DISCUSSION. 



157 



duced from loads to the left. But, in the case of the Cowra 

 Bridge, it is clear from the details shown on Plate 8 that the 

 diagonals fit into pockets and are wedged up to take compression 

 but are incapable of resisting or transmitting tension, consequently 

 Y 3 will be always in compression, and the tensile stresses in the 

 the web will be resisted entirely by means of the verticals. I 

 consider that Mr. Vicars has clearly shown the difference between 

 the Cowra Bridge and that calculated by Mr. Haycroft. 



Mr. Haycroft — I did not consider what the bridge was 

 made of. 



Professor Warren — Mr. Haycroft's calculations appear to be 

 correct for an iron bridge, but are incorrect as applied to the Cowra 

 Bridge. Mr. Dare in his remarks agrees with Mr. Vicars in the 

 calculations given in the paper for the Cowra Bridge, and he 

 differs consequently from Mr. Haycroft. Mr. Dare has also shown 

 clearly the proportion of depth to span in ironbark timber beams 

 in order that they may not fail by horizontal shearing along the 

 neutral axis, his method is correct, and may be applied generally 

 to Australian timbers. I would merely remark that the resistance 

 to shearing along the grain frequently exceeds 2,000 pounds per 

 square inch, and that consequently the depth need nob always be 

 as great as one-ninth of the span even for a distributed load. In 

 the case of a compound beam these considerations do not apply 

 for reasons already given. The remarks made by Mr. Shellshear 

 are very valuable, and will be appreciated by all those who have 

 had experience of timber structures. Mr. Shellshear has clearly 

 shown the advantages of Australian timbers in railway construc- 

 tion over iron and brickwork, his objections to the planking shown 

 on Plates 5 and 8, I fully admit. Mr. Vicars also pointed out 

 that I had not shown in my paper how long compression members 

 should be designed in such a structure as the Cowra Bridge, and 

 in thinking over the question since reading the paper, I decided 

 to prepare the following table which covers every case likely to 

 arise in practice : — 



Comparative Strength of Long and Short Columns of Australian 

 Timbers per square inch of sectional area. 



Local Name. 



NEW SOUTH 

 Eed Ironbark ... 

 Red Ironbark ... 

 Grey Ironbark ... 

 White Ironbark 

 Tallow-wood 



Number and 

 Letter. 



WALES 

 P, 4 

 B, 16 

 B, 17 

 P, 5 

 P, 2 



Ratio of Length 

 to Smallest 

 Dimensions. 



4 to 1 



36tol 



TIMB 

 12,013 



12,308 



10,288 



11,239 



9,511 



EES. 



8,338 

 6,250 

 6,250 

 6,625 

 5,169 



Modulus of 

 Transverse 

 Elasticity. 



3,084,333 

 2,658,433 

 2,806,975 

 3,037,400 

 2,745,333 



