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Author Topic: Building stuff knowledge i don't seem to have  (Read 1845 times)

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Offline Grizzly

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Building stuff knowledge i don't seem to have
« on: December 14, 2017, 04:40:53 PM »
If I've asked before I give you my apologies now. But I'm wanting a roof over the mill and I've got poles in the ground and I'm doing a bit of this and a bit of that building style. I've tried using the calculators to figure out the technical strength of my poles and am hitting a dead end. I've tried using the beam calculator as well and come up with numbers that make no sense at all. I've visited with a few old builders who put things up according to acquired wisdom and whose buildings are still standing. My question is along these lines.

Poles are power poles with minimum 14" base diameter. They are currently rising to the sky some 30ft and will be trimmed to beam height. The poles are 20ft apart by design to allow 16ft lumber to move anywhere anytime. I need a beam to span 80ft total sitting on 5 poles 20ft apart. I've decided against trying to build my own beams as I just haven't found a beam design I can do that I'm happy with. So I can purchase Fir from BC or buy LVL here. Fir from BC I would just get 8"x24" and I'd be good but what about LVL? What do I need with that? I have the idea that a 2 ply 3 1/2"x18" would be enough but that doesn't fit the rule of thumb idea of an inch of beam for each foot of span. What say y'all?
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Offline Don P

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Re: Building stuff knowledge i don't seem to have
« Reply #1 on: December 14, 2017, 06:07:17 PM »
Sizing the beam is straightforward. I'm assuming you are using 20' sticks but if you can do 40's in a built up beam you can weave the joints and make the connections over the posts stronger. What is your design snow load in lbs per square foot?

It shouldn't be too hard to check the posts for buckling under full vertical load.

Where I can't help is with lateral, that is a huge sail on top of tall posts, what is bracing that? Are there walls to provide bracing panels anywhere?

Offline Grizzly

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Re: Building stuff knowledge i don't seem to have
« Reply #2 on: December 14, 2017, 10:31:59 PM »
Snow load - According to jabacus my ULS is 31.4psf & SLS is 28.3psf

Rafters will sit on beams and be braced to poles as well as beam being braced to poles. Side walls will be closed in at some point, but not sure how quick. Roof will be tin. Poles will be trimmed down to create a 16ft ceiling. So approximate pole height is 14'6".
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Offline Don P

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Re: Building stuff knowledge i don't seem to have
« Reply #3 on: December 14, 2017, 11:45:57 PM »
I fired that off too quick, may have done that a couple of times tonight.
 Also think about up. With wind under it, that sail is pulling everything out of the ground and at every connection on the way to the tin, think that through as well. Up can get overall in the 15psf range, Chapter 3 of the codebook has some guidance there, then deduct dead weight from that and somehow tie the rest of that upward load to the ground.

Second thing was building width? ... let me show you, you can modify inputs
Assume 32' truss roof width on the building, 2' overhangs, each line of beams carries half of the weight. A 20 ' long beam is carrying half the 32' width plus the overhang, 18'. 20' of length supporting(x) 18' of width=360 square feet of roof supported by 20' of beam. 360 sf x (31.4 psf snow+10psf dead load) .. 360x41.4=14,904 lbs
I'm a rounder upper if we're that close, 15 kips

Load 15000
Span 240
play with dimensions
Fb #2 dougfir heavy timber 875, #1 is 1300, LVL 2800
E #2 DF 1.3, #1 1.6, LVL 2.0
Fv #2 DF 170, 170, LVL 280

You can change the width in that example to suit your real width and then play with the materials and sizes on the toolbox simple beam calc. Holler if you get stuck. And let me know what your numbers end up being, also the tip dia at 16'.

Offline Grizzly

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Re: Building stuff knowledge i don't seem to have
« Reply #4 on: December 15, 2017, 10:01:34 AM »
The calculator wants to know fiberstress, modulus of elasticity, and horizontal shear. I don't remember any builder I've talked to using those terms and I don't know what values to input. Span of building will be 40ft between beams with 2ft overhang on rafters. I'll use trusses.

Should this stay in thread or should I PM?
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Offline Don P

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Re: Building stuff knowledge i don't seem to have
« Reply #5 on: December 15, 2017, 10:15:48 AM »
I'll shoot you a pm.

