Trusses and lattice girders
To introduce two-dimensional trusses: types, uses, and principal design considerations.
This lecture presents the types and uses of trusses and lattice girders and indicates the members that are most often used in their construction. A discussion of overall truss design considers primary analysis, secondary stresses, rigorous elastic analysis, cross-braced trusses, and truss deflections. The practical design of truss members is discussed.
The truss or lattice girder is a triangulated framework of members where loads in the plane of the truss or girder are resisted by axial forces in the individual members. The terms are generally applied to the planar truss. A 'space frame' is formed when the members lie in three dimensions.
The main uses are:
- in buildings to support roofs and floors, to span large distances, and carry relatively light loads (see Figure 1)
- in road and railway bridges for short and intermediate spans and in footbridges (see Figure 2)
- as bracing in buildings and bridges to provide stability where the bracing members form a truss with other structural members such as the columns in a building (see Figure 3)
The principle of a truss is simple. The structure is composed of top and bottom chords triangulated with diagonals in the webs so that each member carries purely axial load. Additional effects do exist, but in a well designed truss these will be of a secondary nature.
A global moment on a truss is carried as compression and tension in the chords. A global shear is carried as tension or compression in the diagonal members. In the simplified case, where joints are considered pinned and the loads are applied at the panel points, the loading creates no bending moment, shear, or torsion in any single member. Loads applied in such a way as to cause bending, shear, or torsion will usually result in inefficient use of material.
Trusses and lattice girders are classified in accordance with the overall form and internal member arrangement. Pitched trusses are used for roofs. Parallel chord lattice girders are used to support roofs and floors and for bridges although in continuous bridges, additional depth is often required at the piers. In the past, proper names were given to the various types of trusses such as the Fink truss, Warren girder, etc. The most commonly used truss is single span - simply supported and statically determinate with joints assumed to act as pins.
The Vierendeel girder should also be mentioned. It consists of rigid jointed rectangular panels, as shown in Figure 1d. This truss is statically indeterminate and will not be further considered in this lecture although it has a pleasing appearance and is often used in footbridges.
The saving over a plate girder is clear when the webs are considered. In a truss, the webs are mainly fresh air - hence less weight and less wind force.
A truss can be assembled from small easily-handled and transported pieces, and the site connections can all be bolted. Trusses can have a particular advantage for bridges in countries where access to the site is difficult or supply of skilled labour is limited.Read more