146 DISCUSSION. 



a most valuable table of co-efficients for local timbers as well as 

 foreign. 



J. I. Haycroft, m.e., Q.u.i., Assoc, m. Inst., c.E. — Very little matter 

 of a debatable nature, with one special exception, which I will refer 

 to later on, occurs in the paper. The several designs for timber 

 viaducts, though not admitting of much scope for variety, seem 

 to be well thought out, and the rejection of close decking and 

 ballast on such structures, is to be commended for many reasons, 

 the principal ones being saving in materials and consequently cost, 

 reduction in dead weight, increased facility for inspection and 

 reduced liability of timber to decay. Notwithstanding these 

 apparent advantages, however, the Railway Construction Depart- 

 ment do not seem to recognise them, as I notice several of the 

 bridges on the recently constructed St. Leonards line are close 

 decked and ballasted. The remarks in the paper on the use of 

 soft wood decking, on the Hawkesbury Bridge, are, I consider, 

 but just. The explanation of the use of such materials, given at 

 your last meeting, viz ; saving in dead weight, could not I think 

 be an item of great importance, and would entirely disappear if 

 the direct advantages due to the use of suitable colonial hardwood 

 were weighed against it. The mistake however I consider should 

 have been rectified before acceptance of tender, as I surmise, the 

 intended use of this class of material was shewn on drawings and 

 .specified. The special exception referred to is the composite truss 

 •over the Lachlan River, at Cowra ; the stresses in the several 

 members of which will, I think, on investigation be found incorrect. 

 I have from the data furnished in the paper, developed by the 

 method of graphic statics, as shewn on the drawing, the true 

 maxima stresses in the several parts of the truss, and find them 

 •differ considerably from those given in the paper. A complete 

 •comparative list is appended and a few extracted from it are as 

 follows ; the stress in tension boom at centre is given as 172J tons 

 should be 159 tons. The stress in compression boom at centre is 

 given as 137 tons and should be 153 tons. The stress in centre 

 tie F 4 is given as 79f tons and should be 45 tons. In the braces 

 the maximum stress in Y 6 is given as 43 J tons, should be 31^ tons 

 whilst in Y z and Y 8 the stress is given as 21 tons, although I 

 think it will be found no stress can exist in these members from 

 the class of loading under consideration. These braces however 

 should not I think be omitted, as they preserve uniformity in 

 . appearance, and, if properly utilized, are a source of strength to 

 braces Y x 1 and Y G . Without occupying too much of your time 

 in demonstrating what I consider the reasons for these errors, it 

 will I think be manifest to anyone conversant with the subject, 

 that the principle of determining the maxima stresses for live and 

 > dead load, in the several parts of the truss, by multiplying the 



