RESULTS OF TESTING AUSTRALIAN TIMBERS. 



133 



Petersham Viaduct, the plate web girder bridge at Strathfield, 

 and the Penrith Bridge which are similar to those shown on 

 Plates 5 and 7. This deck is also used in the Hawkesbury Bridge, 

 but for some reason or other soft wood timber has been used instead 

 of Australian timber. This very obvious mistake is generally 

 acknowledged, and when the present deck is renewed it is to be 

 hoped that good iron bark or other approved Australian timber 

 will be substituted. 



The advantages of this deck over the ordinary close timber 

 deck are as follows : — The sleepers are spaced sufficiently close to 

 allow the wheels of a carriage to run without sticking in the case 

 of derailment, and they are exposed on four sides to the air and 

 therefore dry uniformly and last a much longer time, moreover 

 any defective sleeper can be readily seen and replaced. 



The timber guard rail is a precaution against the effect of derail- 

 ment and serves also in assisting to destribute the weight of the 

 driving wheels over at least three sleepers. The spacing is large 

 enough to allow hot material from the engine to fall between the 

 sleepers and helps to make the deck fireproof, as was proved a few 

 years ago when a train was on fire upon the Petersham Viaduct. 

 The number and strength of the sleepers gives considerable lateral 

 strength and stiffness to the structure, enabling it to resist wind 

 pressure and oscillations. The 7 feet spacing of the main beams 

 ensures an elastic road, and easy running. 



The strength of the main beams in the 10 feet spans will now 

 be considered. The greatest bending moment which can be pro- 

 duced on these spans will be from the Consolidation engines the 

 driving wheels of which are shown in sketch fig. 2. 



The bending moment at the centre is : — 18 x *5 + 6 x 4-5 = 36 

 foot tons = 432 inch tons. .-. ^ = 36= ™ xl 8 ° xlQ where w equals 

 the equivalent distributed load. .*. w =2*88 tons per foot. 



The moment of resistance of two beams each 12" x 12" on a 



span of 10 feet is :— 2 * 12 x 12 x 12 x 13Q00 =7488000 inch pounds. 



= 3342*8 inch tons. 



