Ronald F. MIDDLEBROOK
Structural Engineer
Middlebrook + Louie (retired)
Past President - SEAONC
San Francisco, @Sonoma, CA, USA
ronfranco@gmail.com
Roumen V. MLADJOV
Structural Engineer
Louie International
San Francisco, CA, USA
rmladjov@louieintl.com
Summary
Built in the 1930’s, the San Francisco-Oakland Bay Bridge reached its capacity to adequately carry traffic in the 1980’s. After the 1989 Loma Prieta earthquake, the bridge’s West crossing was reinforced and its East crossing is currently being replaced in its entirety (Fig.1). Despite these improvements, the traffic capacity of Northern California’s busiest highway link remains unchanged -- clearly inadequate for the current demands of Bay Area Traffic. The authors discuss a creative alternative to the proposed demolition plans for the soon-to-be-retired East crossing elements: reusing these structures as part of a new, separate crossing. This paper describes several potential benefits of relocating and converting these existing steel structures into a new bridge, including nearly doubling current traffic capacity, as well as significant time/cost savings.
Keywords: bridge, retrofitting, reinforcing, reuse, traffic, Oakland, San Francisco, Loma Prieta.
Fig.
1. Bay Bridge with New East Crossing (under construction)
1. Introduction, Historic Significance
The San Francisco-Oakland Bay Bridge opened to traffic in 1936. It connects San Francisco and Oakland, and is the busiest vehicular link in Northern California.
The bridge is actually several bridges with distinctly different structural systems, strung together to form about a 13,7 km cross-bay roadway (7,1 km over water). West crossing: the 3140 m crossing from San Francisco to Yerba Buena Island (YBI) includes a twin suspension bridge with central spans of 704 m (Fig. 2). YBI segment: the 549 m Yerba Buena Island (YBI) segment features a tunnel and short concrete viaduct. East crossing: the 3417 m crossing from YBI to Oakland consists of several different steel truss systems: four short, (approx. 88 m) steel truss spans on YBI followed by the 738 m long cantilever truss structure (Fig. 3), then five deep through-truss spans at 155 m, fourteen deck-truss spans at 88 m, and the remainder on simple land-based steel structures.
The original Bay Bridge is one of the grandest achievements of the American Art of bridge engineering, and is clearly important from a historic standpoint. At the time of completion, the bridge was the longest bridge in the world – 13,7 km, including approaches. Among the bridges of the world at that time, it featured the second longest suspension span (704 m), the third longest cantilever truss span (427 m), the deepest pier foundation (74 m below water surface at low tide), and the largest bored tunnel. The West crossing was the only major bridge with two consecutive suspension spans.
The bridge, with its three major segments, is listed on the National Register of Historic Places (NRHP). The Register’s comment is: “One of the largest and most important historic bridges in the country”. In addition to its place on the NRHP, the bridge received accolades from former President Herbert Hoover, speaking at the ground-breaking ceremony in 1933: “This marks the physical beginning of the greatest bridge yet erected by the human race.” Hoover, who was originally a mining engineer, had followed the development of the design of the bridge during his presidency. He was particularly interested in its effect on unemployment in trying times—the Great Depression.
The entire bridge deserves its exalted historic credentials, from the graceful sweep of the West crossing suspension structure, through the YBI tunnel and viaduct, to the steel Cantilever Truss section, to the through-truss and deck-truss spans.
The bridge was built in just 3½ years, at a cost then estimated at $78 million. It was, and still is, one of the greatest engineering achievements of the 20th century.
Fig. 2. West crossing
Fig. 3. East crossing (currently being replaced)
The Bay Bridge is a double decker. The original design featured 6 automobile lanes on the top deck – 3 lanes in each direction. The bottom deck provided 3 truck lanes and 2 lanes (one in each direction) for an interurban commuter train. Around 1960, the arrangement was converted to five eastbound lanes of traffic on the lower deck and five westbound lanes on the upper deck.
The Bay Bridge was designed and built using state-of-the-art techniques available in the 1930’s. For example, the engineers specified the highest strength steel available for critical elements of the structures. Nickel (Grade 380 MPa) and silicon steel (Grade 311 MPa) for the East crossing making up 62% of the total steel used there and 72% of the Cantilever section. Even the carbon steel used in this bridge was higher strength (Grade 255 MPa) than normal. High-strength cable steel (Grade 828 MPa) was specified for the West crossing suspension cables.
