Replacing the

East Bay Bridge

The new San Francisco-Oakland Bay Bridge features four distinct components - all designed to withstand the next major earthquake.

Man-Chung Tang, P.E., Rafael Manzanarez, P.E., Marwan Nader, P.E., Sajid Abbas, P.E. and George Baker, P.E.

The Loma Prieta earthquake, which occurred in October 1989, measured 7.1 on the Richter scale and heavily damaged the eastern span of the San Francisco-Oakland Bay Bridge, a double-deck truss bridge built in 1937. A portion of the upper deck collapsed onto the lower one, killing one person and taking the bridge out of service for a month. After this event, the California Department of Transportation (Caltrans) launched a thorough study to determine whether the structure could withstand another major earthquake. Since the bridge is just a few kilometers from the San Andreas and Hayward faults, Caltrans and seismic experts agreed that a major earthquake is likely to occur sometime over the next 30 years.

Caltrans concluded that while the west-side suspension span between San Francisco and Yerba Buena Island could be upgraded to meet current seismic safety codes, the east-side truss span between Yerba Buena Island and Oakland is far more susceptible to significant damage from a major earthquake. Caltrans decided it would be safer and more cost effective to build a new east span rather than seismically retrofit the existing one. With a length of 13 km, the San Francisco-Oakland Bay Bridge was one of the longest high-level bridges in the world at the time of its construction. In accordance with the 1930 Uniform Building Code for buildings, the Bay Bridge was designed for seismic accelerations of only 0.10g. Retrofitting the structure was determined to be not only expensive and unreliable but also very difficult to implement while the bridge, which currently carries 280,000 cars per day, was in use.

Four distinct structures will make up the bridge: a low-rise posttensioned concrete box girder near the Oakland shore; a segmental concrete box girder crossing, called the skyway, above; a self-anchored suspension signature span, left; and a doubly posttensioned concrete box girder that connects to the east side of Yerba Buena Island.

The state authorized funding for the design and construction of a replacement bridge 150 m north of the existing east span (see figure 1). The structure will traverse the bay from the east end at Oakland to the west end at Yerba Buena Island (YBI), with parallel decks lying 17 m apart. The new crossing will accommodate five lanes of traffic in each direction and is being designed to carry the light-rail transit (LRT) system that is being considered for each of the 25 m wide decks. In addition, the eastbound structure will support a 4.8 m wide path for bikers and pedestrians.

Four distinct structures will make up the bridge crossing: a low-rise posttensioned concrete box girder span constructed on falsework near the Oakland shore; a 2.4 km long segmental concrete box girder crossing, called the skyway; a self-anchored suspension signature span; and a doubly posttensioned concrete box girder that connects to the east portal of the YBI tunnel. The bridge is being designed by a joint venture team comprising T.Y. Lin International, of San Francisco, and Moffat & Nichol, of Los Angeles. When built, the suspension span will be the largest self-anchored suspension span in the world. The project will also require the design and construction of two temporary steel structures at YBI to accommodate traffic detours during the construction of the YBI transition structure.

The design will be completed in May 2001 and construction is expected to start that summer. When the new bridge is completed in 2004, the existing structure will be demolished. Caltrans is coordinating the design process with input from the Metropolitan Transportation Commission (MTC), the Federal Highway Administration, and the San Francisco Bay Conservation and Development Commission. The MTC, at the request of California's governor, Gray Davis, and Bay Area legislative leaders, became involved to ensure that regionwide interests are appropriately addressed in the project development process.

This collaboration helped the Engineering and Design Advisory Panel (EDAP), appointed by the MTC, to develop a list of recommendations to be considered in designing the bridge. For the overall design of the bridge, the EDAP recommended that two cable-supported design alternatives be taken to the level of 30 percent before a final selection was made and that the possibility of incorporating a path for bikers and pedestrians on the structure be evaluated. The other EDAP recommendations that had a significant impact on the design include ensuring that the bridge remain serviceable after a major earthquake, since it is a "lifeline" route; including 10 traffic lanes, 5 in each direction, with 2 standard 3 m shoulders in each direction; accommodating the possibility of future light-rail service; building on an alignment north of the current crossing; not using a double-deck system; and maintaining visual consistency throughout the structure.

