182 



PRACTICAL STRUCTURAL DESIGN 



Two methods of framing intermediate joints are shown in Fig. 

 107 and Fig. 108. They are very common and yet violate the 

 principle that the carpenter work should be simple. When the 

 strut is not normal to the member it abuts against, the two sur- 

 faces of the indent must be separately investigated and the bear- 

 ing pressure found, for each. The unit bearing pressure having 

 been found the minimum bearing area must then be determined 

 by methods already given. It involves considerable "cut and 

 try " work. It is also imperative that the exact angles used must 

 be marked on the drawings so the carpenters can make the joints 

 in the field and secure the conditions assumed in the design. The 

 angles of cuts having been found so the bearing is correct on each 

 face, the depth of each cut is fixed by the bearing stress on the 

 ends of the fibers, at the assumed angles. In Fig. 107 the cuts 

 are not normal, the stress actually acting along the center line 

 of the strut, or so nearly along the center line that the moment 

 due to eccentricity may be neglected. In Fig. 108 the cuts are 



normal and the total thrust 

 is assumed to act over the 

 face of the normal cut. The 

 unit stress on the fibers of the 

 chord is found as shown in 

 the design of the cut for the 

 end brace. The depth of the 

 cut is then found. At the 

 upper end the normal (right 

 angle) cut is on the lower 



Fig. 108 



side of the strut and at the lower end it is on the upper side. 

 Draw lines through the centers of these normal areas, parallel 

 with the top and bottom of the strut. The eccentricity is the 

 distance between these center lines. 



Multiply the thrust by the eccentricity in inches and get the 

 bending moment in inch pounds. This has a tendency to make 

 the end of the strut move on the face of the normal cut and 

 "jump out." It must be resisted by the friction of the wood on 

 the face of the cut. Divide the eccentric moment by the length 

 of the strut in inches and this gives the force to be developed by 

 friction. Assuming the coefficient of wood against wood, for 

 sliding friction, to be 0.2, multiply the direct thrust by 0.2 and 

 obtain the resistance the wood will offer against being forced out 



