Equilateral Truss Bridges
Name: James R. K.
Date: December 2002
Why do equilateral truss bridges hold the most weight?
I am not sure they do, if you mean what kind of bridge holds the most
weight for the smallest amount of material in the bridge. I would
have thought that a suspension bridge would win that contest.
It is true, however, that three beams connected in a triangular shape
must keep that shape or break. If you connect four beams in a
quadrilateral shape, the quadrilateral can be deformed quite easily,
as you can quickly convince yourself with a little model, if you are
not already convinced.
Best, Dick Plano
I do not think you can say that any one truss type carries the most weight.
Since the mid-19th century, when the basic principles of modern structural
engineering were developed, a multitude of truss designs have been
developed. Each of the successful ones have some advantage depending on the
type of load expected, the span required, and the limitations of materials
and construction available at the time.
That said, the symmetry of an equilateral triangle corresponds to a symmetry
in the forces developed in the truss members. Consequently, this shape has
the potential to spread the load applied to the structure in an even
Andy Johnson, Ph.D., P.E.
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Update: June 2012