The signature suspension span was chosen from among four design alternatives, namely two cable-stayed bridges and two self-anchored suspension bridges. The chosen design gives the east side of San Francisco Bay an aesthetically appealing crossing that complements other Bay Area bridges.

For the skyway section, the EDAP recommended having mostly long spans of equal length for aesthetic appeal and paying particular attention to the design of the supporting pier as it enters the water, including the possibility of submerging the pile cap below water. It also recommended that the tower on the eastern bridge not be taller than the suspension towers on the existing western span and that the main-span tower not have a diamond shape because of that shape's seismic vulnerability. Other constraints in the design include navigational requirements: The minimum horizontal clearance had to be 152 m and the vertical clearance had to be 42 m above mean sea level.

Designers also had to consider the pronounced variations in the site's geology. At the western end of the bridge, near YBI, the piers will be founded on rock. As the bridge alignment progresses east towards Oakland, the bedrock Franciscan Formation drops abruptly and the remaining piers will be in deep bay muds underlain by the layered clays and sands of the Alameda Formation (see figure 3). The main-span tower structure will be sited on relatively shallow, sloping bedrock. The remainder of the skyway will be founded on a significant thickness of sediment.

Site geology and commission recommendations to the contrary notwithstanding, the new crossing's design had to be driven primarily by seismic safety considerations. The seismic risk to the new structure comes mainly from the Hayward Fault, which is located 12 km away and can generate an earthquake as strong as 7.5 on the Richter scale, and from the San Andreas Fault, which is 25 km away and can generate an earthquake as strong as 8.1. Seismic hazard evaluations were performed to define what are called the safety-evaluation earthquake (SEE) and functional-evaluation earthquake (FEE). The SEE in this case corresponds to an earthquake with a return period of 1,500 years, whereas the FEE corresponds to a temblor with a return period of 450 years. In the wake of an FEE the bridge will provide full service almost immediately and there will be minimal damage to the structure. Minimal damage implies an essentially elastic performance and is characterized by a minor inelastic response, narrow concrete cracks, no apparent permanent deformations, and no damage to the expansion joints. After an SEE, the bridge will provide full service almost immediately and repairing any damage sustained will pose only a minimal risk to functionality; it is characterized by reinforcement yielding, spalling of the concrete cover, and limited yielding of the structural steel. The bridge design is controlled by the SEE event.

Wind was also a factor in the design of the signature span. A wind speed of 31 m/s, which has a recurrence of 50 years, was the design wind speed, but the design criteria recommended that a speed with a 100-year recurrence be used for the service load design and that a 10,000-year speed be used for a lower-end, critical-flutter-velocity threshold. To ensure the suspension bridge's wind stability, wind tunnel tests are being performed on a section model and on a full bridge model in Marina, California, at the West Wind Laboratory.

Fig. 1 San Francisco-Oakland Bay Bridge Alignment

The self-anchored suspension bridge consists of a 385 m main span and a 180 m back span. The tower will be 160 m tall and will comprise four steel shafts connected with intermittent steel shear links along its height (see figure 4). The shafts are tapered stiffened box members made of grade 50 steel. The tower will be supported on steel pipe piles socketed into rock with concrete. The east pier will be supported on steel pipe piles founded on the Alameda Formation and the west pier will be supported on 12.5 m deep gravity foundations.

