JOINTS AND CONNECTIONS 137 



as if inverted, putting all the diagonal members in compression 

 and substituting vertical wrought iron rods for the thin boards 

 used in the original design. Before accepting the change, however, 

 the architect attempted to retain his truss by substituting in each 

 joint four f-in. bolts, saying that he had been architect for more 

 than twenty schoolhouses, in which he had used the form of truss 

 shown in his drawings. 



When a member is in compression the joint is not so hard to 

 construct, or design, as a connection at the end of a tension mem- 

 ber. It is an elemental fact that when a piece in compression 

 rests on another piece there must be a bearing area of the proper 

 size to prevent crushing. It is only in joints for pieces in tension 

 that the average draftsman seems to forget elementary principles. 

 He laps one piece over another, specifies that the contractor shall 

 " nail it securely," and passes on to something else. It is either 

 because of laziness or downright ignorance that such things happen. 

 Experienced contractors are often life savers for incompetent 

 draftsmen, for we can hardly call them designers. 



When there is a pull in a board used to transmit tension in a 

 truss the joint must be strong enough to resist the pull. It is 

 necessary to decide on the size of nail to use and then divide the 

 total pull by the resisting power of one nail to obtain the number 

 of nails to use. To avoid splitting the wood the nails should be 

 separated by a space at least twenty times the thickness of the 

 nail. Even this rule must be modified by the sort of wood used, 

 as some wood is brittle and cannot stand many nails in a small 

 space. Wire nails do not cut through the fibers, but spread them 

 apart so it is not necessary to make up the area occupied by the 

 nails, but this must be considered in using cut nails. The proper 

 size of nail to use is governed largely by the length. It must be 

 not less than three times the length of the thinnest outside piece. 

 If it cannot go two-thirds its length into wood, because the wood 

 is not thick enough, the end must be firmly clinched. 



A rule often blindly given in pocket books for the lateral resisting 

 value in pounds for nails is as follows: 

 P - Cd, in which 



P - total number of pounds transverse load per nail, 

 C - a coefficient varying from 4.5 to 12, depending on the nail, 



whether wire or cut, and on the wood, 

 d - the size of nail in pennyweights. 



