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Experimental and Analytical Investigations on the Effects of Live Load on the Seismic Performance of a Highway Bridge
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Current bridge design specifications have few requirements concerning the inclusion of live load in the seismic design of bridges for perhaps two reasons: 1) the likelihood of the full design live load occurring at the same time as the design earthquake is deemed to be very low, and 2) adverse behavior in an earthquake due to live load has not been observed in practice. However, with increasing congestion in major cities, the occurrence of the design earthquake at the same time as the design live load is now more likely than in the past. But little is known about the effect of live load on seismic response and this report describes an experimental and analytical project that investigates this behavior. The experimental work included shake table testing of a 2/5th-scale model of a three-span, horizontally curved, steel girder bridge loaded with a series of representative trucks. The model spanned four shake tables each synchronously excited with scaled ground motions from the 1994 Northridge Earthquake. Observations from the experimental work show the presence of the live load had a beneficial effect on performance of this bridge, but this effect diminished with increasing amplitude of shaking. During the design earthquake, the bridge with live load was essentially elastic whereas the bridge without live load suffered some yielding and the maximum displacement at the top of the column was approximately 35% less in the live load case. Parameters used to measure performance included column displacement, abutment shear force, and degree of concrete spalling in the plastic hinge zones. Results obtained from nonlinear finite element analyses of the bridge with and without trucks confirm this behavior, that live load reduces the dynamic response of the bridge. The most likely explanation for this phenomenon is that the trucks act as a set of nonlinear multiple mass dampers, a variation of tuned mass dampers that are known to be effective at controlling wind vibrations in buildings. Parameter studies have also been conducted and show the above beneficial effect is generally true for other earthquake ground motions and vehicles with different dynamic properties. Exceptions exist, but adverse effects are usually within 10-15% of the no-live load case. Although the above results were obtained for a particular bridge, earthquake loading, and vehicle configuration, they may also apply to other bridges. Further work is required to confirm this observation. The contents of this report are based on the dissertation of the first author.
Report No. CCEER-13-15