Sizing of roof ridges

[Øyvind]

Hello! Hi, I am Norwegian, and I'm wondering if there are specific standards for dimensioning "takåser" in Sweden?
By takåser I mean horizontal timber as shown in the picture:
I am naturally interested in the span width, and if I can calculate on individual beams of round timber?

 

[justusandersson]

I am not aware of any specific standard for roof rafters in Sweden, so one must use the general regulations available for load-bearing structures in the Eurocode. The "träguiden" has a readable text about roof rafters. Although it specifically applies to glulam, much of it is universally applicable. When using roundwood, some specific problems arise. See this link.
https://www.traguiden.se/konstrukti...projektering-av-limtrakonstruktioner/takasar/
The calculation process is first to calculate the load per meter for each rafter in the vertical direction and then divide this into a component in the plane of the roof and one perpendicular to it. The simplest (though slightly underestimated) way is to calculate the roundwood as a square inscribed in the circle of the roundwood.

 

[fahlis]

... The simplest (though a bit underestimated) way is to calculate the round timber as a square inscribed in the circle of the round timber.

Isn't it easier to calculate the round timber as a circle?

 

[justusandersson]

Then you have to delve into a somewhat more complex calculation of the moment of inertia, as it doesn't get any simpler. It's quite a common method to use values that are easier to calculate with. Also, I'm a bit doubtful about the utility of using timber for rafters. It's difficult to secure it in the framework, and it's also challenging to attach roofing material on top. Additionally, you lose some of the stiffening effect of the roofing material.

 

[fahlis]

More complicated? The moment of inertia for a circle is
(pi)*(d^4)/64
What's complicated about that? Pi?

 

[justusandersson]

I was thinking that TS might not know what the moment of inertia is. Otherwise, one formula isn't more difficult than the other. Generally, I think that round timber is underrated, though not in this application.

 

[fahlis]

... Generally, I think round timber is underrated, though not in this application.

I agree ...