138 W. H. WARREN. SOME APPLICATIONS OF THE 



Where c = half span 



x = distance from centre of beam 



• * J ' ~ 4x12 



If sc = o ; F\ = 3*6 tons 

 x - 6 ; F\ = 8*1 tons 



And these stresses are distributed over the section of the beam as 

 before. The maximum shearing stress per lineal inch in the centre 

 of the beam is therefore : — F — |^4i = 0*225 tons. 



The bolts and wedges are spaced 18 inches from centre to centre, 

 therefore the central bolt will be called upon to resist a stress of — 

 18 x 0*225 = 3*38 tons. The area required is consequently §ii? 

 = 0*56 square inches, or say 1 inch diameter. 



The bolts at 6 feet from the centre will be required to resist a 



stress : — F= £s£ - 0*5 tons per lineal inch, or a total force of 



18 x 0*5 = 9 tons, the area required is therefore — f = 1*5 square 



inches or say 1£ inches in diameter. 



The width of the corresponding wedges 6 feet from the centre 

 of the beam is found thus :— 3000 x = 9 x 2240 



. • . x = 6*7 inches 

 hence the wedges may be made 7 inches by 4 inches. 



The sizes of the remaining bolts and wedges may be determined 

 in a similar manner. 



In order to test the foregoing theory, the author had made several 

 scale models which were tested and gave results in close agree- 

 ment with the calculations. Compound beam bridges have been 

 constructed by the Roads and Bridges Department up to a span 

 of 42 feet, the main beams being formed with 3 beams each 12 

 inches by 12 inches. 



Plate 8 shows an example of a composite structure designed by 

 Mr. J. A. McDonald M.inst. C.E., Engineer for Bridges, for a bridge 

 over the Laehlan River at Cowra. Only one of the large spans 

 will be considered in this paper. The trusses are 160 feet from 

 centre to centre of the bearing on piers, and are 27 feet deep 

 between centres of triangulations. The top chord principals and 

 diagonal bracings are of ironbark timber, the bottom chord is of 

 steel formed of stringer-plates and angle iron bars top and bottom. 

 The suspension bolts are of wrought iron, placed in groups of four, 

 and connected with wrought iron washer plates. 



All the timber members are adjustable by means of wedges, so 

 that any shrinkage that occurs may be taken up. 



The cross girders are of steel spaced 20 feet from centre to 

 centre, and constructed with flange plates and angles connect- 

 ing the flanges to the web, they are fish-bellied and project on 



