StrEng007
Structural
- Aug 22, 2014
- 537
I was getting different answers from my FEA and spreadsheets, so I decided to refresh some hand calculation concepts to check my work.
When doing the moment distribution method to a moment frame with lateral loading (or unequal gravity loading to cause sway), we first do a moment distribution assuming a non-sway frame. Next, we do a moment distribution assuming an applied moment that will induce the lateral drift on the frame. For equal column legs with the same base fixity, these fixed end moments are equivalent. When the bases are fixed we take this as M = 6EI∆/L²
What doesn't make sense to me, is that while performing this part of the moment distribution, we assume there is no moment at the cross-member. Why is that?
See the image below:
Why are the fixed end moments for BC and CB assumed to be non-existent? Wouldn't there be an equivalent moment from BA and CD, respectively? I'm not sure I quite understand this part of the moment distribution method (as it applies to the sway portion of the exercise).
I'm good with the rest of the procedure and can get the same answers by hand as the analysis program.
When doing the moment distribution method to a moment frame with lateral loading (or unequal gravity loading to cause sway), we first do a moment distribution assuming a non-sway frame. Next, we do a moment distribution assuming an applied moment that will induce the lateral drift on the frame. For equal column legs with the same base fixity, these fixed end moments are equivalent. When the bases are fixed we take this as M = 6EI∆/L²
What doesn't make sense to me, is that while performing this part of the moment distribution, we assume there is no moment at the cross-member. Why is that?
See the image below:

Why are the fixed end moments for BC and CB assumed to be non-existent? Wouldn't there be an equivalent moment from BA and CD, respectively? I'm not sure I quite understand this part of the moment distribution method (as it applies to the sway portion of the exercise).
I'm good with the rest of the procedure and can get the same answers by hand as the analysis program.