JOINTS AND CONNECTIONS 165 



unless thoroughly connected by shear pins or spikes, is that the 

 ratio of slenderness reduces the load carrying capacity in a greater 

 degree than the load is reduced by the number of pieces. For 

 example the allowable unit fiber stress for solid piece with dimen- 

 sions equal to, or greater than, one-fifteenth the length may be 

 1200 Ibs. per sq. in. Divide the solid piece into four slices and the 

 ratio of slenderness for each slice (plank) is one-fourth of one- 

 fifteenth = fa L. By the above formula the allowable fiber stress 

 is only 400 Ibs. per sq. in. Assume that the whole load is carried 

 on the cross section at 1200 Ibs. per sq. in. provided it is a solid 

 piece. By dividing it into four planks, each carrying one-fourth 

 the load, it is seen the total carrying capacity of the four slender 

 pieces is only one-fourth the total load. That is, the four pieces 

 acting separately are each strong enough to carry one-sixteenth 

 of the load. By nailing them together at intervals they are made 

 to act together to some extent, and if arrangements are made 

 to connect them together rigidly at intervals not greater than one- 

 fifteenth the thickness of each piece the allowable fiber stress is 

 increased. Putting bolts or spikes through several planks is not 

 nearly so effective as spiking them together by internal nailing, 

 that is by " piling " them, by which term is meant nailing each 

 plank to a lower one, the spikes passing through at least three 

 after three are assembled. Two should be spiked together and 

 the spikes clinched. Then the " piling " is done by adding a plank 

 first to one side and then a plank to the other side, so the first 

 two planks form the core. To hold the two outside planks the 

 cross pieces are nailed on the edges. 



Member U 9 Lt has a length of 14.14 ft., measured from center 

 line to center line of the chords, but somewhat less in the clear. 

 The total length, however, will be used for convenience in pro- 

 portioning the piece. Assume a piece 4 in. x 8 in., in which the 



ratio of least thickness to length = -r * 40. This is much 



14 x 12 



too great. Try a 6 in. x 6 in. and the ratio - - " 2S, 



o 



which is still large, but we will investigate and see whether the 

 piece will do. 



First find the fiber stress. This is usually done by using a 

 straight-line formula, but other formulas are discussed in the 



