Pressed 170/45, two editions maximum length?

[Noppe]

I am going to build a simple pergola to hang a sun sail over a patio.

Does anyone know if there could be problems using a pressure-treated 170/45 on only two supports with about 5 meters between the supports? What happens when it dries? Can it warp or sag? It will be unloaded except for its own weight!

Grateful for help!

 

[Mazen]

Why treated wood? My experience is that it warps without multiple anchoring points.

C24 construction timber seems like a safer bet, or preferably glulam.

https://www.beijerbygg.se/store/pri...glar-c24/45x170-plank-hyvlat-c24#.WrYP29DMBnE

https://www.beijerbygg.se/store/pri...-1384327494597-46?artikel=416083#.WrYQWtDMBnE

 

[justusandersson]

If you hang up a 45x170 rule of C 24 class of fresh wood, treated or not, it will bend down about 1.6 mm in the middle from its own weight. What the sunshade can contribute beyond that depends on its weight. I agree with Mazen, there's no significant point in using treated wood in this case. It still needs to be oiled to prevent drying out.

 

[Mazen]

What do you say Justus, C24 45x195, 45x220, or glulam 56x225? My vote is on glulam...

 

[justusandersson]

Compared to 45x170, 45x195 gives about 1mm deflection due to its own weight, and glulam 56x225 about 0.5 mm. Glulam always looks the best but it also needs to be oiled.

 

[Mazen]

Ask justus which program do you use for the calculations or do you calculate "manually"?

 

[justusandersson]

I calculate manually, but I use a homemade Excel sheet with various setups. This way, I can easily change different parameters interactively. Very simple but quite clever. I'm skeptical of programs where you can't see all the variables.

 

[Mazen]

Nice. Any chance you can share the Excel secret with us other mortals?

 

[justusandersson]

Not really. It's personal and not entirely self-explanatory in its layout. I have a need to see certain values even in older units to relate to my experience. It's possible that I could make a simplified version that I can post. I'll have to think about it a bit.

 

[Mazen]

Thanks, it was just a simple question because one wants to learn from the best, and yes, personal experience is invaluable.

I have only worked with pre-made programs myself, and they often leave much to be desired. Send a PM if you have a simpler version to post.