Help me interpret the timber guide's dimensioning table

[Amrich]

I am going to build a garage with a loft. I'm a bit puzzled when I read the timber guide's table for sizing wall studs:

https://www.traguiden.se/konstruktion/konstruktiv-utformning/stomme/vaggar/vaggreglar/

If I interpret the table correctly, there's an enormous difference between a wall frame of 45x120 and one of 45x145. The max house width for the former dimension is 4.7m and a whole 12.9m for the thicker one in snow zone 1.5. That seems like an unreasonably large difference for an additional 25mm of wood?

For snow zone 2, no figures are given at all for 45x120.

In an older thread here on the forum, I find the following quote that seems to refer to the same table but with completely different (more reasonable?) values. This makes me think that there might be something wrong with the current table. Or am I interpreting something completely wrong?

According to the timber guide, 120x45 (c14) studs with c-c 600mm can support a two-story house with a heavy roof, free-spanning trusses, snow zone 2.5, and a maximum house width of 7.5m

I also react to the fact that I don't see that they take the height of the wall studs into account.

Enlighten me

 

[justusandersson]

The possible load on a post is limited by what is known as the risk of buckling. This depends on the cross-sectional area, length, and strength class. For a post with a rectangular cross-section (a typical stud), the risk of buckling is considerably lower along the y-axis than along the z-axis. In a stud wall, the studs are braced along the z-axis through noggins and panels, etc., so it is the post's dimension along the y-axis that determines the possible load. This goes back to the Swiss mathematician Leonard Euler's buckling case that he formulated in the 18th century.
According to the timber guide's table on the load-bearing capacity of posts made of construction timber https://www.traguiden.se/konstrukti...vor-av-konstruktionsvirke/?previousState=0010, a 2.4 m long 45x120 stud in quality C 24 can withstand a load of about 39 kN if it is unrestrained in the y-direction. The corresponding figure for 45x145 is 60 kN. The additional 25 mm gives a 50% better load capacity. It's a big difference even if it doesn't explain the difference in possible house width that you mentioned. The load-bearing capacity increases with the cube of the stud's increase in depth.

 

[Amrich]

As usual, brilliant response from you!

If I understand you correctly, you also believe that there is something wrong with the table?

I am going to build a garage 10.8x7.2m with a loft designed for residential load. It will have a heavy roof in snow zone 3 and a 38-degree roof pitch.

Due to various circumstances including a raised leca-sockel, etc., I want a frame of 45x120 with cross-braced 45x45 or 45x70 on both sides, then boards on the inside.

The question is if this is sufficient? If not, perhaps it would be if I reinforce it with a 45x120 crosswise between the standing wall studs at half the height.

That's why I thought the table might give me a hint, but as mentioned, there seems to be something wrong there.

 

[justusandersson]

There is no reason why you cannot have 45x120 (c/c 600 mm) in the wall framing. I did a quick check and you are not anywhere near a maximum load. The big challenge is to size the floor joists for the loft if they cannot be supported by any wall on the ground floor. That requires glued laminated timber or a similar material. It might be worth checking what it would cost to have trusses manufactured in a factory with lower chords meeting deflection requirements. If the trusses are at c/c 1200 mm, you can use heavier floor timber instead.

 

[Amrich]

Great to have it confirmed that 45x120 in the framework is fine, thanks!

I want roof trusses with a glulam underframe on cc120 with corresponding glulam beams between each so that the floor ends up on cc60.

I've been in contact with a local manufacturer who couldn't (or didn't want to?) offer a solution with glulam. I received a proposal without glulam that was claimed to be sufficient for storage load. The loft ended up a bit too narrow there as well, so I will continue looking for someone who offers the glulam option.

 

[Amrich]

I have looked at some tables and concluded that I need a glulam beam with dimensions 90x315 or 115x315 on cc60 in the floor structure.

 

[justusandersson]

90x315 is on the borderline when it comes to deflection, depending on what you place on top. 115x315 is safer but heavier and more expensive. Don't go for this attic load option; it usually doesn't suffice for residential use. One option is to separate floor joists and rafters. With sufficient dimensions on the high legs, a collar beam + glulam beams c/c 600 mm is enough.

