JOINTS AND CONNECTIONS 183 



of the cut by the bending moment. Nails and spikes offer resist- 

 ance against being pulled out, so if the ends of the strut are "toe 

 nailed " this additional resistance will be good. It seldom happens 

 that the eccentric moment divided by the length of the strut will 

 give an amount exceeding the direct thrust multiplied by the co- 

 efficient of friction, but if it does then spikes or bolts must be 

 used to hold the toe in place. Carelessness in keeping all joints 

 tight reduces the effect of friction, and decay in the joint also 

 seriously affects it. In the joints illustrated there is often a 

 serious loss in the efficiency of the upper and lower chords because 

 of the depth of the indent. Details tending to reduce cutting 

 into chords should be favored. 



It has been said that all forces should act through the center 

 lines of members. All the detailing is done with this in mind. 

 Due to careless detailing, or, if the detailing has been good, then 

 due to careless framing, any variation in the relation of web mem- 

 bers meeting in a panel point may increase secondary stresses to 

 a dangerous amount. The horizontal component of the diagonal 

 thrust acts through the lower chord on a line intersecting the 

 center of bearing of the thrust. The tension in the chord acts on 

 the center line through the 

 uncut portion of the chord. 

 There is a moment devel- 

 oped by the vertical dis- 



tance between these two j ^r -Av 



lines of action. The ver- 

 tical component of the 

 thrust acts through the 

 center of the face of the 

 cut. This forms a couple Fig - m - 



with the tension in the vertical rod. There is a moment developed 

 by the distance between the center of the rod and the line of action 

 of the vertical component of the thrust in the strut. In wooden 

 trusses the secondary stresses are seldom important except when 

 very high unit stresses are used, but we cannot afford to neglect 

 consideration of the possibility of neglect to keep the joints tight 

 and the possibility of rotting setting in. It is very little trouble 

 to investigate the effect of secondary stresses and provide addi- 

 tional material, or to redesign the joint to reduce the secondary 

 stresses to a minimum. It is at least important that secondary 



