Please help me with my wood
July 25, 2007 12:41 PM   Subscribe

How much pressure can I put on a piece of 6x6 lumber across a 6'6" span without deflecting past the standard l/360 figure? The span tables only have figures for 2-by-X lumber. The back of their full book has some technical values for 6x6's listed but I don't know how to use them.
posted by rolypolyman to Science & Nature (9 answers total) 1 user marked this as a favorite
What grade? What species? What level of working? Treated or untreated? Aged or green? Once you answer these questions, the US Forest Service has done the [t]re[e]search. They can even tell you the BTU's of that lumber if you burn it!
posted by Pollomacho at 12:58 PM on July 25, 2007

Also is your load at the center or edges? Equally distributed or point loaded?
posted by true at 1:00 PM on July 25, 2007

Sounds like you're planning a tile installation, yes? You might well be able to get your question answered here if you strike out in AskMe. Looks like PolloMacho might have you taken care of though...
posted by Pantengliopoli at 1:08 PM on July 25, 2007

pollomacho: #2 pine, pressure treated
true: equally distributed
pantengliopoli: no, it's not a tile install

posted by rolypolyman at 1:32 PM on July 25, 2007

I may have a reference book at home - I will look tonight. For a lower bound you can use the demo version of beamchek. You're limited to a 10-11 foot span so it will be pretty far below the value you can actually support with a 6'6" span, but if it's good enough for your planned usage you don't need to look any further.
posted by true at 2:24 PM on July 25, 2007

I actually got a little too into this, but since it's something I've been meaning to do for myself...I put the appropriate values in a google spreadsheet. You should have a shared copy at the email in your profile shortly. (For anyone else there is a viewable copy here). You can adjust the length (in inches) and total load (assuming uniform distribution). Compare the actual values in red against the acceptable values in blue.

In order to pass muster your beam must work in three areas: total load, shear, and deflection. I plugged in some sample values and it looks like you're probably going to hit on total load first at about 2000 lbs, give or take. I didn't know if you were going to put it in a wet area or have a permanent dead load rather than a live load (people + things) - you need to use adjustment values for those things.

You have two things working against you - first that the depth is only 6 inches (really 5.5). First is that strength increases exponentially with the depth, so make it an 8 inch beam and it can easily hold twice as much. Second is that it's #2 pine - select structural would give you twice as much as well.

Caveats: I am not a structural engineer and you should check these calculations with your local lumber store, they can probably toss them in whatever software they are running. Most of the math comes from "Basic Lumber Engineering for Builders", and the wood values come from the site you posted. Let me know if anything looks wrong.
posted by true at 6:40 PM on July 25, 2007

First is that strength increases exponentially with the depth,
It's been a long time since mechanics but I'm pretty sure it only increases as a polynomial, maybe third power or so.
posted by hattifattener at 10:19 PM on July 25, 2007

Right, nice catch. I think it is width*depth^3.
posted by true at 5:34 AM on July 26, 2007

Deflection is a function of stiffness, not strength. Stiffness varies linearly as width. A 6x6 is 3.67 times as wide as a 2x6 (5.5 / 1.5 = 3.67), so it will be 3.67 times as stiff (assuming the same species and grade), and the deflection will be 1 / 3.67.
posted by Bruce H. at 9:29 PM on July 26, 2007

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