Note: Both diagrams are 2d(they might look 3d because I drew them manually.)
I have a prismatic frame that looks like this:
I made a section on the half of the first pillar(the red line represents the section), to study the internal forces in it(Normal force, shear force and moment.) Here is a free body diagram at this section:
A is a fixed support so I calculated its reactions as follows:
- $X_A= 100 KN$
- $Y_A= 200 KN$
- $M_A= 300 KN$
I have also calculated the normal and the shear force at B, but I have a problem calculating the moment at B and here is why:
now let's apply the equilibrium equation of the moments at B, as:
As you see there are the following moments:
- $X_A\times 3$
- and at last $M_A$
I have done it like this:
$$ 3X_A + M_B - M_A = 0 $$ I just need to know what to do with $M_A$ should I multiply it with the distance $AB$ or just leave it.(I tried both cases and I think both are wrong). now can somebody till me what to do with the $M_A$?
I guess the diagrams are somehow misleading,(I am not good at drawing!). The axis of moment B is the straight line perpendicular to the plane and passing through B and the same for A. Now please look at the first picture: imagine there is a force just above the red section, this force is causing the bending moment in B and the reaction moment in A. I want to calculate the internal moment B, and that's why I need the sum of moments at B.