THEORY AND PRACTICE OF CONCRETE-IRON CONSTRUCTIONS. 117 



constructions subject to bending stresses. A plate thus 

 supported becomes, when loaded, subject to compressive 

 stresses in the upper fibres, and tensile in the lower. In 

 order to augment the tensile strength of the concrete, iron 

 was inserted near the underside of the plate in the direction 

 of the span. This iron took the shape of round bars in the 

 first instance, that being the easiest form procurable, and 

 having been found later on to be also the most advantage- 

 ous shape to use, it has become the standard section of the 

 leading systems of concrete-iron. More frequently iu 

 practice the ends of the plates are more or less fixed to the 

 supports or the plates are continuous over the supports, in 

 which cases, tensile stresses occur at or near the upper 

 side of the plates over the point of support, as well as on 

 the lower side at and near the middle of the span, so that 

 it becomes necessary to place iron in those sectional areas 

 subject to tension. 



As the rectangular section of a plate does not permit of 

 the iron being utilised to the best advantage, the applica- 

 tion of this form of construction becomes for economic 

 reasons limited to small spans. For larger spans, and 

 where heavy loads come into play a more favourable type 

 of cross-section was substituted, which dissected, consists 

 of adjoining T girders. 



This type in its most economic form is designated the 

 plate-beam construction, on account of the webs being 

 spaced so far apart that the flanges between same act as 

 plates continuous over the web. The continuous plates 

 being armoured with iron insertions as above mentioned. 

 The webs also have iron insertions embedded according 

 to their having free ends, fixed ends, or being continuous 

 over supports. Further, the webs require vertical or 

 oblique irons, otherwise termed " stirrups " or " struts," 

 to assist the concrete to take up the shearing stresses 



