94 CAMBRIA STEEL. 



LIMITING SPANS AND MAXIMUM LOADS OF 



I-BEAMS AND CHANNELS DUE TO 



CRIPPLING OF THE WEB. 



I-Beams and Channels, when used as beams for very short spans 

 in which the ratio of length of span to depth of beam is small, 

 should be examined for safe strength of the web considered as a 

 column, subjected to crippling due to the shearing strains. 



The Tables of Safe Loads of Beams and Channels are computed 

 with regard to the safe unit stresses due to flexure, and, with one 

 or two exceptions, as indicated by dotted lines and accompanying 

 foot-notes, the lengths of spans tabulated are -such that the limita- 

 tion due to web crippling does not appear. The shearing stresses 

 acting in the web of a beam may be considered to consist of two 

 stresses of equal intensity acting at right angles to each other, and 

 at angles of 45 degrees with the neutral axis. The intensity of each 

 of these stresses is equal to the intensity of the vertical shear, 

 which is a maximum at the points of support for uniform loading, 

 and uniform throughout from the point of loading to the supports 

 for a superimposed concentrated load at the center. 



The vertical shears for different systems of loading may be ob- 

 tained by the use of moments in the usual way, and these are 

 given for various cases on pages 162 to 165 inclusive. 



The shearing stresses which act at angles of 45 degrees with 

 the neutral axis are equivalent to compressive and tensile forces, 

 and the former will tend to buckle the web, which should there- 

 fore be figured as composed of a series of columns of a length 

 equal to its diagonal depth. 



