Lions Gate Bridge
Lions Gate Bridge
Lions Gate Bridge - Vancouver, British Columbia, Canada
Upgrading a Landmark Bridge
The Lions Gate Bridge in British Columbia connects the city of Vancouver to the north shore communities across the first narrows of Burrard Inlet. The suspension bridge, with a main span of 1,550 feet, was completed in 1938 and designed to carry two lanes of traffic. Faced with a serious deficiency in traffic capacity and with structural deterioration due to age and the elements, in 1999 the British Columbia Transportation Financing Authority elected to replace the suspended superstructure with a new, wider structure.
Parsons performed detailed construction engineering services for this complete rehabilitation. Elements replaced included the suspenders, stiffening trusses, and deck system. Because the nearest alternate crossing entailed a 3-hour detour, the owner required that traffic be maintained throughout the project duration, with only limited nighttime and weekend closures.
Segment 2 in the north side span has been replaced and connected. Ready for traffic!
Deck Replacement Sequencing
To enable daytime traffic flow, the structure was replaced in segments. The stiffening trusses and roadway deck were removed in 64-foot-long sections weighing 120 tons each. Sections were cut free from the bridge and lowered to a barge below, then new prefabricated sections were hoisted into place and supported by adjustable extensions attached to the existing suspenders.
The existing bridge sections were connected to the new sections with a “continuity link” that provided live load shear continuity across the work front. The existing trusses were 16 feet deep and positioned with the top chord approximately 11½ feet above the roadway. The new trusses are approximately 6 feet deep and are entirely below the roadway level. These relative configurations made it impossible to maintain the flexural continuity of the trusses, which dramatically increased the live load suspender forces at the erection front—to levels approximately six times that of the suspenders above the uncut trusses. Sophisticated suspender adjustment sequencing and suspender jacking were required to control loads and roadway alignment.
Parsons worked closely with the contractor, American Bridge Company, to develop detailed methods and purpose-made equipment. We performed erection analysis for each of the 54 stages of construction, determining member demands and stresses on both existing elements and new elements.
Readying a north sidespan segment for lowering.
2-D, 3-D, and Geometric Analyses
Parsons performed extensive computer modeling and analysis, including wind tunnel and aerodynamic studies:
Two-dimensional dead load analysis to determine member forces, suspender adjustments, and any additional measures needed.
Three-dimensional analysis to develop properties of the continuity link, member forces due to wind, and mode shapes and natural frequencies of the bridge for stability analysis.
Geometric analysis to ensure roadway crown alignment and critical deflections and to provide information for design and correct positioning of construction equipment.
Several conditions that affected the geometry of the bridge complicated the analyses. For instance:
Some of the original suspenders had been fabricated to incorrect lengths.
The suspenders and the main cables had stretched over time.
The north cable bent had settled, as had the south tower.
During the original construction the main cable slipped through the cable saddle atop the north cable bent by more than two inches.
We had to incorporate these factors into the computer models to account for stresses that were induced into the trusses.
Parsons is a world leader in bridge design and construction, and we have worked on more than 4,500 bridges of all types in more than 40 countries and on six continents. Our suspension bridge experience includes the following projects:
New York