RESULTS OF TESTING AUSTRALIAN TIMBERS. 137 



in position under favourable conditions, and any defect which 

 may be developed can be easily seen and dealt with. From an 

 inspection of the tables of results of testing the shearing strength 

 of timbers, it will be seen that blackwood, ironbark, spotted gum, 

 red gum, flooded gum, grey gum, slaty gum, mahogany, stringy- 

 bark, white box, rose wood, coach-wood, black bean, red bean, and 

 beech may be advantagously used for this purpose. Since the 

 bending moment increases as the square of the span, and the 

 moment of resistance of the beam as the square of the depth, it 

 follows that a span of 20 feet would give the same factor of safety 

 as that found for the 10 feet span, provided the equivalent dis- 

 tributed load remained the same. The equivalent distributed 

 load on a 24 feet span with the heaviest engines does not exceed 

 2 "4 tons per foot run, hence the span may be increased from 20 to 



oqq 



20 x 2|q = 24 feet. The total depth of the compound beam is 



26 inches, but it will be taken as 24 inches. The area is reduced 

 in the centre by the bolt hole, and to a slight extent by the 

 wedges. The corbels over piers will certainly more than com- 

 pensate for these reductions, and the former may be taken to 

 balance the latter with perfect safety. 



The shearing stress over piers is £— = 28 -8 tons or 14*4 tons 



for each beam which is distributed over the depth of 24 inches 

 by 12 inches in the manner referred to. The maximum 

 shearing stress horizontally and vertically is therefore : — 



F= 2x2i~ = ^ ^ ^ ons P er l mea l mcn « The wedges immediately 

 over the corbels are spaced 15 inches centre to centre and will 

 have to resist a horizontal shearing stress of 15 x 0'9 — 14*5 tons, 

 and the corresponding bolts a vertical shearing stress of the same 

 amount. 



Let x — the width of the wedges measured along the beam, 

 the area exposed to shearing along the fibre (neglecting the portion 

 of the wedge which projects beyond the beam for driving) is 12#. 

 The safe intensity of shearing stress may be taken as 250 pounds 

 per square inch in the timbers which are the most suitable for this 

 purpose, therefore the resistance of the wedge is — 12 x 250 x x = 

 3000x . • . 3000a = 14-5 x 2240 



x = 10*8 inches 

 The wedges should consequently be made 1 1 inches wide by 6 inches 



deep. The area required in the bolts is g— = 2*4 square inches 



or 1| inches in diameter. 



The shearing force at any other point may be calculated from 

 the following formula :— F, = w ~^p^ 



° 4c 



Where F\ = the shearing stress 



w = live load per foot run on each beam 