 

[Amrich]

90x315 is borderline when it comes to deflection, depending on what is placed on top. 115x315

The loft will consist of a living room and two bedrooms and primarily be used for guest accommodations and possibly a teenager or two in the future. I assume that 90x315 is significantly better than the variant for storage load and might be a good compromise in this case. The price up to 115x315 was quite high. Or do you think I will be disappointed?

One option is to separate floor joists and roof trusses. With sufficient dimensions on the high legs, a tie beam + glulam beams c/c 600 mm is enough.

I didn't quite follow what you mean here, can you elaborate?

Do you perhaps mean that I use trusses without glulam that do not support the loft floor at all and in addition to these use glulam cc60? That is, glulam cc60 plus trusses cc120?

 

[justusandersson]

I didn't quite follow what you mean here, can you elaborate?

You can ignore support legs if the main legs are thick enough.

Do you perhaps mean that I use trusses without glulam that don't support the loft floor at all but, in addition to these, use glulam cc60? That is, glulam cc60 plus trusses cc120?

Yes.
90x315 is sufficient if you screw-glue sheet materials on top or use stronger floor wood.

 

[Amrich]

Thank you, I'm with you!

I've gotten a bit focused on this solution with glulam at cc60. Am I missing any other alternatives?

There are, of course, Kerto beams, metal beams to place under trusses, etc. I'm not really well-versed in that area. Should I consider another option that is better from an economic and/or practical perspective?

 

[justusandersson]

One should start with the building's plan dimensions. At this point, I don't think steel is an option. Kerto beams and masonite beams are alternatives to glulam. The advantage of glulam is that it is sturdy and you can nail and screw into it. Exposed, they are also visually appealing. If you have plenty of height space, you can use a smaller number of heavier beams (e.g., c/c 2400) as the primary support and then fill in with regular construction timber (c/c 600) recessed across between the glulam/placed on top. You should draw out the options and calculate the costs. The significant price increases over the past year may have disrupted traditional assumptions.

 

[Amrich]

Thank you! Then I think I'll stick to my main plan. I've been in contact with a truss manufacturer who might be able to offer me a glulam solution.

Does this garage+loft with framework and glulam require any special stabilization in other directions than what is achieved with "standard" cross-bracing and later interior panels and exterior siding?

I also envision a cross of perforated strips in each wall.

The floor structure will have sparse paneling and ceiling gypsum from underneath, but it's not certain that the loft will be finished or surfaced with flooring on top until later.

 

[Amrich]

Today I received a response from a truss manufacturer. I am in the process of trying to understand everything in the truss drawing.

I tried to reach the designer to clarify some questions but without success, so I’m trying here in the meantime:

1. I requested 720 cm internally in the garage (span?), 38-degree roof pitch, and a 500 cm wide loft, but the drawing shows 733 cm internally between the garage walls. Perhaps not all requests were feasible together?

2. Another thing I noticed is that the bottom chords + crossbeam are 360 mm high. I can't find a measurement for the width. Is that a lot? Is the fact that they are part of a truss the reason they need to be so much thicker than a regular floor joist? Where we concluded that 90x315 was "almost" sufficient?

3. The drawing also shows a beam going across all the trusses that appears to be attached at the bottom to the struts. What is this called, and what function does it serve?

Thanks in advance!

 

[justusandersson]

1 can't be commented on without a drawing.
2 90x315 was glulam. Regular construction timber has less stiffness, hence the greater height. I guess 60x360. A span of 7.2 m is a challenge.
3 can't be commented on without a drawing.

 

[Amrich]

1 can't be commented on without a drawing.
2 90x315 was glulam. Regular construction timber has lower stiffness, hence the greater height. I guess 60x360. A span of 7.2 m is a challenge.
3 can't be commented on without a drawing.

I was expecting to get a proposal for a truss with glulam 115x315 or similar. I interpret the proposal I received as glulam in dimension 45x360. I attach the drawing, so please feel free to comment on the alternative we discussed versus what the drawing shows.