FEATURED PROJECT: SAN FRANCISCO-OAKLAND BAY BRIDGE
The San Francisco-Oakland Bay Bridge, which connects San Francisco to the East Bay, was originally completed in 1936. It immediately became the favorite way to travel serving nine million vehicles in the first year. It now serves 280,000 vehicles per day. In 1989, the Loma Prieta Earthquake caused a 250 ton section of the upper deck of the Bay Bridge to collapse. After careful consideration, the California Department of Transportation (Caltrans) decided that replacing the East Span, which connects Yerba Buena Island to Oakland, would not only provide critical seismic safety, but would also allow non-standard traffic safety issues to be addressed. These improved safety issues would substantially improve public safety and with the addition of a bicycle/pedestrian path, the public would now have access and additional recreational opportunities. Caltrans designated the Bay Bridge as an emergency lifeline route. As a lifeline, it is imperative that the bridge remain functional post-quake to allow emergency relief access and connect major population centers.
The seismic upgrade of the entire Bay Bridge encompasses six projects. The East Span section of the bridge has four of those projects; the Yerba Buena Island Transition Structure (YBI), the Self-Anchored Suspension Span (SAS), the Skyway and the Oakland Touchdown (OTD). Cadre Design Group, Inc. (Cadre) has provided 3D Integrated Shop Drawings (ISD) to the owner, designers or Contractors on all four projects. In 2004, Cadre began shop drawings on the Skyway portion of the Bay Bridge.
Cadre was tasked with preparing 3D ISD models of various, highly congested areas at each project. The congestion in these areas is primarily the result of the advanced seismic design employed. It was realized that traditional 2D contract plans did not integrate all the disciplines in a manner that would identify all conflicts and that such conflicts would add to construction costs and more importantly, to delays. It was imperative that any delays to this Bridge be minimized since it was identified as a life-line. These 3D Microstation models incorporated all design/construction disciplines from reinforcement, post-tensioning and steel plates to support imbeds, essentially building the Bridge in MicroStation. This innovative approach to design and construction provided the owner and designers an accurate visual of conflicts and the confidence that all the clashes were being detected.
Initially the client engaged Cadre to develop 3D MicroStation models for the sole purpose of identifying and resolving conflict prior to construction. But very quickly, the owner, designers and contractor realized the potential of each model and the scope significantly expanded. The models are now used for variety of purposes, from designers determining conduit re-routing, anchorage locations and shop drawing review to the contractor determining drainage locations, reinforcement spacing and plate clearances. The owner's initial savings, estimated at several million dollars, has now substantially increased and the construction schedule has been maintained due in large part to the expanded scope.
On the YBI portion of the East Span, Cadre modeled the highly congested areas for the entire westbound structure. This structure has some very complicated geometry. The structure varies continually in width and depth while the profile and superelevation has to transition from the YBI tunnels to the SAS structure. The congested areas modeled includes 4 footings and columns, 9 Bent Caps, 2 Hinges and 32 intermediate diaphragms. The modeling began in the design phase identifying and resolving conflicts prior to construction and continued into the construction phase providing the Contractor the ability to develop a construction methodology around the design. Some of these issues were; the routing of conduits through Bent Cap W8L, the stressing jack accessibility and the reinforcement integration between the stressing diaphragm and the closure diaphragm at Hinge K.
On the SAS portion, which is the signature span of the entire project, Cadre was initially engaged in the design phase to prepare a 3D model of Bent W2. This Bent, with all the transverse, longitudinal and vertical PT, high strength rods, Hinge K anchor bolts and anchorages to support the wrap-around suspension cable, is the most complicated and most challenging component of the entire project. During the construction phase, Cadre originally modeled the superstructure to identify and resolve conflicts but as construction progressed the models were also used to review/confirm shop drawings. The number of issues identified in the 3D model and the shop drawing reviews saved the owner millions and several months of construction delays.
On the Skyway portion, integraded shop drawings were developed incorporating all design disciplines and construction imbeds. Cadre prepared ISD's for all 8 Hinge Segments. These plans were used to identified conflicts and construction issues as well as reinforcement fabrication.
Cadre also developed 3D MicroStation models for the OTD portion of the East Span. These models consist of 12 footings, columns and Pier Caps and 4 Hinges. They were also used to identify and resolve conflicts. This portion of the East Span was completed on time and under budget.
Cadre Design Group, Inc. was recently awarded the prestigious Bentley "Be-Inspired" Special Recognition Award in Attaining Return on Innovation for its work on the San Francisco-Oakland Bay Bridge.