The entire bridge required 151 593 tons of structural steel. [1]
The Bay Bridge, and its neighbor, the Golden Gate Bridge (completed at about the same time) represent the culmination of more than 100 years of development of bridge engineering and construction in the United States. To fully appreciate the achievement of completing the construction in just 3½ years, consider the technical level of construction at the time. In addition to the lack of modern devices – heavy construction equipment, vehicles, cranes, etc. - all steel connections were made using rivets, requiring much more time and labor than modern high strength bolting and welding. Compare this achievement with the 12 years it is taking to build the current replacement bridge just for the East crossing!
Amazingly, the 151 593 tons of steel used for the entire Bay Bridge in 1936 is less than the tonnage Caltrans reported for building just the superstructure of the new East crossing replacement [2]. A testament to the wisdom of the design for the Bay Bridge West crossing is that, 62 years later, Japanese engineers chose a very similar design for the towers of the longest bridge span in the world: the Akashi-Kaikyo (or Pearl) Bridge.
Fig.
4. Local damage to the East Crossing
3. Loma Prieta earthquake (1989)
The bridge’s East crossing was locally damaged during the Loma Prieta earthquake of 1989. A 15 m section of the top deck slipped off its support at an expansion joint; that end of the section collapsed onto the lower deck (Fig.4). One motorist was killed.
It was subsequently decided to replace the entire East crossing. Caltrans (California’s Department of Transportation) dubbed the project an Earthquake Safety project, an important decision because it meant only the pre-existing traffic capacity would be restored. After 7-8 years of discussion, planning and design, construction on the new East crossing finally began in January 2002. The current estimate for project completion is October 2013.
4. West crossing improvements and East crossing replacement
The West crossing (and its approach) underwent seismic improvements in a 5-year project beginning in 1999, at a reported cost of approximately $759 million. The improvements included massive rollers installed between the roadway and bridge supports and 96 new viscous dampers inserted at critical points to allow movement. The bridge’s twin suspension spans were strengthened by adding new steel plates and replacing half a million original rivets with almost twice that many high strength bolts. New bracing was added under both decks and all of the “laced” truss diagonals connecting the upper and lower road decks were replaced. In total about 7710 tons of structural steel was added.
The East crossing replacement is currently under construction. It consists of a single tower self-anchored suspension span and a 14 span (140 m. each) concrete skyway. The new crossing has added shoulders and a bicycle lane. (Since there is no bicycle lane on the West crossing, it will not be possible to bike the entire length of the bridge). The cost of replacing the East crossing is $6 417 M. Current plans are to demolish all of the original East crossing structures from YBI to Oakland, and presumably recycle whatever possible. Demolition is currently estimated to cost at least $280 million. [2], [3]
5. Traffic capacity and population demographics
With only 5 traffic lanes in each direction, traffic movement is greatly compromised, especially during commute hours.
In 1936 the Bay Area population was about 1 650 000. By 1990 it was about 6 024 000 and by 2010 it was 7 150 000. The projected population for 2025 is 8 880 000 (47% greater than in 1990).
Traffic growth has been even more rapid. When the bridge originally opened in 1936, the traffic equivalent was 50 000 vehicles. By the late 1990’s, this critical highway link carried about 280 000 vehicles on an average day. So the growth in demand has increased nearly 6-fold over the past 75 years.
Currently during commute hours it can take up to 30 minutes to drive the 7,1 km (from water’s edge to water’s edge) across the bridge. That’s 14 km/h, and sometimes it’s worse, especially if there is an accident on the bridge.
The idea of supplementing traffic capacity across the bay is not new. Numerous studies over the past 60 years have been conducted for new crossings (both bridges and tunnels). None of these studies were pursued, for environmental, political, cost and other reasons. However, these efforts show a great deal of continuing interest in reducing the pressure on cross bay traffic.
6. So what have we gained?
After more than 17 years of planning, design and construction in connection with this new East crossing, along with the improvements to the West crossing described above, we have a seismically improved, 13,7 km long bridge (at a total cost over $7 billion). The entire Bay Bridge remains restricted to five vehicular traffic lanes, essentially as it was when it was born in 1936. The inadequacy to handle traffic demand was well known even when the new East crossing replacement was being designed in the late 1990’s. And as time goes by, the problem only gets worse!
