Seismic Rehabilitation Design and Construction for the Port Mann Bridge, Vancouver, B.C

摘要

The Port Mann Bridge spans the Fraser River and is a vital link in the transportation infrastructure of Vancouver, British Columbia carrying approximately 110,000 vehicles per day. Constructed between 1960 - 64 the bridge totals 6866 ft (2092m) in length and is currently being seismically rehabilitated to safety level standards. The elegant 1920 ft (585 m) long main span is the second longest continuous tied-arch in the world and was the first orthotropic steel deck bridge constructed in North America. The deck is supported by closely spaced transverse floorbeams that span between main tie girders that are, in turn, supported by arch ribs. The tie girders and arch ribs are large steel box cross-sections. The north and south approaches are curved in plan and are, respectively, 3223 ft (982 m) and 1723 ft (525 m) long and consist of three-span-continuous steel girders supporting a composite concrete deck. Three different span lengths (and associated girder depths) are found: 125 ft (38.1 m), 175 ft (53.3 m) and 225 ft (68.6 m). This paper examines both design and construction challenges of the seismic upgrade which consists of rehabilitating elements of the main span, approach spans and north approach soils densification (including soils under the river itself). Major analysis and design efforts were undertaken to optimize the interaction of the superstructure, substructure and soils to arrive at a cost-effective rehabilitation solution. Both subduction and non-subduction earthquakes were considered. Time-history analyses with multi-support excitation were performed, with concrete pier stiffnesses based on push-over analysis results. An iterative process was used to adjust pier stiffnesses to arrive at matching displacement demands between push-over and time-history analyses. Geotechnical challenges were posed by the greatly varying soil profiles along the bridge alignment. For analysis purposes, six" zones were designated, each with its own dynamic characteristics for propogating firm rock accelerations to ground level. Furthermore, foundation types and conditions vary, requiring close collaboration among the geotechnical and structural design team members to ensure superstructure-substructure-soil interaction compatability. This paper examines the rationale behind the various types of retrofits and the interdependence of superstructure, substructure and soil behaviour.