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Topic by dem45133 posted 01-09-2012 01:06 AM 37567 views 0 times favorited 12 replies Add to Favorites Watch
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dem45133

6 posts in 2503 days

01-09-2012 01:06 AM

Topic tags/keywords: question

Good day gentlemen and any ladies.

I am in the middle of a major remodel and am joining two rooms that are separated by a main support wall for the home. This place is a 105 year old “sorta cape-cod but without the dormers (yet)” 11 foot sidewalls to eave, 10.5” pitch. 4 rooms down 2 up centered over the bottom. 24×30 outside footprint. Upstairs “knee” walls are almost full ceiling height. The wall I am removing is holding up the second floor and by extension the roof… so its critical to beam correctly.

We are doing a special open joist ceiling of the bottom floor showing off our Douglas Fir floors upstairs and a brightening of our red oak full dimension joists. The open beam will set mid-span where the joists meet across a 24 ft wide gable. I do not want modern materials unless its hidden under matching rough sawn red oak… and I did research this some.

But… why not use the real thing? I have a woods full and so does my neighbor (who also has a 42” circular mill) where we can cut a solid red oak beam with the matching circular kerf marks just like our hundred year old joists are (as compared to band sawn). The ends of said beam will be inside a 4”wall sitting on 4×8 oak vertical posts set directly on the base plate on top of the center foundation. Being hidden I plan to through bolt 1/2” by 4” steel plate vertically between the support posts and the sides of the beam to lock the beam ends in place to control warping while it dries (yes it will be only air dried outside for some period of time before installing… but it will not be kiln dried. the load span will be 13’8” and I am leaning toward a 8inch wide x 12” (maybe 14”) deep solid beam cut from one of our trees that has a clear 16ft section of trunk. While drying I may center and quarter post it temporarily… but not sure yet… it really should cure under a minimal load or none I believe.

Does any one know of an engineering calculator to calculate bearing capacity of a span… and the modulus of elasticity for red oak?. I grew up on a place with mortise and tendon east coast 250 year old barns that we put better than 50,000 bales in… still strong as it was when they built it… solid beams are certainly nothing new… but thought I’d ask for any opinions and a site for the calculator. Remember…this is all open construction and finished… no hiding places anywhere.

Oh, I am an scientist/engineer kind of guy… just not in civil… I can do the calculus on any of it but I am way way out of practice. Calculus classes were 35 years ago.

Calculator?

Opinions?

Many thanks in advance… Regards, Dave



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dfletcher

128 posts in 3233 days

01-09-2012 02:28 AM

You could get on the net and locate a free beam calculator. These will tell you what engineered lumber will do and you may even be able to find what regular lumber will take.

I would also check with your local lumber yard. (Not Home Depot or Lowes)

Get them to size the beam for you if you were to use 2x material. Trust me, if they can help you with the sizing for 2x material, a solid beam the same thickness and height out of red oak will be twice as strong and probably outlast the house.

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Grandpa

139 posts in 2728 days

01-09-2012 04:57 AM

Just a side note…pound for pound oak is stronger than steel.
go to the lumber yard is good advice.
Have any new oak dried before using it.

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dem45133

6 posts in 2503 days

01-09-2012 12:36 PM

Thanks dfletcher, I was going to calculate what load the standard materials would have done per required code and then size the oak accordingly.

To my knowledge, none of this house was built with dry oak… had to be wet to drive the 1/4” shanked by 6” spikes (no idea what “penny” they’d be) through it where the joist are overlapped. Same with the 16s and 20s that were used all over. I believe in the old days (1907 for this place) they built with green and let the framing dry in place. If I anchor the ends of the beams as I mentioned, any twist will be minimized, I think anyway.

I’ll be asking at a different time on re-anchoring the roof sides at the eaves. Raised leaded seam roof is original and in general in good condition. None of today’s roofs will ever hold up a hundred years like this has, (except where some fool drove a nail through it, that comes out after 25 years and creates a leak that otherwise would not have been there!). If the roof is maintained this old house will last forever.

Thanks, Dave

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Dan Lyke

331 posts in 3878 days

01-12-2012 08:00 PM

My local building department required a PE’s stamp on all calcs that weren’t prescriptive by standard load tables, and I’m not sure how you’d get a piece of wood engineering stamped, but if you’re just trying to be safe and not trying to get an inspector to buy off on it:

The American Hardwood Export Council suggests that the modulus of elasticity for Red Oak is between 12,549 MPa and 15,721 MPa. Multiply 12549 by 145.037738 to get PSI, or 1820078.

I’m too lazy to go back to my calculus, so we’ll just use the approximation and say that the moment of inertia of a solid wood beam of <= 12” is width × width × height / 6, or, in this case 8 × 8 × 12/6 = 128.

