Tuesday, October 18, 2011

RE: Cantilever Balcony

Very good suggestions.  I was happy that an engineer / contractor contributed to this. 
I saw that you were with Madison Concrete, and I saw on your web site that Madision has experience with SCC.  Would SCC be appropriate for something like this?  I know that 5,000 psi in 28 days would be very easy for SCC. 

Regards, Harold Sprague
> Subject: RE: Cantilever Balcony
> Date: Tue, 18 Oct 2011 08:30:29 -0400
> From: rstone@madisonconcrete.com
> To: seaint@seaint.org
> 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
> deflections.
> 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'
> backspan.=
> 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?
> =20
> Thanks in advance=2C
> =20
> Paul Franceschi
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