TtBULAE GIRDEE BRIDGES. 



189 



TABLE 



SHEWING THE PROPORTIONS OF TUBULAR GIRDEE BRIDGES. 



FROM i60 TO 300 FEET SPAN.* 



In the above Table it will be seen that I have adopted a 

 large multiplier for the excess of strength which I conceive 

 necessary to be observed in the construction of a railway 

 bridge. Twelve tons per lineal foot, equally distributed over 

 the surface of the bridge, is a heavy load as a measure of 

 strength; and although I differ with some of my professional 

 brethren in this question, I am nevertheless of opinion, that 

 the difference of cost in effecting this object is inconsider- 

 able when weighed against the additional security obtained. 



In the wrought-iron tubular girder, the difference in the 

 weight of the bridge itself is proportionally less than in any 



* I have generally taken the depth of the girders at J^. of the span; 

 but in cases where the span does not exceed 150 feet, I have found it more 

 economical to adopt 1 of the span. With upwards of 1 50 feet span it is, how- 

 ever, more convenient, on account of the great height of the girder, to adhere 

 to the original proportion of J^, in order to keep the centre of gravity of 

 the girder low, and in order to {»*event oscillation to the passing load. In 

 situations where it is objectionable to increase the depth of the girders, it 

 tken becomes essential to increase the sectional areas of the bottom and the 

 cellular top in the ratio of the depths. 



