huangyhg

cast-in-place vs. post-tensioned
when would you use a post-tensioned conc. floor slab instead of a cast-in-place slab?
predictable loads, repetitive floors in multi-story highrise, slab on grade in residential construction, lack of penetrations(holes) adjacent to columns, minimum floor to floor heights and lesser weights for foundation design.
it hard to be specific without knowing the project specifics, but in general:
post tensioned floors allow for longer spans with thinner slabs. typical span to depth ratios for two way slabs are in the order of:
l/30 to 33 for conventional reinforcing (aci 318 table 9.5c)
and
l/45 for pt slabs (per pt institute)
if your architect is looking for column spacings in the 30 foot range, i would definately consider post tensioning, as these spans would require fairly thick mildly reinforced slabs.
jmho
sounds like you'd like to use post tensioned just about everywhere!
in my neck of the woods, mid to high rise residential is usually cast in place flat plate.
what about multi-level storage building for cast in place vs. post tensioned? live loads about 3 times residential live loads.
no need for long open spaces in a storage building, would expect a reinforced slab with shorter spans to be cheaper.
me too.
mike mccann
mccann engineering
just to correct your terminology, posttensioned floors are cast in place. pretensioned elements are not.
pretensioned elements are usually refered to as planks not slabs.
composite slabs would also be worth investigating.
csd
lppe,
what is the question? is it:
"when would you use a post tensioned conc. floor slab instead of a reinforced concrete floor slab?"
or does it concern pre-tensioned elements.
we have specified pt slabs when cracking was a huge concern (ice skating rinks, skate parks, tennis courts, ect..). my understanding is that the tension really holds the cracks tightly shut.
we have also specified them when the soils conditions were questionable (fill, ect..) and any sizeable cracking/differential settlement was a huge concern.
a pt slab can reduce the overall height and weight of the building, and therefore will reduce the foundation, lateral system, and facade cost. but, i am sure there is a point where paying for a pt supplier/erector costs more than just laying rebar. so for a couple of stories in a rural area it may be cheaper to just use standard reinforced. when longer column free spans are needed pt is sometimes considered also. storage buildings often have the closest column spacings of any buildings i have ever seen, i have used slab on metal deck on these with small hss columns all over the place with steel strap bracing.

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