Suggestions for thin joist for loft

[vectrex]

Reviving the thread... I see that, among others, member Witten seems to be calculating things. I myself built a sleeping loft on minimal 45x70 cc30 with screw-glued flooring. Rigid, but only a span of 4m with a supporting interior wall underneath (i.e. practically only a 2m span).

Now I am building something completely different that I've been able to ponder for several years - the sauna. I have set out to build the thinnest sauna benches in the kingdom. I finally, after significantly more advanced thoughts with standing strips of k-plyfa, metal profiles, etc., landed on the following:

Span of 1.8m with a 45x45 screw-glued frame, 2 transverse noggings at each third of the length, aspen boards 21x90 screw-glued on top and at the front. Finally, and perhaps most importantly - screw-glued perforated straps 1x20mm under each 45x45.

Spontaneously, this should hold for standing and sitting without excessive deflection, what do you think?

Hard to see how the metal straps underneath would be of much use considering the significantly greater thermal expansion of metal compared to wood.

Additionally, I am skeptical about how well the glued joints will hold over time in such an environment (humid and occasionally very hot).

Even apart from the above, I think the thin perforated strap has a very marginal stiffening effect.

 

[Ingenjörssnickarn]

Hard to see that the metal straps underneath would do much good considering the significantly greater thermal expansion of metal compared to wood.

Additionally, I am skeptical about how well the glue joints will hold up over time in such an environment (humid and sometimes very hot).

Even apart from the above, I believe the thin perforated tape has a very marginal stiffening effect.

I believe in the idea, we'll see if it works in practice! Wood glue shouldn't be a problem, once dried it's a cellulose bond and the sauna is a dry space. Regarding thermal expansion, I appreciate the tip, but it shouldn't be decisive here. An increase of 60 degrees over the span of 1800mm gives 60×1,800×0.000012 = 1.3mm. Without the perforated straps, 45x45 would become like a banana and also reach the breaking point under load.

 

[Ingenjörssnickarn]

I believe in the idea, remains to be seen if it works in practice! Wood glue shouldn't be a problem, once dried it's a cellulose bond and the sauna is a dry space. Regarding thermal expansion, thanks for the tip; however, it shouldn't be decisive here. An increase of 60 degrees over a span of 1800mm gives 60×1,800×0.000012 = 1.3mm. Without the perforated straps, 45x45 would become like a banana and also reach the breaking point under load.

I can add myself that my idea is far from approved according to Svenskt Trä's dimensioning guide, not even with 45x120 calculated as a floor joist with 10 kg/m2 dead weight.

If I calculate the deflection for a 1.8m long C24 beam 45x45 between two supports with a load of 100kg, it becomes 36mm. Significantly larger than the normally acceptable L/300 = 6mm. However, the aspen planks 21x90 and any underlying tension reinforcement with perforated straps are not included. This will be interesting!

https://www.byggahus.se/forum/threa...nsregel-innan-det-gar-av.229334/#post-1934197

 

[Snickarkirre]

I don't really understand the point with that "laven"? If I understood your text correctly, you're supposed to have a 21x90 standing at the front, then it looks like any sauna bench. Those will definitely hold even without glue and hole bands. That's basically the way sauna benches are usually built. The bench in my sauna is a frame screwed into the walls of 21x90 (it doesn't matter there whether it's 21x90 or 45x45 since it's attached to the wall anyway) and then one at the front too, and there are three cross-sectional 21x90s evenly distributed over the length, which is 1980mm. We've had three guys sitting simultaneously, the lightest of whom weighs in at 95kg, and it held without a problem

 

[Ingenjörssnickarn]

I don't really see the point with that bench? If I understood your text correctly, you're going to have a 21x90 standing at the front, then it looks like any other sauna bench. Those are guaranteed to hold even without glue and perforated strips. That's essentially how benches are usually built. The bench in my sauna is a frame screwed into the walls with 21x90 (doesn't matter there if it's 21x90 or 45x45 since it's attached to the wall anyway) and then one at the front too and three crosswise 21x90s evenly distributed along the length, which is 1980mm. We've had three guys sitting on it simultaneously, of which the lightest weighs in at 95kg, and it held without any problem

Thanks for the quick response! Just like in your example, however, I want to try with the thinnest profile possible for the sake of appearance, i.e., 45mm. I might have written it unclearly with a standing bench board at the front - it will also be split to a height of 45mm. That makes it a particularly flimsy "support beam" at the front, and the lower bench doesn't have a back wall to attach to.

