Introduction

This pedestrian and bicycle overcrossing spanning Interstate 80 is located about 0.3 km south of University Avenue in the city of Berkeley, California. The overcrossing carries pedestrians, wheelchair users and bicyclists from Bolivar Drive at the north end of Aquatic Park across Interstate 80 and the West Frontage Road to the Bay Trail and the Eastshore State Park. This bridge acrosses one of the most congested freeways in North America.

The previously existing pedestrian access across Interstate 80 on the University Avenue Overcrossing required the use of an existing stairway east of the freeway which leads to the sidewalk on the south side of that overcrossing. The stairway is inaccessible to physically handicapped persons and requires bicycle riders to carry their bicycles up and down the stairs. The proposed Caltrans construction of direct HOV/transit ramps at the University Avenue Overcrossing would necessitate the removal of the existing sidewalk to accommodate the new ramps. The pedestrian and bicycle overcrossing was developed to replace the existing pedestrian access, allow for wheelchair access, and to improve bicycle commuting and recreational access.

Bridge Type Selection

The following considerations were used as basis for the selection of bridge type:

• Structural system issues considered included bridge pier locations, span length arrangement, foundation requirements, vertical and horizontal clearance requirements, and capacity to resist seismic forces.
• Aesthetic issues such as overall bridge appearance, compatibility with the landscape and existing built facilities near the site, user friendliness, and landmark status were considered.
• Construction issues such as erection method, equipment, construction time and the impact on the freeway were considered.
• The relative cost of the various bridge types considered and the relative cost of maintenance were considered.

The following six bridge types could potentially meet the above design considerations:

1. Concrete box girder with median column on a curved alignment over the freeway
2. Concrete box girder without median column on a straight alignment over the freeway
3. Concrete strut frame with median column on a curved alignment over the freeway
4. Single tower cable-stay without median column on a straight alignment over the freeway
5. Steel arch without median column on a curved alignment over the freeway
6. Steel arch without median column on a straight alignment over the freeway

The Berkeley City Council selected alternative 6 as the preferred structure type. For the main span across the freeway, the preferred structure type is a basket handle arch consisting of steel arch ribs supporting a concrete deck girder. The approach structures consist of concrete box girders. Main span and approach columns are circular in section and supported by groups of cast-in-steel-shell pipe piles. This scheme adapts well to the constraints of the site and complements the adjacent parklands to either side of the freeway. The arch form provides a symbolic gateway for the City of Berkeley.

Design

Design Requirements

The primary basis for the design is the Caltrans Bridge Design Specifications, Caltrans Memo to Designers, Caltrans Bridge Design Aids and Caltrans Bridge Design Details. In general, the bridge was designed within the following constraints and concerns:

• It must meet all Caltrans bridge design and performance requirements.
• It must be aesthetically pleasing and economical.
• Bridge foundations could not encroach within a 7.6 m wide easement containing a large diameter East Bay Municipal Utilities District sewer interceptor along the east edge of Interstate 80.
• A total width of traveled way of 4.7 m was specified by the City of Berkeley to allow for dedicated bicycle lanes and a separate sidewalk.
• A minimum radius of 20 m was specified by the City of Berkeley to facilitate bicyclists.
• A maximum slope of 5% was specified by the City of Berkeley to facilitate bicyclists and wheelchair users, and minimize number of intermediate landings. (A maximum slope of 8.33% is allowed under the American Disabilities Act, but would require intermediate landings for every 0.76 m rise.)

Design Loading

The overcrossing was designed for a live load of 4.1 kPa as well as an H10 emergency or maintenance vehicle load. The bridge was designed to meet strict seismic criteria for a near-field M7.1 earthquake on the Hayward fault, which is located about 3 km from the site. Seismic loading was based on site-specific response spectra for a maximum credible event on the Hayward fault. No utilities will be installed on the overcrossing.

