APPENDIX.] SUPPORTS OUT OF LEVEL. 381 



that the piers should be exactly on level or even on line, or even 

 that the differences of level be known. 



As shown iu Art. 121, these differences produce no effect, 

 provided the girder be built to the profile of the supporting 

 points. 



If in any case these differences are required, and it is con- 

 sidered difficult to determine them over water with sufficient 

 accuracy, then the proper reactions at the several piers may be 

 weighed off* and the girder thus left in position under pre- 

 cisely the circumstances for which it has been calculated. 



THE PRINCIPAL ADVANTAGES OF THE CONTINUOUS GIRDER ARE \ 



1st. Ease of erection, where false works are difficult or ex- 

 pensive. The girder may be built on shore, and then pushed 

 out over the piers. 



2d. Saving in width of piers, as compared with width re- 

 quired for separate successive spans. The girder may be placed 

 upon knife edges at the piers. In fact, such a construction is 

 preferable, as better ensuring the calculated strains. Width of 

 piers is undesirable. 



3d. Saving in material usually from 25 to 30 per cent. 



1. Continuous Girder Supports not on a level. In 

 Chapter XIII. we have all the formulae required for the solution 

 of the continuous girder for supports on a level, or all on line, 

 when the deviation from level is small, whatever may be the 

 number or relative length of the spans. If for a continuous girder 

 of constant cross-section, the supports are properly lowered after 

 the girder is placed upon them, we may obtain a saving of 23 

 per cent., or more, in material over the same girder with sup- 

 ports all on level. If, however, the cross-section varies accord- 

 ing to the strain in other words, if the girder is of constant 

 strength no advantage is gained from thus lowering interme- 

 diate supports. Such disposition of the supports may even act 

 injuriously. 



The formulae for shear and moments which we have given 

 are, indeed, based upon the hypothesis of constant cross-section, 

 but the strains in every piece of the girder being found for the 

 shears and moments thus obtained, each piece is proportioned 



* An idea first suggested by Clemens Herschel, C.E.: Continuous, Revolving 

 Draw Spans. Little, Brown & Co., TJoston, 1875. 



