THEORY AND PRACTICE OF CONCRETE-IRON CONSTRUCTIONS. 127 



which circumstances, to a certain extent must have affected 

 the carrying capacity of the construction. With 55*89 

 tons, a slight crack appeared on the side of beam G, 

 about 15 inches from the centre of the span, the crack 

 starting at a point 3 inches above the underside of the 

 beam, and extending upwards about 4 inches. With 59*99 

 tons a small hair crack on the side of beam A appeared 

 similar to that in beam G. That these cracks were not 

 caused by constructional weakness affecting the carrying 

 capacity of the beams is proved, in that they did not 

 increase under subsequent loading. What caused their 

 appearance may be briefly stated as being the effect of the 

 concentration of the iron for economic reasons as near the 

 underside of the beam as practicable, thereby depriving 

 those areas in which the cracks appeared of the qualifica- 

 tion of elongating to such an extent as the sectional areas 

 surrounding the iron were able to accomplish. 



With 64*98 tons small cracks, hardly perceptible, started 

 simultaneously at the edges of the underside of both beams, 

 and extended under increased loading until with 74*96 tons 

 they became clearly visible across the whole underside, and 

 also extended up the sides of the beams. 



The object of the test having been attained and the 

 deflection apparatus removed, it was decided in order to 

 ascertain how much the construction, so damaged, would 

 be able to support before collapsing, to continue with the 

 loading. This was carried on until the supply of bricks 

 was exhausted, by which time the load amounted to 142*10 

 tons. During this loading the cracks had extended from 

 the underside of the beams upward to near the underside 

 of the plate, and had opened out considerably. The beams 

 had deflected 3 \ inches, causing a tilting of the piers, and 

 a concentration of the compressive stresses on the inside 

 edges, effecting great cracking and crushing at those points. 



