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Steel Frame Joist Height?


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#1 Rosanbo

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Posted 31 August 2015 - 07:39 PM

I am wondering if having a steel frame means you can reduce the thickness of the floor?

On a timber joist construction you will use joists 200mm tall, plus 20mm on the bottom for the groundfloor ceiling plasterboard, plus 25mm on the top of the joist for the floorboards total = 245mm

If it was an internal steelframe, and you had a solid floor on the first floor, by using concrete blocks as your floor, would this mean the thickness of the floor could be substantially reduced? i.e. by 50%

The span of the room is 6 meters.

#2 tonyshouse

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Posted 31 August 2015 - 08:33 PM

Unlikely to get below floor 150, ceiling 20 (but more likely 60 ish), floor screeds likely needed at 50mm

#3 tony51

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Posted 01 September 2015 - 08:49 PM

Totally impractical: forget it.

#4 temp

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Posted 01 September 2015 - 09:34 PM

Using a mix of steel UB and timber might work? Perhaps a steel could reduce the span of the timber joists to 3m meaning they don't have to be so deep?

I think the floor on top only has to be 18mm thick although I prefer 22mm.

What do you have to do about Part E these days? Last time I looked you either had to use a standard floor design (aka "Robust detail") or have a sound test done for anything non-standard.

#5 structuraldesigner

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Posted 03 September 2015 - 06:58 PM

Steels can greatly reduce your floor depth.

By the way, for a 6m span you're going to need a lot deeper than 200mm joists, and anything over about 200-250mm gets expensive. You might even have to opt for timber I joists or posi joists which are 300mm x 97 wide or more.

Instead of timbers you could use steel box section of, say, 80mm deep. You can screw your timber floor directly onto it with self tapping screws. To the underside you would need plasterboard for fire protection. Overall this would be 120-130 deep and could easily span 6m.

2kN/m^2 x 0.4 x 6m = 4.8kN total load.
Factored, x 1.5 = 7.2kN
Moment = WL/8 = 7.2 x 6 / 8 = 5.4kNm

So an 80x50 RHS with 3mm wall thickness should do it, as long as the deflection isn't too high.

Another way would be to reduce the span in half by introducing a steel beam to the middle, creating two spans of around 3m. You could perhaps get away with a 203 deep Universal Beam (UB) but that depends on the span.