Ferro-Concrete. 



399 



The value in rectangular cross-section can be calculated as 

 follows : — 



V 



to = 



h-5 



3) 



The value 



xb 



b X to = 



h-^ 

 3 



acting on the outside of 

 also be calculated in a 



shows also the sum of the adhesion strains 



the reinforcement, so that the same can 



simple way. With the cross-section of the plate beam the distance 



of the tensile and compressive centre point is substituted ui place of 



li- 



lt may also be mentioned that the simultaneous presence of 

 a great bending moment the mentioned value for the shearing strain 

 is not altered even if the tensile strength of the concrete is taken into 

 consideration. For in concrete, subjected to great tensions, the 

 difference between neighbouring cross-sections is zero. An increase 

 can therefore not take place for the shearing strains m the part of 

 the cross-section under tension. 



Out of the adhesion strain the number of iron bars is calculated 

 which must still be present on the bearer, and it is therefore evident 

 that it will not do to assume the dimensions by the line of the 

 maximum moments solely. If sufficient reinforcement is not given 

 to the supports the destruction of the girder occurs through the irons 

 being pulled out of the concrete at the bearer ends. The distance 

 and the thickness of the stirrups are calculated out of the shearing 

 strain; it is a rule that 43 to 72 lbs. per square inch can be taken 

 up by the concrete and the rest falls to the stirrups. If we know 

 that concrete is capable of following the extension of the iron, and 

 we do not take into consideration the tensile strains in the calcu- 

 lations of the dimensions. This is, in my opinion, a method 

 of calculation giving the greatest protection against the occurrence 

 of cracks, quite apart from the absolute safety and strength which 

 is obtained thereby. In reality the tensile strength of the concrete 

 is naturally still present, which fact will show only slight deflections 

 of reinforced concrete constructions under loading tests. 



Further, it must be considered that in consequence of the strong 

 and close interlocking of all parts of a ferro-concrete structure more 

 factors take part in the bearing of the load than generally appears 

 in the calculations, and any of the other building systems manifest 

 it. The slight deflection can also be easily accounted for and 

 determined beforehand. 



