Final design has begun on an innovative lift bridge to be built in California’s state capital, Sacramento. It will be the first to be constructed across the Sacramento River in the city’s downtown area in 50 years and will provide an expansive—and iconic—crossing for pedestrians, bicyclists, vehicles, and—in the future—streetcars. It will connect the city to West Sacramento, a similarly named but independent entity.
The bridge will be the world’s first network tied-arch vertical lift crossing designed for vehicular and pedestrian use, according to bridge design firm Modjeski and Masters, which is based in Mechanicsburg, Pennsylvania. The firm is the engineer of record for the structural, mechanical, and electrical design of the main span. Mark Thomas, a consulting engineering company based in San Jose, California, is the prime consultant, and T.Y. Lin International Group (TYLI), headquartered in San Francisco, is leading the aesthetic design.
The design went through an extensive type-selection process, according to Noel Shamble, AIA, NCARB, the lead bridge designer and visualization group director for TYLI in San Francisco. The selected design, with its sweeping lines and contemporary feel, was chosen in part to reflect the city’s modern, forward-thinking vibe, according to Shamble. It was also crucial that the bridge meet the needs of the U.S. Coast Guard and the U.S. Army Corps of Engineers.
Navigation Necessary
Sacramento is one of the most inland points in the San Francisco Bay tributaries, Shamble explains. The river feeds into the bay via the Sacramento–San Joaquin Delta and the tidal estuaries of the Suisun and San Pablo Bays. It is an area in which levees are crucial to maintaining the boundaries between waterways and land. While the lift span will be used only a few times a week for pleasure craft, the coast guard and Corps required a 278 ft wide navigable channel for use when necessary. This access will be particularly important in emergencies when fast repairs may be needed on upriver levees, according to Shamble.
Because of its location on a river bend, the bridge will be unusually long at 900 ft. It will also be uncharacteristically wide because of its three vehicular lanes, two bike lanes, and expansive sidewalks, which extend along either side of the bridge. For these reasons, the bridge will be larger than its sister bridges that cross the river, Shamble says. “That provided a little bit of a structural and architectural challenge for us because we want it to appear iconic and signature, and a lot of that means making things as light and airy and as elegant as possible.”
To accomplish this, the design of the bridge’s 300 ft long main span and towers was visually and physically lightened as much as possible. The lift span will be a basket-handle arch with networked cables and a lightweight aluminum orthotropic deck. The extruded aluminum used for the deck will be insulated from the structural steel of the lift-span arch, tie, floor beams, and stringers to avoid corrosion, according to the engineers.
“The bridge is 125 ft wide at its widest point, which makes this bridge wider than most bridge spans are long,” said Kevin Johns, P.E., in written responses to Civil Engineering. Johns is the movable bridge business unit director of Modjeski and Masters and a project manager for the bridge’s movable span design.
Stress Relieved
For architectural reasons, “the arch tied girder and rib cross sections are parallelograms rather than simple rectangles,” Johns said. This altered how the stress of the elements were calculated because typical equations and commercial software are not designed for such irregular sections. The analysis is instead being performed with a 3-D, second-order, finite element model, he explained.
The lighter aluminum deck will mean that the counterweights used to lift the span can be lighter and smaller, which in turn means the lift towers can be thinner because they will require less mechanical equipment, according to Shamble. This, in turn, will make the arch span more prominent visually in comparison, he says.
The four independent lift towers that anchor the four corners of the main span will be clad in glass so that the mechanical systems within them can be viewed by passersby. The towers have been rotated 90 degrees from typical lift-tower orientation so that they face one another across the bridge’s deck. This will create a gateway-like feel for those passing over the roadway as well as more space for the sidewalks, Shamble explains.
Machinery Hidden
The drive machinery is also strategically placed to give the bridge an uncluttered and airy appearance. “Due to the unique nature of having counterweights hidden within the towers, the drive machinery cannot be placed in the usual locations for vertical lift bridges (either at the tower top or on the lift span), so the machinery will be located beneath the roadway and at the base of the towers,” said Geoffrey Forest, P.E., a mechanical engineer for movable bridges with Modjeski and Masters and the mechanical manager for the bridge design. Forest responded in writing to Civil Engineering.
The span will be raised and lowered by wire ropes and pulleys driven by electric motors and controlled by variable-frequency drives that finely control speeds and provide smooth starts and stops, according to Forest. “This hauling system is not unlike other vertical lift bridges, but it is adapted to the unique arrangement of the … [bridge’s] architecture,” he noted. “This way, the underlying machinery components are familiar to bridge-building contractors and maintenance personnel.”
Drilled shafts will be used for the foundations because of their seismic resistance and ability to carry large vertical loads with low settlement, according to Johns. The bridge piers and the structural elements of the lift towers will be created economically from reinforced concrete, which can be easily cast into the required irregular shapes, he explained.
Communities Connected
The bridge is intended to replace the double-level I Street Bridge, a steel truss swing bridge built in 1911. The swing bridge, located to the south of the new bridge, will be used as a rail crossing on its lower level, while its top level is being studied for potential conversion to pedestrian and bicycle use, according to material from the City of Sacramento.
The replacement bridge will connect Railyards Boulevard in Sacramento—a road located in a former industrial rail yard that is being redeveloped into a massive expansion of the city’s downtown—to C street in West Sacramento. The bridge will turn the boulevard into a main thoroughfare between the cities.
While historically the river was part of an industrial transportation network and cut off from residential areas of Sacramento, in the past few decades the cities on both sides of the river have been working to beautify the riverfront and make it an amenity for residents. “We really wanted the bridge to be a part of that—to celebrate the river, to get people out onto the water—and make it a statement project for the city,” Shamble says.
The goal is to have the design be “a flagship for the future of infrastructure,” he explains.
The new bridge will link to current and planned riverfront bike trails on both sides of the river. “This bridge will be a major part of that riverfront trail network,” Shamble says. The development also includes shaded seating areas on its main span and a riverbank amphitheater.
Construction of the $220-million bridge is set to begin next year, according to Modjeski and Masters.
This news article first appeared in the July/August 2020 issue of Civil Engineering.
