Help with interpretation of dimensioning of ridge roof

[jonmo]

I have used the excellent tool at byggbeskrivningar.se to calculate which dimensions I should have in my construction. What I need is a little help interpreting the result and answers to a few additional questions.

Once you input all the numbers, you get the option to choose a few different variants as shown below. It seems the program calculates all that pass the utilization rate <100% and then highlights in red those that don't meet the deformation requirement:

I choose the smallest black dimensions and generate the following report:

Good. The house will be built on a concrete slab on the ground, which is 100 mm thick according to standard, with a substantial edge beam according to best practice. The question I don't know where to start looking for an answer to is: Do I need to reinforce the slab where the center pillar, which takes the most force, stands? I assume that if so, it's done by simply not laying as much insulation and thereby creating a trench across exactly where the pillar comes down? Or is this usually solved with a lot of extra reinforcement?

 

[justusandersson]

If I understand correctly, the center pillar should not be in the middle but rather 2.4 m from one end and 6.06 m from the other. In that case, the dimension of the ridge beam is incorrect. It should be more like 90x405 mm. However, the support reactions on the pillars match my numbers. The best option if you want a center pillar is to create a longitudinal center beam in the concrete, similar to the edge beams. An alternative is to skip the center pillar and increase the dimension of the ridge beam. 140x495 mm would be sufficient. The additional cost for the beam is under 3000 SEK. At the same time, you avoid the cost of the center pillar and the reinforcement of the bottom slab. The end result is guaranteed to be cheaper. Also, skip the C 18 timber and use C 24. It is unusual for wood suppliers to stock C 18. C 24 will likely also be cheaper.

 

[justusandersson]

Upon closer inspection, I understand that the program assumed the center post is exactly in the middle.

 

[jonmo]

If I understand correctly, the middle pillar should not be in the center but rather 2.4 m from one end and 6.06 m from the other. In that case, the dimension of the ridge beam is incorrect. It should rather be 90x405 mm. The support reactions on the pillars, however, match my numbers.

Hmm, strange that the calculator has such an error. But of course, I trust you more with your extensive forum history.

The best option if you want a middle pillar is to make a longitudinal center beam in the concrete, analogous to the edge beams. One option is to skip the middle pillar and increase the dimension of the ridge beam. 140x495 mm would suffice. The additional cost for the beam is under 3000 SEK. At the same time, you avoid the cost of the middle pillar and the reinforcement of the base plate. The end result is guaranteed to be cheaper.

Great suggestion! Additionally, the proposed loft above the toilet, storage room, and the smaller room becomes more accessible.

Skip the C 18 wood and use C 24. It's unusual for wood suppliers to stock C 18. C 24 will probably be cheaper as well.

Yes, I was surprised that it was mentioned; I had a feeling that C24 was standard. I'll go with that!

One more question while I have you here: Since it will be open at the ridge, I would really like to add a track for a crane while I'm at it. The simplest thing would be to bolt it from underneath to the glulam beam, but it doesn't feel right to burden it without knowing its capacity. On the other hand, a steel beam (H-beam) bolted from underneath should positively impact the load-bearing capacity of the glulam. Or should one place the steel beam on a standing post in the wall and then mount the glulam beam on top?

 

[jonmo]

Upon closer inspection, I understand that the program has assumed that the middle pillar is exactly at the center.

I understand. But it's quite strange then that you have to specify the measurements L1 and L2.

 

[justusandersson]

Either you have made a mistake or there's a bug in the program. The fact that your measurement data is included in the printout points to the latter. The dimension of the end beams should be adjusted partly because they will have an increased load, approximately 27 kN per beam, and partly to fit the ridge beam's width better. 140x140 is an over-dimensioning, but it looks best. The edge beam must also withstand a point load of about 30 kN at the beam attachments. Most of the roof load is snow. If you use the crane at the same time, the beam must be re-dimensioned. The best type of attachment, I think, is through bolts, but it might be a bit bulky in this case. Screw attachments must be calculated.

 

[jonmo]

Great, thanks for the help! I think I'll attach a lighter beam and make sure to only lift things that weigh more than 100 kg in the summer, in other words.