THEORY AND PRACTICE OF CONCRETE IRON CONSTRUCTIONS. 113 



Taking any point on the line of form-alteration of the 

 concrete-iron body, say M, then Mm represents the total 

 elongation and MR the total stress per square inch on the 

 combined material. MP the stress per square inch on the 

 concrete, and PR multiplied by the ratio of the area of the 

 concrete to the iron represents the stress in the iron per 

 square inch. In case the concrete body has hardened in 

 air and thereby become subject to initial internal stresses, 

 viz., tension in the concrete and compression in the iron, 

 these stresses must be added or deducted as the case may be. 



The tensile stress on the concrete from that source must 

 be added to the stresses created by the loading, and the 

 distance OA from O represents the intensity of that initial 

 stress (at the beginning of the test). Using now A as the 

 starting point, we must, in order to get the true tensile 

 stresses in the iron, deduct the initial compressive stress on 

 that material from the stresses created by the loading, which 

 is achieved by the deduction of the distance A U, represent- 

 ing the intensity of the initial compressive stress (at the 

 beginning of the test). 



To now show the total stresses on the materials, it 

 becomes necessary to move the lines FF and ON to pass 

 severally through points A and U, taking up the positions 

 F'F' and U V respectively. The total stresses at point M 

 are then represented by M Q in lieu of M P on the concrete, 

 and Q S in lieu of P R on the iron. 



To illustrate the effect of repeated unloading and reload- 

 ing, let it be assumed that if the continuous loading was 

 interrupted at point on the line OBOGE and the load 

 repeatedly unloaded and reloaded then the effect C D, DC 

 — CD...D 11 O a would represent the lines of form-alteration 

 thus produced. Tests made in that direction brought to 

 light (1) that the concrete, although its elongations 

 amounted to from 10 to 20 times that value which causes 



H-Sept. 7, 1904. 



