Tuesday, October 18, 2011

RE: Cantilever Balcony

Wow. 15 foot cantilever and a 10 foot back span? Sounds like a bad deal,
but if the Architect is really serious about this, what Harold and
Andrew said are good ideas.
If you do elect to PT this, suggest that you look very carefully at the
balancing moments---you will be sorely tempted to overbalance it to
offset tension stresses in the top. That leads to up-camber of the slab
and can be a real issue with drainage. In addition, there may be creep
effects that over time actually increase that up-camber and it might be
worse in 5-10 years!!

In my opinion, if you really do have to solve this as a structure, and
considering the exposure issues, you could consider the following:
1. Design the element as a conventional reinforced concrete slab, and
proportion the top reinforcing for ultimate strength.
2. Set the top surface of the slab at the door on out to be 1 inch
thinner than inside, and slope out to the edge at 1 to 2%--that may mean
a pretty thick section but it feels like it would be thick anyway given
the cantilever distance. Add a drain at the doorway!!!
3. As others have noted, use voids to minimize the DL; these could be
sealed round tubes, or foam blocks. Watch out because if they are too
wide, it is tough to get concrete under them and even if you do, it
might not be well-consolidated so I suggest more, smaller tubes (round)
spaced 1 tube diameter apart. Tell the contractor to fasten them to the
slab form so as not to let them float out of the pour!!!!
4. Set the strength at 28 days at 5000 psi ---use epoxy coated
steel---cover 2 inches.
5. Add PT--Proportion the prestress to be enough to keep top stresses
below about 6 Sqrt F'c under static DL but let the live load take the
stresses higher. Centroid at the cantilever end would be at center of
gravity of the section; centroid of tendon at the support up a little to
help control stresses (but not too high) then back down at the end of
the back span. This could take you to a point where you balance the dead
load alone at transfer, but not too much to overbalance. The chances of
a full balcony LL occurring are very small and since the concrete is
going to gain strength over time, and the first big party the owner
might have will be some time in the future, and you have accounted for
the strength demand using bonded reinforcing, it feels that this will
result in a reasonable amount of prestress to prevent overbalancing. Too
much prestress will also lead to potential cracking issues due to
restraint to shortening. Try a few different tendon heights at the
support (drape) and prestress levels to see what happens to stress and
deflection. One point to consider is that a 25-foot long tendon is tough
to monitor with regards to actual initial stress-the elongations (in
simple terms, PL/AE of the strand) are quite short and measurement is a
problem because the required elongation needed to impart the required
prestress considering losses due to anchor set and friction losses may
be small and tough to measure (we usually measure to the quarter inch)
so watch out during stressing that you do not get wild long elongations
and run the strand past yield.
6. Require the cantilever to remain shored (and make sure they tighten
up the shores after stressing) for a long time to mitigate early-age
7. Detail the slab to beam or column connections at the back span--there
will be uplift!

If the PT is too scary/expensive, maybe do 1 to 4 and add camber --then
use 6.

Just my 7 or 8 cents worth.

I have an exterior concrete balcony with a 15' cantilever and 10'
I expect that no matter how thick I make slab I will get deflection
and a= crack over the support. would it be best to detail a pour joint
over the = support or cut a crack control joint to control the crack?
joint could be = filled with caulk / flexible joint sealer.=20 =20
Also=2C project is located on a bluff about 1/2 mile away and 300' above
th= e ocean (Malibu). For slab exposed as finish=2C is anything
recommend fro = rebar protection? I was thinking 4000 psi conc with w/c
=3D 0.5 max=2C and= 2" cover. should I include something like xypex in
the mix=2C or increase= rebar cover=2C or am I distant enough from
ocean that salt will be a minor= issue?
Thanks in advance=2C
Paul Franceschi

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