Calculation of double glulam beams

[hölivägga]

I am renovating an old two-story house from 1910 where they have supported the ceiling with a steel beam on top of the joists instead of having posts underneath. The steel beam hangs on the tie beams, which have then been reinforced with the help of the rafters.

I had a structural engineer there who did calculations and specified a glulam beam that is 115x495mm. The problem now is that I want to reduce the beam to 405mm, but the engineer I used no longer has time to help, and I can't hire a new one.

I did a calculation on byggbeskrivningar.se and came up with the following for the 115x495 beam:

Number of pillars: 2 pcs
Opening dimension [L1]: 4500 mm
Height [H]: 3000 mm
Width, house: 9000 mm
Distance to load-bearing
inner wall [B1]: 4500 mm
Roof pitch [α]: 42°

- Glulam 115×495 GL30c, deformation 10 mm (L/468), utilization rate 68%

I also did a calculation with a smaller opening to see when I can go down to 405 and came to the following:
Opening dimension [L1]: 4300 mm:
- Glulam 140×405 GL30c, deformation 11 mm (L/377), utilization rate 67% (Interesting that the deformation is higher, and the utilization rate is lower)

The problem is that no one has this width of glulam at home, so it became a special order and gets a bit expensive.

So, I wondered if it might be possible to use double 90x405 mm or even 115x405?? I think I can also glue them together into one beam? That should increase the load capacity further?

Opening dimension [L1]: 4000 mm:
- Glulam 115×405 GL30c, deformation 11 mm (L/376), utilization rate 74%
Opening dimension [L1]: 3800 mm:
- Glulam 90×405 GL30c, deformation 11 mm (L/337), utilization rate 90%

I hope someone can help me with calculations for a lower beam.

 

[Testarn]

You can absolutely place two beams next to each other, as they will share the load. It makes no difference to screw them together.

I don't remember the formula/calculation by heart, but our structural engineer was very clear that two 45 mm joists next to each other (without screwing them together) had the same deflection as a 90 mm joist/beam if the load, opening size, and height of the beam and joists were the same.

Then I remember that when calculating deflection and strength, the relationship is linear for width and quadratic for height, so what he said seems reasonable.

 

[hölivägga]

You can absolutely place two beams next to each other, they will share the load. Screwing them together makes no difference.

I don't remember the formula/calculation off the top of my head, but our engineer was very clear that two 45 mm beams next to each other (without screwing them together) had the same deflection as a 90 mm beam if the load, opening size, and the height of the beam and beams were the same.

I also remember that when calculating deflection and strength, there is a linear relationship with width and a quadratic relationship with height, so what he said seems reasonable.

Awesome!
Thanks for the super helpful info.

I also talked to my supervisor and he confirmed the same thing. Two 90x405 seemed to provide about the same load-bearing capacity as a 115x450.

When I studied to become a guitar builder, I know there was some formula where we calculated the difference if you added 1mm to the height or 1mm to the width, but I can't remember it now.

 

[Bjoen]

Hello, can you place 3 alongside each other? We have an older holiday home with a low ceiling height and the only option as I see it is 3 glulam beams or possibly a steel beam. A steel beam would be a completely different cost and I can't fix that myself, so I'm avoiding it.

 

[Testarn]

Hello, can you place 3 next to each other? We have an older summer house with low ceiling height and the only option as I see it is 3 laminated beams or possibly a steel beam. A steel beam would be a completely different cost and I can't handle that myself, so I avoid it.

There is no limitation on how many you can have in width. It's just the support that needs to be there so that they have support.

 

[Bjoen]

There is no limit to how many you can have side by side. It's just the support that needs to be there so that they are supported.

That sounds good. Thank you for responding.