Offline scsmith42

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Re: Building stuff knowledge i don't seem to have
« Reply #6 on: December 15, 2017, 10:30:12 AM »
If y'all keep it in the thread more of us can learn!
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Offline Grizzly

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Re: Building stuff knowledge i don't seem to have
« Reply #7 on: December 15, 2017, 11:27:45 AM »
Thanks. Feels better knowing I'm not the only one needing an education.

So correcting load numbers to a 40ft span and using Don's numbers I get to this.
Load becomes - 17,000
Span - 240
LVL Beam - I need 6" x 18"
#2 Douglas Fir - I need 8" x 22"

Did I do it right?

LVL I can buy locally but fir I'd need to source from Percy or something and then truck it here.
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Offline Don P

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Re: Building stuff knowledge i don't seem to have
« Reply #8 on: December 15, 2017, 06:59:27 PM »
Hopefully y'all will check me   :)

I didn't label those design strength values very well near the bottom of post #3.
Fb is the Fiberstress value the calc is asking for. It stands for "extreme fiberstress in bending". It doesn't mean "extreme" the way you're probably thinking, the extreme fiber in a beam is the strap of wood running along the top , or what we're looking at here is that strap of wood that forms the bottom edge. The extreme fiber is the furthest fiber away from the "neutral axis", the horizontal plane that would run along the center of the beam.             A bending failure, which is what the Fb equation is looking at, would be when the fibers along the middle bottom of the beam begin to tear apart, splinter down, pop. So this part you must pass, it is a safety check. Yes there is a factor of safety built into that design value, that is why redneck dimensions usually sag rather than fail, they're some unkown depth into their safety and into the next check, deflection, sag.

The modulus of elasticity, E, is a check for stiffness. Rubber has great bending strength but isn't much of a beam, it is too elastic. I would limit deflection here to L/180, 1 inch in every 180" of span. Span here is 240" so limit your deflection to 1.33" (about 1-5/16") that would be under full snow load. As long as your Fb check and shear checks are looking good, on a building like this E is more of a judgement call but the more you let it slide the more those deflections start to become permanently set.

Fv, horizontal shear, as a beam bends the fibers along the bottom are stretching, the ones along the top are being compressed. Along horizontal planes within the beam the beam is trying to slip to accommodate those differing stretching and shrinking lengths. If those stresses get too high the beam splits down its length around the neutral axis usually beginning near or at the end supports but it can be at poorly thought out notches or holes. You'll find this is a concern mostly with short, heavy, heavily load beams. A church belltower support beam is the typical example.

So those were the design value inputs explained. The problem;
Tributary area for each 20' section of beam span is half the 40' span +2' o'hang =22'
Trib area= 22'x20=440 square ft
Design load= 440sf x 41.4 lbs per sf= 18216 .. ah you used the SLS 28.3+10psf dead load, ok, we'll go with the 17 kips

LVL comes 1-3/4" thick per ply, so a 3 ply is 5.25 for width. Ignoring deflection for the moment and rolling down throught the sizes hunting the failure, 14" fails, 16" passes, deflection is around 7/8", your call, it drops to about 5/8 at a triple 18" lvl. Your deflections will actually be a bit less if you buy 40'ers, stagger the seams over the posts and nail or preferably timberlock screw the plies together. This is an ag building I'd go with the LVL's, and they will do this math. Those suppliers of "engineered materials" have engineers on staff to check and/or stamp any of their products designs if you ever need it. I get lvls from my truss supplier who also engineers and supplies post frame buildings, if you do one stop shopping I'd run all this by their engineer. There's the beam math. I need to read up on poles. Bracing, can you weld?


Offline Den-Den

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Re: Building stuff knowledge i don't seem to have
« Reply #9 on: December 15, 2017, 07:14:28 PM »
Don is the guru here and is being very helpful.
I remember from when I studied this stuff a LONG time ago, that beam sizing is easy compared to working out the connection details.  Also that the details are VERY important.
You may think that you can or may think you can't; either way, you are right.

