Orange County Branch Newsletter
Oakland Bay Bridge
By Ted Rigoni, PE, PMP
Secretary, Transportation Technical Group, OC ASCE
Mr. Ignatius (Po) Lam presented geotechnical and political challenges encountered in the new San Francisco-Oakland Bay Bridge, East Span project, on Thursday, September 20, 2012, at Costa Mesa’s Center Club. Mr. Lam’s presentation was part of the Orange County ASCE Branch and Transportation Technical Group’s twice a year joint luncheons, and over 70 professionals in the geotechnical, structural and transportation fields were treated to a mini-seminar on geotechnical issues to be considered during the design of a major bridge structure.
As background to the presentation, Mr. Lam received his Bachelor of Science degree from Ohio State University and his Masters of Science degree from Caltech. As the founding partner of Earth Mechanics, Inc. he has over 35 years of experience working on major infrastructure projects, including serving as lead geotechnical engineer on the San Francisco-Oakland Bay Bridge’s East Span. His presentation included a detailed PowerPoint of the geotechnical challenges, lessons learned and status of the project.
Mr. Lam has been involved with the design and construction oversight of the east span for over 15 years, and commented that the self-anchored suspension bridge structure is about 1/6th of the overall new east bay crossing length, but due to structural and geotechnical issues, about ½ of its overall approximately $6 Billion project cost. Design criteria for the single tower determined that it carry 90% of the structural load, considered unusual for a bridge design; the dynamic seismic response is very different from the adjacent bridge viaduct structures as well. The bridge tower is composed of basically four columns and contains sacrificial shear link beams to enhance seismic safety. The bridge has one end anchored into competent material in Yerba Buena Island, and the other end uniquely anchored to the bridge deck. Caltrans selected that the structure undergo a probabilistic seismic hazard ground motion criteria analysis using a 1,500 year return period, and the structure was designed for six ground motion time histories, three based on the San Andreas fault, and three on the nearby Hayward fault. The design is considered as extremely conservative.
The original bridge type selection was made by a 32 member Engineering Design Advisory Panel. After the design was well underway, the City of San Francisco directed that the bridge alignment accommodate post-closure development plans being crafted for the Naval Training Station Yerba Buena Island Historic Site. The Bay area Metropolitan Transportation Authority (MTA) decreed that the existing truss bridge structure be completely removed, including its sub structural supports to below the bay mud line, to negate the possibility for ship to bridge substructure impacts.
Further, local advocates were concerned about the environmental impacts of construction, and so for the substructure portion of the project, a pile demonstration investigation was undertaken, prior to the construction contract, that created a program for sequencing blasting operations; this program limited bay fish kills. Data gleaned from the blasting demonstration and overall robustness of the design was validated, as no substructure construction cost claims were submitted by the contractor.
The new bay crossing is slated to open to traffic by Labor Day 2013, and will provide 5 lanes of traffic in each direction.