One of the most important design constraints to provide ductility is the upper limit on the width to thickness, b/t, ratio for braces. Prior research has verified that braces that meet the highly-ductile b/t limit for ASTM A500 square HSS braces have the required stable cyclic behavior and strength for SCBFs. In 2013, a new specification for square HSS was introduced: ASTM A1085 which has tighter control on material properties and dimensions. As a result, A1085 HSS have smaller width-to-thickness ratios than the same nominal A500 HSS section. Previous research has used A1085 HSS braces within a full SCBF and identified that they may have increased ductility relative to A500. To understand the full benefits of A1085 braces, a multi-phase, large scale testing program of full-size braces subjected to cyclic loading was undertaken. Ten reference A500 HSS braces were tested along with 31 A1085 HSS braces. In the initial phase of the study, the response of the A500 and A1085 sections were compared; the response of the two brace types was similar. A second phase was conducted to study the impact of geometric and material properties, steel type, HSS producer, and loading protocol for the A1085 sections alone. The b/t ratios ranged from of 9 to 25.7; the global slenderness ratios ranged from 60 to 127. As such, most of the studied braces met the compactness limits for OCBFs, some met the limits for SCBFs and others did not meet either. The structural response of each brace specimen was quantified using the axial deformation range, the equivalent story-drift range, and energy dissipation capacity. The findings indicate that deformability of the brace, and therefore the frame, is determined by the local compactness and global slenderness ratios. Specifically, deformability increased with smaller local compactness ratios and larger global slenderness ratios. Current design provisions do not recognize the interdependence of the story drift capacity on both the local compactness and global slenderness. Evaluation of the current local slenderness limits for SCBFs and OCBFs showed that the current highly ductile limit is sufficient, but the moderately ductile limit is too conservative. The brace response did not depend on HSS producer, toughness, or type (A500 or A1085). The story-drift range was independent of load history, while the maximum deformation in either direction (tension or compression) does depend on the load history.