RESULTS OF TESTING AUSTRALIAN TIMBERS. 131 



tensile strength of Australian timber is greater than can possibly 

 be developed at the joints, and failure is more likely to take place 

 by shearing of the timber along the grain, or from the unequal 

 distribution of stress over a given cross-section due to tension and 

 crossbreaking 1 combined. 



The main difficulty in designing timber structures is due to the 

 joints and connections, and these should be so arranged that they 

 will not loose their efficiency through the shrinkage of the material. 

 In selecting timber for the manufacture of railway rolling stock 

 it is necessary to obtain the lightest and most durable timber, it 

 should not decrease appreciably in volume during seasoning, and 

 should possess considerable resistance to the various stresses which 

 are developed in the truck or carriage when in use. Blackwood 

 appears to the author to fulfil these conditions at least as well as 

 any of the Australian timbers, and it is probably as useful for the 

 purpose as that of any other timber in the world. 



Australian timber differs considerably in its resistance to stresses 

 as well as in various other qualities from that grown in other 

 countries. For instance the results obtained in testing red and 

 yellow pine, and spruce timber, in the very valuable and elaborate 

 experiments made at the Watertown Arsenal, Massachussets, and 

 recorded in the reports of the United States Government, as well 

 as those made with equal skill and care by Prof. Bauschinger of 

 Munich, differ considerably from those obtained by the author for 

 Australian timbers, thus : — In the experiments on spruce beams 

 and red and yellow pine timber referred to, failure took place as 

 often by longitudinal shearing along the fibres, as by tearing of 

 the fibres on the convex side of the beam. Direct experiments 

 on the shearing resistance proved that the beams were just as 

 likely to fail by longitudinal shearing as by tearing of the fibres 

 •on the tension side of the neutral axis. 



The resistance of many of our Australian timbers to shearing 

 along the fibres is about five times as great as that of the pine 

 timbers, so that beams of Australian timber will never shear along 

 the fibre unless there is some defect in the specimen tested such 

 ■as the presence of gum veins near the neutral axis. 



In applying the results of testing Australian timbers to the 

 design of simple beam bridges and viaducts, such as those illus- 

 trated in Plate 5 and Plate 6, it is necessary to make use of the 

 following equations. The bending moment must be calculated 

 and equated with the moment of resistance. For rectangular 



beams loaded with an uniform distributed load we have — 



w i 



w 



(i) 



