BAY BRIDGE

MTC Bay Bridge Rail Feasibilty Study

CHAPTER 3. STRUCTURAL FEASIBILITY ANALYSIS

The structural impact of rail transit on the Bay Bridge is dependent on the type of rail system selected. Figure 3-1 shows the system weight and clearance envelope required for each alternative rail system. The current Caltrain system has also been included for comparison.

FIGURE 3-1: RAIL SYSTEM WEIGHT AND CLEARANCE DATA

Rail System	Loaded Rail Car Weight in lbs. per linear foot of track	Rough Rail Envelope Size (height by width)
		Single Track	Double Track
BART	1,170	14' by 16'	14' by 32'
Light Rail	1,390	18' by 16'	18' by 29'
SEPTA	1,720	26' by 17'	26' by 34'
Amtrak — Acela (high speed)	1,800	26' by 17'	26' by 34'
Caltrain — Current System*	1,900	26' by 17'	26' by 34'
*Included as a basis of comparison.

The train cars in the first systems are individually powered. The Amtrak Acela and Caltrain have locomotives powering the train sets that are considerably heavier. The higher locomotive loads may control the final design of some bridge components but, for this study, the main length of the train sets, the passenger cars, are the basis of comparison.

It is important to note that all of the technologies considered for Transbay rail service are electronically powered. BART's electric power is via a third rail alongside the rails for the vehicle wheels. The other three systems access electrical power from overhead wires referred to as catenaries. Diesel fueled systems were not considered appropriate for this crossing due to the likelihood of an indoor stop at the current or reconfigured Transbay Terminal in San Francisco. While diesel buses do use the terminal, their operation allows for easy engine stopping and starting during waiting times. The diesel locomotives used in train sets do not offer this capability.

Any analysis of the impacts of rail on the Bay Bridge requires three separate analyses:

• Analysis of the west (suspension) spans
• Analysis of the Yerba Buena Tunnel system
• Analysis of the east (cantilever spans)

For the purposes of this analysis, the impacts of rail were studied on the post-retrofit West spans and on the proposed rebuilt East spans. Some additional analysis was done to determine the impact of rail on a retrofitted East span.

WEST SPAN STRUCTURAL COMPONENT DEAD AND LIVE LOAD PERFORMANCE - SEISMIC RETROFIT AND NO RAIL
Structural design capacity calculations completed in 1965/67 provided the suspension bridge dead and live load demands and associated capacities for a representative number of components within the superstructure. With these values, "demand to capacity" ratios (D/Cs) can be developed for each member type. The ratios are valuable for ease in reporting and reviewing the performance of a structure. D/C ratios greater than 1.0 indicate that the structural element in question does not have the code required strength to carry the load. D/C ratios less than 1.0 indicate sufficient code required structural capacity.

Figure 3-2 contains the D/C ratios for the main elements of the superstructure. Note that these are the service load demands. The "1959 Reconstruction" numbers come directly from the 1965/67 investigation calculations. The "1999 Current" numbers reported are determined by factoring the dead and live load demands from the 1965/67 demands by the increase or decrease in loads.

FIGURE 3-2: RETROFITTED WEST SPAN DEMAND TO CAPACITY RATIOS (ASSUMES NO RAIL)

	Seismic Retrofit No Rail	1999 Current	1959
Main Cable	0.94	0.90	0.91
Suspenders	0.74	0.71	0.71
Towers	1.17	—	—
Top Chord	0.94	0.70	0.70
Bottom Chord	1.00	0.76	0.76
Diagonals	1.12	0.94	0.47
For service loads only. Seismic loads will alter these results

These values are a worst case scenario for members in each category. These numbers in no way signify that each diagonal for the "Seismic Retrofit" scenario will require strengthening. Rather, the worst case will be to increase the strength of only a few members by 12% and the remainder by less or not at all. In very general terms, a 10% increase in total load would likely be tolerated for service load conditions. However, more in-depth analysis is required to determine the structure's tolerance to added loads.