In the design, the 0.78 m diameter cable is anchored to the deck at the east bent and looped around the west bent through deviation saddles. Unlike traditional suspension bridge schemes, these deviation saddles are fixed to the west bent and the cable force on either side are balanced during construction using a jacking saddle. These saddles are supported by a prestressed cap beam that is designed to carry the differential stresses arising during service and seismic loads. The weight of this cap beam is designed to balance the dead load uplift at the west bent that results from the asymmetry of the bridge. The cables at the tower do not cross and are secured in a single saddle. The saddle at the east pier is supported by the box girders and is designed to move so that it can balance the cable forces on either side. The suspenders are splayed to the exterior sides of the box girders and are spaced 10 m apart.

The superstructure consists of dual, hollow orthotropic steel boxes. These boxes are in compression (supporting the cable tension forces) and are a part of the gravity load system. Diaphragms spaced 5 m apart support the orthotropic deck and distribute the suspender loads to the box. The box girders are connected together by 10 m wide by 5.5 m deep crossbeams spaced 30 m apart. These crossbeams carry the transverse loads between the suspenders (at a span of 72 m) and ensure that the dual boxes act compositely during wind and seismic loads.

The eccentric load caused by the pedestrian path on the south side is balanced by a counterweight on the north side. At the west bent, the box girders frame into the cap beam, which serves as the box girder at that location. The connection between the orthotropic steel box girders and the concrete cap beam is subjected to the compressive forces of the cables. Additional prestress is added through looped posttension strands connected at each rib to ensure that the steel box yields before the connection fails.

The east piers are reinforced-concrete columns with added prestressing to avoid shear failure in tension when subjected to seismic loading. They are supported on 16 steel shell pipe piles 2.5 m in diameter. These piles are 100 m long and are filled with earth up to 55 m from the top; the rest is filled with concrete. The west piers are reinforced-concrete columns enclosed by a steel shell to improve ductility. The columns are made monolithic with the prestressed cap beam, forming the west bent, and are supported on rock through a 12.5 m deep gravity footing.

At the west pier, a tie-down system, designed to resist a seismic uplift, consists of 28 stay cables, each with 61 strands 15 mm in diameter. The stay cables are anchored into the footing. At the east piers, the box girders are supported on bearings. Shear keys and tie rods are provided to carry lateral loads and uplifts, respectively. The box girders are supported at the east and the west pier for lateral loads and are "floating" at the tower.

Fig. 2 Typical Pile Cap

The transition spans between the skyway, the suspension bridge, and the YBI structure each have a hinge. These hinges are designed to allow the structures to move relative to each other in the longitudinal direction only, which helps to ensure that the four elements of the bridge will maintain their different dynamic responses to a seismic event, thus improving the bridge's integrity.

Balanced-cantilever cast-in-place (CIP) and precast segmental construction erection is being evaluated for the skyway. Concrete of normal weight will be used for the superstructure except for the side inclined panel or webs, where lightweight concrete will be used. The 28-day concrete strength for both the lightweight and normal-weight concrete is 50 MPa. The box girder is a single cell with two main vertical webs. The width at the bottom slab, or soffit, is 8.5 m, a dimension that is optimum from the seismic standpoint of pier column design. The 25 m width accommodates deck overhangs of 8.3 m on each side. Inclined girder panels or webs posttensioned both longitudinally and transversely were considered the most effective design to carry the eccentric bike path loading. Vertical posttensioning is used to control the shear stresses in the main webs of the girder.

The 160 m spans are arranged in frame units of three or four piers per frame with a girder depth of 5.5 m at the midspan and 9 m at the pier. Midspan hinges between the frames allow longitudinal expansion and contraction caused by creep, shrinkage, and temperature changes. An internal steel beam assembly at the hinge provides shear transfer and moment resistance in addition to controlling deflections at the cantilever end of each frame. These beams are rigidly connected to the box girder at one end and slide on bearings at the box girder at the other end of the cantilever.

