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RC frame - terrace

RC frame - terrace

RC frame - terrace

(OP)
I have to design a terrace that is 9 x 9,50 m. Slab is 200 mm. I have 2 questions for you guys and I hope for some help.

1.) There is no column in the middle of a slab (where two middle beams are crossed)so the dimension of beams are quite big. I made a quick calculation and I got 350/650 mm (I also checked deflections). I don't have a 'feeling' since I normally don't design beams with a span like this. Is this normal dimensions or should I expect more/less?

2). Columns are on spread footings. If I model columns as pinned at base I get pretty big forces in columns (moments), but in this case there are only axial forces acting on footings. But if I select fixed support at base, then I get huge footings since they have to take care of moments that are transfered from column to footing. I think I should model bases as pinned. Do you agree? Since in reality there will be some moment transfer, should I design footings bigger than calculated from axial force?


RE: RC frame - terrace

That all seems reasonable. You might give consideration to having only one of the interior beams and one way slab running perpendicular to that. You'd eliminate two beam/column moment connections and the crossover connection in the middle where the flexural bars will have to be layered. You'd also simplify the slab reinforcing even if it comes at the cost of a slightly thicker slab. Often, simplicity = economy in concrete. And you'd get nearly the same mid-structure deflection control out of a single 700/650 beam I'd think. A little less because you'd lose the flexural contribution of two columns I suppose.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: RC frame - terrace

Back to this... by treating it as a 2-way slab, you can get by with a 150 slab since you have a configuration that lends itself to a 2-way system (save 50mm concrete and weight). Alternatively, you can span the beams one way breaking the slab spans into 3 @ 2.8m+/- and using a 125 or 150 slab. over. For appearance I'd do the former and for economy and simplicity the latter. Also, the latter avoids congestion of rebar at the beam intersection.

As for design, I'd use the dead load moments for a fixed base column and use that unfactored moment at the base for factored DL+LL design.

Dik

RE: RC frame - terrace

You're right that in reality, the footings will act somewhere between fixed and pinned. Most designers I know default to a pinned connection and then make some consideration of fixity happening. What that consideration is varies.

A major factor there is what your subgrade looks like. A softer subgrade will rotate more and act more like the pinned base. Bedrock brings you closer to a fixed condition.

----
The name is a long story -- just call me Lo.

RE: RC frame - terrace

(OP)
Appreciate your help guys!

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