LOCATION OF RAIL SERVICE ON THE WEST SPANS
Another key factor in analyzing the structural impact of rail on the suspension spans is the location of the rails relative to the traffic lanes and structural elements. After a preliminary analysis of a number of options, three proved to be viable:

• Adding Lanes Below Deck
• Adding Lanes Alongside the Lower Deck
• Adding Lanes Alongside the Upper Deck

It should be noted that lanes were added in all cases to ensure that capacity not be removed from the bridge. While auto lanes could be removed in favor of rail operation, the growing demands for Transbay travel projected by MTC suggest that removing auto capacity would not be desirable.

1. Below Deck
This option adds two lanes to the bridge below the current lower deck. Either traffic lanes or bi-directional train traffic could utilize this deck, created under the existing stiffening truss by hanging a level below, as shown in Figures 3-3 and 3-4.

In this option, it would also be possible to run rail service on the lower deck of the bridge and to use the lanes below the bridge for auto traffic. Such a design would reduce the headroom required for the new deck, but would not substantially alter the structural impacts of the alternative at this level of analysis. If two directions of rail service are placed on the bridge deck in any alternative, a minimum of three lanes of traffic would be displaced. The remedial action would be to replace at least three traffic lanes above, below or beside the West span stiffening truss.

The option of adding structure below the existing deck requires at least some reduction in the clearances for shipping. The United States Coast Guard must approve any reductions in vertical clearance. Preliminary indication is that they would be very concerned about reductions in clearance due to the placement of a radio beam for navigation and because vessels continue to get larger over time. This issue would need considerable attention in further studies.

Lower Side by Side
This option adds bi-directional rail service by symmetrically widening the stiffening truss on either side of the lower deck of the suspension span to accommodate rail or vehicles on the outside of the existing suspension cables as shown in Figures 3-5 and 3-6. Again, it should be noted that additional study is required to determine the advantages of using the new lanes for rail or auto service. Should auto traffic be placed outside of the current truss, a total of four lanes would be replaced, to allow for balanced loads.

While either of the side-by-side rail options have some advantages, a key problem is that they are unable to drop down to the basement level of Transbay Terminal, which is the preferred location for a rail station, connecting Peninsula service to downtown and the bridge.

Upper Side by Side
This option is similar to the Lower Side By Side option, except that the new travel ways would be at the upper deck levels. This option is shown in Figures 3-7 and 3-8.

NON-STRUCTURAL CONSIDERATIONS
As part of the feasibility analyses, the study team and peer advisor panel were able to draw a number of conclusions about each alternative that go beyond strictly structural issues. These non-structural considerations may ultimately prove as important as structural feasibility or cost. They include construction safety and impacts, aesthetics and opportunity for connections within Transbay Terminal. A summary of findings is presented in Figure 3-9. The below deck option appears preferable because of the flexibility it offers at Transbay Terminal. It is the only option that can access the basement, unless rail were placed on the bridge deck, moving auto lanes outside the truss on a side-by-side option. The below deck option will also minimize impacts to ramps on Yerba Buena Island and in San Francisco. However, additional study is required to determine whether the reduction in maritime clearances that would be required under this alternative are acceptable to the Coast Guard. Therefore, all three configuration options are carried forward in this analysis.

