Bridges made easy
“Simple” construction generates fabrication and construction savings
A two-span research bridge over I-680 in Omaha, Neb., makes use of a new trend in steel bridge construction developed by the National Bridge Research Organization (NaBRO) and the Nebraska Department of Roads (NDOR). Using this technique, designers specify steel girders in simple-span configuration (abutment-to-pier or pier-to-pier). Later, the contractor connects them with a lateral concrete diaphragm at the pier to form continuous girders. Aside from Nebraska, bridge designers in Colorado, New Mexico, New York, Ohio and Tennessee have begun to employ this concept, called “simple for dead, continuous for live load.”
The technique greatly simplifies the formation of continuous girders, which would otherwise be bolted together in the field by the contractor. Continuous girders are desirable because they share live traffic loads across spans. Girders in simple-span configurations must independently bear live traffic loads, requiring them to be considerably more robust. For this research bridge in Nebraska, the simple-made-continuous concept reduced the cost of the erected steel superstructure by about one-third compared to a field-spliced rolled beam steel bridge.
The concrete diaphragm connecting the steel girders on the research bridge has been detailed and instrumented by NaBRO. Strategically located strain gauges embedded in the concrete diaphragm will monitor the behavior of the girder connection for several years.
NDOR is widening I-680 to six lanes. The new bridge, which was completed in August, replaces a four-span bridge that carries Sprague Street over the interstate. The elimination of two piers and a grass median facilitated the widening process. The median is now a concrete barrier.
The new bridge has two spans, each 97 ft long. The clear roadway width is 32 ft, including the 4-ft shoulders on either side. In addition, one side has a 7-ft cantilevered pedestrian walkway.
Design of the steel bridge superstructure follows AASHTO LRFD Bridge Design Specification provisions, second edition. The supporting superstructure consists of wide-flange rolled beam girders, specified as W40 x 249 grade-50 weathering steel. The four steel girders within each span are widely spaced at 10 ft 4 in. on center. An 8-in.-thick, 4,000-psi concrete deck poured over stay-in-place steel forms tops the girders.
The bridge requires no bearings or expansion joints. Steel piles within the integral abutments at each end flex to accommodate girder expansion and contraction.
At the center pier, the girders rest on simple 1.75-in.-thick bearing pads. Sponge rubber joint filler surrounds the pads so that the concrete diaphragm matches the bottom of the girder flange.
Designers called for K-type cross frames, with a simple bent plate alternative at the discretion of the contractor. The contractor picked bent plates. The plates bolt to stiffeners on the girders to connect them laterally.
Live traffic loads impose a large negative moment on a continuous girder at the pier. This means the continuous girder tends to deflect in a frown-like manner about the center pier. The result is the generation of large compressive forces at the bottom flanges of the girders near the pier. These compressive forces must be transferred through the concrete diaphragm connecting the steel girders. Full-scale tests carried out at NaBRO confirm the possibility of crushing the concrete in the diaphragm near bottom flanges.
In the Nebraska research bridge, the steel girder detailing has been conservatively modified at the pier to accommodate the transfer of these large compressive forces between spans. The girders, when placed on the pier, sit 4 in. apart. The fabricator has welded 1.5-in.-thick steel end plates to each of the girders to be joined. A 2-in.-thick brick-shaped plate also is welded to the bottom of each end plate so that the girders are physically touching. This eliminates any concrete between the bottom flanges that might otherwise be crushed by compressive forces.
The four holes aligned vertically behind each girder bearing stiffener accommodate reinforcing rods running the length of the diaphragm across the width of the bridge.
From this point the contractor will add the reinforcing rods, stringing them through the holes and between the girders. Loops of reinforcing rod will wrap around these rods, forming a long cage-like structure.
Wooden forms enclose the girder ends and rods, creating a cavity that can be filled with concrete.
The contractor first pours concrete into this cavity until it’s about two-thirds full, making the girders partially continuous. The concrete diaphragm becomes fully continuous when the contractor pours the deck, filling in the remaining third of the cavity. Additional reinforcement in the slab over the pier—similar to what has been practiced in the case of pre-stressed concrete bridges for years—provides enhanced continuity for live and super-imposed dead loads. Reinforcing consists of epoxy-coated, grade-60 steel rods.
The simple-made-continuous concept for bridges developed by NaBRO offers the following design efficiencies:
- Reduces the negative moment near the pier compared to a continuous for dead- and live-load design;
- Increases the positive moment at mid-spans;
- Creates more balanced negative and positive moments. Often the same section will carry the loads;
- Eliminates field splices; and
- Increases the moment of inertia of the heavier final section, reducing the live-load deflection of the bridge.
The simple-made-continuous concept also improves fabrication and construction efficiencies, including:
- Minimal detailing of the steel beams;
- Smaller, less expensive cranes to erect the bridge;
- Minimal time for erection of steel on site compared to continuous girders with bolted field splices. Generally, conventional girder field splices are made at low-stress inflection points about one-third of the way across the span from the pier;
- No need for temporary shoring or holding cranes for making field splices; and
- Minimal disruption to any traffic below the bridge while making field splices.
The result is considerable fabrication and construction savings compared with conventional continuous-girder construction. The cost for in-place erected steel for the Sprague Street bridge over I-680 amounted to only $0.52/lb. This compares with a rule of thumb estimate of $0.75/lb for erected rolled steel bridges having conventional bolted field splices.
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