Ferro-Concrete. 395 



For given values of dc and de, h and fe are obtained propor- 

 tionate to the square root of M. 



It frequently occurs that in the cross-section also the compres- 

 sive zone is reinforced, for the purpose of obtaining- a higher com- 

 pressive strength, or to have a sufficiently strong reinforcement for 

 changing moments. 



Thrust. 



Apart from ordinary deflection, the deflection by thrust can come 

 into operation, as it usually occurs with bridge arches. If there 

 are only compressive strains on the whole cross section, then the 

 calculation can be made in the same way as in a cross-seciion of a 

 homogeneous material, if the cross-section of the reinforcement is 

 multiplied by 



_ Ee _ elasticity of iron _ lo _ 

 Ec elasticity of concrete i 



The case is different if the compressive force shows such an 

 eccentricity that tensile strains appear on the opposite side of the 

 cross-section. We then obtain for the calculation of the distance of 

 the neutral axis by a given cross-section and reinforcement, an 

 equation of the third degree, or the small tensile strength of the 

 concrete should exceptionally be taken into account, whereby the 

 calculation for the homogeneous cross-section could be made. 



In reinforced concrete arches, or, as they were formerly called, 

 the IMonier-Arches, there occur in most cases no tensile strains, 

 provided that the arch is given a serviceable shape. The employ- 

 ment of the reinforcement only serves to ensure a greater safety 

 and to lend a greater compressive strength to the concrete. Should 

 any tensile strains still occur, the following method may be applied 

 provided one does not want a very intricate calculation. The strains 

 of the pure concrete cross-section are first ascertained, and then the 

 total of all tensile strains of the concrete put to the reinforcement 

 on the side of tension. 



The above-mentioned methods of calculation are meant for the 

 rectangular cross-section only, and are therefore essential for the 

 calculation of the ceilings-plates and arches. It is evident that the 

 plates can either rest free or be fixed on two or more supports, and 

 that accordingly special types for the shape of the reinforcement 

 must be chosen. In these works the reinforcement is placed in the 

 direction of the tension in the lower part of the plate, and in such 

 a manner that the iron is still sufficiently covered by the concrete. 

 The case is a different one with fixed plates, or with plates resting 

 continually over several supports. The moments are in this case 

 negative near the supports, and the moments of the supports proper 

 are rather greater than the positive maximum moments in the centres 

 of the plates. The reinforcement must therefore be placed over and 

 near the supports. 



