142 



PRACTICAL STRUCTURAL DESIGN 



truss shown in Fig. 85s. It being assumed that wire nails and 

 common screws merely push the fibers aside, no additional area is 

 required on account of the holes. Lag screws and bolts, however, 

 cut the fibers, so it is necessary to add to the members a width 

 equal to the diameters of the holes. By the time the student has 



worked through the 

 problem of detail- 

 ing all the joints he 

 will probably learn 

 that there is not 

 area enough for 

 fastenings. The 

 problem will serve 

 to show why tim- 

 ber should not be 

 used for members 

 in tension unless 

 the load from other 

 It is best when using 

 Use wrought iron or 



10 20 



30 40 50 60 

 Value of 6 in Degrees. 



Fig. 88. 



70 60 



members can be transferred by end thrust, 

 wood to have all joints in compression, 

 steel for tension members. 



Fig. 89 illustrates several types of joints and fastenings used in 

 framing timber. Bolts in carpentry should be used, when pos- 

 sible, only to hold abutting portions of timber together. It is not 

 always possible to so use them, but the hint is enough to set a man 

 to studying seriously every joint he makes in order to hold to the 

 rule if possible. Straps are more expensive than bolts and are not 

 so good. A detail that is too common and which should never 

 be used is shown at (a); what happens when the wood shrinks 

 is shown at (6), for it is impossible to tighten the joint. Water 

 getting into the toe of a joint often causes the fibers to decay and 

 this added to shrinkage ruins the truss. A preferable method is 

 shown at (c) . The bolt may be tightened from time to time. This 

 form of joint, however, should never be designed with part of the 

 load carried by the abutting end and part by the inclined bolt. 

 Many experiments have demonstrated that under test the two do 

 not act together. The weaker system will act first and give way 

 before the other gets into action. It is best to design for the entire 

 load to be taken by the dap and use the inclined bolt only for the 

 purpose of holding the pieces in contact. All bolts will be screwed 



