Assessment of ssi effects on a seismically isolated multi-span bridge under bi-directional seismic excitation

Elevated highway bridges are important elements in modern society infrastructure. Due to their strategic importance, loss of functionality in an earthquake is not an acceptable performance criterion. Observed performance of highway bridges following recent earthquakes suggested that conventional design methods might not provide the desired performance levels. Seismic isolation is an efficient alternative for mitigating earthquake effects for bridges. Recent seismic events have shown that if not designed properly, seismically isolated bridges that are thought to be safe may suffer sever damage (e.g. Bolu Viaduct, in Duzce Earthquake 1998) if not total collapse. Extensive research has been carried out in the past 30 years regarding the effects of soil structure interaction (SSI) on the seismic response of civil engineering structures and until recently it was believed that neglecting SSI effects would lead to a safe and conservative design. Case studies conducted after the Kobe (1995) earthquake reveal that for non-isolated bridges, SSI will affect the dynamic properties of the structure that in turn may be beneficial or in many cases detrimental to their seismic response. Herein the effect of SSI on the seismic response of a seismically-isolated bridge, Bolu Viaduct, founded on soft non-liquefiable soil is investigated. Accounting for the nonlinear behavior of the seismic isolation system, the inertial interaction between the deep foundation and the superstructure is studied. It is shown that for certain conditions this interaction might increase the displacement demands imposed on the structure, which in turn might lead to loss of functionality of the structure. The results show that SSI is an important factor in the earthquake response of seismically isolated bridges and that these effects need to be considered in the design and detailing of the isolation system as well as in the evaluation of the overall performance of the structural systems during an earthquake event. Copyright © (2010) by Earthquake Engineering Research Institute