362 



NOTE TO CHAP. I. 



[APPENDIX 



and diagram with ease and accuracy any form of truss which 

 rears in riiirineering practice. 



11. In the bow-string girder represented IP Fig V. it is 

 evident that the bottom flange serves merely to resist the thrust 

 of the bow and keep it from spreading. It adds nothing to the 

 supporting power of the combination. We might remove it 

 entirely and replace it by abutments which would equally well 

 Mistain this thrust, and if we then introduced a horizontal flange 

 at crown, and inserted diagonals between for stiffness, we should 

 have the form of braced arch given in Chap. I., Fig. 5 (c). If, 

 however, we should resist the thrust of the bow by an inverted 

 arc, it would answer the same purpose as the bottom flange, 

 and we should, in addition, double the supporting power. 



We have illustrated this in Fig. VI. 



The span is the same as before. The lower apices only are 

 supposed to be loaded, for comparison. [Properly, we should 

 have distributed the load over both upper and lower apices.] 

 The rise of each arc is one-half as great as before, or 5 ft. only, 

 thus making the total depth the same as in the preceding 

 case. 



By means of two strain diagrams, we find the strains due to 

 . Thus: 



Then, precisely as in the preceding Art., we can fill out our 

 table of strains. This the reader can now easily do for himself. 

 We thus find, for a uniform dead load ths the live load, the 

 total maximum strains below. 



Comparing these with the corresponding strains for the bow- 



