Earthquake-Resistant T-Shaped Concrete Walls with High-Strength Steel Bars

Abstract

This study examined the effects of the mechanical properties of high-strength reinforcement on the seismic behavior of concrete walls. The primary variables were the nominal yield strength fy, 100 ksi (690 MPa) or 120 ksi (830 MPa), and the tensile-to-yield strength ratio ft/fy, nominally 1.2 or 1.3. Two large-scale T-shaped structural walls were subjected to reversed cyclic loading to assess their strength and deformation capacity. Test results were compared with data from four walls tested by Huq et al. (2017) at The University of Kansas to evaluate the influence of the uniform elongation εsu, and the fracture elongation εsf, in addition to fy and ft/fy, of high-strength reinforcement on the deformation capacity of concrete walls subjected to reversed cyclic displacements. The four walls tested by Huq et al. (2017) had nearly identical geometry, detailing, test setup, and loading protocol to the two walls of this study, but had different reinforcement mechanical properties.

Two walls were tested, one with Grade 120 (830) reinforcement (Wall T5), the other with Grade 100 (690) reinforcement (Wall T6). Confined boundary elements were provided at the three tips of the T section consisting of the main flexural reinforcement (No. 6 or 19 mm bars) enclosed by No. 3 (10 mm) hoops. Outside the boundary elements, No. 4 (13 mm) bars were used as longitudinal and transverse reinforcement. The nominal concrete compressive strength of 8 ksi (55 MPa) and wall dimensions were kept constant in both specimens. The walls had a thickness of 10 in. (25 mm) and height-to-length ratio of 3. Wall stem and flanges were 100-in. (2540-mm) long. The axial load was only the self-weight and the weight of the testing apparatus. The T-shaped cross section allowed a shallow neutral axis depth (within the flange) at flexural nominal strength and induced high tensile strain demands in the main flexural reinforcement (within the stem). The walls were designed such that flexural behavior controlled their strength inducing a maximum shear stress of approximately 4√f’c, psi (0.33√f’c, MPa). The design complied with the ACI Building Code (ACI 318-14) and incorporated the additional detailing recommendations in ATC 115 for Grade 100 reinforcement.

Wall T6 with Grade 100 (690) reinforcement had similar strength and deformation capacity to the four walls tested by Huq et al. (2017) at The University of Kansas with Grade 60 (420) reinforcement in T1 and Grade 100 (690) reinforcement in T2, T3, and T4. These walls had a drift ratio capacity not less than 3% if the tensile-to-yield strength ratio (ft/fy) of the flexural reinforcement was greater than 1.18, the uniform elongation (εsu) was greater than 6%, and the fracture elongation (εsf) was greater than 10%. Wall T5 had a drift ratio capacity of 2.3% with Grade 120 (830) flexural reinforcement having ft/fy = 1.32, εsu = 5.3%, and εsf = 8.6%.

Moment-curvature analyses were conducted to support the development of closed-form solutions for estimating the deformation capacity of the walls and strain demands on reinforcing bars and concrete. Formulations were derived to include deformations due to shear and strain penetration (or bond slip) to provide conservative (safe) estimates of deformation capacity and strain demands.