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Re: Post Tensioning and Distributed Loads

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The column reactions and moments from the prestress case should equal the
secondary (parasitic) moments of prestress. In a normal design with tendons
profiled logically, these reactions and moments should be relatively small
compared to the applied load effects.  They should be taken into account in
your estimation of the column reactions etc.

I would make sure that you have modelled the prestress load case correctly
in SAFE. Also, compare the figures with the 2D prestressed design program,
RAPT,  that you use. It gives the correct figures in each direction and
provides figures which should be comparable to the SAFE results if
everything is modelled correctly.


At 08:06  17/09/99 +1000, you wrote:
>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
>
>

Regards  Gil Brock
Prestressed Concrete Design Consultants Pty. Ltd.
5 Cameron Street Beenleigh Qld 4207 Australia
Ph +61 7 3807 8022		Fax +61 7 3807 8422
email:	gil(--nospam--at)rapt.pcdc.com.au
webpage:	http://www.home.aone.net.au/rapt.pcdc/