Span Capability of an 8x8 Timber

      Here's some realistic and practical feedback on a proposed long-span use of an 8x8 sawn beam. September 27, 2012

Question
I am getting ready to have a pine sawed into an 8x8 by 20 or 25 feet long. How long can a span be without support?

Forum Responses
(Architectural Woodworking Forum)
From contributor J:
As written, this question is unanswerable. Hire an engineer.



From contributor I:
20 to 25 feet is a serious span - an 8X8 won't even come close.


From contributor P:
Figure an inch per foot of span. 20' would require 20", 25' would require 25".


From contributor L:
Too many things missing! Are the 8x8's used as joists? How far apart? What is the deck? What is the live load? Any codes involved? The width of a joist/beam is not nearly as important as the depth. Much of what the width is doing is keeping the beam from rolling sideways (why a 2x12 that is only 1.5" wide can span 16'). There are tables for stress rated lumber that show allowed sizes. The tables are usually based on allowed deflection, not ultimate strength.


From the original questioner:
The plan I imagine is that the 8x8 will be the center beam of a pergola with bamboo covering the top from the beam to the end of the pergola. So the load would be low on the beam.


From contributor A:
Get a copy of the UBC or code that is relevant in your area. There are span tables based on load and species.


From contributor B:
Seems to me you're trying to talk yourself into a full span of twenty feet or more. An 8x8 (even with no load) would sag under its own weight at that distance. Stop agonizing over it and add a mid-span post. Any professional carpenter will tell you the same thing.


From contributor A:
A 6 x 8 will span 12' for flooring. Even with your light load, you are pushing at 20' plus the uplift and side shear for the wind. Maybe if you integrated some arched gussets.


From contributor M:
You can pick up Ted Benson's book "Building the Timber Frame House." In the back of the book there are all the formulas, tables, and example calculations for sizing sawn beams. There is always one caveat - you will have to trust in yourself as the lumber grader. This will never fly in an area where work is to be inspected by the building department, as they will require an engineer for any such application and the engineer will then require the lumber to be graded, which will likely not be possible if you are having the material sawn.

That said, if the building department doesn't apply, you can simply calculate your beam based on a low grade of material to cover the worst case scenario.

The formulas and tables list nearly every span imaginable along with all the various design loads. As has been mentioned, there is simply no way an 8x8 is going to carry 20', even carrying its own weight. It will undoubtedly sag over time. You are looking at a pretty massive beam to carry any 20' span.



From the original questioner:
All great suggestions. I will take a look at the codes and the other suggestions, especially the mid span post, arched gussets, and Ted Benson's book. Thank you.


From contributor S:
I did a partial post and beam house and there were three areas that had support beams. The longest spanned 34 feet and I was told I would need an engineered beam 5" by 27" and laminated. The next spanned 23 feet, and it was 3" by 18". The last ones were all 8 foot span, and I was using 3" by 12" solid fir. The engineer rejected 2 of the 8 foot ones because they had knots in them. It was easy to deal with because they weren't installed yet.


From contributor M:
The hard part about it (it becomes political) is it all falls back to the grading. The engineer technically has no right to refuse anything if the material is graded. Each step in the process is simply a cover-your-butt situation for each trade. The engineer is supposed to do his calculations calling out a specified size member for a specified span based on a given grade of material. He may even call out several different size members based on a number of material grades readily available in a given area. The builder/customer then has to decide which they can integrate into the project based on size and cost. Beyond that the engineer has no jurisdiction over refusing a piece of lumber, as he is simply not a lumber grader and likely holds no grade stamp or education in the matter. He is actually opening himself up for future liability in that he acted as the lumber grader in that situation. Knots in a member doesn't inherently indicate a thing.

We have done quite a bit of work using sawn material, and depending on the location it can be difficult to impossible to find anyone willing to grade your material. It's next to impossible to pay for large graded timbers in all but the highest end work. For that reason, most large timber frame operations carry their own in-house grading and engineering and then of course the liability coverage to protect themselves.

Most any time we have tried to work large structural members into a project where any building department is involved, those elements are quickly phased out of the project due to cost. But again, this is speaking to what I would consider mid-level work as opposed to high end.



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