is PERCY ALLAN. 
by splaying principals, the introduction of iron in lieu of timber 
side braces, the more general use of sawn timber free from heart, 
and the more effective splicing of chord. 
In 1823 to replace the 1886 type of truss bridges, Mr. Hickson, 
M.I.C.E., Engineer-in-Chief for Public Works for New South Wales, 
approved of the author’s designs for new standard type of truss 
bridges for 70 feet and 90 feet spans, Plate 3. The’ 90 feet 
span is designed for a traction engine of 16 tons and a distributed 
live load of 18°8 cwts. per foot run, or 74 cwts. per lineal foot 
more than for the 90 feet truss bridge previously in use. 
The truss is divided into nine panels of 10 feet with a depth of 
13 feet measured between centres of triangulations, and carries a 
15 feet carriageway and two 5 feet footways. 
The timber generally employed is tallowwood for the planking 
and ironbark for the stringers, floor beams and trusses, the timber 
in trusses being cut free of heart ; but in many districts the local 
timber, with a somewhat shorter life could be used for the 
planking and stringers. 
The flat decks in the old type of bridges, resulted in water lying 
in pools on the surface of the planks, more especially in the 
centre of the roadway, where the wear is greatest ; 3 inch scupper 
pipes along kerb line, even if not choked up, are therefore of little 
‘service. 
To provide against this a camber of 13 inches in cross section of 
deck is provided in the new type of bridge, permitting of the quick 
escape of water through the large gratings at ends of spans. 
Unlike previous designs, 4 inch transverse planking spiked to 
longitudinal stringers has been adopted, thus permitting of shorter 
lengths being obtained at a cheaper rate and of their being replaced 
with less trouble than diagonal planking. 
Longitudinal stringers varying from 103” by 5” to 9” by 6” (to 
give camber in deck), pitched 3 feet apart, are bolted to the floor 
beams, in lieu of the intermediate floor beams used in all previous 
designs, effecting a saving in cost, and permitting of the conceD- — 
