172 



PRACTICAL STRUCTURAL DESIGN 



direction of the bolts. The line BC is perpendicular to the bed 

 cut into the top of the chord. The line BC intersects the line 

 AC at C. The line AC to scale gives the tension in the bolts. 

 This is found to be 45,500 Ibs. 



Deciding to use two bolts the tension in each = 22,750 Ibs., 

 which calls for two If-in. bolts. The holes must be filled so there 

 is no reason to use bolts with upset threads, and the size of the 

 bolt is determined by the area at the root of the thread. The 

 25,000 



shear = 



= 13 ins., therefore the clear distance between 



2 x 8 x 120 



the edges of the cuts to receive the bolts cannot be less than 13 ins. 

 The distance from the edge of the first cut to the end of the chord 

 must be equal to or exceed 13 ins., for each bolt washer is assumed 

 to carry half the total shear. 



The allowable pressure on the side of the wood is 350 Ibs. per 

 sq. in. and the washers on the face of the end brace are dimen- 



sioned as follows: 



22,750 C1 . 

 r^ = 8.1 ins. 



Make them 8 in. x 8 in. 



8 X350 



The thickness may be found by the rules mentioned earlier. The 

 washers on the lower end can be smaller, for they bear on the wood 

 at an angle. The allowable pressure, the angle being 45 degrees, 



= 1200 x 0.45 = 540 Ibs. per sq. in. The width of the washers 



22 750 



= ?r rr?: = 5.25 ins. The vertical cut into the bottom of the chord 

 8 X 540 



to permit these washers to bear properly is 3f ins. deep, which leaves 



a space of 3 ins. be- 

 tween the top of the 

 cut and the bottom 

 of the brace seat for 

 the tension in the 

 chord. Counting the 

 width as being 6 ins. 

 the strength of the 

 chord = 3 X 6 X 1600 

 = 28,800 Ibs., so the 

 chord is safe. When 



C.L.of Reaction 



Fig. 101 



this form of joint is 



detailed so part of the load is carried by thrust against the chord 

 and part is carried by the bolts, the weaker detail will give way 

 before the other is brought into play, as they do not act together. 

 Therefore an end joint should be designed so all the load is carried 



