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Reinforcing bar physical properties are main determinants for reinforcing-bar seismic demands. Consequently, seismic codes set appropriate single upper or lower limits on reinforcing bar yield strength and tensile/yield ratio, but they do not consider the variable-parameter effects of the shape of the reinforcing-bar stress-strain curve on what tensile/yield ratios and ductilities should realistically be required of reinforcing bars in seismic-resistant structures. Therefore, a theoretical study was performed to evaluate the effect of range of allowable steel yield strength, shape of steel stress-strain curve (strain and tangent modulus of elasticity at onset of strain hardening), and beam slenderness (S/d, where S is the clear span length and d is the effective depth to the reinforcing bar centroid, Figure 1) on the minimum values of steel tensile/yield ratio and useful ductility that are necessary to accommodate 2% seismic drift by plastic hinging at the end of beams, Figure 1, of concrete rigid frames reinforced with Grade 60 steel reinforcing bars.

The application of headed reinforcement as a replacement for conventional reinforcement was investigated in two projects relating to the seismic design of bridges. In the first project, a test unit composed of a column, cap beam, footing and knee joint was designed entirely with headed reinforcement, and in footing and knee joint was designed entirely with headed reinforcement, and in the second project a test unit representative of a multi-column bridge bent was investigated, having a cap beam design utilizing both headed reinforcement and a mechanical coupler system. In both investigations the use of recently developed reinforcement products facilitated simplified detailing, particularly in the cap beam/column joint region, resulting in reduced reinforcement congestion the joint zone and improved constructability. The design and performance of the test units under simulated seismic loading are presented.

Headed reinforcement uses one or more anchorages, called heads, attached to the ends of steel reinforcing bars. Such heads serve to develop a bar in a relatively short distance, and can also better confine the interior concrete. For over a decade, headed reinforcement has had extensive field use in major structures subjected to cyclic fatigue and dynamic loading, as well as thorough laboratory testing on both bare steel bars as well as on concrete members with headed reinforcement. Such test have also demonstrated the superior performance of headed reinforcement under seismic loading conditions, even in high moment zones, and joint regions. This paper addresses both: (I) aspects of design and detailing with headed reinforcement for seismic resistance, and (ii) aspects of the concrete material performance as it is modified by headed reinforcement. Specific advanced design tools are discussed including empirical equations, strut-tie modeling procedures, a new membrane stress theory, and a new cyclic reinforcing bar bond-slip theory, together with design examples for bridge structures. Currently, ACI 349, CSA 474, and several overseas codes provide design rules for headed reinforcement. Where necessary these rules may be supplemented by experience, engineering judgment, empirical guidelines, and test results. New standards, regarding the use of headed reinforcement in concrete, are pending with both ASTM and ACI 318; which when incorporated should further facilitate the design process.

Theoretical moment-curvature analyses were performed in this study for analyzing the effects of cyclic behavior of reinforcing steel on seismic performance of reinforced concrete members. Cyclic stress-strain relations for reinforcing steel were estimated from an analytical model proposed in the literature and considering the onset of buckling of a steel rebar defined according to an approach proposed in this study. The ACI318-95 provisions for evaluating probable flexural strength are used for relating interstory drift and strain demands in longitudinal reinforcement of typical sections of reinforced concrete members subjected to earthquake loading.

The use of high strength reinforcement in seismic zones 3 and 4 dates back to 1979, in the design and construction of the Continental Plaza Building, Seattle, Washington, a 37-story concrete building. The design team comprised of Whitley-Jacobson Company, engineer of record; Neil Hawkins, Professor of Engineering, University of Washington; the fabricator, supplier of WWR, and the architect. A combination of shear wall and moment frame design was selected as the most economical lateral force resisting syst4em. Grade 75 WWr was used as horizontal reinforcement to confine the column cores.