Say we have the following indeterminate beam:
We can make it determinate by adding a pin at B, and then adding back the bending moment at B which was destroyed by the pin:
My lecture handouts then go to say that we can consider the beam as two separate spans, and then use a compatibility condition to calculate $M_B$:
However, there are several things about this step that are not clear to me. How/why are we able to a split a multi-span beam into its individual spans? Is there any sort of rule that we need to follow when doing so?
And also, what is going on with the supports? We start with B being a pin over a roller support, and then we split into two spans, we get a roller support at B (looking at AB) and a normal pinned support at B (looking at BC).
Any clarity on how exactly and when we can consider a multi-span beam as separate spans would be very much appreciated.