Why is the shear capacity of a steel member higher in the minor axis?

Question:

Why is the shear capacity of a steel member higher in the minor axis?

I have only applied a point load in the middle of a simply supported beam ("I" section) member and find that the shear capacity is higher in the minor axis than it is in the major axis. Why is that the case?

Answer:

The reason the shear capacity is higher in the minor axis for some members is due to the calculated shear area (A_v). When an "I" beam is loaded in the major axis you have the web resisting the majority of the shear force as opposed to two flanges when loaded in the minor axis.

Example

In this example we will look at a UC203x203x46 section. This section has the following properties which are used to calculate the shear areas.

The key variables are:

A - Area of section
b - width
h - height 
t_w - web thickness
t_f - flange thickness
r - root radius

 

Major axis shear area

 

Minor axis shear area

 

Summary

As can be seen from the two screen shots above, the shear area is evidently higher in the minor axis for this section. UC sections usually have comparatively thicker flanges than UB sections, therefore this behaviour is greater in UC sections as two flanges with an overall larger area will provide more resistance to vertical shear than a single beam web.