04 - 043004 INTERIM EAST BAY RETROFIT



FEBRUARY 4, 1999





PROJECT CONTACT PERSONS
PROJECT MANAGERKen Terpstra(510) 286-4679
PUBLIC INFORMATION OFFICEGreg Bayol(510) 286-4444
CONTRACTORCalifornia Engineering Contractors(650) 965-8600

CONTRACT INFORMATION
CONTRACTOR'S BID$ 12,878,088.00
NUMBER OF WORKING DAYS460
CONTRACT AWARDEDFebruary 5, 1998


PROJECT SCOPE HIGHLIGHTS

INTRODUCTION
The proposed work is in lieu of all previously-proposed seismic retrofit projects for the east span of this bridge. The project is part of a legislatively mandated seismic retrofit program adopted January 1, 1991.

LOCATION

The project limits are from Bent 46 of the Yerba Buena Island East Viaduct to E23, the last pier supporting the truss spans, inclusive.


Construction began on the San Francisco-Oakland Bay Bridge in the 1930's and was completed on October 23, 1936. The upper deck of the bridge originally carried six lanes of automobile traffic with three lanes in each direction. The lower deck carried three lanes of truck traffic. The lower deck also carried two railroad tracks for use by the Key System and two other railroad companies that provided commuter train service. In the early 1960's the bridge and its approaches were extensively reconstructed to convert to one-way traffic (mixed vehicles) on each deck, with five lanes each, establishing the lane configuration that we have today. That contract also strengthened the upper deck structure to carry truck traffic and eliminated the vertical columns between the decks at the San Francisco approach and at Yerba Buena Island.


The east spans of the San Francisco-Oakland Bay Bridge consist of a series of steel trusses with an approximate 2,500-foot long cantilever truss system over the channel near Yerba Buena Island. A tunnel through Yerba Buena Island connects the east span to the bridge¹s west suspension span. East of the tunnel is a concrete viaduct with column bents 39 through 52. The upper and lower deck structures consist of steel girders with non-composite cast-in-place concrete decks, supported on bents composed of steel columns (bents YB1 through E16, except E1) or reinforced concrete pedestals (bents E1, and E17 through E23).


None of the piers or superstructures were originally designed to withstand the transverse and longitudinal forces that would be applied to the bridge during a maximum credible earthquake. For the last three years Caltrans has been working in conjunction with private engineering firms and research universities on the assessment of seismic risk and the subsequent seismic retrofit design for the bridge. When the designs were sufficiently developed for the seismic retrofit, estimates determined the cost to be approximately $909 million for the east span alone. In light of this high cost, Caltrans prepared a detailed study analyzing a replacement rather than retrofit option; it was estimated that the replacement cost of the proposed skyway option would be $1 billion: the initial cost of retrofitting the 60-year-old structure was estimated to be at 91 percent of the replacement cost. Additionally, the life cycle costs of a new structure compared with the retrofitted one would render the new bridge ³less² expensive than the retrofitted one. Other significant benefits of replacing the east span are: the increased seismic safety of the new bridge cannot be matched by a retrofit, the new bridge would provide a higher post-earthquake serviceability level, the maintenance burden would be less on the new bridge, and traffic would not be impeded as much during construction.


Since the replacement structure would require at least two additional years to complete over the original retrofit scheme, including the design phase, the risk to the public during the interim can be reduced by implementing this interim retrofit project.



PROPOSALS

The San Francisco-Oakland Bay Bridge (SFOBB) is a major transportation link and a major interregional freight crossing between San Francisco and the East Bay cities. It currently carries approximately 280,000 vehicles each day. A closure of this bridge due to a seismic event, and subsequent repairs, would create a major economic impact to the region, as was experienced during the Loma Prieta earthquake of 1989. As such, the SFOBB qualifies as an "Important Bridge" as defined in "Seismic Performance Criteria for the Design and Evaluation of Bridges", which was authorized by the Caltrans external Seismic Advisory Board.

