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P-Delta

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Sorry in advance if this gets too long:
I am using the RAM software to model a complex 8-story, cast-in-place
concrete, rigid frame building.  RAM gives the user the option to have it
calculate the p-delta effects on the members of the frames.  In order to
better understand how RAM calculates everything, I modeled a simple
rectangular, 3-bay, 2-story building with a rigid frame on each end in each
direction. When looking at individual frames in the small model, with only
the wind loads applied in one direction (also the innate mass load, which
would be the "P" of P-delta), the horizontal reactions at the base are
greater than the applied story forces for each direction.  I modeled one of
the end frames using Visual Analysis 4.0 and did not get the same results as
I did with RAM despite using the P-delta analysis.  The reactions added up
to the applied loads in VA.  I contacted RAM to see if I was interpreting
their program incorrectly and the representative stated that since I used
the P-delta calculation, the building is no longer in static equilibrium and
will have base horizontal reactions greater than the applied lateral loads.
My concrete textbook discusses this topic very well and does illustrate an
additional load of P*delta/length of column at each level, but it only shows
those as additional loads at each level, and never really discusses whether
or not the load is resisted at the foundation or if it is just an additional
load to determine additional member forces due to this second-order effect.
It is somewhat better that RAM is giving a greater reaction than the applied
load, because at least it will be more conservative (slightly), but I do not
fully grasp the idea of the loads not being in equilibrium.  I am hoping
someone can explain to me why (or if) the reactions are greater than the
applied loads when using a second-order analysis such as P-delta because it
would seem based on pure statics/dynamics, that the building is no longer
standing still.


John Peterson, E.I.T.
HCDA Engineering, Inc.
619 N. Cascade Ave., Ste 201
Colorado Springs, CO 80903
P: (719) 633-7784
F: (719)471-3173
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