DESIGN OF FIXED ENDED ARCHES BY THE ELASTIC THEORY 158 



to reduce compressive intensities and to bind the concrete masses 

 together. 



In conclusion it may, therefore, be stated that for properly 

 formed ribs, the amount of steel section required need rarely exceed 

 from 1 to 1.5% of that of the concrete. 



The various steel ribs and the flanges of each rib should be 

 thoroughly held together by light steel work. Where heavy flanges 

 must be employed it is desirable to hold them together by a stout 

 web system. 



In this article it has been assumed that the greatest stresses are 

 produced by a live load covering five-eights of the span. To make 

 the study complete, computations like the above should also be made 

 in turn for a live load covering one-half, three-quarters and the 

 whole span. The greatest abutment pressures occur with the span 

 wholly loaded, but the accompanying bending moments in the rib 

 are small. The greatest interscepts between the arched rib's neutral 

 curve and the true equilibrium frame are found for a live load cover- 

 ing about flve-eights of the span. But the smaller the live load the 

 smaller is the pole distance. Hence for the partial live loading the 

 bending moments in the rib are increased by the larger interscepts 

 but decreased by the smaller pole distance. The exact amount of 

 live load to give the greatest bending moments cannot be stated. 

 Fortunately the live loads are relatively small when compared with 

 the dead loads especially of masonry structures so that a small change 

 in the position of the live load does not greatly change the stresses in 

 the ribs. It is, therefore, generally sufficient to make a calculation 

 for one position of the live load. 



End abutments should be designed for the arch completely 

 covered with live load. The corresponding arch thrust in the line 

 K,K, equals then very nearly the pole distance, ZD = 204,800 pounds, 

 multiplied by the total load, 289,760 pounds on the arch for a full 

 live load, and divided by the total load AB = 275,130 pounds for a 

 five-eighths live load. Thus the pole distance for a full live loading 