The replacement bridge will be designed to allow immediate service level and minimal damage for the Functional Evaluation earthquake. It will be designed to allow for immediate service and repairable damage for the Safety Evaluation earthquake.

The goal of the interim retrofit of the existing bridge is to protect the traveling public during a smaller magnitude probable earthquake until traffic is transferred to the replacement bridge. A specific level earthquake ground motion was not applied in the interim retrofit effort. Rather, the most glaring vulnerabilities, as identified in the previously studied Safety Evaluation earthquake retrofit, will be eliminated. A push analysis was used to establish the threshold for the first failure, and each subsequent one, in vulnerable members, joints, and connections. One exception is that a scaled, non-linear time history analysis was performed on the complex cantilever truss. Demands from this analysis were compared to capacities of truss members and joints to identify local vulnerabilities. Retrofit strategies were determined from the demand/capacity evaluation.

The proposed seismic retrofit work consists of the following:

BENTS 46 - 52 OF THE EAST VIADUCT APPROACH

Retrofit work in this area consists of strengthening the following columns by constructing partial-height column collars:

Bents 46-47 south columns

Bents 48-49 north and south columns

Bent 50 north column

Bents 51-52 north and south columns


The concrete collars will surround the existing column, extending beyond the existing faces by eight or nine inches; reinforcing steel will be drilled and bonded to strengthen the interface between new and existing concrete. The collar height will be approximately seven feet.

Existing shear walls will be extended to the bottom of the arch beams at bents 51 and 52. This shear wall infill will close the areas between columns, resulting in a continuous shear wall along the length of the bents, with the exception of a pedestrian access opening through bent 51. The new concrete will connect to the existing concrete by drill and bonded reinforcing steel.

Even though structural excavation will be completely contained underneath the existing roadway, soil testing results show that portions of the proposed excavations contain contaminants; as such, and since the excavation quantities are relatively minimal with respect to the overall project cost, all structural excavation will be considered to be Type H.

PIERS YB1 - YB4: THE 288-FOOT SPANS AT YERBA BUENA ISLAND


PIER YB1:

There will not be any work at this pier.

PIERS YB2 and YB4:

The east and west faces of both vertical tower legs will be covered by half-inch perforated plates. The varying lengths of cover plates will be spliced together by nine-foot plates, over the existing splice plates. The existing battens will be removed and the new plates will be connected to the tower leg by high-strength bolts, or by replacing existing rivets with said bolts.

PIER YB3:

Four hollow, square (18" x 18") steel kickers will connect the lower truss chords to the tower legs in order to assist in transferring longitudinal thrusts to the tower legs, thereby fully utilizing the capacity of the truss-shoe connections on the top of the tower legs. The pinned kicker is approximately 48 feet long, and will attach to the tower leg and lower chord with base plates. The lower truss chords at two adjacent panels of Pier YB3 will be strengthened by replacing existing lacing with perforated cover plates.

SUPERSTRUCTURE WORK:

The end truss verticals at each span will be strengthened by adding cover plates and perforated cover plates, replacing rivets with high-strength bolts at the floor beam-to-vertical connections, replacing the bracket assemblies to strengthen the vertical-to-upper floor beam connection, adding stiffener plates to the verticals and adding new splice plates on the outside face of the verticals to strengthen the connection between verticals and gusset plates.

PIERS E1- E4: THE CANTILEVER SPANS

PIER E1

A doweled, concrete shear key between the bearing shoe for the Yerba Buena Island 288-foot spans and the anchor for the cantilever spans will be constructed on the north and south sides of the tower tops.

PIERS E2 AND E3:

Piers E2 and E3 will require minimal work, consisting of rivet replacements with the gusset plate connections between the diagonal bracing and the tower legs.

PIER E4:

Tower E4 will require substantial work along the entire height of the twin columns. At the base of the towers additional anchor bolts will be added while the existing anchorage brackets will be substantially strengthened. The diagonal bracing/column connections will be strengthened by replacing selected rivets within these connections with high strength bolts. At the top of the towers, the top strut/column connection will require strengthening. A "bumper" system will be installed to absorb longitudinal impact between the towers.

