shear stress before and after cut

In the diagram bottom of the image uploaded , it's clear that width of the section after and before the cut are the same , right , which is 25x3=75mm , why the author stated that the width of section after cut become 25mm ? I cant understand

• Look at the cut and the picture on the top right. There were 3 sections that the shear was acting on. After the cut, the other two sections are no longer solid at that point. There's no way for the shear to act on the cut sections, so it all must go through the centre.
– JMac
Dec 1 '16 at 13:48
• @JMac , can you draw a diagram ? i couldnt imagine and understand what you said Dec 1 '16 at 13:59

1 Answer

As @JMac pointed out in a comment, the cut in the beam causes the following cross-section to occur (exaggerating the size of the cut):

Originally, the shear stress can flow from the top to the bottom along the three webs. Therefore, the effective width to calculate the shear stress was equal to the sum of the three webs. After the cut, however, the shear can only flow through the central web. This therefore triples the maximum shear stress.

• So, the author only cut 2 sides only , but not the center ? Dec 1 '16 at 14:16
• @kelvinmacks, correct. As can be quite clearly seen in the question's drawing.
– Wasabi
Dec 1 '16 at 14:17