TUBULAE GIRDER BRIDGES. 185 



At a recent discussion on this subject, which occupied 

 two successive meetings of the Institution of Civil En- 

 gineers, Westminster, it was maintained that m^^ formula 

 was not correctly applicable in cases of girders of more than 

 one span, and that I had neglected in the calculations the 

 great increase of strength which was derived from the gir- 

 ders being continuous. 



This continuity of the girder was estimated by some to 

 add not less than one-third, and by others one-fourth, to 

 the ultimate strength of that part of it which formed a single 

 span, when viewed simply as a beam supported at the ends, as 

 exhibited in the model now before you. On this question, 

 I observe in a note appended to ray paper read at the Insti- 

 tution of Civil Engineers, " that the doctrine of continuity 

 is doubtless true to a certain extent ; and, although I admit 

 the fact, I have purposely neglected in the calculation any 

 auxiliary support of that kind as a counterpoise," &c. — I 

 think it safer to do so, as any admission of increased strength 

 in that direction, might lead to serious practical inconveni- 

 ence, if not dangerous results. I have therefore freely given, 

 as additional security, those advantages of strength, what- 

 ever they may be, rather than adopt refinements in the cal- 

 culation, which, if exercised by the general practitioner, 

 might lead to serious error in reducing the ultimate strength 

 of the bridge. To give to a tubular girder bridge, of more 

 than one span, the full benefit of the extra strength derived 

 from the counterpoise of the girders on the opposite side, 

 the girders would require to be differently constructed ; and, 

 in place of the joinings of the plates being prepared to resist 

 compression throughout the whole length of the girders, the 

 cellular top would require to be constructed for about two- 

 thirds of the span in the middle of each girder on the 

 principle of compression — and for a distance of one-sixth on 

 each side of the pier on the principle of tension. In fact, it 



would require a complex series of constructive operations, 



2 B 



