i 1 1 



GIRDERS AND TRUSSES 111 



it is here the pieces should join and the pins be driven. Between 

 the pins should be vertical bolts with larger washers to hold the 

 pieces together. The spacing of the pins will be determined in 

 like manner as the pitch of nails is determined when reinforcing 

 planks are used on the side. First determine the bearing value 

 of the wood and the - - - - - - 7? 



shearing value with the 

 grain. Divide the shear 

 where a pin is placed by 



the allowable bearing Fig ' 70 ^^^ 



times the breadth to obtain the depth of the hole, half of which 

 will be cut in each half of the beam. The shear divided by the 

 breadth times the allowable unit shear with the grain gives the 

 minimum distance allowable between pins. When the computa- 

 tions are completed it will be discovered that the pins get farther 

 apart as the middle of the span is approached. 



Flitch plate girders, Fig. 71, are seldom used to-day, although 

 very popular at one time. The only reason for referring to this 

 type of compound girder here is to show wherein it fails. A flitched 

 girder consists of a plate of steel, or wrought iron, between two 

 planks, the whole construction being firmly bolted together. 

 The writer, in wrecking old buildings, found a number of such 

 beams evidently put together on a basis of relative fiber stresses, 

 with no thought for relative deflections. He worked once in the 

 office of an architect who tried to get him to design such a beam 

 in this way and the man was greatly surprised when the proper 

 method was shown to him. The method used is as 

 follows: Assuming a maximum bending fiber stress of 

 1300 Ibs. per square inch for wood and 16,000 Ibs. per 

 square inch for steel, the relative areas of wood and 

 - steel will be 16,000 -5- 1300 - 12.5, or a J-in steel plate 

 between two f-in. planks makes a girder having the 

 strength of four J-in. planks. 



Referring to the deflection formulas it is seen that for a fiber 

 stress of 16,000 Ibs. in steel the deflection in inches on any span 



while for a fiber stress of 1300 Ibs. in wood the de- 



flection - rr- Therefore yellow pine deflects 60 + 41-1.46 



times as much as steel, under the respective fiber stresses given. 



