STRUCTURAL ASSESSMENT OF RAIL ON THE BAY BRIDGE
|Rail Option||Weight in lbs. Per linear foot of track||Rail Envelope Size
|Light Rail||1,390||18 by 29|
|BART||1,170||14 by 32|
|Commuter Rail (SEPTA)||1,720||26 by 34|
|High Speed Rail (Acela)||1,800||26 by 34|
|Caltrain (diesel)||1,900||26 by 34|
Note: The Caltrain service as presently operated is shown in the Table above for comparison purposes. It is not recommended and may not be feasible for diesel locomotives to be operated into the Transbay Terminal and on the Bay Bridge.
The structural assessment examines the retrofitted west spans, the proposed new east spans currently being designed and the Yerba Buena Island tunnel. It should be noted at the outset that rail services could not be returned to the decks of the bridge spans as they were operated in the past without significant structural and seismic strengthening, safety considerations and operational impacts (loss of vehicular traffic lanes). Therefore, this analysis considers restoring rail on the bridge decks and other potential alignments for placing rail on the Bay Bridge and assesses the impacts and requirements for each of these alignments. The major findings of the structural assessment for the west spans, east spans and Yerba Buena Island tunnel are provided below.
Caltrans is currently conducting a seismic retrofit of the west spans of the Bay Bridge. As a result of the seismic project, the west spans will be retrofitted to withstand the design earthquake as defined by Caltrans.
Six options were examined for placing rail on the west spans:
1) Rail on the Bridge Deck
2) Vehicle Lanes Outside of the Truss
3) Vehicle Lanes or Rail above the Truss
4) Rail Suspended Below the Lower Deck
5) Rail Cantilevered from Lower Deck
6) Rail Cantilevered from Upper Deck.
Based on the analysis, it is recommended that three of the options (Rail on the Bridge Deck, Vehicle Lanes Outside of the Truss, and Vehicle Lanes or Rail above the Truss) be eliminated from further consideration. This analysis shows that each of these three options have major construction or operational impacts which are considered as fatal flaws, as follows:
|Eliminated Alignment Options||Fatal Impacts|
|Replacing vehicle lanes with rail||
|Vehicle lanes outside of the truss||
|Vehicle lanes or rail above the truss||
The remaining three options have advantages and disadvantages and a number of major issues and considerations. Some of the major issues and considerations for these options are as follows:
|Alignment Options Pursued||Major Considerations/Issues|
|Rail Suspended Below the Lower Deck||
|Rail Cantilevered from Lower Deck||
|Rail Cantilevered from Upper Deck||
Adding any of the proposed rail options would exceed loading requirements for the west spans of the Bay Bridge and require significant additional seismic retrofitting measures to meet current codes. Therefore, significant strengthening will be required to accommodate adding rail to the structure. The extent and costs of the required improvements are considerable because even after the current retrofit of the west spans there will not be significant reserve capacity in the structure. The addition of rail to the structure will increase the dead loads and live loads and change the seismic behavior of the spans, which requires substantial modifications and strengthening. Given that the improvements are being made to an existing structure, implementation of the needed improvements (e.g. foundations) are complex and costly.
In summary, the structural improvements required to accommodate rail on the west spans include:
Caltrans has completed approximately sixty-five percent design of a replacement east span project. The current design includes side by side bridge decks with each carrying five lanes of traffic and shoulders in each direction. The current design accommodates on deck rail on the inside lanes of each of the spans reducing the vehicle lanes to four lanes on each deck without shoulders. According to Caltrans, additional strengthening would be required for the east spans to accommodate light rail on the inside lanes of the spans and maintain five lanes of auto traffic without shoulders, at an additional cost of $50 to 100 million.
Based on this analysis, the heavy rail options could not be operated on the inside of the bridge decks because there are inadequate clearances between the east and west bound roadways as they stack entering the east side of the Yerba Buena Island.
This analysis finds that the most feasible method to transition rail services from the east spans through Yerba Buena Island is to place the rail on the outside of each of the east span decks. This would allow splitting the rail tracks off of the main decks permitting alignments through new tunnels in Yerba Buena Island.
Based on the analysis, except for BART or a similar low clearance rail options, rail on the outside of the decks and provision of standard vehicle lane widths would require widening each of the east spans from its current design by 3 to 5' to provide adequate clearance for the suspension cable system.
Based on this analysis, the introduction of rail services on the Bridge should avoid impacting the existing Yerba Buena Island tunnel. Rail on the upper deck of the bridge through the tunnel would significantly compromise the floor beams of the upper deck, which would be costly and operationaly difficult to remedy. Separate bores to accommodate rail services through the Island are possible and preferred. This alignment requires separate rail structures with support columns on both the north and south sides of the new east span transition structures on Yerba Buena Island.
The total estimated cost of the structural requirements and improvements for placing rail services on the bridge spans and through Yerba Buena Island range from approximately $3.06 billion (rail below the lower deck of the west spans) to $3.33 billion (rail cantilevered on west spans).
The project cost estimates include the costs of strengthening and modifications to the east and west spans of the Bridge to accommodate the rail services, tunneling through Yerba Buena Island and tunneling or building structure to Harrison Street in San Francisco. The costs do not include structure to carry rail beyond the Toll Plaza on the East Bay side of the Bridge, structure or tunnel to the Transbay Terminal in San Francisco and do not include the costs of the rail tracks or other infrastructure to operate the rail options.
Please note that the costs presented are order-of-magnitude cost estimates. These estimates are based on costs gathered on bridge retrofit and construction projects that are planned or underway in the Bay Area. Significant further analysis will be required to determine the actual costs. In general, the costs presented are appropriate for all of the potential rail options.
As scoped, the analysis performed is a "first cut" feasibility analysis of the structural requirements for placing rail on the Bay Bridge. To further determine the structural requirements and costs, it is recommended that the next steps necessary to complete the analysis are as follows:
a) Develop a three dimensional computer model of the bridge in sufficient detail to perform service load and seismic load analysis.
b) Perform both service and seismic analyses using the structural configuration and section properties of the retrofitted west spans.
c) Using the preferred alternative, construct the rail corridor in the computer model and determine the demand impacts on the structure.
d) Produce standard strengthening details;
e) Cost each major increment of work in the systematic construction of the corridor.
Without a defined design, construction, and operation budget established, feasibility can only be assessed in limited terms of structural feasibility. There are several hurdles that must be crossed prior to deeming the whole of rail on the Bay Bridge feasible. Namely, the non-structural issues of operations, routing, systems costs, ridership, navigational impacts, right-of-way acquisitions and environmental clearances must be resolved. In the context of structural feasibility only, rail on the Bay Bridge is possible.
However, restoring rail to the Bay Bridge is very costly based on this 'first-cut' study. More accurate cost estimates can be made after the design has been progressed. Note that further design and greater accuracy could translate into cost decreases or increases. Further analysis is required to better define the structural and operational modifications needed for rail.
It should also be noted that adding rail to a structure (west spans) that is more than 60 years old complicates the project and reduces the ultimate life of the entire project. Therefore, alternate means of crossing the Bay with rail should be evaluated and compared with adding rail to the Bay Bridge.
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This page was last modified Friday February 27, 2009
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