Joists meeting across a 24’ span says you’ve got… uh… I dunno, lump live and dead load together and call it 40 lb/sq.ft on a second floor, or 480 lbs/ft of beam (because that load is supported on the sides, too)? Or 40 lbs/in,

I can then put a bunch of load numbers into a calculator like this one, and I think, if I’m entering the numbers right, that’s claiming 1.62” of deflection. My gut feel says that’s pretty high, on the other hand 480 lbs/ft on a nearly 14’ span is significant. At any rate, not nearly the l/360 sort of limit that my city’s building department would want to see…

(And if I were getting paid for this I’d spend the time to go back and double-check the assumption that the “y” in that calculator is half the beam depth, and probably actually do the calcs myself rather than trusting someone’s random JavaScript…)

I’d be tempted to build a box beam around a steel I beam and let the steel supplier do the engineering calcs for me…

-- Dan Lyke, Petaluma California, http://www.flutterby.net/

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Grandpa

139 posts in 2728 days

01-13-2012 04:51 AM

I believe an architect has the PE stamp that you need. They are engineers too.

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Dan Lyke

331 posts in 3878 days

01-13-2012 05:16 AM

The architect I talked to for my workshop was not a Professional Engineer. He was very helpful on some fronts, but sent me down some rather mundane paths on other fronts. My general impression (and this is coming from the sort of person who will dive into a question like this with an answer like you see above, so take that with the appropriate grain of salt) is that the PEs are the people who really understand the trade-offs and options at a level that’s interesting to me, and generally take a concept from the architects and say “okay, now here’s what you can actually build from that vision”.

-- Dan Lyke, Petaluma California, http://www.flutterby.net/

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dem45133

6 posts in 2503 days

01-14-2012 04:52 PM

Hello Dan…

Thank you for your kind workup… but upon working that calculator I believe it has issues. I located another and a neat book on beam design as well as other floor joist, column supports etc. and the tech spec on a whole host of species. Its called “Wood Structural Design Data” by American Forest and Paper Association. its located at http://www.awc.org/standards/wsdd.html if anyone is interested. A calculator is there too.

The design of this house… has the center support foundation and 1st floor center support walls carrying just under 3/4s of the roof structure down through the high knee walls to mid span on the second floor joists which then divides it in half to each end of its span… translates to the center support wall carrying in the neighborhood of 11000 lbs accross the 13’8” beam span. That’s quite a bit more than the average – I think. So yes, its imperatively critical to build to spec. Any deflection also translates clear up to the roof peak… on essentially 1/2 the house’s length.

My first thought was steel and had a couple ideas for that. One was to use two 9”x2” 1/4” wall rect tube (similar to that I’ve used on an 12000 lb capacity tandem axle trailer mains…it has three 5×2x1/4wall main frame members)... weld the two together (creates a 9×4 with a full inch of web) set that under my existing top plate (leaving it intact) and then boxing in around it with 1” rough sawn red oak to match the rest of the structure. This idea appealed because it will be exactly 4” wide which is the same as my top plate and be easy to box and the max depth will be 11”. I was going to set this pair of 9×2s on 4×4x1/4 square tube in the end walls down through to the first floor base plate with a 4×8 ” 1/2 steel plate on the end of the square tube to prevent fiber crush (although it’s well cured after a 100 years, but I’ll spread it out anyway). In lieu of the rec tube pair I could set an I beam under it the top plate and then build out its width to match for boxing.

But I will not see a wood beam… I’ll see a box built with 1” planks around a steel beam trying to imitate a wood beam. I’m real bad that way. So then I got on the idea of staying with wood… in 1907 they wouldn’t have had steel easily and would have used what they had… lots of red oak, white oak, hickory, elm and chestnut back then in this area.

The calculator says I can use a 9×12 red oak to support the above load and meet l/360 as well as the other other shear and stress limits for the red oak although it quotes a deflection of only something e-7 (that’s 10 to the minus 7 power for those not familiar with scientific notation, or something like 0.000000367”) which makes me question that calculator too. l/360 requirement was 0.35” or so if I remember.

Oh, we do not have any building inspection requirements here, or permits, let alone any PE sign-off requirements. But as I said in another post on electrical… one does need to learn and research. I have a personal issue with PE sign off as in my industry (environmental restoration), scientist like me who didn’t happen to get an engineering degree, were no longer allowed to design restoration systems due to an EPA requirement for a PE sign off… Eventually that caused the private sector to require a PE even for the Project Managers. This displaced a lot of us who got their degrees before there was anything called environmental. I graduated with my BS in 1981 in Ag, (basically the Earth Sciences)... environmental was in its infancy and curricula designed around it didn’t exist yet. I also have a MS is Environmental Management… but its limited to waste management and regulatory compliance essentially (which is what I do BTW). Ohio codified only a handful of engineering degrees to qualify to even take the fundamentals exam. To me if you can learn it through self study and night classes and can pass the exam one should be able to do so… and then do the EIT portion, but not here! My grandfather did… took him 20 years starting out in drafting and kept studying and taking classes. Can’t do that now.

