RESULTS OF TESTING AUSTRALIAN TIMBERS. 139 1 



either side of the main trusses forming cantilevers for carrying the- 

 foot-ways. The minimum carriage way is 20 feet in the clear and 

 the two foot-ways have each a width of 5 feet. The longitudinal 

 girders resting upon the cross girders are of sawn timber, and the 

 deck is formed with tallow wood boards laid diagonally on the 

 carriage way, and transversely on the foot- ways. The two trusses 

 are braced together in a horizontal plane between the top and 

 bottom booms with timber transverse struts and wrought iron, 

 diagonal tie rods. 



The piers are composed of wrought iron cylinders 6 feet in. 

 diameter, stiffened with wrought iron diaphragm bracing, and 

 founded on cast iron cylinders tilled with cement concrete. The 

 pressure on the rock foundations is 14*6 tons per square foot, and 

 on the clay foundations 6 tons per square foot. The bridge has 

 been designed to carry a live load of 84 pounds per square foot of 

 carriage and foot-way. The cross girders and longitudinal girders 

 have been designed to carry a traction engine weighing 16 \ 

 tons. 



Plate 9 shows the method adopted for determining the stresses- 

 in the various members of the truss. The dead load concentrated 

 at each of the apices of the triangulation due to the weight of the 

 the structure itself is 14 tons on each truss. The live load at each 

 of the apices is 11*25 tons on each truss. The four reciprocal 

 diagrams of stress have been drawn for the dead load of 14 tons 

 concentrated at each of the four apices in the half truss separately. 

 The length of the lines in the four diagrams represent the stresses 

 in the various members of the truss to scale, they are measured 

 and tabulated in the manner fully illustrated on Plate 9, so that 

 the maxima stresses for the dead load may be written down for 

 each bar. The stresses ascertained in this manner have also been 

 found independently by the method of sections (which will be more 

 fully explained) and the mean of the two methods written down 

 in the column Mean Maxima Stresses. Since the live load dia- 

 grams would be simply a repetition of the dead load diagrams- 

 referred to, the stresses due to the live load are found by multi- 

 plying the maxima dead load stress by the ratio of live to dead 

 load thus — ■ — -j- = 0*8, and each of the stresses in column " Mean 

 Maxima Stresses '' is multiplied by 0*8 and the result written 

 down in column "Distributed Live Load Maxima Stresses," The 

 sum of the two gives the greatest stress than can be developed in 

 the members of the truss. The column '• Concentrated Rolling 

 Load " contains the stresses developed when a traction engine of 

 16^ tons passes over the bridge. It will be seen that they are 

 considerably smaller than those due to the distributed live load. 



