Lion’s Gate Bridge Seismic Retrofit

KCB was retained by American Bridge/Surespan JV to provide design-build engineering for the seismic retrofit of the North Approach Viaduct. This was the first major structure to receive a seismic retrofit using the design-build contract format. The viaduct structure is 670 m long and comprises 25 simply supported plate girder spans composite with an orthotropic steel deck supported on braced steel bents. The retrofit scheme had to be designed during a short bid period.

KCB determined the most economical and robust retrofit strategy was to dissipate energy by allowing the braced steel support bents to rock by uplifting at the baseplate level. This innovative approach was taken as a means of limiting the loads in the steel structure.

Impact, rocking, and other non-linear response characteristics of the retrofit structure were taken into account using explicit time history modeling. The retrofit included strengthening of the steel superstructure, additional bracing, concrete tie beams between existing footings, and piles in locations where large liquefaction-induced displacements were anticipated.

An independent check of the north approach sidewalk widening was provided. KCB also provided engineering assistance to the contractor for the suspension span deck replacement, which was replaced in indifidual segments during night closures while maintaining all daytime traffic.

The project won the 2004 CEBC Award of Merit for Transportation Engineering.

Services:

• Structural analysis and retrofit design
• Liquefaction mitigation design
• Complex non-linear analysis
• Construction engineering assistance