Understanding Reaction Forces: Problem 6.8 & 6.82

[fayan77]

I am having trouble recognizing reaction forces. for example, problem 6.8 on pin C there is no y component, the argument there was because there is no vertical member attached to C. That is understandable, but if we follow the same logic then there should not be an x component on pin A in problem 6.82

 

[fayan77]

Sorry, ABCD should be one piece.

 

[CWatters]

Not really my field so I might be wrong but...

I don't think you can determine the vertical components at A and C in the first drawing. I think some assumptions would have to be made. For example if the frame was made slightly too short in the y direction wouldn't it tend to pull A and C together? Likewise push them apart if made too tall. So I don't see how you can work out the share of the vertical load each point carries. Perhaps there is an approach to solving this that I'm not aware of.

Perhaps it's resolved by convention? Eg if no vertical beam at C then it must be designed so that C carries no vertical load?

 

[CWatters]

In the first drawing there is a joint in the middle of the lower horizontal beam. If that is a pin does that imply no vertical force can be transmitted by that beam to C?

 

[CWatters]

Sorry, ABCD should be one piece.

So the frame cannot bend at B and can transmit a horizontal component to A.

However I think the vertical components at A and B are indeterminate.

 

[CWatters]

In the first drawing there is a joint in the middle of the lower horizontal beam. If that is a pin does that imply no vertical force can be transmitted by that beam to C?

More I think about it the more I think that this is the answer. Try just thinking about a single element pinned at each end either horizontal or at an angle...