Moments -- sign convention in beam

[PhanthomJay]

is my working of RB(5) -30.71 - 15(5)(2.5) = 0 incorrect ?

It is correct. Whenever you are summing moments about a point to find reaction forces, you can assume applied clockwise moments, and applied clockwise moments from forces, all as plus, or all as minus, and you get the same answer. I suggest however to assume clockwise moments as plus.

For moment about A , i have M(A) -RB(5) +30.71 + 15(5)(2.5) = 0 , thus , moment about A = RB(5) -30.71 - 15(5)(2.5) = 0 , RB = 43.64 up

I did in this way because for the left end of span , clockwise moment is positive ( cause the beam to bend upwards) ,

Forget about upwards or downwards in this step when determining reaction forces. Just use cw as plus and ccw as minus.

similarly , when finding moment about B , I assume anticlockwise as positive .(because anticlockwise moment causing the right span to bend upwards) Moment = M(B) -RA(5) + 30.71-15(5)(2.5) = 0 , 31.36 up

Or should I be consistent , keeping clockwise moment as positive and anticlockwise as negative or vice versa?

Once again, when determining reaction forces, keep it a simple set of rules: Consider clockwise moments, whether an applied moment couple or an applied 'force times distance moment about a point', as PLUS. And counterclockwise as MINUS.

I will try to sum up the signage rules later with a sketch, when I get a moment.

 

[fonseh]

It is correct. Whenever you are summing moments about a point to find reaction forces, you can assume applied clockwise moments, and applied clockwise moments from forces, all as plus, or all as minus, and you get the same answer. I suggest however to assume clockwise moments as plus. Forget about upwards or downwards in this step when determining reaction forces. Just use cw as plus and ccw as minus.Once again, when determining reaction forces, keep it a simple set of rules: Consider clockwise moments, whether an applied moment couple or an applied 'force times distance moment about a point', as PLUS. And counterclockwise as MINUS.

I will try to sum up the signage rules later with a sketch, when I get a moment.

https://www.physicsforums.com/threads/pinned-end-supported-span.912159/https://www.physicsforums.com/threads/moment-in-beam.912156/

Can you help me in this 2 threads ? It seems like you are the only one who is familiar with the structural engineering problem