Earthquake-Resistant T-Shaped Concrete Walls with High-Strength Reinforcement

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 yield strength f_y and the tensile-to-yield strength ratio f_t/f_y. 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 walls recently tested by Huq et al. (2017) at The University of Kansas to evaluate the influence of the uniform elongation ε_su and the fracture elongation ε_sf, in addition to f_y and f_t/f_y of high-strength reinforcement, on the deformation capacity of concrete walls subjected to reversed cyclic displacements. 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 to concentrate 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 with a wall 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 limited to 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 capacity and induced large tensile strain demands in the main flexural reinforcement (within the stem). The specimens were designed such that flexural behavior controlled their response inducing a maximum shear stress of approximately 4√(f_c^',psi) (0.33√(f_c^',MPa)). The design of the specimens complied with 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 (f_t/f_y) of the flexural reinforcement was greater than 1.18, the uniform elongation (ε_su) was greater than 6%, and the fracture elongation (ε_sf) was greater than 10%. Wall T5 had a drift ratio capacity of 2.3% with Grade 120 (830) reinforcement having f_t/f_y=1.32, ε_su=5.3%, and ε_sf=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.