 

[Snickarkirre]

Alright, I'm in! Then it gets a slightly different style no exactly, that's how it is on my lower shelf too, but it's 21x90 both front and back, so that's a different thing. But your idea surely works. Couldn't you bend a metal angle like 40x40 and screw/glue it into the 45x45, so you have both at the front edge and underneath and then hide it with the standing 21x45 at the front edge? I think that would make it even stronger

 

[Ingenjörssnickarn]

Okay, then I'm on board! Then it gets a slightly different style no exactly like that on my lower tier as well, but it's 21x90 both front and back, so it's a different thing. But your idea will surely work. Wouldn't it be possible to bend a metal angle bracket like 40x40 and screw-glue it to the 45x45 so you get both the front and underside and then cover it with the standing 21x45 at the front? I think it should become even stronger

I've looked at solutions with angle brackets in the inner corners of the frame, but I don't think they're needed. Screw-gluing should keep the corners at right angles, the main challenge should be to reduce the calculated deflection (36mm with 100kg on a bare 45x45 beam, length 1.8m).

How much the overlaying screwed aspen boards reduce the deflection is unclear to me. Likewise, how much the underlying perforated strips can contribute...

 

[Bernieberg]

Beyond the long sides of the frame, you can attach a 45x45 underneath each or at least some lav boards (lengthwise). If they are also glued to ensure good collaboration, they will form a fairly stiff T-profile.

 

[Ingenjörssnickarn]

In addition to the long sides of the frame, you can attach a 45x45 on the underside of each or at least some lav boards (lengthwise). If they are also glued for good interaction, it creates a fairly stiff T-profile.

Smart, thanks for the tip, I haven't considered that solution. Right-angle nogs in the middle become more cumbersome (a bunch of stubs), and the lav becomes significantly heavier. Maybe first try a minimalist solution and see how much the lav bends down in practice, and in the worst case glue on additional longitudinal 45x45 under each aspen board if needed.

 

[Bernieberg]

Blocking helps to achieve cooperation between the boards to reduce deflection for those with pointy behinds, but the shelf as a whole does not become stiffer.

 

[Ingenjörssnickarn]

Kortlingar help to provide cooperation between the boards so that there is less flex for those with a pointy rear, but the overall laven does not become stiffer.

That's correct. Kortlingar also distribute the load between the longitudinal beams (not just the boards), whether I use only 2 in the frame or add 2 in the middle for a total of 4 - so that the weight does not rest on just one or two of the beams at a time.

 

[Snickarkirre]

I was thinking that you should bend a metal angle that's as long as the entire ladder, which you then screw-glue all the way on the 45x45 at both the front and back edges, and then cover it with the standing 21x45 that you're going to use. I thought you should use that instead of perforated tape. Go to a metal shop and ask them to bend them in about 1mm sheet metal. That should significantly reinforce the construction.

 

[justusandersson]

Incorporating metal into sauna benches seems quite inappropriate to me, not to mention self-indicating as they used to say in the military. One must start by discussing what is an acceptable deflection. The deflection requirement that applies to floors, I think, is exaggerated; the idea is to sit down. It doesn't matter much if it flexes a little when you sit down. I choose to set the requirement that a 100 kg man sitting in the middle should not cause a greater instantaneous deflection than L/600, i.e., 3 mm. This is not achievable with just 2 45x45 C 24 carrying the load. The simplest solution is to increase the modulus of elasticity by manufacturing 2 mini-glulam beams; you screw-glue two 22 mm boards (of good quality). To meet my goal, they need to be 70 mm high. If they are 95 mm, it will be even better.

 

[Ingenjörssnickarn]

Mixing metal into a sauna bench is quite inappropriate, to say the least, self-marking as they said in the military. You need to start by discussing what is an acceptable deflection. The deflection requirement for floors, in my opinion, is exaggerated; the intention is to sit down. It doesn't matter much if it springs a little when you sit down. I choose to set the requirement that a 100 kg person sitting in the middle should not cause a greater instantaneous deflection than L/600, i.e., 3 mm. This can't be achieved with just 2 pieces 45x45 C 24 to bear the load. The simplest way is to increase the modulus of elasticity by manufacturing 2 mini-glulam beams, screwing and gluing 2 pieces of 22 mm boards (of good quality). To meet my goal, they need to be 70 mm high. If you make them 95 mm, it's even better.

Thanks for the tip. It worked with 45mm, though in the form of 2 pieces 45x21 with flat iron 40x3 in between. PS. L/300 is usually used for floor joists.

 

[Ingenjörssnickarn]

The final result exceeded expectations!
45mm thin free-hanging frame with a 5mm deflection when 2 men stand in the middle
The downside is, of course, the time consumption with all custom constructions, but this was my journeyman's test after 8 years of house building, now finally fully completed.