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Firefighter Safety Report: Truss System Failures

NIOSH | John Howard M.D.

What is a Truss?

A truss can be defined as structural members (such as boards, timbers, beams, or steel bars) joined together in a rigid framework. They are most often in the shape of a triangle or series of triangles.

Some trusses are rectangular. Trusses can be built of wood, steel, wood and steel, or aluminum. Concrete trusses are not common but do exist, usually in very large structures (see Appendices B and C for descriptions of different truss types).

The truss framework is usually arranged in a single plane so that loads applied at points of intersecting members will cause only direct stress (compression or tension). Three-dimensional trusses (space frames) are very light in weight.

The design of a truss, which separates compressive and tensile stresses, allows for a minimum of materials to be used, resulting in economic benefit.

The top and bottom members of a truss are called chords. The top chord of a truss is in compression, and the bottom chord is in tension.

The inner members are called webs and give stability to the truss system. The unique characteristic of a truss is the inherent stability of the triangle. Web and chord members arranged in a triangle are much more stable than the same members arranged in a square.

The square configuration requires diagonal bracing, which then produces multiple triangles.

Truss Types

Although many types of trusses exist, three typical truss construction methods are most commonly used:

• Heavy timber roof and floor truss systems • Lightweight wooden roof and floor truss systems • Steel roof and floor truss systems

Each of these construction methods is described in detail in Appendices C and D, along with causes of failure for each under fire conditions.

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