THEORY AND PRACTICE OF CONCRETE-IRON CONSTRUCTIONS. 119 



considerably less than that of an equally strong steel girder. 

 This property is of great importance in dealing with con- 

 structions of large spans, subject to heavy and rapid moving 

 loads as in the case of bridges. 



Many bridges with one or more openings up to 70 feet 

 span have been carried out for road and railway purposes, 

 and the experience gained therewith has been highly satis- 

 factory. Within certain limits, according to circumstances, 

 plate-beam bridges are more advantageous than arch 

 bridges, but in spans of considerable size the arch bridges 

 are more economical. Arches similar to plates and beams 

 require to be strengthened in those areas subject to bending 

 moments by means of iron insertions. Arches of parabolic 

 shape, carrying an equally distributed load, need only be 

 reinforced with iron near the intrados ; but to carry varying 

 loads it becomes necessary to reinforce the arch near the 

 extrados as well as the intrados. These iron insertions 

 while effecting a great increase in the factor of safety, 

 and elasticity, give important economic advantages. 



In constructions of circular shape subject to equally distri- 

 buted internal pressures, as occurs in pipes, reservoirs, etc., 

 filled with water, concentric iron insertions are necessary, 

 but when they are subject to unequal external pressures 

 as in the case of pipes under earth pressure, it is necessary 

 to use either eccentric iron insertions, extending from the 

 outer surface at the horizontal diameter to the inner sur- 

 face at the vertical diameter, or else to use concentric 

 rings near the inner and outer surfaces respectively. 



It has been already stated that round iron bars have 

 become the standard section of the leading systems of con- 

 crete iron. Other shaped irons have here and there been 

 substituted for the round bars, such as I irons, X iron, 

 twisted iron, and many other sections, for each of which 