FIGURE 3-9: NON-STRUCTURAL CONSIDERATIONS - RAIL ON WEST SPAN

Issues	Below Deck	Upper Side by Side	Lower Side by Side
Construction Safety	Pro: Work is below traffic.	Pro: Work is outside of traffic	Pro: Work is outside of traffic
Traffic Impacts during construction	Con: Individual lane closures are likely.	Con: Individual lanes closures are likely.	Con: Individual lanes closures are likely.
Catastrophic derailment — Train attempts to leave the rail corridor and collides with a portion of the bridge	Pro: New framing would be damaged. The main truss would remain stable	Likely elements damaged would be the suspender cables and main cables. Pro: Loss of suspender cables does not produce an unstable condition. Con: Damage to main cables would be difficult to repair.	Con: Loss of truss chords, diagonals, and verticals would lead to possible collapse of the truss.
Aesthetics	Con: Adds depth to the stiffening truss	Con: Adds width to the stiffening truss and minimal depth	Con: Adds width to the stiffening truss and minimal depth
Access to all levels of the Transbay Terminal	Pro Access to the Transbay Terminal is not restricted. Entering Rincon Hill through the anchorage is possible	Con: Can not drop to basement level. Dropping sufficiently below grade for a Rincon Hill tunnel is not possible without hanging the decks off the existing truss well east of the approach to SF. Aesthetically undesirable.	Con: Can not drop to basement level. Dropping sufficiently below grade for a Rincon Hill tunnel is not possible without hanging the decks off the existing truss well east of the approach to SF. Aesthetically undesirable.
Yerba Buena Island crossing	New tunnel bore below the existing.	New tunnel bores on each side of the existing.	New tunnel bores on each side of the existing.
Impacts on shipping clearance	Con: Reduction in shipping clearance by approximately 24 feet	Pro: No reduction in shipping clearance.	Pro: No Reduction in shipping clearance.
Final configuration traffic flow	Pro: No impact on the bridge or YBI.	Con: Will require reworking some YBI and San Francisco off ramps	Con: Will require reworking some YBI and San Francisco off ramps
Caltrans Maintenance Operations	Con: Providing moving maintenance clearance below the existing lower deck will further encroach on shipping clearances.	Pro: Shipping Channel encroachment is considerably less than "Below Deck". Side access maintenance gantries can remain.	Maintenance vehicles can use upper deck of widening. Side access maintenance gantries can remain provided the bridge rail is suitably outboard of the existing truss.
Steel Truss long-term Service Life	Pro: Member demands will generally be a function of the vertical load and are not as likely to cycle from tension to compression.	Con: Member demands will be a function of vertical and torsional load cycles and cycling is possible. Additional joint strengthening may be required.	Con: Member demands will be a function of vertical and torsional load cycles and cycling is possible. Additional joint strengthening may be required.
Conflicts with adjacent structures.	Pro: No conflicts provided rail enters a Rincon Hill Tunnel.	Con: Air rights have been given for a building 10 feet north of the SF anchorage. The building will be as tall as the bridge.	Con: Air rights have been given for a building 10 feet north of the SF anchorage. The building will be as tall as the bridge.

WEST SPAN STRUCTURAL COMPONENT DEAD AND LIVE LOAD PERFORMANCE - WITH RAIL, STRENGTHENING AND SEISMIC RETROFIT

While the location of rail on the bridge would ultimately be an important consideration in a more detailed structural analysis, the three location options were not independently analyzed at this feasibility stage.

Load Impacts of Rail on the West Spans
Adding 34 feet of deck either as a widening or by adding a level below the existing lower deck is the minimum width required to permit bi-directional train operation. The added structural weight would require strengthening of the stiffening truss, which in turn would add further structural weight. Finally, there would be additional seismic retrofit required due to the weight and stiffness changes, which would again increase the weight. Figure 3-10 summarizes the weight impacts of adding bi-directional rail service on the West spans of the Bay Bridge. The impact of each of the four rail technologies (BART, Light Rail, Commuter Rail and High Speed Rail) would differ because the live load of each rail system is different, as previously discussed.

FIGURE 3-10: LOAD IMPACTS OF RAIL ON THE WEST SPANS

Incremental Increase	Description	Weight in pounds per linear foot
1. Additional Deck Area	Steel Superstructure	5,920
	Normal Weight Concrete Deck, 34 feet wide, 8 inches thick	3,400
2. Rail Equipment	4 Rails, fasteners, -plinths, power and control systems, etc.	300
3. Rail Live Load — Two tracks	BART Light Rail Commuter Rail (Septa) High Speed Rail (Amtrak)	2,340 2,780 3,440 3,600
4. Strengthening to accommodate rail	Approximately equal to the current seismic retrofit	800
5. Second Seismic Retrofit	Approximately equal to the current seismic retrofit	800
Total Added Load Per Rail System	BART Light Rail Commuter Rail (Septa) High Speed Rail (Amtrak)	13,560 14,000 14,660 14,820

Although there is some weight variation among the four rail technology options, any one of these options would have a negative impact on the main span and side spans of the Bay Bridge. The changes in load values on these elements is seen in Figure 3-11, assuming a uniformly distributed load. The figure shows that loads on the main span would increase by between 55% and 61%, depending on the technology selected.

