JOINTS AND CONNECTIONS 



171 



whereby the stress can be computed in these bolts. The size is 

 fixed by judgment based on experience. For a truss with such 

 light loads as the one under review, f-in. bolts might be used, 

 but designers generally prefer to use nothing smaller than a 

 f-in. bolt, this being good practice. Since they carry a very 

 small stress, if any, it is not necessary to cut a seat in the lower 

 chord, which would weaken it, but cast iron washers of the 

 form shown are used, they being seated in the timber about half 

 an inch. 



The detail in Fig. 99 is not good, for the reason that the sloping 

 face on the 16-degree angle cannot be accurately cut on account 

 of the depth to which the piece must go into the chord. The 

 back edge of the sloping face should meet the top of the chord. 

 These details must be worked out on the drawing board to a large 

 scale in addition to being computed. The computations and draw- 

 ing to scale must go together. In all the end joint details a block 

 is inserted through which the bolts go. This is to diminish as much 

 as possible the effect of secondary stresses set up by the bolts. In 

 Fig. 99 the slope of the bed might be carried to the top of the 

 chord without weakening it and a block fitted in the space, but it 

 increases the labor cost. The designer understands, of course, 

 that he should make several designs and choose the one that costs 

 the least and will do the work. 



In Fig. 100 is illustrated a very common form of joint and it is 

 not a good one. This joint 

 is used in an attempt to 

 get rid of the long end pro- 

 jection caused by design- 

 ing to resist shear. The 

 end of the brace is dapped 

 into the top of the chord 

 merely to hold it in place. 

 The diagonal bolts take 

 all the stress and the shear 

 is resisted close to the bot- 

 tom of the chord. Note 

 the line diagram in the 

 upper left-hand corner of 



SVxS'Woshtn 



Fig. 100 



the figure. The line A B is parallel with the brace and is drawn 

 to a scale to represent the load. The line AC is parallel to the 