7. Proposed plans to increase traffic capacity
Seeking a rational solution for this “problem”, some of the possibilities are:
• Expanding BART’s (Bay Area Rapid Transit’s) underwater, cross-bay tunnel. BART has already reached maximum capacity of its twin tube system during peak commute times. Enlarging the underwater portion (and the above ground track system) seems unrealistic.
• Adding more ferries. The Bay Bridge ended the ferry system era long ago. And ferries imply more automobiles to get to and from the water’s edge. Quite impractical.
• Adding a second bridge parallel to the existing Bay Bridge, a more practical idea already proven in other cities around the world (Fig. 5).
Considering these limited options, a second bridge seems to be the most logical approach to solve the restricted capacity of the existing bridge. And, given the fact that the replacement of the old East crossing structure is nearing completion, now is the time to begin planning and designing a completely new 2nd SFO Bay Bridge alongside the present one using the retrofitted, old East crossing structures. There are two major parts to this proposal:
• The efficient retrofitting and realignment of the original, historic East crossing structures.
• The design and construction of a new West crossing, alongside and complementing the existing suspension bridge.
Fig.
5. Existing and proposed second bridge
8. New East crossing (second bridge)
The current East crossing replacement project, now nearing completion, includes about $280 million for demolition of the replaced original steel structures. With the exception of the local damage caused by the ’89 Loma Prieta earthquake, the original structure has served well for 76 years carrying ever increasing volumes of traffic.
The most questionable elements of the original (1936) East crossing are the foundations supporting most of the piers. These are timber piles driven into questionable soil and should not be considered safe for reuse. It is believed, however, that the idea of salvaging the 55 000 tons of already fabricated structures is a worthy one. The best option for reuse of the existing steel structures would be to build new foundations and piers alongside the current ones, barging and lifting or sliding the existing superstructure on to the new substructure and then reinforcing the existing superstructure to meet current codes.
Fig.
6. Old truss spans – concept for creating continuity
Even if some of the existing structures need to be strengthened or reinforced, there is a good deal of value seen in the reuse of that steel, not to mention the possibility of converting significant demolition costs to a positive use – that of creating added bridge traffic capacity. In contrast to past studies, this proposal differs in that it looks to take advantage of the “abandoned” East crossing structures by retrofitting and reusing them. This represents some 55 000 t of fabricated steel that served very well for decades, and is doing so currently. Coupled with eliminating a good portion of the $280 million demolition budget, this could easily represent in the range of $500 million “in the bank” to be credited to construction of the new SFO Bay Bridge second crossing.
There are various options for increasing the structural capacity of the systems involved. One method is to interconnect the adjacent simple spans, converting them to a continuous system (Fig.6). Members can be strengthened by a judicious addition of plates, channels or other shapes to the existing members, connecting them with bolts and/or welds. If necessary, compromised rivets can be replaced with high strength bolts, as in the West crossing retrofit.
Fig. 7. Cantilever truss section – reinforcing cable-stayed system
An interesting option would be to add a cable-stayed type of reinforcement system to the Cantilever truss section introducing “up-lift” forces at the tips of the cantilever arms (Fig.7). Another ingenious way to “increase” capacity is to reduce dead load by replacing existing heavy reinforced concrete slabs with lighter systems such as orthotropic steel decks. This approach was used very successfully in the early 1980’s on the redecking of the Golden Gate Bridge. This can reduce deck dead loads by up to 50-60 percent, “buying” extra structural capacity or otherwise relieving overstress situations. This technique would also make foundations more economical to construct. Of course, combinations of such approaches would be in order. It is recognized that the use of orthotropic steel decks and other “lightening” techniques would require demolition of the concrete slabs now existing on the structures planned for demolition (approx. 102 000 sq. m of 160 mm slabs) thus reducing the potential savings from foregoing demolition. But the other benefits, such as having to deal with much lighter elements to be relocated, may well more than offset this issue. It is obviously very important that decisions be made and initial activities undertaken before the afore-mentioned demolition is initiated. That demolition is scheduled to begin in late 2013.
