With the foundation complete, one of the 220 ft high towers begins to take shape. Work was done between 8 PM and 5 AM to keep the roadway open during peak travel times. © Backus Aerial Photography, Inc.
VDOT is replacing the aging Gilmerton Bridge, a twin-bascule drawbridge, on the same site, while keeping the existing bridge open.
December 18, 2012—Early in 2013, a team led by the Virginia Department of Transportation (VDOT) plans to float a massive 250 ft long, 90 ft wide steel truss lift span—weighing approximately 5.2 million lb—down the Southern Branch of the Elizabeth River from a marine yard in Norfolk, Virginia, to Chesapeake, Virginia, where the span will be positioned between two 220 ft tall steel frame towers.
The installation will complete phase two of the Gilmerton Bridge replacement, a $175-million project made considerably more complex by a tight site that is currently occupied by the aging 1938 twin-bascule drawbridge that the new span will replace, as well as a parallel bascule bridge owned by the Norfolk Southern Railway (NS) that is just to the south of the existing roadway. The low-clearance bridges open a staggering 7,500 times per year over the active waterway.
“We couldn’t build the new bridge to the north because just north of this bridge there is a 90 degree bend in the waterway,” says Ricardo Correa, P.E., VDOT’s design manager. “Oceangoing vessels come through here. And right now, as it is, they’ve got a very difficult challenge to make that maneuver around the bend and then through the 125-foot horizontal channel that we have through the bridge. We also have a constraint to the east to tie in to the existing roadway before an existing grade-separated interchange that is within a quarter mile of the bridge.”
The current Gilmerton Bridge averages 35,000 vehicle crossings per day, and is often at capacity during rush hour and in the summer. The vital crossing had to be maintained, but significant deterioration of the substructure ruled out rehabilitation for the span, which heavier emergency vehicles must bypass because of weight restrictions.
The new bridge will have 35 ft of clearance when closed, greatly
reducing the 7,500 openings per year of the current bridge.
© Backus Aerial Photography, Inc.
This left VDOT with a significant design challenge that almost sounds like a riddle. How do you build a new bridge to replace an existing bridge, on the site of the existing bridge, while the existing bridge remains operational?
“Initially, a double leaf or four-leaf bascule bridge was being considered,” Correa says. “When it became clear that the Coast Guard absolutely would not allow any alternate alignment—[and] the new bridge was going to be built over the existing bridge—[that] ruled out a bascule bridge. A bascule pier, being fairly solid, could not straddle the existing roadway [and] bridge.”
The solution VDOT developed—working with engineering firms Modjeski and Masters, of Mechanicsburg, Pennsylvania, and Gannett Fleming, of Camp Hill, Pennsylvania—is an elevated lift span bridge, built in three phases. During phase one the existing bridge was reduced from four to two lanes and 30 ft of the approach roadways and bridges were constructed in the tight space between the current bridge and the railway. The lift span and the full support towers were also constructed in phase one.
Phase two is the installation of the lift span and transfer of the two lanes of traffic from the existing bridge to the new span. Phase three is demolition of the existing bridge and construction of the rest of the approach roadways and bridges on the north side. The new bridge is wide enough to accommodate future widening of Route 13 to six lanes.
The lift span towers are 38 ft long by 90 ft wide and 220 ft high. Each tower has 1,009 steel members, each ranging from 5,000 to 150,000 lb. The towers were assembled exclusively at night, with VDOT closing the bridge from 8 PM to 5 AM. This meant the team worked closely with building information modeling (BIM) information to plan beam lifts and installations.
The bridge is a vital link in the area, with 35,000 vehicles crossing
per day. © Backus Aerial Photography, Inc.
“The time constraints are the tough thing,” says Marc Papini, project manager for Parsons Brinckerhoff, New York City, which is providing construction management services on the project. “You close the roadway at 8 PM and you have to open it at 5 PM. You really have to have your operations down to a T. What you are going to accomplish within that time frame?”
“VDOT had a detailed process for sweeping both the towers and the roadway in the early morning hours before the roadway opened to ensure the safety of motorists traveling on Route 13,” says Bud Morgan, P.E., M.ASCE, area construction engineer at VDOT. The team never missed a 5 AM opening during the tower construction phase of the project.
The towers are founded on a total of 8 enormous 12 ft diameter drilled shafts. Because the shafts had to be placed close to the substructures of the existing bridges, and to a greater depth, vibration was a concern. PCL Civil Constructors, Inc., part of the Denver-based PCL construction conglomerate, utilized an oscillator to drill the 124 ft long shafts, minimizing vibration to the foundations of the existing bascule bridges, which could become inoperable if their foundations were to move as little as a fraction of an inch. The team used highly sensitive seismic monitors to check vibrations and encountered only false alarms from passing trucks.
“As the steel casing of a drilled shaft is oscillated down, you are excavating the interior,” says Papini. “The drill shaft rebar cage is then placed inside the 12 ft diameter casing. Then you put in your tremie pipe, and you start your tremie pour of concrete, and as your concrete is being placed, you are oscillating and removing the top portion of the casing. So you are getting 65 feet of permanent casing at the top and then the rest is uncased, enabling a concrete and soil interface. So you get the appropriate skin frictions that we need for the towers.”
Soon, the approximately 5.2 million lb lift span will be floated into
space between the towers. © PCL Civil Constructors
The 4 shafts of each tower are connected by 9.5 ft by 8 ft concrete perimeter strut beams to create a rigid diaphragm. Prestressed concrete bulb T girders, 45 in. deep, are placed 8 ft on center all the way across the diaphragm, topped by a 3.5 ft thick concrete slab. Complicating much of this diaphragm construction work was the fact that it was done beneath the existing bridge.
“Many have asked why we built with these unusually large 12-foot diameter drilled shafts,” Correa says. “The reason for that is it provided the smallest footprint of a foundation that could give the amount of capacity that we need in that foundation. With a more traditional foundation, the footprint would have been too large to fit within the geometric constraints.”
The constraint of building the replacement bridge on the same alignment as the existing bridge presented another challenge.
“Typically, a lift span bridge would have the lift span resting on the diaphragm—the cap of the drilled shafts—when in the down position,” Correa says. “But because of the maintenance of traffic constraints we have here, that diaphragm is actually poured below the existing bridge, much lower than the support of the lift span. So the lift span in this case is going to be supported on steel ‘shelves’ on the tower front steel columns. It just made the design of the tower frame even more complex, because in this case we can have a five-million-pound lift span supported on the steel framing instead of on the concrete diaphragm.”
The new bridge will accommodate the eventual widening of Route 13.
Courtesy © Virginia Department of Transportation
“Wind loads at the site added another complexity to tower design. An aerodynamic study of the main span revealed that some of the tower bracing members may be susceptible to wind-induced vibrations,” Correa says. To solve this problem, the flat plates that would normally make up the flanges of the I-shaped members were replaced with channels in order to increase their out-of-plane stiffness.
“The busy Hampton Roads waterway required large vessel collision design loads,” Correa adds. A vessel impact study was performed and the results controlled the diameter of the drilled shafts.
“The fenders and dolphins that protect the bridge from a vessel impact are designed for the full vessel collision load, and the tower foundations are designed for half the load per VDOT’s project specific requirements,” Correa says. “Large reinforced concrete struts and a stiff diaphragm connect the four shafts at each tower to distribute the vessel collision forces to all shafts in the group.”
The hard work will pay off handsomely for VDOT. The new bridge can accommodate greater traffic. And by increasing the span’s height from the current 10 ft to 35 ft, the number of openings for the span will be greatly reduced.