112 PRACTICAL STRUCTURAL DESIGN 



This question of deflection does not take into consideration the 

 thickness of the material, for deflection is governed by the span 

 and the depth. 



The statement about relative deflections means that if the 

 thicknesses are proportioned by the relative stresses, then the wood 

 planks must be 1.46 times as deep as the steel plate between them. 

 This will not do in practice, so it is necessary to obtain the rela- 

 tion between the stresses when the plate has a depth equal to the 

 depth of the inclosing planks. The fiber stress in the wood divided 

 by the fiber stress in the steel must equal the modulus of elasticity 

 of the wood divided by the modulus of elasticity of the steel; 

 that is, 



fw _ E w 



f. ~ E.' 



v* 16,000 x 1,500,000 

 Then / = -- = 30,000,000" = <>0 Ibs. per square inch. 



This is a low stress for yellow pine, so a softer wood can be 

 used. Assume a wood having a modulus of elasticity of 1,000,000, 



, 16,000 x 1,000,000 _ _ ., 

 then / = -- 30 QQQ QQO -- = P 61 sc l uare mch - 



If a steel fiber stress of 18,000 Ibs. per square inch is assumed, 

 the fiber stress in the wood = 600 Ibs. per square inch. The com- 

 putations show that for a flitch plate beam a soft, cheap wood is 

 the kind to use. It is wasteful to use a wood in which a high 

 fiber stress may be permitted. 



To design a flitched girder the fiber stresses are first found. 

 Then assume the depth and thickness of the steel plate. Find how 

 much it will carry as a thin deep beam and deduct this load from 

 the total load to be carried. The difference is to be carried by the 

 two wood planks of which we know the depth and the fiber stress, 

 so it is easy to find the thickness. The bolts are figured to trans- 

 mit the shear. It is an interesting exercise to design a flitch girder, 

 but a rolled steel beam or a trussed wooden girder will usually be 

 cheaper. 



Plate Girders 



Plate girders are compound girders made of wrought iron or 

 steel, the latter material being generally used to-day, for it may 

 be used with a higher fiber stress, thereby reducing the weight. 

 When rolled beams are not obtainable in a large enough size a 