SUPERSTRUCTURE WORK:

A review and analysis of the deck-to-floor beam connections for both the upper and lower decks determined the most vulnerable elements to be the rivets which connect the floor beams to the main truss. As such, all of the rivets which connect the upper deck floor beams to the main truss verticals will be replaced. For the lower deck floor beams, those rivets which connect into the vertical posts/hangers will be replaced. The lower deck floor beams are substantially tougher than the floor beams that support the upper deck, allowing greater discretion in selecting which rivets need replacing.

PIERS E5 - E9: THE 504-FOOT SPANS

PIERS E5 - E8:

Most tower cross bracing connections will be strengthened by replacing existing rivets with high-strength bolts. Angles will be added to the upper bracing on towers E5 through E8.

PIER E9:

Provide transverse shear restraint at the top of the west legs by bolting a guide plate to the tower using threaded rods. Elastomeric bearing pads will be placed between the column and new guide plate. Remove alignment struts at the west end of tower E9. The struts currently rigidly attach the 504-foot truss train to the tower. A system of dampers will be installed in the space vacated by the west alignment struts; the damper system allows ductility in the connection, but will prevent excessive longitudinal displacements at the shoe locations. Remove a portion of the bolts connecting the east shoes to the east legs to avoid a catastrophic collapse of the legs and provide ductility afforded by the existing dampers.

SUPERSTRUCTURE WORK, 504-FOOT SPANS:

The end and one-third-span truss verticals between decks will be strengthened by replacing lacing with perforated plates. The end lower floor beam and all upper floor beam connections will be strengthened by removing and replacing rivets with high-strength bolts. The upper truss lateral bracing system will be supplemented by connecting lateral diagonals (at midpoint) to outer chord members with double extra strong pipe.

SUPERSTRUCTURE WORK, 50-FOOT SPAN:

Increase seat width and length (west and east end) to significantly reduce risk of longitudinal motion drop failure. This will be accomplished by providing additional seats between existing seats. The additional seats will be installed by attaching plating to the existing floor beams, attaching channels to the plating, and supporting a W section on the channels. Because the seat will be continuous (transversely), drop failure due to transverse motion will not occur.

PIERS E10 - E16: THE CONTINUOUS 288-FOOT SPANS

PIERS E10, E12 - E16:

The cross bracing and strut gusset plate connections will be strengthened by replacing existing rivets with high-strength bolts and adding angles at the tower-to-gusset plate connections.

PIER E11:

There will not be any work at this double-tower pier.

SUPERSTRUCTURE WORK: The end (from L0 to L2 and L6 to L8) lower truss chords between piers E15 and E17 will be strengthened by replacing the lacing (at the tops and bottoms of the members) with perforated cover plates. These chords are critical to the anchor trusses connecting spans E11 through E16 to pier E17. The end truss verticals in each span will be strengthened by: adding cover plates and perforated cover plates, replacing rivets with high-strength bolts at the floor beam-to-vertical connections, replacing bracket assemblies to strengthen the vertical-to-top floor beam connection, and adding stiffener plates.

PIERS E17 - E23: THE 288-FOOT EXPANSION SPANS

PIER E23:

The north and south hollow towers of pier E23 will be filled with reinforced concrete. An eight-foot wide seat extension will be constructed at both westerly shoes. High-strength rods in cored holes will further strengthen the integrity of the seat extension to the pier. Related work to facilitate retrofit construction includes realignment of the 12-inch water line, and installing a new exhaust fan in the utility vault.

SUPERSTRUCTURE WORK:

The end truss verticals at each span will be strengthened by: adding cover plates and perforated cover plates, replacing rivets with high-strength bolts at the vertical-to-floor beam connections, replacing bracket assemblies to strengthen the vertical-to-top floor beam connection, and adding stiffener plates.