Evaluation of the Seismic Response of a Typical Concrete Bridge Structure In Zones IIB and IV Due to Variations of the Foundation Soil Types

摘要

Concrete bridge structures, supported on tall columns, play an important role in the transportation and economical development of the Southwest region of the United States. They are highly vulnerable to damage due to strong earthquake motions. In this paper, the dynamic response of a typical intermediate size and width concrete highway bridge ramp structure is investigated. The alternate bridge designs should allow its construction in Southern Nevada (Seismic Zone IIB) or Southern California and part of Northern Nevada (Seismic Zone IV). The design of the bridge is highly sensitive to the variation in its foundation soil type, to the intensity of an earthquake, and to the proximity of the structure to the earthquake source. Variation in seismic zone or in the soil type as defined by the seismic provision of the Uniform Building Code 1997(UBC97) and AASHTO (The American Association of State Highway and Transportation Officials) result in substantial changes in the dynamic forces and lateral displacements that the mass of the bridge will be subjected to during a strong motion earthquake. Such forces and displacements have a profound effect on the seismic response and the subsequent design and total construction cost of the bridge. The bridge under investigation consists of a concrete box girder, 26.5 ft(8.08 m)wide, 5 ft(1.52 m) deep and 250 ft.(76.22 m) long. The bridge is supported by two columns 20.5ft(6.25 m) and 18.7 ft. (5.7 m) high place approximately at third points along its length. The bridge is subjected to UBC 97 zone IIB or IV intensity earthquakes, while the foundation is assumed to be of UBC97 soil types SA( hard rock) or SD( stiff soil) profiles. Site specific spectral acceleration response curves are constructed and shown for this study. A complete response spectral analysis study of the bridge and its piers, show that the foundation base shears of the piers and shear demand at the pier tops increase dramatically between zone IIB and zone IV intensity earthquakes and when the bridge piers are supported by a stiff soil profile instead of hard rock.