Construction of the signature span of the self-anchored suspension span of the San Francisco-Oakland Bay Bridge required the entire deck to be in place prior to transferring its weight to the hangers and main cable. Consequently, the deck needed to be supported on an independent, temporary structure during the significant period of time required for the assembly of all the components and the load transfer. While supported by the temporary structure, the deck remains vulnerable to earthquakes. For the design of the temporary support structures Caltrans specified a response spectrum based on a 92-year return period, and required the steel support towers to have a minimum ductility of two. The erected portions of the permanent structure were to remain undamaged in the design earthquake. To meet this requirement Klohn Crippen Berger designed twin trusses supported on eccentrically-braced frame (EBF) towers constructed using tubular sections. Tubular EBF’s, based on recent research and development at the University at Buffalo in New York, do not require lateral support at the yielding shear link, thus allowing the towers to be flexible in the longitudinal direction of the bridge and ductile in the lateral direction. This paper details the analysis and design of the Tubular EBF temporary towers for the erection and assembly of the bridge deck for towers ranging from shorter ones over land, to taller marine towers on piles sunk deep into the bay sediments. The interplay between design for wind and the design for earthquake is described.
Dowdell, D.J., B.W. Hamersley, M. Bruneau and S. Tudor. 2015. “Tubular Eccentrically Braced Frames for the San Francisco Oakland Bay Bridge Temporary Works,” in Proceedings of the 11th Canadian Conference on Earthquake Engineering (11CCEE), Facing Seismic Risk, Victoria, British Columbia, July 24-24.