Truss Structural Concept
Truss Structural Concept
A truss is a type of structural framework composed of straight members connected at their ends by joints. It is designed to efficiently support loads and distribute them across its members. Trusses are commonly used in engineering and construction to create stable and rigid structures such as bridges, roofs, towers, and cranes. Trusses are typically made of slender members, such as steel bars or wooden beams, arranged in a triangular pattern. This triangular arrangement provides stability and ensures that the loads applied to the truss are evenly distributed. The basic components of a truss include top and bottom chords (horizontal members), web members (diagonal or vertical members), and joints (points where the members are connected) as shown in Figure 1.
A truss structure behaves similar to a beam, but with excess material removed to reduce weight. The chords of a truss correspond to the flanges of a beam and carry the internal couple that handles the applied load’s moment. The vertical and diagonal members of a truss primarily transfer vertical forces to the supports at the ends of the truss. Trusses are often considered more cost-effective than steel beams in long-span structures because they use materials more efficiently. By using a truss instead of a beam, engineers can design lighter and stiffer structures at a reduced cost as well as provide unobstructed space for utilities such as ducts, pipes, and electrical conduits.
Designers have the flexibility to vary the area and depth of truss members to reduce weight. In regions with high bending moments, such as the center of a simply supported structure or the supports in a continuous structure, trusses can be deepened to handle the increased loads as Figure 2. The diagonals of a truss typically slope upward at angles ranging from 45° to 60°. To limit the unsupported length of compression chords, long-span trusses should have a distance between panel points of 5 to 7 m. Increasing slenderness in compression chords raises the risk of buckling, while slenderness in tension members should be limited to reduce vibrations caused by wind and live loads.
If a truss has equal or nearly equal loads at all panel points, the direction in which the diagonals slope determines whether they carry tension or compression forces. Figure 3 illustrates the difference in forces between the diagonals of two identical trusses, except for the direction of their slope (T for tension and C for compression). Trusses are stiff within their own plane but flexible in other directions, so they need bracing or stiffening for stability. Trusses can be connected together to create a rigid-box structure, especially when used in pairs or spaced side by side. Figure 4 depicts a bridge made from two trusses.
Trusses can be classified into various types based on their configurations, such as the Pratt truss, Howe truss, Warren truss, and many others. The specific truss configuration used depends on factors such as the span length, expected load conditions, and aesthetic considerations. Truss analysis involves determining the internal forces in each member of the truss to ensure that the structure can withstand the applied loads without failure. This analysis can be done using methods such as the method of joints or the method of sections.
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