We have a bedroom with another bedroom above. We are renovating the ground floor and would like to raise the ceiling in the bedroom as it is only 2 m.
The current joists are 3550 mm long and have a dimension of 45x190 with a spacing of 400 mm on center.
My question is: can I cut the joists lengthwise to make them 45x120 and compensate for this by adding additional studs of 45x120 so the new spacing becomes 200 mm on center?
I would say that you CANNOT do that. The stiffness of a floor joist is cubically proportional to the height of the joist.
See the calculation principle below.
If you do a quick calculation on the deflection, you can also see that it becomes way too large, which can cause significant issues with bounce even if it may meet strength requirements.
You can get glulam beams 115x225 and lay them on edge c/c 600 mm and screw-glue chipboard on top of that. You can use steel, but it's probably more complicated and expensive.
You can get glulam beams 115x225 and lay them flat c/c 600 mm and screw-glue chipboard on top of that. You can use steel, but it's probably more complicated and expensive.
Thanks for the tip.
However, I would appreciate avoiding breaking up the floor or doing anything else on the upper floor. If I leave the existing chipboard on the upper floor and "just" glue when I lay the glulam beams, would that give a sufficient effect?
The particle board needs to be glued to the laminated beam to limit the flexing. 115 mm is otherwise too poor in height concerning the span. If you can build from below without breaking up the particle board, it is mostly a practical issue. It is, of course, possible to reinforce the existing beams with flat steel on both sides with through bolts, but the dimension should rather be around 10x120 mm, which would likely be quite problematic. The flexing is the major issue, not the strength.
Is there anyone who has another suggestion on what I can do to raise the ceiling?
Honestly, it's just about accepting that there's nothing you can do that even remotely justifies the cost and trouble for a single gained cm.
Lifting the entire house and raising the walls is the only sensible option.
Lift 400mm
Well theoretically it's possible. I did the calculation assuming:
C24 construction wood.
Service class 3 (on the safe side)
Safety class 3 (on the safe side)
Freely supported beam.
Live load 2 kN/m2 and density of 420 kg/m3 for wood.
Assuming no risk of buckling as the floor stabilizes the beams.
Design load: 0.627 kN/m (load combined)
Bending capacity: 76%
Shear capacity: 23%
Compression perpendicular to grain (at support): 11%
Initial deflection: 12.5 mm compared to L/300 = 11.8 i.e. a minor exceedance but no problem in reality.
Final deflection: 21 mm compared to L/150 = 24 mm.
theoretically it's not possible at all. The deflection requirement of 1.5 mm/1kN is not met. Deflection will be the biggest issue. The breaking point is almost never decisive in shallow wood floors. 21 mm long-term deflection is also too much unless you're talking about a ceiling.
I agree with Justus that screw-gluing might be the only practically possible economical solution, it does a lot as you basically get a series of equivalent H-beams.
However, I also agree with Mikael that it might be doing too much to gain only a few centimeters.
Is it possible to reinforce downwards in some way (in the ceiling of the downstairs)?
The house is an old summer studio, i.e., a summer cottage, built in 1929. It has, of course, been extended at various times with different added structures, making it difficult to lift the upper floor without needing to replace roof trusses, etc.
We are doing a major renovation on the ground floor, where we plan to remove a wall and open up the kitchen into a bedroom. In connection with this, we think that since we already have all the ceilings down and the existing floor structure accessible, we should do what we can to raise the lowest parts of the house.
I have started tearing down the insulation now and discovered that on top of the beams I see so far lies rough sheathing with air above it. I need to bring in the carpenter and possibly an engineer.
theoretically it's not possible at all. The deflection requirement of 1.5 mm/1kN is not met. The deflection will be the biggest problem. The breaking limit is almost never decisive in shallow wooden floor structures. A 21 mm long-term deflection is also too much unless we're talking about a roof.
Very true, I was far too quick. If you were to install noggins, how would you account for the stiffness? Are you thinking of a fictional cross-section (moment of inertia), or that the beam is no longer freely supported?
In the British annex to EN-1995, methods are provided to account for the effect of noggings. With a 22mm chipboard, you can certainly reduce the point load on the joist by ~50%. Regardless, the natural frequency must be above 8Hz to be able to apply the rules in EC.
In the British annex to EN-1995, there are methods to account for the effect of noggins. With a 22mm chipboard, you can certainly reduce the point load on the joist by ~50%.
Do you have any experience with how we do it in Sweden?
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