PROBLEMS IN DESIGN OF BEAM 101 



pression sometimes appears, 



f W M 



'+ Ar 

 in which S - section modulus. 



Buckling of Beams 



The author has been careful in calling attention to the fact that 

 beam formulas and tables of carrying capacity of beams assume 

 the beams to be stayed for lateral stiffness. If a beam is too long 

 the upper half acts as a slender column having a least dimension 

 equal to the breadth. When a floor is fastened to the upper sur- 

 face along the length it is usually a sufficient stay. It is best, 

 however, to have a stay as well for the lower edge of the beam. 

 A familiar illustration is the cross bridging between wood floor 

 joists placed at intervals of about 24 times the breadth of the joist. 

 In steel beams the lateral stays should be spaced at intervals not 

 exceeding 40 times the width of the flange. All stays prevent 

 a sidewise buckling, and the stay at the lower edge prevents a 

 blow from pushing the beam to one side, which would cause the 

 loading to become eccentric and thereby increase the stresses. 

 The effect of lateral deflection and eccentric loading is to set up 

 the simultaneous action of a direct thrust plus bending. 



Stiffness of Wood Beams 



The following formula was evolved by Thomas Tredgold, a noted 

 British authority on carpentry in the last century. A beam 

 designed according to this formula will deflect less than 3 | T the 

 span. 



6 = breadth of beam in inches, 



h - depth of beam in inches, 



L - span in feet, 



P =* concentrated load in middle of span, 



W - uniformly distributed load - 0.625P, 



, ,/L'PC ,/LWC 

 "NTT" ' \~~5~ 

 . UPC UWC 



- 



* ft* 



C - a constant - 0.010 for fir and yellow pine. 

 - 0.013 for oak and white pine. 



