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Post Tensioning and Distributed Loads

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I am currently designing a 12 condominium structure which will utilize a 2
way post tensioned slab bearing on walls, exterior SMRFs and interior
columns (no band beams for interior spans).

In order to design the frame/wall/interior column elements, I of course need
an idea of the gravity load on them.  This is particularly important because
the structure needs to conform to zone 4 requirements, and I am concerned
about expected axial loads on the columns when they form plastic hinges at
the base level.

Now for the question - the post tensioning, due to the parabolic trajectory
of the tendons, can be modeled as a distributed upward load on the spans of
the slab, with downward reactions at the supports (the sum of the dist loads
and reactions being zero).

--> Should I be accounting for these loads and their effects on the
distribution of loads to the various vertical elements?

I know that the ultimate yield limit state of the slab will basically be the
same as a conventionally reinforced slab (i.e. the tendons will act as
"rebar"), but during regular service, won't the effective loading of the PT
cause the distribution of loads to be different from that of a
conventionally reinforced slab?

To investigate this, I obtained the effective upward and downward reaction
loads from a PT design run. I then modeled the structure on SAFE, and loaded
the slab with self weight, misc. DL, LL and the PT loads (checking to see
that the PT loads in fact added up to zero).

The net result was a 20% increase in loading on the interior columns when I
account for the PT loads as opposed to when I don't account for them.  This
increase is causing some concern for me, because when I check the structure
for maximum expected displacements (using a displacement spectra) these
columns WILL be yielding at the base, and a heavy axial load is not going to
help the performance of these columns.

Anyone have any thoughts on this matter?  Any insights would be much
appreciated.


T. Eric Gillham PE
GK2 Inc.
PO Box 3207  Agana, Guam  96932
Email - gk2(--nospam--at)kuentos.guam.net
Ph:  (671) 477-9224
Fax: (671) 477-3456