elements benefit from the controlled conditions inherent in prefabrication. The additional cure time allows the structural elements to be placed on-site at full strength. Prefabricated systems consist of “an entire superstructure, an entire superstructure and substructure, or a total bridge that is procured in a modular manner such that traffic operations can be allowed to resume after placement.”3 A bridge consists of a superstructure and a substructure. The superstructure consists of the bridge girders, deck and any parapets or walls resting on the deck. The substructure consists of the bridge columns, abutments, and foundations. Prefabricated ABC systems can be “rolled, launched, slid, lifted or otherwise transported into place” through various methods. A separate construction phase is generally not needed after the bridge is in place.3 Some of the ABC prefabricated systems recently used in Utah are bridge slides, SPMTs, and bridge launches. Slide-In Bridge Construction (SIBC) can be used with prefabricated systems, but the Federal Highway Administration considers SIBC to be a different type of ABC technology. SIBC can be a cost-effective method for using PBES and often has minimal impacts on traffic. A new bridge is built parallel to the existing bridge so that traffic can proceed as normal on the existing bridge. When the bridge construction is completed, the existing structure is demolished or removed so the newly constructed bridge can be slid into place. Bridge slides need a horizontal force to push or pull the bridge on a skid track with Hilman Rollers or Teflon pads with lubricant.4 Bridges that are moved using rollers must be moved on and off the rollers using jacks before they are finally lowered to their final elevation. Bridges moved with Teflon pads need to be constructed on elastomeric bearing pads that rest on top of the Teflon pads. With either method, the construction team often slides the bridge in stages. The construction team economically reuses rollers and pads during the move by transferring rollers and pads from behind the bridge to the front of the bridge. Although bridge slides can happen in a few hours, weekend closures for ABC are fairly common. The additional time is used to tie in the bridge and pave the surrounding area. The Utah Department of Transportation (UDOT) estimates that the cost of a bridge slide is approximately 20% less than the cost of using SPMTs. However, bridge slides are often built over the traffic like traditional construction and require the bridge to be constructed adjacent to the existing bridge.4 Prefabricated systems with SPMTs can provide efficient solutions for bridge replacements on high-traffic roads. They reduce traffic disruptions and increase safety for the workers and the traveling public due to bridge construction occurring in a staging area. Using SPMTs requires a staging area within close driving distance of the bridge location, and the fewer obstructions or grade changes, the better.5 SPMTs are “multi-axle, computer-controlled platform vehicles that can move bridge systems with precision to within a fraction of an inch.”5 A complex hydraulic jack system that ensures each axle is equally loaded allows vehicles to lift bridges and maintain the geometry while moving.5, 6 These paired axle wheels can pivot 360 degrees so that the vehicles can move horizontally in any direction to bring a bridge superstructure to its final location. The SPMTs can be controlled by a single person using a “joystick-like, portable control panel … to drive, lift, steer, or brake” the large vehicles.6 SPMTs are often used in Utah due to the short construction season and the “need to avoid rush hour traffic impacts during bridge replacement projects.”7 As a result, UDOT has created standardized ABC detail to improve the efficiency of bridge replacements for the local design and construction firms.7 Photograph 1. SPMTs transport the 4500 South bridge in Salt Lake City. Photo: Michael Baker International. 34
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