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Re: Post Tensioned Slab Attachment

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This is more than scary.  I've just finished a job for a cell carrier who
wanted to put a shelter up on a pt roof slab and drill a lot of holes for
electrical and mechanical supply.

If I understand correctly, your roof top units are to bear on the existing
pt slab.  Check through this carefully.  Typically in our area of the world,
the roof slab was the same design as the other slabs just for simplicity of
construction.  Roof loadings are typically much less than floor loadings, so
you may have the capacity in the slab to handle those loads.

If you run into problems though, I'd recommend a steel grid installed to
transfer the load to thet nearest columns.  I'd suggest this in any case
actually.

I disagree strongly with the contractor's thoughts that drilling through is
not an issue.  I don't know what your post-tensioning force is but I doubt
it's trivial.  Damaging the strand, or cutting the strand can send a missile
out the side of your building at the anchorage point, and who knows how many
buildings it can go through before it stops.

Non destructive testing is a good idea, but I always follow this up with
hand chipping to verify the test results.  By the time the drillers get to
site each location is confirmed, and there is little drilling left to do.

PT systems are great for architects because they can move columns off grid
lines and create more useful space.  But for M and E it is not as nice a
system.  Check to see if there isn't some utility shaft next to the elevator
core or at some other location.  Some buildings I've worked on had cast in
place beams to support a concrete slab that was not pt but standard
reinforced concrete.  These are usally much better places to drill through
if you have to create your chases.

With respect to suspending ceilings, I see this as less serious.  But I
would try to tighten up the spacing of the supports such that self-tapping
screws could be used.  Screws that are shorter than the cover provided in
the pt slab.

I have never cut a pt strand.  I don't intend on doing so.  Let me think on
how I would if I had to though.  Generally I try to make sure that there is
1.25" of cover around the strand when the slab is drilled.

regards,

Hans