BEAMS AND STRUCTURES 77 



Therefore, since the appUed unit shearing stress of 10,900 lb. per sq. in. is less than 

 the critical buckling stress of 11,250 lb. per sq. in., the web will carry the 4,000-lb. 

 shear load without buckling. 



VERTICAL STIFFENERS FOR SHEAR-RESISTING WEBS 



An approximate formula for computing the required moment of inertia of the 

 stiffener is 



^ 2.29d (VhY 



For 24 ST aluminum, E = 10,500,000, this equation becomes 



2.29d ( Yh 



Lt = 



\3i6, 



t \3i6,500,000. 



where d = distance between stiff eners h = distance between centroids of upper and lower chords 

 t = thickness of stiffener 



Note: Best practice is to make the stiffener thickness equal to that of the web and then compute the required 

 moment of inertia by the above equation. 



DIAGONAL TENSION WEBS 



To determine when a web should be designed as a shear resisting web and when 

 it is to be designed to carry the shear load in diagonal tension, calculate 'SJV/h, 

 where V is the applied shear, in pounds, and h is the depth of the beam, in inches. 

 Usually, if this ratio is less than 7, the web should be designed as a diagonal tension 

 member. If this ratio is more than 7, a shear resisting web should be used. If the 

 ratio is 7, or nearly so, both types of web members should be investigated to determine 

 which is the more economical. 



The diagonal tension stress St in a tension field web is 



^ ht sin 2a 

 where h — distance between centroids of upper and lower chords 

 For a = 45 deg., 



Ot 



2V 



ht 



Theoretical maximum allowable St is equal to ultimate tensile strength of the 

 material. An allowable St equal to about 0.7 ultimate tensile strength is recom- 

 mended for calculations. 



Vertical Stiffeners 

 Compression load P' in the stiffeners can be determined from 



P'= -(:^^)tana 



