Second-order analysis
Second-order analysis
(OP)
When is it really required?
I keep hearing a lot about people performing second-analysis on all their structures these days. We use RAM Advanse and performing a second-order isn't much of bear, but when I have done it, the difference has almost always been negligible. Maybe, at most, it increase the moments in my columns by a percent or two. Now, the firm I work for mainly works on low-rise buildings.
Do second-order effects really become significant for larger buildings? Can anyone shed some light on the trends here??
I keep hearing a lot about people performing second-analysis on all their structures these days. We use RAM Advanse and performing a second-order isn't much of bear, but when I have done it, the difference has almost always been negligible. Maybe, at most, it increase the moments in my columns by a percent or two. Now, the firm I work for mainly works on low-rise buildings.
Do second-order effects really become significant for larger buildings? Can anyone shed some light on the trends here??






RE: Second-order analysis
RE: Second-order analysis
Taller buildings are examples.
Low-rise building, a little on the tall side, with non-trivial gravity loads and a loose drift limit.
Tall wind column that also has axial load.
I agree, though, that there are many, many counter-examples. I used to design lots of 5-10 story hospitals that had rigid frames for the LFRS and I seem to remember hte second-order amplifications being small, like 5-8% or less.
RE: Second-order analysis
You should also break your columns along the length to capture P-little delta effects.
RE: Second-order analysis
Theta = P*Delta / (V*h)
Where:
P = total applied vertical load (at or above the story level)
Delta = the elastic story drift between that floor level and the level below it.
V = The applied lateral force at that level.... NOT the sum of the shear forces from all the floors above that one.
h = story height.
ACI 2002 (section 10.11.4.2) has a similar definition that they use to distinguish between a SWAY frame and a non-sway frame. With a sway frame requiring a more sophisticated P-delta or second order analysis. To some extent 2nd order effects are still required to be considered for non-sway frames.... But, only for very slender members.
Personally, I like the ACI definition better, because their cutoff is a a 5% amplification rather than the 10% that ASCE is recommending.
RE: Second-order analysis
Can you explain this? I may be missing a point. Decrease in lateral load = decrease in Δ = decrease in P-Δ effects?
RE: Second-order analysis
RE: Second-order analysis