112 F. M. GUMMOW. 



parallel, or if unequal deformations take place on account 

 of unequal application of the force, or unequal texture of 

 the cube, matters which any test in spite of the greatest 

 precautions may more or less be subject to. 



On the other hand, in a concrete-iron construction subject 

 to bending moments, there is an even transmission of 

 pressure at an increasing rate from the neutral axis. The 

 elasticity imparted to the construction by the iron rods 

 counteracts the permanent set, and by doing so has an 

 equalising effect on the stresses, thereby preventing unequal 

 deformations, and consequently preventing those secondary 

 shearing stresses which cause destruction of the cubes, 

 when determining their compressive strength. 



Since great differences between the direct stresses and 

 calculated stresses of the materials when used in concrete- 

 iron construction do exist, as is evident from the above, 

 it becomes necessary to obtain special data of permissible 

 compressive stresses for concrete-iron constructions with 

 the various percentages of iron and different mixtures of 

 concrete. 



Concerning the tensile stresses it is necessary to examine 

 the distribution of the same over the two materials. In 

 order to do this, reference must be made to the line of 

 form-alteration of concrete-iron in tension, viz., the curve 

 OBCGE plotted in the ratio of stress to elongation from 

 results obtained from direct tensile test. The apportioning 

 of the stresses was arrived at by measuring the elongations 

 of the iron under various loadings, calculating the stresses 

 and deducting same from the test load in question, whence 

 the balance would represent the stresses on the concrete. 

 The stresses on the combined material as plotted represent 

 the stresses per square inch, and the stresses in the iron 

 represent the stresses per square inch shortened in the 

 ratio of the areas of the iron to the concrete, the iron being 

 represented by the dotted line of form-alterations FF. 



