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

Abstract

The object of this study was to determine experimentally the influence of selected reinforcing steel mechanical properties on wall deformation capacity. Four large-scale T-shaped reinforced concrete wall specimens with different types of reinforcement were subjected to reversed cyclic displacements. The primary variables were the yield strength (𝑓𝑦) and the tensile-to-yield strength ratio (𝑓𝑡/𝑓𝑦) of the reinforcing bars. The study also aimed to identify the minimum uniform elongation (𝜀𝑠𝑢) and fracture elongation (𝜀𝑠𝑓) required of high-strength reinforcement for use in earthquake-resistant concrete structures.

Test data are presented from four walls, T1 with conventional Grade 60 (420) reinforcement and T2, T3, and T4 with high-strength Grade 100 (690) reinforcement. The flexural reinforcement consisted of No. 6 (19) bars inside confined boundary elements and No. 4 (13) bars elsewhere. Confining reinforcement in boundary elements consisted of No. 3 (10) hoops and crossties of the same grade as the flexural reinforcement. Wall T1 had 𝑓𝑡/𝑓𝑦 of 1.34 and 1.39 for the No. 6 (19) and No. 4 (13) bars, respectively. Walls with Grade 100 (690) reinforcement had 𝑓𝑡/𝑓𝑦 of 1.15 and 1.10 for T2, 1.23 and 1.21 for T3, and 1.36 and 1.20 for T4. All walls were loaded with a shear span-to-depth ratio of 3 and had the same nominal dimensions and concrete compressive strength (8 ksi or 55 MPa). Axial load was limited to the self-weight of the wall and testing apparatus. The walls were designed to have nearly the same nominal flexural strength. Flexural yielding controlled the lateral strength of the walls, inducing an average shear stress up to 3.5√𝑓𝑐′, psi (0.29√𝑓𝑐′, MPa). To ensure large tensile strain demands in one of the loading directions, the neutral axis depth at nominal flexural strength did not exceed the thickness of the flange. Design of the walls complied with ACI Building Code (ACI 318-14) requirements for special structural walls with additional detailing requirements applied based on ATC 115.

Walls designed for a target flexural strength using Grade 60 (420) or Grade 100 (690) reinforcement, with similar 𝑓𝑡/𝑓𝑦 for the primary flexural reinforcement, had similar strength and deformation capacity (defined as the drift cycle completed before a 20% loss of lateral strength). The limited test data indicate that walls with low axial force and reinforcement that satisfies tensile-to-yield strength ratio (𝑓𝑡/𝑓𝑦) ≥ 1.2, uniform elongation (𝜀𝑠𝑢) ≥ 6%, and fracture elongation (𝜀𝑠𝑓) ≥ 10% exhibit a minimum drift ratio capacity of 3%. Walls T1, T2, T3, and T4 exhibited drift ratio capacities of 3.7, 1.8, 3.0, and 3.9%, respectively.

Moment-curvature analyses were conducted to evaluate the use of the plastic hinge model for estimating the deformation capacity of the walls and the maximum strain demands. The use of the plastic hinge model was conservative for estimating wall deformation capacity with simple rules for the plastic hinge length depending on whether deformations due to shear and strain penetration are considered. However, the plastic hinge model did not consistently provide conservative estimates of the maximum strain demands.