The different support reactions at B and C

I know that pins prevent horizontal and vertical translation and that rollers prevent only the vertical one. Therefore, what I do not understand here is that while the support at B reacts with two forces C support has only one, even though both look like pins to me.Where am I going wrong here?

(I could not find anything on the site about whether I can upload images this way. I hope there are no copyright issues related to my post because I do not own the images. If there is, please notify me and I will immediately take them.)


2 Answers 2


It's simple: while B is a pinned support, C is a roller. You can see that in the figure, where B is drawn clearly bolted to the "ground" while C isn't. In fact, the contact between C and the "wall" seems to be a wheel of some sort.

Rollers don't have to "prevent only the vertical [translation]". They have to prevent translation in one direction while allowing it in another. The most common case of a roller is resting on a horizontal surface, therefore preventing vertical motion but allowing horizontal motion. In this case, however, the roller rests on a vertical surface, therefore horizontal movement is restricted but vertical movement is free.


The support at C looks like a roller.

The reaction from roller is always perpendicular to the surface on which it rests. Hence it will be from left to right, since the surface is vertical here. So the figure shows the correct direction of the reaction, and yes there is only one reaction at C.


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