80 PRACTICAL STRUCTURAL DESIGN 



designer seldom has to deal, it being part of the work involved 

 in the design of bridges. Deflection also affects appearance and 

 a camber is given to trusses to hide deflection. 



A beam may be amply strong so that it will not fail by bending, 

 shearing, or crippling, and yet the deflection may be so great that 

 it will not be suitable for use in the proposed location. The amount 

 of deflection must then be found and if it exceeds the allowable 

 deflection a deeper beam must be substituted. When using steel 

 it is often possible to secure a deep beam which will weigh less 

 than a beam of less depth of practically equal strength in bending 

 and shear. For timber, experience indicates that the most eco- 

 nomical beam, considering the two factors of strength and stiffness, 

 has a breadth equal to two-thirds or three-fourths the depth. 



Deflection Formulas 



Deflection formulas as usually presented are formidable in 

 appearance, so tables are given in the steel handbooks which 

 enable the deflection in inches to be found by dividing a factor in 

 the table by the depth of the rolled section in inches. 



Similar information for wooden beams was not so readily 

 obtainable until in 1913 the Yellow Pine Manufacturers' Associa- 

 tion issued a book entitled "A Manual of Standard Wood Con- 

 struction," following the lines laid down previously by the steel 

 manufacturers in their handbooks. Copies of this book may 

 be obtained from the secretary of the above association in 

 St. Louis, Mo. 



The " Structural Timber Handbook, for Pacific Coast Woods " is 

 issued by the West Coast Lumbermen's Association, Seattle, Wash., 

 and valuable books on the subject of wood design may be obtained 

 from the National Lumber Manufacturers' Association, Chicago, 111. 



The complicated formulas for deflection are made to appear as 

 follows, after certain substitutions and transformations of factors : 



30 /L 2 



D = 



Eh 



in which D = deflection in inches, 



/ = allowable maximum fiber stress in bending, 



L = length in feet, 



E = modulus of elasticity, 



h = height of beam in inches. 



