A balanced layout is an important goal in bridge design, as it tends to produce an economical solution. When laying out multiple-span steel beam bridges, there are a few ways to achieve balance. For two-span bridges, equal span lengths are preferred. For three or more spans, end spans are typically designed to be 75-85% of interior span lengths.
In a perfect world, all bridge designs would be balanced. However, sometimes site constraints can dictate an unbalanced layout. This could be a two-span bridge with unequal spans or a three-span bridge with unusually short end spans. When this occurs, a conventional continuous steel beam design may produce uplift at one or more ends supports. One way engineers can help eliminate uplift is to use a simple-made continuous (SMC) design approach.
How it Works
In the SMC approach, beams are designed and erected as simple spans for self-weight and the weight of the concrete deck slab, and as continuous spans for superimposed dead loads and live loads. This approach has been used for decades in the design and construction of precast concrete bridges but has only recently been considered for steel bridges.
The SMC approach produces increased reactions at end supports, compared to conventional continuous design. Other solutions to counteract uplift include tie-downs or additional ballast at or near the end supports. However, in some instances, additional ballast may not be desirable or economical, and tie-downs have the potential for corrosion and need to be inspected and maintained regularly.
SMC designs are also beneficial because they reduce the negative bending moments at interior supports, which is usually the location of highest moment demand for continuous structures. These designs also simplify girder fabrication and eliminate in-span bolted field splices, which allows for faster and less expensive girder erection, resulting in shorter interruptions to traffic below.
In SMC design, there are several methods of detailing the beams at the interior support(s) to achieve continuity for superimposed dead loads and live loads:
- At the top of the composite section, bolted splice plates at the top flange or longitudinal reinforcing steel in the deck slab, or a combination of the two, may be used as the tension-carrying component(s) of the negative bending moment at the interior supports.
- At the bottom of the composite section, the compression-carrying component of the negative bending moment may consist of bolted splice plates at the bottom flange, an integral concrete diaphragm, or a direct bearing connection of the bottom flanges of each span, either using “wedge” plates or by extending the bottom flanges to meet.
Above: a direct bearing connection of bottom flanges at an intermediate pier support is shown.
- In some instances, bolted splice plates may not be desirable because they increase fabrication cost and erection time. If a concrete diaphragm is used to transmit compression, an assessment of the stresses in the diaphragm must be performed. If the bottom flanges are extended to create a direct bearing connection, attention must be given to the type of weld used.
Want to learn more about how to use SMC to eliminate uplift in bridge design? Contact the bridge experts at B&N.