78 



HANDBOOK OF MECHANICAL DESIGN 



For a — 4,5 deg., tan a = 1, 



P' = - ^ 



Vb 

 h 



w 



•0.9 



0,8 



£ 07 



0.6 



c 0.5 



E 0.4 



o 

 .i^O.3 



Ql 



Because the web in diagonal tension tends to hold the stiff eners straight, prevent 



bowing as a column, the stiff eners need not be 

 designed for the full column length, but only 

 to the equivalent column length as given by 

 the curve in Fig. 7. The design of a vertical 

 stiffener is the same as for any pin-ended 

 compression member. 



Stiffeners must not be spaced farther 

 apart than one-half the depth of the beam. 



Chord Load 



At any point distant A^ from the applied 

 load (Fig. 8), the total chord load is 



M 



0.2 



0.4 0.5 0.8 1.0 

 d 



1.2 



1.6 



1.8 2.0 



(-D 



cot a 



For a = 45 deg., this reduces to 



M _V 



"^ h 2 



h (cot a- cot /3) 

 Fig. 7. — Equivalent free buckling length for 

 tension field stiffeners is obtained by multiplying 

 the total actual length by the factors from this 



"""'"'• where M = XV 



The web is always neglected in computing the section modulus of the beam, for 



it has no resistance to compression. 



I 



Sfiffeners / Upper chord cent raid 



35" 



//x li X 0.040 "chords 



H 





^ Lower chord cenfroid 



Fig. 8. — Diagonal lines represent the diagonal field 

 tension in a thin web. 



^---1 Jequol spaces- 



^l2>fl2>^8 chord angles 



-^ I"x$k0.05I" 

 '^ stiffeners-^ 



._L 



I 



FiG. 9. — Example of a tension field web beam. 



EXAMPLE 



Assume two loads and dimensions as given in the accompanying Fig. 9, 



27 



St ^= 



■where a = 45 deg. 



V = 10,000 lb. 



Or — 



2 X 10,000 

 20 X 0.025 



ht sin 2a 

 h = 2Q in. 

 t = 0.025 in. 



= 40,000 lb. per sq. in. 



