130 W. H. WARREN. SOME APPLICATIONS OF THE 



on the convex side of the beam, in some few cases owing to the 

 presence of gum veins, failures occurred by shearing longitudinally 

 near the neutral axis. The reason of these peculiarities may be 

 seen by inspecting the tables of the results of testing in tension, 

 compression and shearing, which show that the tensile strength is 

 about double the compressive, while the modulus of rupture is 

 slightly less than the tensile strength. For instance, the com- 

 pressive strength of the best iron-bark timber is about 10,000 

 pounds per square inch, the tensile strength 22,000, and the 

 modulus of rupture 18,000 pounds per square inch. The resistance 

 to shearing along the fibre is much greater in Australian timber 

 than in pine timber or oak, and frequently reaches 2,000 pounds 

 per square inch, it is not proportional to the tensile compressive 

 transverse strengths. 



The shearing resistance of red gum timber is not much below 

 that of good iron bark, but the compressive, transverse and tensile 

 strengths are not much greater than one half those of iron-bark. 

 Blackwood timber is inferior to ironbark in compressive, transverse 

 and tensile strength but superior in its resistance to longitudinal 

 shearing along the fibres. Former experimenters on Australian 

 timber have confined their attention to the determination of the 

 constants for transverse strength and elasticity, leaving the tensile, 

 compressive and shearing strengths and elasticities untouched, 

 excepting Mr. Laslett whose results as pointed out by the. author 

 in a paper read before the Society in 1886, are inaccurate so far 

 as tension is concerned. A few experiments have been made by 

 Mr. Campbell, C.E., Melbourne, on the tensile strength of timber 

 which agree, as far as they go, with those made by the author. 



In deciding as to the value of timber for a special purpose the 

 following considerations may be necessary. Its durability under 

 the conditions to which it will be subjected in practice, its weight 

 per cubic foot, seasoned and unseasoned, its loss of weight and 

 decrease in volume during seasoning, its transverse, tensile, com- 

 pressive and shearing strengths and elasticities. For floor beams 

 and simple beam bridges the durability, transverse strength, 

 modulus of transverse elasticity are the most important data. 

 For compound beams consisting of ordinary beams connected 

 together by means of wedges and bolts, in addition to the consider- 

 ations necessary with plain beams already referred to, the shear- 

 ing resistance of the wedges and the shrinkage of the timber must 

 be considered. In timber roof principals and timber bridge 

 trusses we have to consider in addition to the foregoing the 

 compressive resistance of the timber when used as long and short 

 columns, the force which will cause failure by buckling. The 

 tensile stresses which will be developed in the main tie beams and 

 bracing of the roof and bridge truss need not concern us, as the 