Using the structural capacities discussed previously and the loading capacity required for the four rail systems, it is possible to develop demand/capacity ratios for key structural members with each of the four rail technology options.

FIGURE 3-12: DEMAND/CAPACITY RATIOS FOR WEST SPAN MEMBERS WITH RAIL AND ADDED DECK AREA

	2000
	Lt. Rail	BART	Commuter Rail	HSR
Towers	2.01	1.89	2.20	2.25
Top Chord	1.19	1.11	1.31	1.34
Bottom Chord	1.25	1.17	1.37	1.40
Diagonals	0.85	0.79	0.94	0.96
Main Cables	1.40	1.38	1.42	1.42
Suspenders	1.14	1.12	1.16	1.16
For Service Loads only. Seismic Loads will alter these results.

As expected with such large load increases, the structure would require strengthening. Note that the values shown in Figure 3-12 do not suggest that rail is infeasible, they do suggest that significant effort will be required to avoid impacting the main cables.

Insufficient capacity in the main cable is one of the greatest strength obstacles to overcome in a suspension bridge. Adding rope to strengthen the existing main cable is usually complicated by protective cable wraps, suspender saddle, tower saddle and anchorage constraints. When all other solutions are exhausted, adding a second set of cables is an option. The following section briefly describes alternatives that could reduce structural weight and therefore lower demand.

Options to Reduce Structural Demand
Immediate structural weight reduction could be achieved by replacing the existing concrete decks with lighter materials. Three deck options were assessed for weight savings: replacing the normal weight concrete on the lower deck's south side with lightweight concrete, orthotropic steel deck and composite fiber decks. Lightweight concrete and orthotropic steel are common decking materials. Composite fiber systems have only recently been developed and are in limited experimental application on far less essential structures. While the composite fiber may become an accepted decking material in time, it is not currently used by Caltrans and was not considered in detail in this study. The weights per square feet of area for 8" thick decks are given in the following table.

FIGURE 3-13: WEIGHT REDUCTION POTENTIAL OF LIGHT WEIGHT ROADWAY DECKS

System Type	Weight in Pounds per Square Foot	Weight Savings in Pounds per Linear Foot of Bridge
Current: 8" Lightweight concrete throughout except normal weight concrete on the south side of the lower deck	90 (Average)	-
8" Lightweight Concrete Throughout	83	770
Orthotropic Steel Throughout	45	5,220

To provide a protective wearing surface for the concrete slabs on the West spans, Caltrans has installed a 3/4-inch epoxy overlay. For the proposed deck systems, this report assumes an equivalent surfacing. Fully developed deck designs could increase or decrease this thickness. The resulting change in added load would not change the conclusions of this feasibility study.

Combining the weight savings of the deck options presented above with the loads from the various rail options changes the load impacts of rail on the main cable as seen in Figure 3-14. The range of load increase on the main cable and suspender cables, assuming an orthotropic steel deck, can be reduced to an increase of between 25% and 31%, depending on the technology selected.

It should be noted that no significant study has been completed to determine the life-cycle implications of using an alternate decking material on the Bay Bridge Roadway.

FIGURE 3-14: WEIGHT IMPACTS OF RAIL OPTIONS ASSUMING LIGHT WEIGHT DECKING MATERIAL

Revised demand/capacity ratios with various decking options are shown below:

FIGURE 3-15: IMPACT OF LIGHTWEIGHT DECK MATERIAL ON MAIN CABLE DEMAND

	2000
	Lt. Rail	BART	Commuter Rail	HSR
Normal Weight Concrete	1.40	1.38	1.42	1.42
Lightweight Concrete	1.33	1.32	1.35	1.36
Orthotropic Steel	1.14	1.12	1.16	1.17
For Service Loads only. Seismic Loads will alter these results.

FIGURE 3-16: IMPACT OF LIGHTWEIGHT DECK MATERIAL ON SUSPENDER CABLE DEMAND

	2000
	Lt. Rail	BART	Commuter Rail	HSR
Normal Weight Concrete	1.14	1.12	1.16	1.16
Lightweight Concrete	1.09	1.07	1.10	1.11
Orthotropic Steel	0.93	0.91	0.95	0.95
For Service Loads only. Seismic Loads will alter these results.

