Darn Architects!

[XR250]

This is the wish of the Arch/Owner on a screened porch. This is the typical eave side of a gable roof. The members support a beam that supports the roof framing. The demand on the 2x6's is about 1,000 lbs. ASD. The 2x6's actually check out (with gravity loads and a 200lb horizontal point load from the guardrail) assuming they are braced weak axis by the guardrail.
They also sorta check out un-braced with just gravity loads. I am assuming everything is concentric etc. The (3)2x10 horizontal beam almost makes it strength-wise for a clear span but fails on deflection so I don't feel there is a safety issue here. Am I being too much of a Cowboy?
I can probably also use PT LVL's which are 1 3/4" thick and are significantly better. The beam will be 3 ply with 2 -layers of PT plywood so it matches the thickness of the supports to help with my concentricity. I might have other options such as using a deeper beam that does require intermediate supports or maybe a flitch beam. But for discussion sakes, what are y'all's thoughts about the 2x6's?

 

[JAE]

All 2x6's have some bow to them so your calculations might show they work in compression as "straight" members but if they have a bow - second order effects may change their capacity significantly.

Using LVL's where they can get wet in a rainstorm also is cause for concern.

A small steel rectangular tube might work here (2" x whatever) to get the same look. Otherwise I'd insist on 4x4's.

 

[GaStruct]

I don't have any issues with this. You only need 1% of axial load to brace the 2x6's so it's basically zero and the guardrail will suffice as a brace.

 

[XR250]

Using LVL's where they can get wet in a rainstorm also is cause for concern

PT LVL's. I'll explore the tube steel though.

 

[Greenalleycat]

Theoretically looks fine but seems like a sitation for a 4x4 instead?
The 2x6 is going to look like shit in a year or two after some sun gets onto it
And, as JAE said, second order effects will kick in as it moves with the sun - I can imagine you'll get significant sag in the beam above as the 2x6 bows

 

[EngDM]

Using LVL's where they can get wet in a rainstorm also is cause for concern.

Agreed with this. In my conversations with Weyerhauser, they don't even provide values for deeper beam LVL for applications that even could get wet, and rather recommend the designer use LSL. PT or not, it's not about the wood rotting its about how these are glued together and how that process reacts to being wet and the deformation that occurs due to this.

 

[kissymoose]

Remember Weyerhaeuser's response to their PSL?

Sagging Exterior PSL Deck Beam - Structural engineering general discussion

I have a project where a 10-year-old exterior 3 1/2x16 PSL beam is sagging. The beam spans 18’-8” and supports 10 foot span floor joists. The beam is measured to be sagging 11/16”. I calculated it should be sagging no more than 5/16” supporting dead load only using a modulus of 1.4E6 per...

www.eng-tips.com

I don't think I've specified treated LVL or PSL ever since you posted that. Probably a combination of being overly cautious, mad at Weyerhaeuser, and having adequate alternatives, but still...

I can't imagine that 2x6 looking good after a few years exposed to weather. It might look sleek on paper, but will look trashy with even a slight curve/twist. From a structural engineering perspective, which I guess I should focus on, if the numbers work then so be it. If it's adequately attached at the top, bottom, and restrained at the guardrail, why not?

 

[EngDM]

This would look really slick using cold formed thats panted black, with a finish similar to how Simpson does their outdooor accents.

 

[bones206]

I’ll chime in a thought: if future homeowners ever decided to ditch the screens and railing, there’s a good chance they would assume those 2x6’s are not load bearing posts and rip them out without much thought.

I’d try to make a beam that can span between the 6x6 posts with reasonable deflection, without considering the interior 2x6 as supports.

 

[Tomfh]

No way. There’s nothing to restrain them when they bow.

If they want slender then use metal posts.

 

[jhnblgr]

Would deflection criteria apply to a screened wall? Typically isn’t deflection more finished based? Could you use a steel t section?

 

[ANE91]

Typically isn’t deflection more finished based?

There’s still the “what will later sell / what could freak out a homeowner” deflection criteria…

 

[Ron247]

I would consider running a 4x6 through a planer a few passes to make a 3x6. That should give a "leaner look" and also give me more confidence in its longevity. I would also use Yellawood if possible. Exposed to water tends to become damaged eventually.

 

[phamENG]

I'd go for the PWT Treated LVL that can span without the "posts" and sleep well.

For those decrying using LVLs here, note that PWT is (as far as I know) the only manufacturer who manufactures a wet service rated LVL.

 

[TRAK.Structural]

Personally I would size the beam to clear span.

I have mixed feelings on LVL that may get wet; I see them specified for porch roofs all the time in my area. Also see the link below from Weyerhaeuser. Still a little on the fence about it though.

https://www.techsupport.weyerhaeuser.com/hc/en-us/articles/360056303814-Can-Microllam-LVL-Be-Used-in-Porch-Roof-Framing

 

[phamENG]

Also see the link below from Weyerhaeuser.

How is this link applicable to to a product from a different manufacturer who treats their LVLs with preservatives and rates them for exterior, damp use?

 

[TRAK.Structural]

How is this link applicable to to a product from a different manufacturer who treats their LVLs with preservatives and rates them for exterior, damp use?

It's not.

PT LVL's aren't common where I live, and untreated LVL for this application is.

The link shows that there can be some debate about whether treated products are required in this application.

 

[phamENG]

Got it. Sorry. Took your post as saying an LVL shouldn't be used because it might get wet.

 

[XR250]

I think clear span beam is the way to go.
Thanks for the input.

 

[gte447f]

Would deflection criteria apply to a screened wall? Typically isn’t deflection more finished based? Could you use a steel t section?

Yes, of course deflection criteria would still apply. Deflection criteria in the building code are listed for the member under consideration, i.e., floor member, roof member, etc.