I was wondering if I could get some help from someone knowledgeable about timber dimensioning.
We are planning to extend our house and I'm working on a drawing for an extension. The extension will connect to the gable of the existing house structure and we want to make a fairly large opening there:
The facade of the existing house structure is built with standing planks (about 70 mm thick). If I'm thinking correctly, you would need to make two load-bearing structures in the same opening:
1. One that absorbs the load from the plank frame of the existing house structure (green beam in the drawing). Perhaps the plank frame can be attached to the floor structure on the upper floor?
2. One that absorbs the load from the trusses of the extension (pink beam in the drawing)
The span of the opening that needs to be made is 3155 mm. The span of the trusses is 3860 mm.
The offset in the plank wall (3-inch plank = 75 mm) can be made simple. That wall does not handle any roof or floor loads. The planks are skew-nailed with very coarse nails quite closely, so the offset mainly has a practical and aesthetic function. At the same time, it is a long stretch. I suggest you use a glulam beam with dimensions of 78x180 mm. For the offset in the newly built section, which is affected by roof loads, you also need glulam, preferably 90x270 mm.
The offset in the plank wall (3-inch plank = 75 mm) can be made simple. That wall doesn't handle any roof or floor loads. The planks are nailed with very coarse nails quite closely, so the offset has mainly a practical and aesthetic function. At the same time, it is a long stretch. I suggest you use a glulam beam with dimensions 78x180 mm. For the offset in the newly built part, which is affected by roof loads, you also need glulam, preferably 90x270 mm.
Thanks for the input! Suspected that the offset on the plank wall isn't as necessary as on the newly built part.
I just found out that an acquaintance has some IPE120 beams that I can have for free. Will this work as offset on the newly built part, or is it too weak? I've been googling a bit on bending resistance and such but can't find a good source to compare glulam and steel beams...
No, IPE 120 is a bit too weak. You compare beams of different materials and dimensions by comparing the product of the modulus of elasticity and the moment of inertia, commonly called flexural rigidity and abbreviated EI, for each beam. Bending resistance, abbreviated W, is relevant for ultimate limit state calculations, and is for a beam with a rectangular cross-section b*h^2/6.
Isn't it simpler to size that particular roof truss according to the conditions? Then you wouldn't need to reinforce anything, and the roof truss rests as usual on the exterior walls and then carries the current load. The other roof can be attached to the truss if it is sized accordingly.
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