Polymorf - Knowhere - Howe truss bridge

Howe truss bridge

The Howe truss bridge, patented by William Howe in 1840, used diagonal wooden struts to

stabilize the Queenpost design in a pattern that could be repeated for any length bridge.

Note that the red colored diagonal struts in the following images point away from the mid-

point of the bridge's span.

	Fig. 166 - Howe truss bridge
	.
	◄ 13 = 2 ( 8 ) - 3
	therefore stable
	(static demonstration models)
	click image to enlarge

This is done so that when the bridge is loaded these struts experience compressive stresses

induced by the force of the load and the shear forces it generates between the top and

bottom chords of the bridge. As demonstrated in Fig. 167 a) below, the diagonal rubber

bands highlighted in red collapse when a vertical load is applied to an idealized model of


click image to enlarge
a) Compressive loading of truss diagonals	b) Shearing of top and bottom chords
.
Fig. 167 - Load induced stresses in the Howe truss (training aid model)
.

the Howe truss. This indicates they are reacting to compressive stresses. The shearing

action caused by the top chord, which is in compression, attempting to slide past the

bottom chord which is in tension, is detailed in b).

In the early nineteenth century, when wood was plentiful and iron was relatively expensive,

the Howe truss design was popular because iron was used only for the vertical struts which

are subjected to tensile stresses mainly. Heavy wooden timbers were used for the diagonal

struts. However, because most of the dead load on a bridge is concentrated towards the

mid-span, as the span increases the weight of the heavy wooden beams becomes more

costly to support due to the large bending moment exerted by their weight. As iron became

less expensive and more readily available, truss bridge designs that made greater use of its

unique properties became more popular.

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