16.3 STABILITY OF TRUSSES

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Trusses are composed of beams and pin joints. Generally, if the frame is constructed of only triangles (internally) it will be stable. (not collapse when pushed the wrong way)

Quite often additional members are added to frames to stiffen it (make it stable), but not to carry loads. We can see such members in a bridge,

[picture]

We can predict stability using a basic topological relationship where we look for equality. The equation shown below requires that the truss be composed of simple pinned beams, and when drawn to scale on paper the beams must not cross or touch except at the pinned joints

We can also verify stability by making nodes that are held in place either by supports, or triangular structures.

As an example,

[working model file]

16.3.1 References

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Beer, F.P., Johnson, E.R., Statics & Mechanics of Materials, McGraw-Hill, 1992.