BAY BRIDGE

MTC Bay Bridge Rail Feasibilty Study

APPENDIX A. DEFINITIONS AND NAMING CONVENTIONS

Live load and dead load are terms used widely within the discussion of structures. In general terms, dead load is any weight the structure must support that is unchanging. The bulk of the dead load is the self-weight of the structure and permanent fixtures on the bridge such as roadway wearing surface, sidewalks, guide railing, railroad track, utilities, signing, and signals.

Live loads are considered the weights the bridge must support which are not fixed. Live loads are the weights of cars, trucks, trains, pedestrians, and bicycles.

Two general types of loading are referenced in discussion of bridges; service and seismic loads. Service loads are the common loads the bridge will carry. For the Bay Bridges, these include live, dead, wind, tidal flow, vehicle braking, vehicle centrifugal and maintenance. Seismic loads are a combination of dead load, a percentage of the live load and lateral and vertical acceleration loads due to seismic ground motion.

Later in this paper there are references to "demand" and "capacity" of the bridge as a whole and for individual structural elements. Demand is the seismic or service load that is placed on the bridge or structural element. Capacity is the load the bridge or structural element is capable of carrying at a designated stress level. The capacity load is usually considered to be the maximum design load. A simple example is occupancy of an elevator. With only one person in the elevator, the demand load is one person. However, the placard in the elevator may state that a maximum of 8 people is permitted. In this case the capacity of the elevator is 8 people.

Certain names and references are specific to transportation structures and suspension bridges. Figures 2 through 5 provide the naming convention for the main structural elements of the West spans.

FIGURE A-1: PARTIAL THREE-DIMENSIONAL MODEL - BAY BRIDGE WEST SPANS

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FIGURE A-2: WEST SPANS PARTIAL ELEVATION

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FIGURE A-3: STIFFENING TRUSS - TYPICAL SECTION - SUSPENSION SPANS

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FIGURE A-4: STIFFENING TRUSS - ELEVATION