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Composite Bridge
"Composite" means that the steel structure of a bridge is fixed to the concrete structure of the deck so that the steel and concrete act together, so reducing deflections and increasing strength. This is done using "shear connectors" fixed to the steel beam and then embedded in the concrete. Shear connectors can be welded on, perhaps using a "stud welder", or better still on export work, by site fixing nuts and bolts:
Shear connectors correctly spaced to resist the loads, make the concrete work "Compositely" with the steel.
Usually, the steel carries its own weight; and that of the wet concrete. But when the concrete is "cured", and has acquired its full strength, then all future loads (traffic, surfacing, wind, water pressure, seismic loads) are shared by the Steel/Concrete Composite.
The concrete is good in compression, while the steel is good in tension and compression.
This composite bridge design can be used in the following ways:-
- Simple Beam Bridges. On short spans, (8m, 10m, 12m, 15m and then more expensively up to 24m) bridges can be made from a number of beams under the roadway straight across the gap. These bridges benefit the most from composite action. Exaggerated deflected shape showing bending, with concrete carrying some of the compression and steel carrying all of the Tension T.
- Over truss Bridges. These are similar in design to simple beam bridges but for longer spans, from say 18m up to 100m or so. The deck is still on top of the steel structure, so goes into compression when deflected. As with the simple steel beam bridge, the concrete can share the compression load C with the steel trusses.
- Transoms on any type of bridge. Transoms go from side to side across a bridge and carry the deck loads to strong steel frames on either side. This is usually on "open-top" through truss bridges or "closed-top" through truss bridges. Again the transoms bend down under load, and the concrete contributes compression strength.
- Steel Bridge Decking. In a normal steel building, the concrete can act together with steel decking, the steel decking taking the place of regular steel reinforcing bars in the concrete. However, in a bridge, the underside of the decking is liable to corrode in time, so is not counted as part of the reinforcement. It is just used as "lost formwork".
