Propping and formwork for suspended RC slab

[hazymat]

I'm planning my first concrete slab. Structural engineer has spec'd a 200mm slab with 2 layers of rebar that bends upwards when it meets the walls (2 walls are poured concrete, the other 2 are concrete blocks), with a minimum 100mm bearing onto all walls.

The area is 4.7x4.8 = 22.56m2, giving total mass of concrete 14,415Kg. I have 36 Acrow props spec'd for a 3m soffit height, these will be spaced at approx 940mm apart. I have greatly over-spec'd the propping.

However I want to know if my timber primaries and secondaries will be adequate. Here's my plan.

I have 22 lengths of C24 timber, 4x2 inch (47x95mm). The primaries will be laid across 6 rows of props edge to edge. These will be screwed to squares of ply which are screwed into the Acrow props. (i.e. 940mm apart)

Then I will lay the remaining 16 in the other direction at 400 centres.

For additional strength I will lay the timbers in an upright position (as opposed to flat). I don't have U channels / forks for the Acrows (i.e. for primaries) but I'll improvise with blocks of wood at either end of the lengths, and of course they will be tied by the secondaries which will be screwed into the primaries.

My structural engineer agreed with this plan over the phone, however he was vague and I'd like to double check by making my own calculations for the load across these timbers.

How do I calculate whether these 2x4s will be sufficient with these spacings, for the weight of concrete on top?

 

[rssteve]

I guess about 660kg per m2. Your props are slightly nearer than a meter so about 500 kg per m2. I guess you got ply ontop of the timber?

 

[hazymat]

I guess about 660kg per m2. Your props are slightly nearer than a meter so about 500 kg per m2. I guess you got ply ontop of the timber?

Thanks. Yes I'm just using basic cheap shuttering ply, 18mm.

When you say 660kg per sq m, do you mean that's the weight (i.e. pressure) of the concrete? Or do you mean that's what the timbers can cope with? I worked the concrete out at 638Kg per sq m, so roughly the same.

I just need to know if those timbers are sufficient, and if anyone has any bright ideas about how to make sure they stay upright (e.g. using blocks either side of the timber to stop them twisting)... hope that makes sense?

 

[Roger928]

The primaries would be like 6 x3 every 4 feet apart with the props spaced every 4 feet.
Then the 4 x 2 secondaries every 18 inches.
That will give you a really strong deck.
You want to use as few fixings as possible as it all has to be stripped down.
Most acrows have holes in the end plates so you can screw those to the primaries.