10 NOTES ON PLATE-GIRDER DESIGN 



stress intensity is therefore assumed to act along the 

 clement of the web having a length of / in Fig. 76, and 

 this must be less than the permissible unit stress for a 

 column of this length. Assuming that a safe unit stress for 

 a column is given from tin- formula 



P- 16,000 -70p 



the notation for which needs no definition, the 16,000 

 should be reduced to 12,000 if the average shearing unit 

 stress is used, as the maximum unit compression. The 

 formula then becomes 



P = 12,000 -52.5-. 



Substituting for / and r their values, 



, and r = .29J, 



where /the t hickness of the web. 

 The formula then is 



P = 12,000 -52.5 = 12,000 -257.5y . . (a) 



As the tension in the web below the neutral axis will 

 prevent huekling, the value of d should be ^, and as the 



tension in the upper half of the web helps to prevent buckling 

 the constant may be divided again by 2, the formula then 

 being 



P- 1 2,000 -G5j- ...... (10) 



