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# calculating reactions with a known upward force already in place watch

1. Hi Everyone.

I'm struggling with this question.

I can calculate reactions fine but there is an upward force in the middle and I just can't get my head around it.

I'm not asking for the answers just how to calculate it.

Question,
A beam AB is supported at the locations shown. Masses of 12, 8 and 6 tonnes are placed on the beam at the positions shown. If the length of the beam is 6 metre, find the reactions at the supports.

Diagram attached

Thanks in advance
Attached Files
2. reactions question.docx (11.4 KB, 35 views)
3. Take moments about A, paying close attention to the sign of the moment. Don't forget that reactions can be negative
4. (Original post by EwanWest)
Take moments about A, paying close attention to the sign of the moment. Don't forget that reactions can be negative
I'm still not getting my head around it sorry
5. (Original post by duckydarren00)
I'm still not getting my head around it sorry
What equations have you got so far?

You need to resolve forces vertically and then, as said above, take moments about one of the supports,
6. The moment caused by the upwards force in the middle will be the opposite sign to the ones caused by downwards force. Then take moments about A, with anti-clockwise being positive. The equation should take the following form
0 = |Moment caused by reaction at B| + |Moment caused by upward force| - |Moments caused by weights|

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