Subject: RE: notional load method of second-order analysis
From: Christopher Wright <chrisw(--nospam--at)skypoint.com>
Date: Fri, 26 Jan 01 10:02:11 -0600
>Some software programs "do" second-order analysis. Some may even do it
>correctly. Second-order analysis involves both the P-big Delta effects
>(frame deformations) and P-little delta effects (member curvatures). Since
>computer programs only track the nodes, the P-little delta effects may be
>lost or neglected. I've "tricked" software into considering P-little delta
>by placing an extra node at the mid-height of the columns in the model.
I think you need to trigger what we dark-siders call stress stiffening
(actually softening), even if you use more nodes for a member. Some
software also can update node coordinates after an ordinary static run.
Then you can make a second run which will provide the required P-delta
effects so you can apply wind and seismic loading to a deformed shape.
This skirts a lot of non-linear analysis, and is probably close enough
for the usual engineering purposes.
One thing I am curious about--how SE's deal with the P-delta analysis
given the incremental application of dead loading in multi-story
construction. Frame analysis usually applies dead loading as though it
goes on in one bunch after the frame is completed. In reality the thing
is built story by story with columns plumbed irrespective of how columns
in lower floors deform under load. Same for beams to a degree. So each
new load increment is applied to an already-deformed structure. I'd guess
there's not much effect on beam loads, but it seems like there would be
an effect on column loading.
Then when it comes time to add interior appointments and equipment, all
this stuff is added to an initially deformed structure, not an initially
undeformed structure as we assume. Moreover the initial deformation isn't
the same as if the structure dead weight were applied suddenly. I don't
get into this a whole lot because service loading dominates most of my
stuff, but is this really any big deal? Did someone actually go through
this for all the skyscrapers built before FEA got so big?
Christopher Wright P.E. |"They couldn't hit an elephant from
chrisw(--nospam--at)skypoint.com | this distance" (last words of Gen.
___________________________| John Sedgwick, Spotsylvania 1864)