Basic Layout, Span Arrangement, and Articulation

The bridge layout was developed to meet the following requirements:

• Function of the structure to facilitate pedestrians, bicyclists and wheelchair users
• Clearance requirements over the freeway which includes taking into account the future westbound widening of Interstate 80 to accommodate future HOV lanes
• Aesthetics, economy, and constructability
• Foundation requirements and limitations
• Seismic resistance

The bridge consists of five spans totaling 215.3 m, with an 89.4 m main span. The east approach is 78.1 m in length and located adjacent to the eastbound shoulder of Interstate 80 to the west of the Aquatic Park Lagoon. The west approach is 114.8 m in length and located west of the West Frontage Road. The bearing of the main span is at a 6° 39' skew angle to the freeway in order to provide users with a direct view toward the Golden Gate Bridge.

The articulation of the bridge with expansion joints was kept to a minimum. Expansion joints are located at the extreme ends of the structure only. The four bridge columns are fixed to the girder and footings with monolithic, moment-resisting connections. The abutments support the girder on sliding bearings with vertical restrainers to prevent uplift during a seismic event.

This combination of few expansion joints and high column fixity was evaluated for serviceability under the required seismic excitation, as well under the influence of creep and shrinkage in the concrete, and temperature change in the structure. The columns and foundations provide enough flexibility so that the time-dependent strains and temperature strains of the concrete, accumulated over the entire girder length between expansion joints, cause acceptably small bending stresses in the columns and axial stresses in the girder.

The preliminary design made use of simple and rational analytical tools that have proven reliable for many structures. The following design tasks were performed to determine the dimensions of the main structural elements of the bridge:

• Cross section design
• Longitudinal system design
• Column design
• Footing design
• Pile design

Superstructure Design

Detailed design and analysis made use of more refined analytical tools in order to verify the preliminary design. The following tasks were performed:

• Development of structural models for service load, seismic load and time-dependent analyses
• Evaluation of design values for moment and shear
• Evaluation of transverse and three-dimensional considerations
• Evaluation of pier moments, shears, and axial forces
• Push-over analyses

The cross section for the main span consists of a cast-in-place prestressed concrete edge girder with intermediate floor beams supported by vertical suspenders from two 0.9 m diameter steel pipe arch ribs. The arch thrust is tied by prestressing tendons in the deck. The main span is 89.36 m long with a girder depth of 1.0 m, deck width of 6.56 m and suspender spacing of 4.8 m.

The cross section for the approach spans consists of a cast-in-place prestressed concrete box girder with girder depths varying from 1.0 m to 1.35 m and deck width of 5.64 m. The approach spans vary from 25 m to 39.86 m in length. The minimum depth to span ratio is 0.034.

The main span and approach span superstructure consist of lightweight concrete in order to reduce structural mass and thus foundation costs. The roadway surface has a maximum 2% cross-slope. The traveled way consists of a 1.68 m wide by 0.076 m high sidewalk and two 1.22 m wide bicycle lanes with 0.30 m outside shoulders for a total width of 4.7 m. Level landings 4.6 m in length are provided at least every 122 m to meet access compliance requirements. Curb cuts are provided at every landing location. Guardrailing 1.1 m in height at the sidewalk and bicycle railing 1.4 m in height are provided at the edge of traveled way. Chain link fencing with a minimum height of 2.5 m is provided on the main span and on a portion of the east approach adjacent to the freeway.

Substructure Design

At the east approach, the first 78.1 m of approach is on fill for an abutment height of 4.0 m. At the west approach, the first 114.8 m of approach is on fill for an abutment height of 5.6 m. Seat abutments are provided at each approach. Lightweight fill was used for the east approach in order to minimize slope instability. Normal weight fill with a settlement period of 5 months was used for the west approach in order to minimize embankment costs. To provide a smooth transition from the approach pavement on embankment to the bridge superstructure, reinforced concrete approach slabs are provided at each abutment.

The approach ramps are supported by single column bents. The main span is supported by a pair of double column bents. Each column consists of a circular reinforced concrete column of 1.2 m diameter. Column cladding consisting of reinforced concrete columns that taper from a 1.676 m x 1.9 m oblong with semicircular ends at the top to a 1.676 m x 2.95m oblong with semicircular ends at the bottom are provided at the arch supports. A polystyrene detail was used to limit the effective size of the arch support columns to 1.2 m diameter under seismic loads. Foundations will consist of reinforced concrete pile caps supported on groups of 24" diameter cast-in-steel-shell pipe piles.

