Wind Load - Which method is correct

[dkrpink]

Per the attached sketch and using basic statics you obtain different answers for the moment about A for an open structure.

The sketch shows an open structure analyzed using ASCE 7-05.

The method on the left uses a distributed load and then transfers the moment.

The method on the right breaks up the load into components and then analyzes the structure.

Both seem to meet basic static requirements but which is correct?

I lean toward the uniform load and then transfer the moment because ASCE 7-05 applies the load as a uniform load not as a force on projected vertical and horizontal surfaces.

 

[PMR06]

Either would give you the correct answer, but the one on the left incorrectly assumed Ma=Mb.

At B you have Mb, but also the vertical and horiz components. The vertical does not add moment, but the horizontal does. 91800 + (1594*15) = 115710, which basically matches the answer on the right.

 

[Lion06]

If you use the sketch on the left, you still need to break it into components. As PMR06 noted, summation of moments about A does not equal sum of moments about B, because the line AB is not parallel to the load. As a result, the perpendicular distance between the load and points A and B (or the moment arm for each case) is not equal.

I also am having a hard time with the sketch. I think it's the dots - that symbolizes a pin to me. If the dots are pins, then the I hope the FBD is not complete.

 

[dkrpink]

Thanks PMR06
I obviously need to ensure my FBD is complete before I run my numbers, and confuse myself.

Lion06
The sketch was intended to indicated fixed connections at A and B and the dot was for locating the point of interest.