Truss beams have a criss-cross web design and are used in bridges, mining tunnels, and as reinforcement for shotcrete. They were used in the Eiffel Tower and are still used in tunneling as they can be easily erected and embedded in shotcrete. They can also provide load-bearing support for floors and curved walls.
A truss beam is a type of beam with a criss-cross web design, as in gardening trellises, between the two edges of the beam. The diagonal lines of the steel give support in all directions, helping to prevent bending of the girder, which is one of the main supporting elements in a bridge design. Often found on bridges or older buildings, truss beams are also used extensively in mining tunnels for roof support during excavation and can be erected quickly. They are also used for reinforcement when applying shotcrete, a form of concrete or mortar applied pneumatically at high speed from a hose to building supports.
One of the best known examples of truss girders is the French Eiffel Tower which was built of lattice iron in the 1800s. Truss girders are no longer used as much in buildings or bridges as they have been replaced by solid steel plate girders . Tunneling operations, however, make good use of them as they are light in weight and can be easily erected over uneven floors and walls with just a few workers. A truss girder can be used as-is or covered with shotcrete for added load resistance. Modern truss beams are most often used in 3- or 4-bar configurations and diagonal stiffening provides loading along the entire length of the bars for strength.
In tunneling operations, a truss girder is often chosen over a sheet steel girder as it can be fully embedded in a shotcrete lining which allows easier molding of walls within a tunnel. It can adapt well to different ground levels and provide a cover with the shotcrete that keeps water out. Static load studies have shown that even when not encased, the truss has a high bearing capacity; it can handle loads well even when the shotcrete is still cured and soft. The ability to easily deform beams to fit the walls and flooring of a tunnel without affecting loading is why lattice girders are so reliable in mining projects.
Lattice beams are also used as a component to provide load-bearing structure support for floors that can handle heavy loads. They are used to create so-called lattice slabs for high load capacities. In curved wall designs, their surfaces can be bonded with polystyrene forms to reduce wall weight loads. A truss girder can be a component to accommodate large arched openings when using these void formers, due to low weight loading.
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