Bridging the wood-steel gap

[theonlynamenottaken]

I'm looking for everyone's thoughts on subjecting engineered lumber to exterior exposure. On more than a few occasions now I've encountered situations where conventionally available dimensional lumber (<= ~3-ply 2x12) is not structurally adequate in an exterior exposure setting. LVL, PSL and Glu-Lam's aren't typically rated for exterior exposure, and even when they are I've heard that their resistance to exterior conditions is similar to exterior rated plywood, i.e. that its good for short-term exposure during construction but needs to be completely protected in the long run. I've seen my fair share of poorly/improperly spec'd PSL and LVL beams that soaked up moisture like a sponge and lost over 50% of their cross-sectional area in just a few years.

Most residential and small-time commercial contractors shriek when the dreaded "steel" word comes up, so in the past I've given the contractor preliminary engineered lumber beam sizes and recommend that they look into engineered beams that the manufacturer will back up as rated for continuous exterior exposure. Without fail the contractor returns to say either, "they aren't available", or, "they're so expensive lets just use steel".

What does everyone else do? Flitch beams aren't very structurally efficient, and they're labor-intensive and costly on the engineering/detailing side of the coin. Heavy, solid sawn timber beams are expensive and have long lead times, at least in this area. It's easy from our end to just spec steel and be done with it, but most of these project's budgets can't afford more than a semi-skilled carpenter with a chop saw. Anyone know anything about engineered lumber beams with continuous exterior exposure rating? Do they even exist? Does anyone have any other reasonable alternatives for these situations?

 

[SteelPE]

I've been told they exist, at least that is what my colleagues tell me. Although I have never had many problems specifying steel though. In fact, I just got done with a house that had all of the girders and columns done out of steel.

You might want to call Boise Cascade to see what they recommend and what might be available in your area. I find that most of the contractors you are talking about are resistant to change and might be scared by what is a minor specification.

 

[XR250]

I spec pressure treated glulams all the time. They are easy to get here from Rosboro.

http://www.rosboro.com/index.php?action=products.treatedxbeam

 

[manstrom]

Anthony PPG

PT Glulam. Strong, cheaper than PSL.

http://www.anthonyforest.com/treatedbeams.shtml

When I am working on a problem, I never think about beauty but when I have finished, if the solution is not beautiful, I know it is wrong.

-R. Buckminster Fuller

 

[BUGGAR]

I spec steel all the time but I make sure I design it with holes in flanges and webs for bolting on nailers all around. That seems to make the framers happy. And nothing heavier than a forklift can carry. Don't let OSHA come around when carpenters start erecting structural steel.

 

[TLHS]

I don't do wood, but there are engineered lumber products all over the place in projects I see around British Columbia. It doesn't seem to be unusual at all.

 

[msquared48]

Transparent Aluminum.

Mike McCann, PE, SE (WA)

 

[mike20793]

There is a treated version of Parallam PSL called "Parallam Plus" that has a warranty of 30 years. It cannot be resawn in thickness or depth, but it can be cut to length. I spec this at all balconies or exterior walkways where 2x lumber doesn't have enough strength.

 

[Splitrings]

I used a two ply LVL continuous beam for my exterior porch in VT. It has been in place for 12 years with no signs of decay, delamination or other signs of failure. It was stained with quality solid exterior stain when installed. It is not exposed to direct rainfall.

 

[Triangled]

I've used glulams manufactured from decay resistant species such as Alaskan Yellow Cedar Fb=2000 psi for exposed girder rafter tails and trellises and pool buildings etc. The trick is for AYC, as I recall (it's been a few years, the sapwood (not decay resistant) looks similar to the heartwood (decay resistant) and therefore must be specified (all heart) and assembled correctly. Verify with your glulam guy.

Otherwise, definitely check with Scotty on the transparent aluminum.

 

[theonlynamenottaken]

Thanks for all the input. I think we've got a clear path ahead now thanks to all of your help.

@ XR250 / manstrom / TLHS
I have seen countless pavilion structures with glu-lam frames but didn't know that the exterior rated beams were commonly available. I have since found a somewhat local manufacturer that supplies our area.

@ BUGGAR
This specific project isn't new construction, its repair and rehab of several large occupied apartment buildings. There is no forklift or wheeled equipment access, and any work that starts at 9AM must be finished by 4PM. The install crew is already onsite and they are all carpenters. So steel really isn't feasible.

@msquared48
I really wanted to poke fun and make some Star Trek references, but then a quick web search showed me that transparent aluminum now actually exists. Actually, its transparent alumina and not aluminum...
Wow, transparent aluminum

@ mike20793 / Splitrings
We're hesitant to specify PSL or LVL, even if they are manufacturer or installer treated. This specific project is to replace 8 year old PSL and LVL beams that we could scrape away ~75% of cross-sectional area with our fingernails. We're not in a marine environment here, but we are in the worst area otherwise for decay potential according to AWPA (see map in the attachment). Vermont is in the "Moderate" decay potential area, while we are in the "Severe" area.

 

[dhengr]

Theonlynamenottaken:
You’ve answered your own question on the wood vs. steel issue. Give them a choice and they quickly come around. Carpenters do know how to use a nut-n-bolt and a socket wrench when they have to. Detail the steel cleanly and have the fab. shop punch flg. and web holes, when and where needed to pad the stl. beam out, when that’s needed. They can do it when they have to, or when it costs less to accomplish what you want, they just don’t do it every day. Minimize the steel work they have to do and keep the pieces fairly light, and they are usually o.k.

I am quite sure that you can get treated GlueLams, but they aren’t an off-the-shelf item, or even made by every GlueLam fab. shop. I also believe a few suppliers have done some work of preservative treated LVL’s and PSL’s, etc.. And, I suspect they don’t take treating very well because of the glue, and there are thousands of surface cells and fibers which tend to suck up moisture and causes them to swell and go to hell. One important detail on exterior wooden beams, and built-up members (post tops too) is to flash the beam tops with peal-n-stick tape or even ice-n-water guard self sticking/self healing rolled goods. Cut it so .5" to .75" hangs over each side of the beam, and bend it over a bit, but don’t stick it down tight to the side. Rather let it hang down so it acts as a drip edge on the top of the beam. Do this on deck joists and multi-ply 2x beams too, keep the water off the tops and out of the plys, where it tends to never dry out. Watch out for some of today’s treatments and hardware corrosion.