Consider an I section beam shown below, which is unsymmetric about the x-y plane.
I apply a load P, at the centroid of this section. The textbook I follow, says that the member will twist apart from bending. I don't understand exactly how did we conclude it is twisting?
To better understand my point consider that I cut the beam through a section to expose its intermediate c/s as shown in (a). If twisting is to occur there must be forces developed in the cross section such that they form an internal resistive twisting moment about some axis. This internal twisting moment, should be equal and opposite to the twisting moment produced by P, about the same axis.
You know like how we get an internal resistive torque in a shaft and the applied torque is opposite to it (as shown in b)