RESULTS OF TESTING AUSTRALIAN TIMBERS. 143 



3. It is found in truss-bridges that the bottom chord or tension 

 member is the first to decay, consequent on the number of joints 

 and keys required to give the necessary tensile strength, and the 

 chance thereby given for water to find its way in. Where any 

 part of this chord decays it is most difficult to renew it, without- 

 expensive staging under the whole truss, and again, on the com- 

 pletion of the life of the truss, scaffolding has to be erected, the 

 whole of the old truss removed and the new truss erected completely 

 in its place. This work causes considerable and lengthened inter- 

 ruption to the traffic. To overcome this difficulty to a great extent 

 a steel chord has been provided which maybe regarded as permanent. 

 When any part of the truss requires renewal it can be done with 

 only a very slight interruption to the traffic. And when the 

 removal of all the timber in the truss is necessary only a very 

 slight staging will be required to support the chord during re- 

 erection, the traffic going on all the time. 



4. The timbers in the top chord are made double with a space 

 between them, so that if it is desired to replace a defective piece 

 of timber, there is sufficient room to insert a temporary piece 

 while the defective piece is removed without stopping traffic. 



5. Each of the struts in the bracing can be removed and replaced 

 in a similar manner wnenever it is necessary to do so. 



6. This bridge is an exceedingly economical one, and reflects the 

 greatest credit on its designer, and the author submits it as an 

 example of the many advantages to be derived by preparing designs 

 within the colony to suit the materials obtainable. 



The author has considered it desirable to give every detail in 

 connection with the design of this important structure, because it 

 may be taken as a model for similar works in the future, and 

 because the strength and elasticity of the timber under the various 

 stresses developed, have been arranged in strict accordance with 

 the experimental data supplied by the author. 



Plate 11 shows a timber truss bridge of 90 feet span, and Plate 

 12 an arched bridge in timber with three hinges, one at centre and 

 one on each abutment. 



Plate 13 shows a skeleton diagram for the truss and for the arch 

 with the stresses developed in the various members which have 

 been derived in the same manner as in the case of the Cowra 

 Bridge which has been fully considered. 



The author desires to acknowledge the kindness of Mr. R- 

 Hickson, M. Inst. c.E., Commission and Engineer-in-Chief for Roads 

 and Bridges in supplying him with the particulars of the designs 

 illustrated in Plates 6, 8, 9, 10, 11, 12, 13, and also that of Mr. 

 J. A. McDonald, m. inst. c.E., Engineer for Bridges, for his help 

 on a variety of matters referred to in the paper. 