For transverse seismic loads, the piers behave as cantilevers with significant moments developed at the bottom only. For resisting longitudinal seismic excitation, the main piers behave as members in a rigid frame. The creep, shrinkage, and temperature deformations of the superstructure are adequately accommodated in the monolithic piers by the flexibility of the foundations and the pier shafts.

Foundations consist of 0.61 m diameter steel pipe piles filled with concrete and reinforcing steel. The piles, up to 38 m in length, act as friction piles with some limited end-bearing support.

Peer Review

The design was peer reviewed by the City of Berkeley Seismic Technical Advisory Committee consisted of Professor Vitelmo Bertero and Professor James Kelly of the University of California, Berkeley, and L. Thomas Tobin, past executive director of the California Seismic Safety Commission.

Analysis

Structural Modeling and Analysis

All important structural features of the bridge were modeled, including the superstructure, the columns, and the foundations. The superstructure was modeled as a line of frame elements representing the entire cross section, oriented along the centroidal axis of the prototype girder. The columns were modeled similarly, including semi-rigid elements in the beam-column intersection. Analytical studies of the bridge were performed to support design under service and extreme-event conditions.

Stage-by-Stage Analysis

A stage-by-stage analysis for construction and long-term conditions was performed to validate the time-dependent behavior of the bridge that included the following:

• Development of a time-dependent structural computer model for construction procedure.
• Evaluation of stresses and deflections at critical construction stages.
• Evaluation of stresses and deflections over the service life of the bridge.

SFRAME/S3D©, OPAC's time-dependent bridge analysis program, was used to perform time-dependent analysis. The time-dependent analyses provided accurate assessment of camber requirements for the arch span (where creep effects deflect the deck upward) and the tightly curved ramps.

Construction

Construction of the arch span took advantage of California contractors' familiarity with cast-in-place post tensioned bridge construction. The foundations, piers, and girders of the entire bridge were built using established conventional methods on falsework.

The steel-tube arch ribs were erected on temporary bents atop the partially completed concrete deck of the main span. With the arch ribs and suspender ropes in place, the deck tie was post tensioned, shortening the deck and springing the arch ribs. As the arch rib profile rose, the deck weight was transferred to the suspenders and the system became arch-supported.

Upon completion of the arch span, the approach girders were completed, providing a unique structural solution to the challenges of building a bridge with the plan layout of a pretzel.

Mobilization for the construction of the bridge began in the fall of 2000. In general, construction followed the sequence outlined below:

1. Site preparation
2. Construction of the west approach embankment
3. Pile installation
4. Construction of the footings
5. Construction of the columns
6. Construction of the abutments
7. Construction of the east approach embankment
8. Erection of the falsework and temporary supports
9. Construction of the main span superstructure
10. Construction of the approach spans superstructure.
11. Erection of the arch ribs
12. Construction of the superstructure closure east of Bent 3
13. Construction of the approach slabs, curbs and sidewalks
14. Installation of the railings, lighting fixtures and approach pavement

To allow the contractor to build the bridge without the need of construction engineering consultants, OPAC provided arch fabrication data in the contract documents, including fabrication geometry, arch splice locations, suspender cut length and jacking protocol. The bridge was completed in February 2002.

Conclusion

The Berkeley pedestrian and bicycle overcrossing is an innovative structure that meets the basic requirements of the project and coordinates well with the existing site geography. The clean, uncluttered lines of this arch bridge design provide a landmark structure marked by simplicity and purity of design. The relative proportions of its structural elements provide a sense of balance and strength to the bridge. The structural system is truthful with respect to expressed function. The aesthetics of the bridge should stand the test of time, providing a bridge whose appearance will be appreciated for many years.

The pedestrian and bicycle bridge has become an important landmark for the city of Berkeley, and helps to reduce traffic congestion and improve air quality by encouraging the use of alternative modes of transportation.

Credit

Owner		City of Berkeley
Designer		OPAC Consulting Engineers and Professor T.Y. Lin
Contractor		C. C. Meyers, Inc.
Steel Erector		Adams & Smith, Inc.
Service Date		February 2002