Well, its time to get busy…

Thanks all and I appreciate all your thoughts on the matter. Dave

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Dan Lyke

331 posts in 3878 days

01-15-2012 06:36 AM

Yeah, I didn’t double-check the calculator.

However… So 11,000 feet for the 13’8” span is 800 lbs/ft. A little warning here: Beam and joist span includes “live load” and “dead load”. Some of the worry with a beam is that flex can be more of an issue than overall load bearing weight, for things like floor squeak (and freaking out when the floor moves) and wallboard or plaster cracking. So it’s important to separate the two, if only because you may in fact have way more dead load than live load.

I believe that l/360 is just the length over 360. 13’8’ is 164”, so l/360 for your span is .45”.

Back to calculators…

The American Wood Council Maximum Span Calculator says says that red oak 2x12 with 24" joist spacing can carry 50lbs psf live load and 20 lbs psf dead load over a 14'1" span.

But because that’s psf for the supported floor, we can double that for load per linear foot of beam, that’s 120 lbs per linear foot. If you assume that weight carrying capacity scales linearly with width (and it’s gonna be pretty close to that for a lot of reasons), this suggests that your 8×12 could carry 480 lbs/ft over that span. If you could really go 2” more depth you may be getting into the right territory, but that calculator won’t go there.

And I am totally with you on PEs and certification.

-- Dan Lyke, Petaluma California, http://www.flutterby.net/

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dem45133

6 posts in 2503 days

01-15-2012 04:03 PM

Hello again all…. and Dan

Since beam depth is an issue (head clearance) logic indicates I can use a shallower beam if in effect I shorten its effective span by using a 45 degree member (its got proper post and beam name I cant think of at the moment) say at the 3 ft mark in from each end… if these are slightly notched into the bottom of the beam and the side of the 4×9 end posts (0 shear slippage) a portion of the beams load will go down through the 45s and into the posts (posts must be positively anchored at their base as there will be a lateral moment to them now as well as vertical)... what then would the effective span be? I’d venture to swag that it would be about 11’8” then (or so)... There would also be a minor stiffening effect on the deflection for impact forces. I’d have to go find my old engineering trig and calc books but they’re all boxed up as part of this remodel and out in the shop… no idea what box they’re in. Its been a while since I worked out math problems like this.

Yes I am aware of both dead (static) and live (dynamic) loads. Were high on both these numbers as we are book hounds (120 linear feet of book shelves on this load – that’s just upstairs on the north half of the house, there is another 130 lf downstairs) as well as pack-rats (eaves are full too but better than 1/2 of this goes down the side walls). With a 10.5:12 pitch and on top of a ridge we get a lot of wind forces too.

Another thing I could do is incorporate some decorative inner posts at the 2 or 3 foot mark… center span then would only be about 8 feet (on 3s), 10ft (on 2s)... which is likely the best way with the loads we have. I really would like it clear spanned if I can… one front room full width of the house facing our north view out over a valley. Might still have to go with steel. Humpht.

Dave

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Dan Lyke

331 posts in 3878 days

01-15-2012 11:27 PM

I believe (and I are not a professional engineer, I’m just a self-taught college dropout computer geek) that it’s fairly simple trig: The sum of the forces in a triangle add up per Pythagoras, so on a 45° strut holding up n lbs vertically, the strut will be taking 1.4×n (1.4 being approximately the square root of 2) in compression.

But as long as you don’t get near buckling, cosmetic sizes should be way over what you need in compression (with any sort of sheathing, I think I remember that prescriptive code says you can put 3000 lbs on an 8’ 2×4 as a stud in compression, over 8,700 lbs on an 8’ 2×6.). In fact, maybe thinking about a strut at an even shallower angle would give you your center clearance, and some more sense of width.

-- Dan Lyke, Petaluma California, http://www.flutterby.net/

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rockwill

2 posts in 2453 days

03-02-2012 12:52 PM

Hi,
Many people who rely on calculators forget how to perform even simple operations over the years! Do you really want an engineer who can’t divide decimals, or who gets trig work done by fiddling with plastic buttons? You need to understand the heart of the math to survive and thrive in mathy fields like engineering.


lawn maintenance

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dem45133

6 posts in 2503 days

03-02-2012 01:53 PM

Very true, especially when you only use those skills sporadically. I fall into that group. I’ve stressed the same point to my nieces who are science and engineering oriented, and any other child if I get a chance.

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