Size of the bearing beam?

[matkin]

I have purchased a conservatory kit from Willab, size approx. 5000x4000. The inner support beam is intended to be fixed to the facade, but that won’t be possible, so it will only rest on two posts at each end. Therefore, I thought I’d check here if I should reinforce it to compensate for not being attached to the facade.

The inner support beam has dimensions of 42x225x5161 with a little overhang at the sides. The outer support beam has dimensions of 56x270x5161, so it's a bit thicker. Both are laminated beams. On this rests a roof of multiwall polycarbonate. I live in Östergötland, so no extreme snow conditions.

Should I increase the strength of the inner support beam, or is it sufficient as it is? I'm a bit concerned since the outer beam is so thick.

 

[Rickard Ohlin]

Is the outer one intended to rest on two posts at the edges just like you intend to do with the inner one? If so, you have your answer there. It won't break, but the key issue is usually how much it bends down. I assume it's < 1200 cc on your joists that go between the bearing lines which the channel plastic rests on.

Otherwise, Beijer has an online tool to calculate and simulate different systems.

 

[justusandersson]

Since it is a roof construction, it is the bending strength of the beams that determines the dimensions. The inner beam is weaker because it is intended to be fastened to the wall at several points. The easiest solution is to place a pillar in the middle to support the inner beam. This gives you very large margins.

 

[matkin]

Since it is a roof construction, it is the bending strength of the beams that determines the dimensions. The inner beam is narrower because it is intended to be attached to the wall in several places. The simplest solution is to place a column in the middle that supports the inner beam. Then you have created very large margins.

Thanks for the reply. The reason I cannot attach the construction is that I have to extend it from the facade about 30 cm to make room in height. This also makes me want to avoid placing a post in the middle as it will take up too much space.

 

[matkin]

Is the outer one intended to rest on two posts at the edges just like you plan to do with the inner one?
If so, you have the answer there. It will certainly not break, the determining factor is usually how much it bends down. I assume it is < 1200 cc on your beams that go between the support beams which the channel plastic rests on.

Otherwise, Beijer, among others, has an online tool for calculating and simulating different systems.

Thanks for the response. It is cc 756 on the outer beams and cc 1235 on the three in the middle, so it varies a bit. The outer one also rests, as you say, on two posts at the edge. I'll test Beijer's tool to see. But as you write, now the construction will be the same on both support beams, so maybe one should reinforce the weaker one to make it the same dimension.

 

[Rickard Ohlin]

You don't want sagging because sliding doors can get stuck or fixed windows can crack if you get a heavy snow load.

What happens if you attach posts along the facade and place a diagonal beam higher up to transfer the load from the main beam? Say from 1800 mm height? The attachment needs to be quite robust, but you can calculate it.

 

[justusandersson]

If you don't want a central post under the inner beam, you need to change the beam dimension. I did a quick check, and 56x270 meets the bending strength requirement in both places but not the maximum deflection requirement of 1/300 of the span. To meet that requirement, you need to go up to the 56x360 dimension. This might have the most significance for the outer beam. One option is to change the inner beam to 56x270 and place an extra post under the outer. Another option is to return the 42x225, place a 56x270 on the inside, and buy a new 56x360 for the front edge.

Depending on the roof design and how you attach the inner posts to the house, the entire structure might need stabilization.