A lattice girder is a type of girder with a criss-crossed web design, such as in gardening lattices, between the two edges of the girder. The diagonal lines of steel give support in all directions, helping to prevent the girder, which is one of the main support elements in a bridge design, from bending. Often seen on older bridges or buildings, lattice girders are also widely used in mining tunnels for roof support during excavations and can be erected quickly. They are also used for reinforcement when applying shotcrete, a form of concrete or mortar pneumatically applied at high velocity from a hose to construction supports.
One of the most well-known examples of lattice girder design is France's Eiffel Tower, built in latticed iron in the 1800s. Lattice girders are not used so much in building or bridges anymore as they have been replaced by solid steel plate girders. Tunneling operations, however, make good use of them as they are lightweight and can be easily assembled on uneven floor and wall surfaces with few workers. A lattice girder can be used as is or covered in shotcrete for additional load strength. Modern lattice girders are most often used in the 3-bar or 4-bar configurations, and the diagonal stiffening delivers load along the entire length of the bars for resistance.
In tunneling operations, a lattice girder is often chosen over a steel plate girder as it can be completely embedded in a shotcrete lining that allows for the easier molding of walls within a tunnel. It can adjust well to differing ground levels and provide a covering with the shotcrete that keeps out water. Static load studies have shown that even when not encased, the lattice girder has high load-bearing capability; it can handle loads well even when the shotcrete is still curing and soft. The ability to easily deform the girders to match the walls and flooring of a tunnel without affecting the load-bearing is why lattice girders are so trusted in mining projects.
Lattice girders are also used as a component to provide structure load bearing support for floors that will handle heavy loads. They are used to create what are called lattice girder slabs for high capacities. In curving wall designs, their surfaces can be bonded with polystyrene void-formers to reduce wall weight loads. A lattice girder can be a component in accommodating large arched openings when using these void-formers, due to the reduced weight load.