Development of built-up shear links as energy dissipators for the seismic protection of long-span bridges

Abstract

The proposed new east bay crossing of the San Francisco – Oakland Bay Bridge (SFOBB) will use steel shear links as energy dissipators for added protection during strong earthquakes. These large-capacity, built-up shear links are embedded at various heights in the tower, which comprises four vertical steel shafts that are cantilevered from the pile cap. Although A709 HPS 70 steel is used throughout the bridge, the shear links use A709 Grade 50 steel, due to a lack of data regarding the inelastic performance of the HPS 70 steel at large plastic strains. Inelastic shear links have been used in building construction for many years but their application to bridge structures is still in its infancy. For this reason, the California Department of Transportation (Caltrans) funded both full-scale and half-scale tests of the proposed SFOBB shear links. The full-scale test was carried out at the University of California in San Diego, and the half-scale test, which included sections of the adjacent tower shafts, was carried out at the University of Nevada in Reno (UNR). Both tests demonstrated the adequacy of the design by T.Y. Lin, with rupture occurring at about 11% shear strain, well above the minimum required value of 3%. This paper presents the results of the UNR tests and describes a subsequent study which compares this performance with that obtained from a series of follow-on experiments using links fabricated from a high-performance steel (HPS 70) and two different low-yield-point steels (LYP 100 and LYP 225). It is shown that the HPS link performed as well as the Grade 50 link with 32% less overstrength at high inelastic strains. Failure in both cases was initiated at the welds between the stiffeners and the web. It was also found that the stiffeners may be eliminated altogether if thicker webs and low-yield-point steels are used (for the same nominal shear capacity). These LYP links demonstrated a very high capacity for inelastic strain, due to the significant reduction in welding in areas with high plastic yield strains. Failure occurred at about 20% strain. These results also appeared to be independent of the load sequence used to reach failure. Overstrength was observed to be of the same order of magnitude as for the Grade 50 links. The paper concludes that inelastic shear links are very efficient energy dissipators and can be designed to have a high tolerance for inelastic strain. Applications to long-span bridges are attractive because of their low cost and negligible maintenance requirements.