The figures show that while demand/capacity ratios are improved, the amount of weight savings is insufficient to eliminate strengthening or supplementing the main suspension cables.

West Span Seismic Considerations
Added weight is not the only impact to the Bay Bridge Structure. The addition of rail service would have significant seismic impacts, requiring further retrofit. In very basic terms, the properties that control a bridge's behavior under a defined seismic event are:

• Mass - how much the structure weighs and where the weights are in the structure.
• Stiffness - how much the structure moves when subjected to a given force.

More complex aspects of a structure's seismic performance include the lateral framing system's material properties, as well as internal or external devices that dissipate energy and reduce sustained cyclic motion.

The added structure for two rail tracks would certainly alter these characteristics of the seismically retrofitted West spans. Adding rail would result in the following changes:

• The weight of the stiffening truss would increase by roughly 50% with the introduction of new members and deck.
• The stiffness of the truss would change due to the new structural members, the added deck width, and any changes to the existing roadway deck materials.
• Strengthening the towers would change their stiffness.
• The energy dissipating dampers designed for the current retrofit would be inappropriate for a revised West span.

One of the greatest consequences of retrofitting the new structure would be the difficulty in working on the foundations and cable anchorages. Work on these deep-water foundations is very complex and costly. The current retrofit does not require work on the tower piers beyond increasing the capacity of the connection between the steel towers and the concrete piers. With the proposed new rail decks and the altered seismic performance, increased foundation demands and subsequent foundation and anchorage retrofits are a likely possibility.

Some benefit may be derived from the rail scenario that widens the deck and adds tower legs and cables. With the added weight of the deck, foundation improvements are likely, as are tower leg improvements. By widening the piers and foundations to fit the new tower legs, much of the work can be designed to resist the added seismic demands. Likewise, up at the tower level, properly detailed connections between the existing and new tower legs offer opportunities to protect the existing towers from increased seismic demands.

THE IMPACT OF RAIL OPERATIONS ON THE PROPOSED EAST SPANS

The Proposed East Spans
Following the Loma Prieta earthquake, Caltrans and its consultants undertook a number of studies related to the options for seismic strengthening of the Bay Bridge. While a retrofit plan was developed for the west suspension spans, it was determined that the East spans would be replaced. Replacing the East spans could be achieved at about the same cost as seismic retrofits, while providing a new structure with a long life-cycle. This study assumes the proposed replacement spans as the condition on which rail is imposed.

The width of bridge is sufficient to allow for an additional lane with rail although the design does not provide for the needed structural capacity. Geometrically, there is sufficient space to have five lanes and no shoulders, provided a lane width of 11'10" is acceptable to the reviewing agencies. As reported earlier, the West spans currently operate with 11'7" lane widths. The potential East span 11'10" lanes could possibly win approval from Caltrans. Also, depending on which train set is used, the rail width could be reduced to allow for both greater lane width and/or additional narrow shoulder. Structural capacity must be added to accept this configuration.

Load Impacts of Rail on the Proposed East Spans
Structurally, there are both service load and seismic concerns with five lanes and rail. The current East span design criteria call for live loads representing four lanes and one track at 1,400 pounds per linear foot.

Percent increase in live load from the current East span criteria of 1,400 pounds per linear foot of rail and four lanes, compared to the heaviest bridge rail alternative and five lanes of traffic, is 27%. With the lightest rail option, it is an 8% increase. Again, using this simplified comparison is appropriate for the level of study reflected in this report. More accurate conclusions require detailed analysis to fully document the increased demands of each structural component.

Greater opportunities to maintain the existing ramps and simplify rail and vehicle operations would exist if rail were moved to the exteriors of the East span decks. The main benefit is on the eastern side of Yerba Buena Island. Exterior tracks can be readily split off the main decks permitting many possible alignments through new tunnels in Yerba Buena Island. With alignment options available, placing track outside of the West spans remains an option and would permit keeping five lanes of traffic together in each direction on these spans.

Figure 3-19 shows alignments for rail on the exterior dropping down to a bore below the existing roadway tunnel. Alignments that parallel the existing tunnel are shown in Figure 3-20.

