Should MA be included in calculating moment about A?

[goldfish9776]

Homework Statement

Lets say i have a structure like this , at A and B are fixed support ( it resists moment about , force in x and y direction) I want to find the moment about the A , should I include the MA as well in my calculation or no ?

Homework Equations

The Attempt at a Solution

moment about A = MB +MA + W(L/2) -RBy(L) = 0 ?
or moment about A = MB + W(L/2) -RBy(L) = 0 ?

 

[PhanthomJay]

Suppose this beam was a simple cantilever fixed at A but free at the other end. Same applied loading. Would you include the moment at A when summing moments about A in your moment equilibrium equation?

 

[goldfish9776]

Suppose this beam was a simple cantilever fixed at A but free at the other end. Same applied loading. Would you include the moment at A when summing moments about A in your moment equilibrium equation?

So, we don't have to consider MA when calculate the moment abour A?

 

[PhanthomJay]

How did you come up with that conclusion?

 

[goldfish9776]

Based on your tone

 

[PhanthomJay]

Based on your tone

Oh, bad tone. Try the simple cantilever case, say a beam A-B of length L fixed at A and with a load P applied at the free end B. Now sum moments about A = 0. Does MA show up in that equation or not?

 

[goldfish9776]

Oh, bad tone. Try the simple cantilever case, say a beam A-B of length L fixed at A and with a load P applied at the free end B. Now sum moments about A = 0. Does MA show up in that equation or not?

no, how is it relate to the question i asked?

 

[PhanthomJay]

In your problem, I cannot tell you whether or not to include MA in your equation when summing moments about A , because that is against forum rules . BTW, the problem is statically indeterminate. But anyway, as a hint, if you consider a simple cantilever of length L fixed at end A with the unknown moment M_A at that end and with a load P applied at the free end, then summing moments about A for equilibrium yields M_A - PL = 0. Thus M_A = PL as you would expect. The moment of any couple about any point is the couple itself. So back to your original problem about whether to include M_A in your equation...what say you now?

 

[goldfish9776]

In your problem, I cannot tell you whether or not to include MA in your equation when summing moments about A , because that is against forum rules . BTW, the problem is statically indeterminate. But anyway, as a hint, if you consider a simple cantilever of length L fixed at end A with the unknown moment M_A at that end and with a load P applied at the free end, then summing moments about A for equilibrium yields M_A - PL = 0. Thus M_A = PL as you would expect. The moment of any couple about any point is the couple itself. So back to your original problem about whether to include M_A in your equation...what say you now?

yes , i should include MA when i want to calculate the moment about A

 

[PhanthomJay]

yes, correct.