Economical Proportioning of Trusses

Figure below shows the common types of simple-span bridge trusses. By varying the depth of a truss throughout its length (fig. c), forces in the chord members can be more nearly equalized and the forces in the web reduced.

Trusses of economical proportions usually result if the angle between diagonals and verticals ranges from 45⁰ to 60⁰. However, if long-span trusses are made deep enough for adequate rigidity as well for economy, a suitable slope of the diagonals may produce panels too long for an economical floor system.

Some bridge trusses are shown in figures above:

a) Pratt truss through bridge

b) Pratt truss deck bridge

c) Curved-chord Pratt truss

d)

d) Warren truss with verticals

f) Subdivided Pratt truss; known as Baltimore truss

g) Petit or Pennsylvania truss

h) K truss

Baltimore truss

The subdivided panels of Baltimore and petit trusses (fig. f and g) solve this problem. Certain objections to subdivided panels were overcome with the invention of the K truss (fig. h).