380 NOTE TO AST. 128. [APPENDIX. 



The above will serve to illustrate the point in question quite 

 as well, perhaps, as an extended theoretical discussion. We see 

 that the saving increases rapidly with the length of the span, 

 and may easily rise as high as 30 or 4:0 per cent., while in some 

 cases even 50 per cent, may be realized. 



TUB DISADVANTAGES OF THE CONTINUOUS GIEDEE ABE: 



1st. The fact that the various pieces, especially the chords, 

 undergo strains of opposite character. 



This, in wrought-iron structures, we venture to think of little 

 importance. The extra work and cost of chords and chord con* 

 nectious necessary to secure the flanges against both compressive 

 and tensile strain, can hardly amount to 10, 18, 30, or even 50 

 per cent, of the cost of girder 1 



2d. Difficulty of calculation. 



We have, we trust, in what precedes, and in Chapter XIII., 

 succeeded in removing this objection. 



The opinion is widespread among engineers that the deter- 

 mination of strains in the continuous girder is impracticable 

 and involved in mystery. No opinion could well be more un- 

 founded. The accurate and complete calculation for all pos- 

 sible loading, live or dead, is precisely similar to and offers no 

 more difficulty than the simple girder itself. 



The formulae for moments and shears are, as we have seen, 

 simple and easy of application. 



The graphic method here developed offers also a thorough 

 solution. In view of both, and of the extensive literature upon 

 the subject (which seems, by the way, to have been so generally 

 ignored), we can finally pronounce the problem to be fully 

 solved. 



3d. The changes of strain, unforeseen and often considerable, 

 which a small settling of the piers, or change of level of the 

 supports may occasion. 



This, be it observed, is only of importance when the piers 

 settle after the erection of the superstructure. If piers are to 

 he considered as settling indefinitely, or continuously during a 

 succession of seasons, continuous girders are not to be thought 

 of. If, however, as is generally the fact, the piers' take their 

 permanent set during the first soson, and afterwards are im- 

 movable, the above objection has no weight. It is not necessary 



