Expert: Veto Varma - 3/15/2007

Question
sir,
could you kindly explain to me the difference between stong axis,weak axis,major axis,minor axis of a beam/column and why are they termed so?

Answer
Hello Dency,

For any cross section there are inertia about two axis and one polar moment of inertia. We will talk about these two inertia. Lets say these are Ix and Iy. Unless it is a solid square section usually one of these value is considerably more then other. Lets say Ix >>Iy. So if the load causes bending about x axis it will be said that it is bending about strong axis.

Usually this is important in column design where you have lateral loads (wind). You have to design that column so as to use the strong axis bending. Also consider lateral support for column to consider that bending. Lateral support reduces effective length and increases bending capacity. This is one way of reducing weak axis bending.

For beam design usually the top of the beam is laterally restrained because of decking or bracings hence it may not pose problem for the week axis bending, but it is a must check in crane girder design where you have to consider horizontal loading on beam/girder.

Hope this helps.

Regards
Veto