Experimental Investigation to Compare the Cyclic Response of A500 and A1085 HSS Braces

Abstract

HSS are often used as bracing members in special concentrically braced frames (SCBFs) in areas of high seismicity. These systems efficiently resist seismic forces through the stable and ductile inelastic yielding and buckling of braces. However, the current requirements for braces in SCBFs have reduced their practicality and usage in recent years, in particular due to the stringent local slenderness requirements for ductile members and the Ry factor. Traditionally, ASTM A500 HSS sections are used in SCBFs. However, AISC 341 now permits the use of ASTM A1085 HSS sections. The ASTM A1085 specification requires tighter control over the material and geometric properties of HSS, allowing for more efficient designs, i.e. the same section has a smaller local slenderness ratio for an A1085 HSS section than an A500 HSS section. Additionally, the Ry factor for A1085 members is smaller than that of A500 members, thus reducing the brace force demands used to design the remaining components of the SCBF. Previous research has indicated an increase in ductility associated with the use of A1085 HSS in braced frames, which could make SCBFs with A1085 braces a more commonly used seismic force resisting system. The objective of this research is to investigate the behavior of A1085 square HSS members subjected to inelastic cyclic loading and compare their structural response to that of conventional A500 HSS members. A test frame was designed and constructed to accommodate testing, with a maximum specimen length of 20 feet. Two hydraulic actuators acting in parallel were used to apply forces up to 1000 kips in tension and 650 kips in compression, with a displacement range of ±10 inches. Ten different square HSS shapes were tested in both A500 and A1085 steel using the same symmetric quasi-static displacement history. Study parameters such as brace local and global slenderness ratios, steel type, and steel producer were investigated in this research. The local slenderness ratios of the test specimens ranged from 9 to 25.7, and the global slenderness ratios ranged from 60 to 127. The results of this experimental program indicate that the seismic performance of both the A500 and A1085 HSS members depends considerably on the local and global slenderness ratios, as expected. The performance of the A500 and A1085 HSS sections were evaluated based on their strength, deformability, and energy dissipation capacity. Material testing revealed some differences in strength and toughness between the HSS members from different steel producers. Minor differences in structural response between the pairs of A500 and A1085 HSS sections were observed through component testing, but no significant differences in deformability or energy dissipation capacity between the types of steel were observed across the entire test series. Brace damage in the form of local deformations and striations typically developed at similar levels of axial deformation in the A500 and A1085 HSS sections. Plots of brace deformation range prior to fracture and energy dissipation capacity showed that the A500 and A1085 HSS sections exhibited similar seismic performance across a wide range of width-to-thickness and global slenderness ratios. This result might be due to the symmetric displacement history used in this test series; this history does not replicate the displacements that a brace in a braced-frame test or building would experience. It is well established that low-cycle fatigue depends on the applied displacement history. Therefore, the next phase of the research will investigate the impact of the tube producer as well as the displacement history on structural response.