Consider a beam built up as shown here.  To find the spacing of nails 1, we separate the pieces at A, but then we must cut at B also.  Note that there are no nails at B.  We calculate Q from the area separated, then after finding the shear flow, q, we divide by 2 to get the shear flow over one of the cuts since only one cut has connectors.

 

 

 

 

For the nails at 2, we would cut at A and B as shown to separate an area to get Q and again find q and divide by 2.

 

 

 

 

 

 

 

 

The author of the text, in section 5, also discusses the calculation of shear stress in the flanges of thin walled members.  You should look at this, but it is not of tremendous importance to us in this course.  In more advanced courses, the stress in the flange is needed to find the shear center as discussed in section 6 when the cross section does not have a vertical axis of symmetry.  We will not be covering that topic in this course.

Lecture #20, page 2                 Back