Concrete cantilever

IMO most people in here is being way over-concerned with this. It is a table at the end of the day and you can probably use really high performance concrete in it; high cement content, some GGBS in there too (adds strength and makes it a nice white colour). Using a really high performance mix will help a lot as it can reduce cracking and essentially makes your lever arm bigger.

To improve cracking resistance, you want to have lots of smaller diameter bars, rather than a few big ones, but don't make the minimum clear spacing. With this in mind here is a back of the envelope calc...

  • 1.655m + 100mm (conservatively assuming support 200mm) = 1.755 cantilever

  • Ignoring self weight but assume 2 fat people sit on the end: P=2kN

  • Med=3.51kN

  • As=M/(fyd z)

  • z = 0.95d

  • d = 100-20-4-4/2=74mm (assume using 4mm bars and have the outer layer of bars going in the direction perpendicular to the direction of the cantilever as this will help cracking... having the main bars on the inside) (Also assume allowing 20mm clear cover to your first bar... in practice, if youre using a sand-only concrete, you can make this a bit smaller, to say 15mm and improve cracking resistance, but past that, it may begin to hinder you again.

  • z= 70.3mm

  • As = 3x106 /(434 x 70.3) = 99mm2

  • Using 4mm bars you need 99/12.5= 8 bars to limit cracking to 0.3mm

  • Another option instead of steel bars is to use slightly bigger fibreglass or glass fibre bars and then use chicken wire everywhere else. Chicken wire is great at controlling cracking as it has really small spacing.

HOWEVER, you can get way, way more than that in - it looks around a meter wide or more?. I would put 4mm rebar at 35c/c in both directions.

That said, a few things to bear in mind...

  • You will get cracking, it is fundamentally unavoidable, but you can make them smaller.

  • polishing can help make cracks look a bit better; waxing and other finishes can make them so that you can't feel them.

  • Consider a crack inducer. This may feel counter intuitive, but if you place something and make it architecturally deliberate, you will essentially be able to conceal the crack. This table will crack on the top surface where it meets the support. if you can put a metal strip flush with the top of the concrete you can potentially hide the crack under this strip but you will have incorporated this into your design.

  • Don't lap any bars at the support - this can make cracking worse.

  • Make sure you cure it really well. keep it nice and hydrated for a couple of weeks at least, if not a month, to get extra performance. This is particularly important as weak concrete can crack and spall more easily.

  • vibrate it or expect bubbles.

  • http://www.coastconcrete.co.uk/expectations/

/r/AskEngineers Thread