# Replace a support

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1. Jan 22, 2017

### fonseh

1. The problem statement, all variables and given/known data
In i ) I was told that for the pinned support , we can assume to replace it with a roller , while for the cantilever , we can assume it to be supported by double roller .

For the cantilever , why we cant replace it with roller instead of double roller ? What 's the difference between double and single roller ?

2. Relevant equations

3. The attempt at a solution
In iv ) I was told that we can use roller guide , so from the figure , it's single roller , right ? So , IMO , the ii) was wrong , for the cantilever , we should always use
single roller

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2. Jan 25, 2017

### haruspex

Took me a while, but I think I get it.
The question posed is what happens to the rest of the system if the point A is somehow displaced "vertically".
If A is only a pin, the beam can accommodate the displacement by rotating about A. So the effect is the same as if A were just a roller in the first place.
If the joint at A is a cantilever then the beam cannot rotate about it. So displacing A vertically is like the support at A being a double roller.

3. Jan 25, 2017

### fonseh

Well , thanks for your explaination .But ,If A is only a pin, rotating about Av has the same effect as if A were just a roller in the first place???

Secondly , why placing a double roller at A will not allow rotation ?

4. Jan 25, 2017

### haruspex

No. A roller is like a pin that can also slide in one direction.
The actual support at A is a pin. We are looking at the consequences of movement of that support in the vertical direction.
Pin + 1 dimensional movement = roller.
Similarly,
Fixed (cantilever) + 1 dimensional movement = double roller.
That's what a double roller does. Each roller is constrained to slide in a track. Rotation would tug one of them out of the track.

See if http://www.engr.uky.edu/~gebland/CE 382/CE 382 Four Slides per Page/L4 - Support Reactions.pdf helps.