9. New West crossing (second bridge)
It is envisioned that the new West crossing would be a double decker like its neighbor so that it would “flange up” with the retrofitted (old) East crossing. That would mean it would have 5 lanes in each direction (or 4 lanes plus a shoulder) - the same width as the retrofitted East crossing. One scenario would feature twin suspension structures placed end to end from San Francisco to YBI very similar to the existing West crossing, but with two main spans of approximately 740 m, one central span of approximately 760 m and two side spans of 370 m. A central anchorage separating the twin bridges as in the 1936 bridge probably would not be necessary. Using the sophisticated analytical tools available to today’s engineers, it is felt the bridge can be designed and built without a central anchorage.
While the original bridge featured a tunnel and viaduct through Yerba Buena Island separating the West and East crossings, the idea for the new bridge would be to create a pier near the southern tip of YBI. This pier would serve a combined function: anchorage for the east end of the new West crossing and support for the west end of a new, slightly curved, transition section between the pier and the west end of the retrofitted, relocated cantilever truss structure.
An alternative to the above could be a cable-stayed bridge with similar spans. There may be other solutions, however, suspension and cable-stayed designs are the only ones that have been shown to reasonably span farther than 700 m. Consideration of other systems would likely require shorter spans necessitating more supports. This bridge should be designed to the highest level of today’s achievements in bridge engineering and construction using all modern technical ingenuity including orthotropic decks, high strength steel and concrete, composite steel/concrete and the like. It is the authors’ strong opinion, the way to achieve the best result in terms of beauty, efficiency, cost optimization and construction time is to invite international concept competition by the best bridge designers, worldwide. The winner of such a competition should be invited to participate in design-build competitions for the elements of the project.
Fig.
8. Traffic lane concept for second bridge
10. Discussions, Final Comments and Conclusions
Because of the relentless increase of population in the San Francisco Bay Area, and the resulting pressure on traffic crossing the Bay, it is inevitable that a new bridge will be needed to relieve that congestion. The abandonment of the old East crossing offers an opportunity to double, or nearly double, the capacity of the renovated SFO Bay Bridge. This paper lays the ground work for doing that cost effectively by realigning and retrofitting the 55 000 t of structural steel destined for abandonment and demolition, sitting out there alongside the new and reconditioned bridge. It has been demonstrated here how this could be done. But decision making and planning must begin very soon—especially the decision not to demolish the steel of the old East crossing.
Advantages of the proposed Second Bay Bridge crossing include the following points:
1. It will resolve the extreme traffic congestion problems associated with the original 1936 and improved current bridge, whose capacity has remained essentially the same for 76 years.
2. With four or five traffic lanes in each direction, the Second crossing will increase traffic capacity between San Francisco and Oakland by 80% to 100% (Fig.8). (Note: several options exist; 4 or 5 traffic lanes, plus a shoulder and/or a bicycle lane with the ability to adjust structural capacity by using orthotropic decks, etc.).
3. It will provide reserve crossing capacity should unforeseen problems such as serious accidents, earthquake or other damage, occur.
4. It will redistribute traffic flow within San Francisco by way of more entry and exit approaches, reusing previous routes from the removed in the 1990’s Interstate 280 to the Embarcadero.
5. It will greatly reduce the time required to cross the Bay especially in peak traffic hours.
6. It will result in environmental savings from fuel saved by reducing “stop and go” traffic during commute hours.
7. It will result in environmental savings by the reuse of 55 000 tons of steel bridge structures – by reducing the need to produce, manufacture and transport this quantity of new steel.
8. It can result in the possibility for bicyclists to ride from San Francisco to Oakland and vice versa.
9. The large scale reuse of the existing East crossing structures will help in the development of pioneering techniques to be used for the retrofitting of many of the 150 000 deficient bridges in the U.S.
10. It will capitalize on converting demolition costs to new construction, and reusing already fabricated structures to the tune of $500 million or more.
11. With the ingenuity required to adapt the older structures to today’s requirements and the anticipated high level of design criteria and modern technique for the new West crossing, it will contribute to the reawakening of the Art of American Bridge Engineering.
11. References
[1] UNITED STATES STEEL, “The San Francisco – Oakland Bay Bridge”, 1936
[2] CALIFORNIA DEPARTMENT OF TRANSPORTATION, Bay Bridge Project / Fact Sheets, baybridgeinfo.org
[3] CALTRANS “2nd Quarter Report, 2012”, www.dot.ca.gov/baybridge/2012-2QReport.pdf
