110 



To reinforce with a steel plate on the bottom use a moment 

 arm = X 14 = 9.333 ins. The fiber stress in the steel will be 



16,000 Ibs. per square mch. Then area of plate = ^ 



y.ooo x 16,000 



= 0.783 sq. ins., and 0.783 -=- 7 = 0.118 ins., the thickness of the 

 plate. Use lag screws to fasten the plate to the beam, the proper 

 pitch being determined as in the last example, using the full depth. 

 The objection to the use of the steel plate is that the compression 

 in the upper half of the beam is increased, although the effect 

 of adding* the plate is to lower the neutral plane. The proper 

 method for reinforcing a beam, or girder, in place is to add planks 



on one side or on both sides, but 

 when fixtures or wires are in the 

 way it may be best to use a steel 

 plate on the bottom. 



Compound beams have been made 

 consisting of two shallow beams 

 superimposed (Fig. 69). If not care- 

 Fig. 69 - Compound Beam fully f aste ned together they act 

 singly, because the line between them is in the position occupied 

 by the neutral axis of a solid beam, having a depth equal to the 

 combined depth of the two pieces. Several methods have been 

 used to cause the two pieces to act together, one of which is 

 shown in Fig. 69, the other in Fig. 70. 



No matter how thoroughly the pieces are fastened together 

 the strength of such a compound beam is only about 70 to 75 

 per cent of the strength of a beam of equal dimensions made 

 from one piece of timber. The deflection of such a beam under 

 load is much greater than the deflection of a beam of equal dimen- 

 sions made from one piece of timber. 



The diagonal side pieces shown in Fig. 69 should be preferably of 

 a harder wood than the beam, and each should be not less than one- 

 eighth the thickness of the beam, thus making a beam 25 per cent 

 wider than the width of the pieces of which it is composed. The 

 pieces should be diagonal and slope in opposite directions on the 

 sides of the beam. Plenty of nails must be used. 



In Fig. 70 the pins should be of hard wood or of metal. It is 

 best to use two pieces in each hole, wedge-shaped, so they may be 

 driven tight and have a bearing against the wood the full width 

 of the beam. The shear being greatest along the neutral axis, 



