AISC 13th Ed. Figure 13-3(d) question (Uniform Force Method)

[jeg1976]

In Figure 13-3(d) from AISC 13th Ed., is there a moment due to Rb? The only moment shown is from Mb = Hb x eb. Isn't there a couple generated by Rb? Say we take moments about the w.p., isn't there a moment due to Rb equal to Rb times the distance between the w.p. and the cut/section?

 

[JoshPlumSE]

Rb is just the regular beam end reaction. It doesn't include any force from the gusset plate in this case.

So, there could be a moment due to Rb, but it is already taken care of in the regular beam design forces. Nothing gets added to it from the gusset plate or brace connection. The Mb moment is the ADDITIONAL moment that needs to be accounted for in the beam design due the the eccentricity of the Hb force coming from the gusset plate.

 

[DCBII]

Summing moments about the workpoint at the upper left corner of the beam gives:

Sum M = 0 = + M - (Rb)(x) where x is the distance to the point at which M is located.

Rearranging gives:

M = (Rb)(x)

Equilibrium is satisfied.