DISCUSSION. 155- 



bark sleepers should have the same strength they would require 

 to be 8 inches wide by 6 inches deep, since the modulus of rupture 

 of ironbark is at least twice as great as Oregon timber. The 

 weight of the present Oregon deck is about 60 tons, and the iron- 

 bark deck would weigh 80 tons, as the weight of ironbark is 

 about twice that of Oregon. The total load on one span of the 

 bridge is : — 



Steel = 960 



Timber ... = 60 



Live Load ... = 944 



Total ... 1964 tons. 



Therefore one ton of steel carries about two tons of load, hence 

 in order to carry the extra 20 tons due to the ironbark deck, 

 the total weight of steel in main trusses, cross girders, and 

 longitudinals would not require to be increased by more than 

 10 tons. Taking the cost of this steel erected at £35 per ton, 

 the extra cost of steel in one span of the bridge is £350. The 

 life of a steel bridge is usually taken at 150 years, and the iron- 

 bark deck at 25 years, while the Oregon deck would not exceed 

 7 years. Hence, in order to cover interest at 4 % on prime cost 

 and depreciation, we have the following annual charges :— 

 Steel would require 4*01 % per annum. 

 Ironbark ,, 6*40 ,, ,, 



Oregon ,, 16-66 „ ,, 



The price of Oregon timber is 2s. per cubic foot, and of ironbark 

 timber 2s. 3d. per cubic foot, hence the cost of the oregon deck 

 would be <£376, and the cost of the ironbark deck £282. The 

 capital values for the extra steel, the ironbark, and the Oregon 

 are as follows : — - 



Steel = ®^m = ^ 351 



Oregon = 376x f 66 = £1566 



Ironbark = 2 -^i° = £451 



The difference in favour of the ironbark deck is therefore £764 

 per span. If the life of the oregon deck were 10 years instead 

 of 7 the saving would have been £232 per span. The saving 

 duf 1 to the less frequent renewal of the ironbark deck, and the 

 interruption to the traffic every 25 years instead of every 7 years, 

 would represent something more, hence I conclude that Mr. 

 Burge is incorrect if he supposes (which I really doubt) that 

 there is any real advantage in the oregon deck. 



Mr. Burge — I did not intend to convey that impression. 

 Professor Warren, in his paper the other evening, said he did 

 not know for what reason the oregon deck was adopted. I 



