CHAPTER IV 

 Girders and Trusses 



A METHOD frequently used by carpenters to strengthen 

 joists and beams is shown in Fig. 64. Two pieces are 

 nailed as indicated, the presumption being that they exert 

 an arching action because the ends abut at the middle of the span 

 and the nails hold the pieces in place when thrust is exerted. 



Wood shrinks 

 when it dries, so 

 the close contact 

 is lost, and then 



Fig. 64 A Poor Method for Reinforcing Joists 



considerable deflection must take place before the ends again meet, 

 the bending being sufficient to cause failure in many instances. 

 Provided the hoped-for arch action does occur there will be such 

 a pushing against the nails that the wood is bound to split. How- 

 ever, assuming the arch action does take place and the nails do 

 not split the pieces the reinforcement is not effective. For effec- 

 tive arch action there must be substantial abutments provided. 

 If there are no substantial abutments a tie rod is necessary to tie 

 the ends together and take the thrust. There being no tie rod, 

 it is evident that the lower part of the joist will have to act as 

 a tie. We know that when a beam is loaded the lower fibers are 

 stressed in tension and the upper fibers are stressed in compres- 

 sion. To increase the tension in the bottom by adding to it the 

 amount required to take care of the thrust 

 in the diagonal reinforcing strips is not 

 helpful. This old-time method is, therefore, 

 based on a fallacy and should be abandoned. 

 In Fig. 65 is shown another method, the 

 reinforcing being spiked along the top edge 

 to make a beam of T-section. This raises the 



Fig. 65 T-beam of 

 Wood 



neutral surface so the increased area in compression is supposed 

 to be offset by an increased area in tension. 



Assume a joist 3 ins. X 14 ins. of wood in which a fiber stress of 



102 



