BAY BRIDGE

MTC Bay Bridge Rail Feasibilty Study

CHAPTER 1. INTRODUCTION

BAY BRIDGE HISTORY
The longest bridge in the world at the time of its construction, the 1936 opening saw the Bay Bridge carrying 6 lanes of passenger vehicles on the upper deck and three lanes of trucks and two tracks of rail on the lower deck as shown in Figure 1-1. The rail system, known as the "Key System" was ultimately replaced by rubber tire buses. Under the increasing pressure for vehicular capacity, the bridge was converted by 1963 from a mixed use bridge to one accommodating vehicle traffic only. The conversion removed all of the rail systems and replaced the lower deck with a precast concrete slab.

During this conversion, the tunnel at Yerba Buena Island was reconfigured, removing the lower level wall between the rail and roadway that supported the upper level deck, requiring the upper deck to span the bridge without intermediate support.

Although traffic on the bridge has increased dramatically, live structural loads have not increased over time. In fact, the removal of trains actually reduced live load requirements.

Between 1963 and 1988, the Bay Bridge was not seismically changed. Following the Loma Prieta earthquake, Caltrans began a study of alternatives for seismic strengthening of the bridge structure. After a number of years of study, Caltrans has developed a plan for seismic retrofit of the west suspension spans of the bridge, between Yerba Buena Island and San Francisco, and replacement of the East span, between the Island and Oakland. Retrofit and replacement activity is already underway and will be completed by 2010.

STUDY PARAMETERS
As a feasibility study, the intention is to provide knowledgeable judgements as to the structural and economic possibilities of placing rail on the bridge. The work within this context is very "broad-brush." In the sequence of engineering work to follow this feasibility study, the next level of effort would be preliminary design followed by final design. Each step takes the design and cost estimates to the next level of detail and accuracy.

Limitations, therefore, do exist within this study. First, computer modeling was not undertaken. Short of a rigorous, three-dimensional, non-linear computer analysis, any lesser effort would provide only partial answers. Such preliminary answers are more readily obtained through existing calculations.

FIGURE 1-1 RAIL ON THE WEST SPANS IN 1959
Courtesy of Caltrans

Lane load assumptions were derived from a 1965/67 Caltrans study that placed the live load from all 10 lanes in varying arrangements along the entire span. For this study, the West span¹s structural demands under proposed lane and rail roads are established by proportioning the 1965/67 values. Such proportioning implies that the proposed train may extend the full span length of the bridge. Since actual train lengths are much less than the bridge span, the demands on some structural components will be reported higher than actual impacts.

Calculations for the East spans are currently underway and will not be available prior to the completion of this study. Proportioning similar to the West span work will not be possible. The basis for assessment of the East spans will be strictly based on the design criteria.

These qualifiers are meant to place this study into context. Suspension bridges are simple in form yet complex to analyze. That complexity is expanded when seismic performance is studied. Such detailed work would be undertaken in a preliminary design effort that would follow this completed feasibility study.



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