4 TIMBER BRIDGE CONSTRUCTION IN NEW SOUTH 
WALES 
By Percy ALLAN, Assoc. M, Inst. C.E. 
[With Plates 1-8.] 
4 [Read before the Engineering Section of the Royal Society of N. 8S. Wales, 
: September 18, 1895.] 
_ Tue necessity for the economical designing of timber bridges in 
the Colony of New South Wales, may be gauged from the fact, 
a that in the last ten years some 680 timber structures have been 
erected at a cost of £634,000, so that any economy in a type 
- design means a very large saving in the annual expenditure of 
_ the Colony. 
For many years Plate 1, was the type of truss for 69 feet, 
75 feet, 90 feet, and 100 feet spans. The defects in this design may 
e briefly summarised—weakness in suspension rods, no means of 
taking up slackness in braces, caused by shrinkage of timbers, | 
and tendency of top chord to incline inwards owing to want of 
lateral stiffness. 
Although this type of truss has for many years carried the 
traffic without accident, yet in view of the increase in settlement 
and the greater risk of the structures being subjected to heavier 
oads, it was thought desirable in 1886 to adopt the design shown 
in Plate 2 for 65 feet, 75 feet, and 90 feet spans. 
_ These structures were designed for a distributed live load of 
& bs. per square foot of roadway and a traction engine weighing 
16 tons, on a 10’ 4” wheel base having 94 tons on the leading 
Wheels, 
The improvements in these trusses are the increased sectional 
‘Mea of suspension rods, the providing of means of taking up 
slackness i in braces, increased lateral stiffness imparted to truss 
1—Sept. 18, 1895. 
