By: Brad Miller
Bridges at the lower end of the size/cost spectrum lend themselves to accelerated construction by almost exclusive use of prefabricated timber, concrete, or steel. Prefabrication saves construction time, a key benefit in regions with short construction seasons; remote settings such as county roads, U.S. Forest Service or Bureau of Land Management property, and private land; and where road closures must be kept to a minimum.
Most prefabricated bridges consist of a single span over small streams, typically in the 20- to 60-foot range, although they can reach 140 feet. Materials are often selected according to context-sensitive design guidelines, construction time, and economics.
LONGITUDINAL LAMINATED TIMBER
Timber bridges come in all sizes and configurations, but the longitudinal laminated timber deck bridge — in which the deck serves as the main structural element for the superstructure — is especially well-suited for accelerated construction.
The deck may be made of individual timber pieces stressed together with transverse steel rods through holes at mid-deck. Or, glued and laminated panels may be laid flat and connected with transverse steel distributor channels fastened to the bottom of the deck. With this configuration, spans 25 to 30 feet are common. Longer spans are possible by varying the flat slab concept and using T-sections or timber box beams.
Longitudinal laminated bridges are also used as temporary or construction bridges because they are easy to remove and install.
In one instance, a bridge consisting of four 10¾-inch by 3½-foot glued laminated panels was 27 feet long. The exterior deck panels came to the site with curbs blocked up off the deck for drainage. A bridge railing and approach railing weren't required because it was a private bridge with low traffic volume. When the panels were placed and fastened together, they formed an economical single-lane bridge 14 feet wide with a 12-foot clear distance between curbs.
Though precast concrete footings were used for the structure, gabions are fast and economic alternatives to concrete footings for low-volume or pedestrian bridges. The timber deck could have been lag-bolted to a timber cap that was bolted to a 3x3x18-foot-long gabion footing using 1-inch-diameter threaded rods and square steel plates located at the bottom of the gabions. If gabions are used for abutments they may be stacked to provide a deeper abutment. It is important to protect gabions from direct stream action since materials moving with the stream will erode the gabion wire's galvanized coating, resulting in rusting and eventual failure.
Installation time including simple abutments can be as little as one day using a backhoe loader. Construction costs are $100 to $120/square foot. It is important to specify wood members that have been treated in accordance with best management practices for the use of treated wood in aquatic environments. This ensures that excess surface treatment doesn't contaminate sensitive environments.
STEEL AND CONCRETE
Prefabricated steel bridges come in a variety of configurations that lend themselves well to accelerated construction. They typically use wide flange girders. In addition to their use as permanent bridges, they are also used as temporary, emergency, or contractor work bridges because of their rapid installation and removal time. They can be installed in one day. Girders are often self-weathering steel for low maintenance, durability, and economy. Spans are typically in the 40- to 80-foot range.
Decks may be timber or corrugated steel bridge decking, with asphalt, concrete, or timber running surface. A single-lane bridge is typically made up of four girders and comes to the site in two longitudinal components, often stacked on one truck, with the deck and railing or curbs already attached. The cost is $120 to $160/square foot for bridges with simple precast abutments.
Small precast prestressed concrete girder bridges with integral decks come in a variety of configurations. Girder types include flat slabs, void decks, rib decks, deck bulb-tee girders, and others. When combined with precast concrete abutments, these bridges eliminate or greatly reduce the need for placing concrete onsite. This is very important for remote sites where concrete haul times can greatly exceed one hour.
Precast concrete girders are typically connected with grouted keyways and weld ties or transverse rods. Flat-slab 12-inch girders are good for spans up to 25 feet; rib-deck sections 16 inches deep are good for spans up to 35 feet; tri-deck girders 27 inches deep are good for spans up to about 60 feet. Deck bulb-tee girders are good for spans from 50 to 160 feet, depending on girder depth.
Simple rectangular section abutments are often used for low-scour-risk applications. Precast abutments may be used with piling for deeper foundations. Weld plates may be installed in the bottoms of caps for welding to steel piling, or grout block-outs may be used to accommodate tops of piling.
Construction time varies with project complexity and contractor's methods. Actual construction times of four to five weeks are common, but shorter construction times are possible. Costs vary from $140 to $180/square foot, depending on bridge complexity.
In addition to the bridge itself, there are lots of opportunities to save construction time and money through the use of simple designs and construction details. Use your supplier of bridge systems to help develop a solution that meets your unique needs.
— Brad Miller, PE, is a senior bridge project manager with HDR Inc. based in the firm's Missoula, Mont., office.