No, it only shows how many times greater the deflection becomes compared to today.P ptlf said:
No, but the original poster wants to use weaker trusses than recommended. And you suggested tighter spacing...Nissens said:
And I meant that he might be able to keep the dimension he has by reducing the spacing between the studs. I understood it as he was satisfied with not increasing the dimension, not that he wanted to decrease it compared to what is there now.B BSOD said:
If you use a sizing program, you quickly realize that even with cc 60, 225*45 is faaar from what is recommended.P ptlf said:
/Krille
Proper dimensioning is like a seatbelt - completely unnecessary until the "accident" happensP ptlf said:
Personally, I would never dare to dimension so lightly in Västerbotten to save "a few" thousand kronor. I was glad about that last winter when we were abroad for three weeks while it snowed a lot and drifted up almost 2 meters of snow on one side of the carport roof./Krille
Do it like this.
Design according to Svenskt trä.
Calculate the total second moment of inertia for the construction, which is the number of beams multiplied by the second moment of inertia for one beam.
Then calculate the second moment of inertia for your section (beam) and ensure you reach the same total second moment of inertia as in Svenskt trä's design.
Search the internet for formulas for the second moment of inertia, and you should find them quickly. You can neglect the radius if your timber has such in the corners of its cross-section.
Design according to Svenskt trä.
Calculate the total second moment of inertia for the construction, which is the number of beams multiplied by the second moment of inertia for one beam.
Then calculate the second moment of inertia for your section (beam) and ensure you reach the same total second moment of inertia as in Svenskt trä's design.
Search the internet for formulas for the second moment of inertia, and you should find them quickly. You can neglect the radius if your timber has such in the corners of its cross-section.
If I'm not losing my way in the brain office now, the maximum deflection for a distributed load (snow) should have a double quadratic dependence on the beam's length, meaning there's a hell of a difference between a 3 and 5 m span.
I haven't dealt with this type of simple calculations for many years, so this is straight from memory.
I haven't dealt with this type of simple calculations for many years, so this is straight from memory.
That clarified everything... not...yoloboi said:
Deflection = constant * length^4
Anyone who wants to understand better needs to study beam deflection. Anyone who wants to design seriously needs to understand the applicable standards.
The reason your construction holds in practice but not in theory is because the dimensioning tool and dimensioning in general according to standards are based on taking into account safety factors and other partial coefficients to account for various types of uncertainties. This may involve uncertainties regarding material quality, load-bearing capacity with regard to a myriad of factors such as moisture and load duration, the load in question, etc.
Things can go really wrong if you remove all these margins. One day it might be unfortunate that the structure has suffered moisture damage and there is a significant snowfall. That said, 45x220 holds for 5 m and c/c 600 as well as for snow zone 2 in the ultimate limit state according to all the rules except for deflection. There you get 20 mm deflection in the middle, which is considered a bit high compared to recommendations if you, for example, compare with L/300 ~ 17 mm. But in this case, I have, as mentioned, included quite a lot of safety margin, so 45x220 will likely hold for 5 m.
Things can go really wrong if you remove all these margins. One day it might be unfortunate that the structure has suffered moisture damage and there is a significant snowfall. That said, 45x220 holds for 5 m and c/c 600 as well as for snow zone 2 in the ultimate limit state according to all the rules except for deflection. There you get 20 mm deflection in the middle, which is considered a bit high compared to recommendations if you, for example, compare with L/300 ~ 17 mm. But in this case, I have, as mentioned, included quite a lot of safety margin, so 45x220 will likely hold for 5 m.
Thanks for the good information! Moisture damage can probably be avoided if you make sure the roof is tight, and you don't need to use the beams with the worst knots, and the amount of snow is something we're used to keeping track of up here, so good with your input.S scorp1on said:
Note that I counted with snow zone 2.