Will this hold?

[Maximilian Wicen]

Rudimentary sketch of a very low sleeping alcove.
140cm bed + two people + div (under 100kg). Total maybe 3000-4000N load spread over 75% of the area.

What type of joists should I use underneath? Will 120x45mm joists hold? The picture might be misleading as the joists used there are 170mm, but I had hoped to get away with 120.

https://i.imgur.com/2HeHP1K.jpeg
https://i.imgur.com/wpltNuW.jpeg

All the best and happy continuation /M

 

[KnockOnWood]

Rudimentary sketch of a very low sleeping alcove.

How high is the alcove's floor from the subfloor?
What is underneath, i.e., how is the joist structure below built?

 

[Maximilian Wicen]

How high is the "alcove's" floor from the subfloor?
What is underneath, i.e., how is the joist system constructed below?

Well, it's barely an alcove, more like a raised platform. It stands on solid concrete. It's about 50 cm above the floor.
The only strength issue is really the joists, whether they are dimensioned OK.
120 x 45mm or if larger joists are needed for a span of 2.8m x 2.4m

 

[petterovski]

It should hold with 120, but it might wobble a bit. But if you have a concrete floor underneath, can't you just add some extra "legs" to the construction?

 

[Maximilian Wicen]

It should hold with 120, but it might flex a bit. But if you have a concrete floor underneath, you could just add a few extra "legs" to the construction?

True, I could do that (extra legs). Do you come from a construction background or are you just making an educated guess?

 

[petterovski]

I would definitely put an additional leg in the middle of the two support beams. Then, if you feel that the floor joists themselves are flexing (try standing on them before laying the floor), it should be sufficient to screw a leg on the side of them in the middle of the span. I am training to become a civil engineer, but this is a bit of an educated guess based on the fact that I've done some renovations.

 

[scorp1on]

Is the span of the beams 2.4 or 2.8 m?

 

[Maximilian Wicen]

Is the span of the beams 2.4 or 2.8 m?

2.4m.
The load-bearing beams are 2.8m.

 

[Plupp 184001]

With 120x45, you get a deflection of 10 mm with C24 at 100 kg/m2, which is approximately 300 kg on the surface roughly. With 145x45 C24, you get 5 mm.

However, the simplest solution is a support leg in the middle of the span, then you can easily use 120x45.

Best regards, Joakim