I have a deck over my 2-car garage facing the south so the it isn’t really usable space due to the constant sun or rain (Seattle). I want to build a trellis/pergola that spans from the house to the edge of garage without a post in the middle (deck surface is floating floor Apiant pavers). That’s 22 feet long! There won’t be loading on it other than its own weight, and some 2X6″ on top spaced at 16″ on center to give it a nice pergola look.
I really don’t want to make this a massive beam, but can’t seem to find a wooden beam long enough for the job that isn’t massive, and risky with twists in future, and needing to be sent for pressure treating.
I plan on putting tracks on these beams where I can slide some fabric overhead coverings in and out depending on weather conditions. See Shade Tree product.
I thought about aluminum, or steel I-beams, etc… but don’t know if this is even necessary. I would hold these beams up with 6X6″ wrapped in poly columns. Any suggestions?
Thx.
Replies
Conventional wood isn't going to clear span that without sagging. Steel would work, as wood a pressure treated Glu-Lam. Aluminum would be very expensive. It depends on your style, but I would probably get a steel I-beam (called a W-section, for Wide flange).
Conventional wood isn't going to clear span that without sagging
Since there's virtually no load, 2x12's would span with a deflection of only 1/4" (assuming a total load of 12 pounds per foot). My only concern is that they might start to sag with time, being exposed to the weather and such. Do you have any opinion on that?
You're right, it would span just fine. I don't know what I was thinking. It was early. I just checked your numbers and even adjusting your value for I (2x12's are 11 1/4" wide), using your value for w the deflection is still 0.27".
The weather would affect it, you could add a wet service factor of say 0.8 and bring the deflection all the way down to 0.34". Still perfectly acceptable.
you could add a wet service factor of say 0.8 and bring the deflection all the way down to 0.34"
Is "wet service factor" a "real" term?
http://www.southernpine.com/adjfactors.shtml
Check out table A-2. Wet service factor is a real term that accounts for the fact that wet wood fibers are more flexible than dry wood fibers.
Thanks for the link, woodguy. There's some valuable information there.
I don't know why conventional wood wouldn't. I would use 3x10sThis attachment shows with 3x8 spanning up to 18'
I don't liek two bys for this because they twist and are not proportionate looking.
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Egg on face, see above.
I agree about the proportions. Visualizing a 1.5" x 22' as a pergola member just didn't seem right but the numbers work, and there's virtually no load. 3x's would look better. A 22' span will still look wrong IMO without greatly increasing the scale of all the members, but that's not the question.
Your profile says you're an engineer -- do you still remember your equations for simple spans? ;)
Making a few assumptions, it looks like a 2x12 would span that 22' with only a 1/4" of deflection (one pound per inch of loading). The only problem is that I'm not sure you'll be able to find something that long. But you could try calling some lumber yards. What part of Seattle do you live in, by the way?
Also, I'm concerned that plain 2x12s would eventually sag with time.
You could make your own "I-beam" out of wood by screwing and adhering 2x4s to the top and bottom of the 2x12 "web". That would add tons of bending strength. If you got creative, you might be able to design a truss that looked nice; the inherent "airiness" of a truss might look nice in this application. Either of these approaches would allow you to use something wider than 16" on center, too.
As far as species, cedar would be a good choice for its resistance to rot, etc. Fir would work, too, provided that you applied some sort of finish. I'd avoid hemlock, however.
Hope that helps!
Edited 4/30/2007 12:16 pm ET by Ragnar17
Not sure what gave it away, but I am an engineer. However, electrical in this case, so not much help with structural stuff. I think I will go with steel I-beams (W) and wrap in wood. I am thinking 4X7 I-beam. Is there a chart that one could reference for loading on these beams (other than consulting a structural engineer)? Thx.
Couldn't you get a couple open webbed parallel chord trusses? Seems like the airiness of such trusses would lend them to being a pergola. Seems like either wood or steel would work.
Not sure what gave it away, but I am an engineer -- says so in your profile. ;) It also says you live in Seattle (like me), so that's why I reference Limback Lumber below.
I'm an ex mechanical engineer, so I took a couple of courses dealing with beam spans, etc. But nowhere near the amount that structural guys do.
However, I just used the general equation for a uniformly loaded beam,
max deflection = 5wL^4 / 384EI
where w is loading (pounds per lineal inch), L is span (inches), E is elastic modulus (estimate of 1.3E6 psi for fir), and I is the moment of inertia (in^4) specific to the cross section. (For a 2x12, the value for I is 190 in^4.)
Assuming a load of about one pound per inch, I calculated a deflection of 0.27" at midspan, leading me to conclude that 2x12's would be adequate, with the caveat that long term exposure to weather might cause them to sag (just a guess on my part; I've no direct experience to know for sure). By the way, I checked with Limback Lumber (Ballard) and they have 2x12s in up to 24-foot lengths.
As someone already mentioned, wide flange beams are referred to as W-shapes, and "regular" I-beams are called S-shapes (S for "standard", I guess). Just for reference, a "light" 4x7 I-beam (S7x15.3) would be be 15.3 pounds per foot and have a value for I of 36.7 in^4. (E for steel is about 29E6 psi). So for the unloaded (no wood covering) steel, you'd get a deflection of only about 1/16".
Note that the S7x15.3 will weigh over 300 pounds for the span you're working with.
If we assume that the 1x wrap will add another 200 pounds (just a wild guess), then the deflection will be something like 1/8". This is well within typical design parameters of 1/360 or 1/480 of the span (which in your case are 3/4" and 1/2", respectively).
Anyway, that should give you an introduction and allow you to fiddle around with different steel sizes if you want to. You can probably find the values for E and I somewhere on the net, a library, or maybe your local steel yard could supply them.
Ragnar
Edited 5/9/2007 11:15 pm ET by Ragnar17
If you do this right, the only thing the 22' span has to support is itself, dead load of maybe 5-6 #/ sfthat and wind load.Rest those long span pieces on the end beam which can hae added pillar under it and itr generally looks better to pair them anyways.
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Piffin,
Good suggestion regarding the 3x's: they'll look better and (hopefully) twist less. If you're curious, the 3x10 has only slightly less "I" value than a 2x12, so the deflection is similar.
You posted 3x10s spanning 18 feet; how long have they been there? Have you seen any long-term sag?
What are the chances that he'll be able to purchase them in 22- or 24-foot lengths?
Depends where they are. I got these cut in doug fir and can get it up to 40' long easy. sixty feet if I order enough of it, not that I would want 60' long 3x10.I started rebuilding that house in '91 and finished that upper pergola in '96 or '97 I think. that would explain why I don't have a bunch of digital photos of it since I went digital about '97Anyways - no sag whatsoever. We laid plywood on that to use it as a base for staging to shingle that gable end wall and it was as solid as any floor for the way it felt.
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Thanks, Piffin. If the choice were up to me, I'd use 3x's instead of steel.
My problem with the steel boxed in or any of the other laminated choices is how do you weatherproof it all to keep it from rotting
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The other problem with the "box" around steel is how it looks. With solid stock, you can cut nice decorative tails, like it seems you did in the photo posted.