JS^otes on Iron Arches. 87 



to that originally present, and a consequent inequality 

 of stress and diminution in the power of the struc- 

 ture to endure a load. Thus the bridge will be best 

 able to bear its load at a certain calculable temperature 

 somewhat higher than that at which it was first put 

 together, and its strength will fall off as this temperature 

 is departed from in either direction. Hence we draw the 

 inference that it is desirable to complete the erection of 

 such an arch at a comparatively low temperature, in order 

 that it may attain its maximum strength at or near the 

 mean temperature to which it will be exposed. The 

 engineer of the great St. Louis Bridge over the Mississippi 

 enveloped the arch ribs in a kind of gigantic poultice of 

 ice, in order to effect the final junction at a temperature 

 sufficiently low. 



The effect of a live load extending over a portion of the 

 span will be the same as in the preceding case, the maximum 

 effect being produced when the bridge is half-loaded and 

 half-unloaded ; the extra stresses due to the partial distri- 

 bution of the live load being, of course, cumulative upon 

 those due to temperature.* The most appropriate section 

 for the rib will, as before, be a girder section; but we 

 cannot say, as in the preceding case, that the deeper the 

 rib the better, for great depth in the rib, while it will reduce 

 the extra stresses due to partial loading, will increase those 

 due to temperature, and a compromise will have to be made 

 avoiding each extreme. 



Having thus briefly detailed the considerations to be 

 borne in mind when designing an iron arch, I will conclude 

 by supplying a few particulars relative to a structure of 

 the kind referred to, erected some time since by my friend 

 Mr. T. E. Rawlinson, C.E., and which is, as far as I am aware, 

 the only wrought-iron arched bridge in this colony. 



This bridge is situated at Heidelberg on the River Yarra, 

 and consists of a central opening originally occupied by a 

 laminated wooden arch of 100 feet clear span and 17 feet 

 rise and two lateral openings of smaller size. About three 

 years ago the laminated arches gave way through decay 

 of the timber ; and Mr. Rawlinson, to whom the work of 

 reconstruction had been entrusted, requested me to deter- 

 mine by computation the stresses on the proposed structure. 



* This is not mathematically correct, but is practically so for arches of 

 the proportions commonly adopted by engineers. 



