Does anyone have a source for a conversion table to compare teh strengths of Douglas Fir #1 6×6 posts/columns, to Glulam columns, to Versa Lams?
Any preference on this board for one material over another? Any rought price comparisons?
Yes, yes, yes, I know .. consult your supplier. But for others wishing to banter and or willing to give me a heads up, I would be grateful.
Thanks to all the friendlys
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Replies
Yes
I could do that on StruCalc if I were at home. I'll try to remember to do it. There's more to it than crossection, though. What's the column height? And honestly, what's your loading? It's not often in residential work that column strength is a limiting factor.
Thanks and loading
Thanks Andy,
The loads are pretty big, or seem big to me. My county says build to 60 pound snow load. I know snows can be even bigger so I am looking at 80 pounds. I have one large beam in a great room so the designer has spec. 15,000 pounds on a 6x6, DF #1, 20 foot, column, as an example. I think there might be one, point where two beams come togather on one column and the total load is 28,000 (I need to check on this).
Secondly, I should expand teh discussion to beams. I have a number of beams which are not to been seen and if VersaLams are cheaper I would like to consider them where he has specified Glulams.
My designer is pretty good, but I think he is a bit limited in thinking about alternative materials, so I am not sure how to work out hte chose of materials on my own. And it might be that the cost of GluLam vs VersaLam is a push anywhy. Thoughts?
Thanks
Failure in compression
Assuming your load is even across the end of the post Doug fir has a compressive strenght of upwards of 6000 lb/sqin. So a 6x6 will support a load somewhere around 165,000 lbs if the beam sits on top of the post. If you are hanging the load then there could be some shear forces involved.
Slenderness ratio
Mike, did your calcs consider the slenderness ratio of a 20 ft. column? To me, that sounds potentially dicey, particularly with sawn wood.
Andy, no.
It's not really a calc. Just compressive load. There are lots of other factors involved. Obviously a freestanding 20' column would be absurd. There has to be some latteral restraint and resistance to bending. Also shear considerations. The real point was that 28,000 pounds may sound like a lot, but with the compressive strength of wood, really isn't.
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Supplier?
Did you talk to your supplier about these beams and loads? They should be able to do the engineering on the VersaLams and the Glulams, usually for free. And they'll give you a report you can take to the building department.
Columns
Are these columns (and beams) to be for exterior or interior use? If exterior, make sure that any engineered material / product is applicable for exterior use. Even wrapping them is no guarantee that they will stay protected over the long haul.
Also what is the finish?
If it looks cheap, it was over priced regardless of the cost.
If you google 6x6 load table you can get an idea from pages like this:
http://www2.wwpa.org/Portals/9/docs/PDF/TN9.pdf
According to it you're going to need something larger than a 6x6 even if it's a structural select doug fir with those loads and lengths.
edit: The boise cascade versalam guide has load tables for posts.....again 6x6x20' isn't going to cut it.
http://www.bc.com/...versa-lam.../western%20versa-lam%20guide%20060...
I'm primarily a finish carpenter so I'll tell you what I'd do with 20' columns that were visible and I'm assuming will be either wrapped in wood or painted over a layer of sheetrock.
Kiln dried posts in larger sizes are expensive and then are a special order so you'll be lucky if they are straight to begin with and will remain straight as they finish drying once installed. Green posts are even worse!
If the price was reasonable I'd go with the versalam post simply because it won't twist or bow nearly as much, although it will still shrink a bit.
If you have to go up to something like an 8x8 I'll bet it's cheaper to go with a metal post - it will definitely be thinner.
I'm having a hard time imagining 20' posts being anything but an eye sore in a great room. I would factor that in with the additional cost and installation cost of the posts when deciding on a broken up beam over a larger single beam. Under those posts will be a load path down to a separate footer so that's an added expense as well. Posts seem like a good idea, but unless they are in an area where they add rather than detract from the feel of the room I don't think it's worth the savings.
It's not an uncommon problem in larger houses in snow country and you can get a number of other ideas by picking up any log or timber frame oriented magazine since they often have large great rooms, high ceilings, large snow loads and whatnot. I've seen a number of designs that look good by using a few steel rods as rafter ties - an engineer would have to spec them out, but they still allow an open ceiling and avoid those darn posts.
Here's a load table for glue lams:
http://www.aitc-glulam.org/column.asp
Long column strengh for a given cross section (e.g. 6x6) has zero to do with the strength of the wood, the load supportable is proportional to the modulus of elasticity and the free length and how the ends are either fixed or free or pinned.
DF = 1.6E6, not much else is better in common woods except longleaf pine. Hemlock& soft pine are 60% less,
Lam column strength totally dependent on type of wood.
Update
Teh 6x6 is for interior use. It would be embeded in a stud wall and not free standing, thus laterally supported.
The designer I am working with runs all the engineer calculations. I have ssen the work for teh beams but not for the column. He says all is well. I am just being thorow (spelling ? complete).
Lateral bracing was concerning me. With it, end grain has pretty amazing compressive strengh.
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