Fig. 3 Foundation Types Investigated

The foundation and piers are designed to resist loads arising from elastic and plastic deformations of the superstructure, including creep, shrinkage, and temperature drop. This loading is critical for the proposed concrete superstructure for the sections with short piers near the Oakland shore. Steel tubular piles were found to be best suited for the skyway foundations because of their strength and ductility as well as their potential ease of installation. These piles will be driven to the lower Alameda Formation and are 90 to 100 m in length.

The piers of the skyway are about 50 m in height and stand in water depths of 15 m. This, added to the presence of a 15 m layer of young bay mud, establishes a very flexible structure. The design approach for such piers was to adopt a stiff foundation system, thereby confining the elastic displacements of the pile caps to acceptable levels and minimizing the potential for permanent offsets. A relatively stiff foundation system for the tall, flexible piers was achieved by using large-diameter battered steel piles filled with concrete.

The thickness of the pile shells was determined by the required flexural capacity of the pile, the ductility capacity of the pile, corrosion allowances, and drivability. In particular, the flexural demands governed the thickness at the upper regions of the piles, and drivability governed the thickness in the lower regions of the shells. The thicknesses ranged from 40 to 70 mm. To ensure the proper transfer of loads from the concrete fill to the steel shell, shear ring plates will be welded to the inside of the steel shell in the concrete-filled section.

Fig. 4 Tower Elevation

The pile cap will be approximately 6 m deep, with 2 m exposed above mean sea level. The pile cap's structural system will be a moment-resisting steel frame interconnected with the steel piles and the pier reinforcement (see figure 2).

The YBI transition structures are technically and logistically challenging because of the roadway geometry, the positioning of future on- and off-ramps, the island's topography, structures of historical importance, and temporary detours. The eastbound and westbound transition structures connect the main-span structure to the existing viaduct section near the Yerba Buena tunnel's east portal. The two structures are carried on separate single-column bents, except near the viaduct end, where they are supported on outrigger bents. The separate transition structures currently merge into the existing double-deck viaduct structure.

The length of each transition structure is approximately 467 m. The westbound structure has an overall deck width of 25 m and varies in depth from 1.6 m at the viaduct end to 4.5 m at the main-span hinge support. The height of both structures varies from 8 m at the viaduct end to 46 m above grade at the main-span end. Most bents are located in overburden sand and are supported on cast-in-drilled-hole piles.

The superstructure of the transition structures consists of cast-in-place reinforced-concrete box girders near the outrigger bents and cast-in-place prestressed-concrete box girders over the single-column bents. The shape of the box girders has been determined primarily by aesthetics to match the streamlined girder shape of the main-span superstructure and skyway.

The Oakland shore approach presented its own set of challenges. The westbound structure extends from the tip of the Oakland Mole to the point where the proposed westbound Route 80 joins the existing Route 80, connecting the Oakland shore to the skyway. The total length of the westbound approach structure is about 660 m, which is subdivided into two sections, an elevated structure and one that is essentially at grade. The eastbound approach structure is an elevated two-span frame 105 m long. The eastbound structure and the westerly portion of the structure are elevated and consist of a cast-in-place, prestressed-concrete box girder superstructure supported on reinforced-concrete piers, reinforced- concrete footings, and prestressed-concrete piles or small-diameter pipe piles. For the westbound at-grade structure, a flat-slab-on-piles option was selected.

The final design of the new east span of the San Francisco-Oakland Bay Bridge had to be innovative to deal with complex seismic and wind-load criteria, difficult site geology, and community requirements. The result will be a bridge that not only complements the many other outstanding Bay Area bridges but also offers East Bay residents their own signature span.

Man-Chung Tang, Ph.D., P.E., is the chairman of the board of T.Y. Lin International, San Francisco. Rafael Manzanarez, PE, is a vice president at T.Y. Lin, and Marwan Nader, Ph.D., PE, and Sajid Abbas, Ph.D., PE, are both principals of the company. George Baker, PE, is a senior engineer at Weidlinger Associates, Inc., in New York City.

Source: CIVIL ENGINEERING  MAGAZINE, September 2000