FIGURE 3-17: CURRENT PROPOSED EAST SPAN TYPICAL SECTION - 5 LANE DESIGN

Courtesy of Caltrans and T.Y. Lin International/Moffat & Nichols, a Joint Venture

FIGURE 3-18: CURRENT PROPOSED EAST SPAN TYPICAL SECTION - 4 LANE DESIGN WITH LIGHT RAIL

Courtesy of Caltrans and T.Y. Lin International/Moffat & Nichols, a Joint Venture

FIGURE 3-19: RAIL ALIGNMENT - EAST SPAN'S EXTERIOR RAIL TO BELOW THE YERBA BUENA ISLAND TUNNEL

FIGURE 3-20: RAIL ALIGNMENT - EAST SPAN'S EXTERIOR RAIL TO EACH SIDE OF THE YERBA BUENA ISLAND TUNNEL

Currently, for the East spans, the suspension cable arrangement slopes inward from the connection at the exterior of the deck up to the central control tower as shown in Figure 3-21. Rail and catenary wire clearances required for all the train scenarios, except BART, would result in interference with the suspender cables unless the shoulders are reduced for the vehicles. Ideally, the deck needs to be widened to move the cables outward and provide the necessary clearance.

FIGURE 3-21: EAST SPANS SECTION

Courtesy of Caltrans and T.Y. Lin International/Moffat & Nichols, a Joint Venture

From a traffic perspective, the deck should also be widened to accommodate the full five lanes. The current design width is 59'4" which is greater than the 58'0" on the West Side. For proper 12'0" lanes and one-foot shoulders, the East span decks need to be widened by 2'8".

YERBA BUENA TUNNEL
Revising the usage of the upper and lower decks to a mixture of cars and trucks in 1959 required reconfiguring the support system of the upper deck. Structurally, allowing trucks on the upper deck increased the load on that deck. There was also a headroom problem which limited clearances for trucks on the outside lanes of the upper level in 1959. As a result, the columns between the vehicles and trains in the lower level, which supported the upper level, were removed. The upper deck was reconstructed with a prestressed floorbeam system that spanned from tunnel wall to tunnel wall at a lower elevation to provide headroom for trucks.

Live load code requirements for the deck in the tunnel have remained unchanged since the 1959 reconstruction. The lower level is a reinforced concrete slab constructed on a grade. Placing rail on this structure should not require alterations to the lower level support system. Rail on the upper deck, however, would compromise the floorbeams as the load changes from 2,400 plf today to over 3,000 plf with three lanes and one track or 3,680 plf with two lanes and two tracks. These values are for BART, the lightest system.

Reconstructing the tunnel to support the additional loads is not a reasonable option since construction could not occur without closing the bridge for extended periods of time. A more feasible option would be to create new rail tunnel bores for rail so that the existing tunnel remains undisturbed.

Note that the limited length of Yerba Buena Island at the likely tunnel elevation would not permit rail to enter one side of the island under the existing tunnel and depart up alongside the existing tunnel. This requires that the rail alignments at both the east and west face of the island are either side-by-side or below, but not a combination. While either option is feasible, there are significant engineering concerns about an option that requires rail on the East spans to drop down to a new Yerba Buena rail tunnel located below the existing roadway tunnel.

Immediately east of Yerba Buena Island, the bridge decks must transition from a side-by-side alignment to a stacked alignment to meet the double-deck roadway tunnel. The vertical and horizontal geometry is extremely tight to accomplish this weave. Splitting off rail would require ramping down and under the planned weave, which may not be possible in the limited distance available.

Vehicle Access to Yerba Buena Island
The current access to Yerba Buena Island is:

• Eastbound traffic exits to the north side of the bridge on the west side of Yerba Buena Island and to the south side on the eastern side of the island.
• Entrance to the eastbound direction is from the south on the East Side of the island.
• Westbound traffic exits on the southeast.
• Entrances to the westbound lanes are on the northeast and northwest.

Westbound rail on the south side of the westbound structure would require new vehicle exit ramps to Yerba Buena Island. With eastbound rail on the north side of the eastbound structure, the vehicle exit ramp on the east side of the island would be eliminated. These impacted ramps could be relocated with sufficient funds and the needed environmental clearances.