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TIMBER BRIDGE CONSTRUCTION IN N.S.W. XVII, 
that is the more exposed parts, required renewal in 11 years, and 
_ that the whole had to be rebuilt in 22 years—£100 is provided. 
£100 
The permanent work is assumed to cost ... 
The timber work one-half, viz. ... 0... 0 see ee 50 
Balance remaining, should the timber bridge be 
WOR occper vee ahaa vee Ne te ee eee 
Of this balance £7 is placed at compound interest at 47/ pro- 
ducing in 11 years £11, with which about one-fifth of the bridge 
is renewed—£43 remains which, at same interest in 22 years, 
becomes £102. The whole bridge is now rebuilt for £50, leaving 
balance of £52 by which the renewal fund is more than kept up. 
The depreciation is therefore about 4}7/ per annum on the actual 
capital expended. 
If, therefore, the permanent work costs not more than twice 
the timber work, it should be preferred from a financial point of 
view, independently of the objection, in railway work, to renewal 
as interfering with traffic. This is about the proportion of cost 
of small openings at 3ft. to 4ft. between concrete or brickwork 
3 and timber. 
Mr. Sma said—one thing he noticed in the new design, was 
that any member of the truss could be renewed without staging 
below, while with the old. style of bridge when this was being 
done a temporary one had frequently to be built. The speaker 
mentioned that in 1876 he erected a bridge of the 1866 design, 
in which the very best timber procurable was used. It occurred 
to him then that from the way the bridge was designed, if any 
part had to be renewed, practically a new bridge had to be built. 
He thought it would be far better to build small bridges, 
Culverts, &c., with stone or concrete. 
Mr. BarractouaH said there was one point he would like to 
refer to, and it was the method shewn in one of the diagrams of 
treating the top member as a long column, and he would like to 
know if Mr. Allan had compared the method which he had 
employed with the theory of long columns, as the variable load 
2—Sept, 18, 1895. 