Offline Don P

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Re: Building stuff knowledge i don't seem to have
« Reply #10 on: December 15, 2017, 09:30:13 PM »
Guru, no, I'm just a carpenter who asked questions, feel free to comment on anything. You just made my sawmill lumber argument to the building inspector, usually the failure is at connections. This thing is big and open, and I saw the lake when I went to pm you, if you are looking across open water the wind has more fetch on you.

Offline Grizzly

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Re: Building stuff knowledge i don't seem to have
« Reply #11 on: December 15, 2017, 10:34:01 PM »
I checked manufactures and suppliers up here and I can get LVL in 3" thicknesses. I can make a 2 ply beam bringing that to 6". That is how I got it through the calculator to give me a pass on all 3 measures. I'm also able to get it in 80ft lengths if I'm willing to wait and pay the trucking. So if I can avoid the joinery at a reasonable cost that looks good to me. We have our own truck and a friend has a truss trailer so who knows what we'll choose to do. I'll need to add a  bit of load on for the overhang. Forgot to do that. Regarding that wind lift; I will be closing in 2 sides of this as soon as possible. Our only wind risk here is when it comes from the east and that is not often. Still have to plan for it though. If I'm not mistaken 2 sides closed creates pretty good dead air space?

Thanks Don for your patient help getting me through the calculator and providing me with values I had no knowledge of. I'll come back to this as I progress.
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Offline 711ac

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Re: Building stuff knowledge i don't seem to have
« Reply #12 on: December 16, 2017, 05:23:15 PM »
I have had suppliers spec the beam for me before. This was with LVL's and or TJI's.
Didn't I read in your first post that 20' was the max. span? (5 poles @ 20' - 4 spans)
My local snow load (code) for trusses is 100#/sf.

Offline Grizzly

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Re: Building stuff knowledge i don't seem to have
« Reply #13 on: December 18, 2017, 11:11:29 AM »
Suppliers up here seem to always ask what the engineer has speced. This ain't engineered so he said he'd go with what i requested.
Yep, total length of 80ft. Not sure if i understand what your getting at?
4 hours north of me your in tundra country. No trees, no snow, not really much of anything. We are not in a high snowfall zone. We just have snow and cold for most of the year.  ;D
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Offline 711ac

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Re: Building stuff knowledge i don't seem to have
« Reply #14 on: December 18, 2017, 08:51:00 PM »
The poles are 20ft apart by design to allow 16ft lumber to move anywhere anytime. I need a beam to span 80ft total sitting on 5 poles 20ft apart.

Grizz, your "talk" about getting an 80' beam and it's cost/trucking etc. through me off and you need to span 20'.... 4 times ;D
Having used LVL a bunch, but not being an engineer, my "seat of the pants" feeling is that a 3 ply (std, 1-3/4" X 18") 18" tall beam would do the trick for a 20' span with your light snow load. I would buy 40 footers to "tie" the beam/post's, alternating the laps. (1st layer use 2-40 footers, 2nd layer start with a 20 footer, 3rd layer use 40 footers again) I would also (if your posts are wide enough at the beam location) notch the post to create a 5" (5-1/4" would be ideal) "seat" for the 3 layer beam to sit on with a 18" tall section of post on the inside to fasten to, further tying everything together. (make an "L" cut in your post top).

Offline Grizzly

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Re: Building stuff knowledge i don't seem to have
« Reply #15 on: December 19, 2017, 09:25:01 AM »
Now i gettcha. And yes, that will be the plan if a single 80ft proves to be more money than i like. One carpenter had the thought of using a steel cap on the pole with a saddle to place beam in. I like the idea but haven't really chased it yet.
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Offline Don P

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Re: Building stuff knowledge i don't seem to have
« Reply #16 on: December 19, 2017, 05:37:46 PM »
Bracing the poles and beams is going to be the trick both in the plane of the beam and also in the plane of the trusses

Offline Grizzly

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Re: Building stuff knowledge i don't seem to have
« Reply #17 on: December 19, 2017, 05:44:43 PM »
For bracing on the rafter plane i was thinking of a brace that goes from about 4-6ft down the pole up across the rafter to about 1/3 - 1/2 way up the top slope. Is that about right?
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Offline Don P

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Re: Building stuff knowledge i don't seem to have
« Reply #18 on: December 19, 2017, 07:57:39 PM »
That is beyond what I know. On smaller ones we've bolted through the post with 2@ 1/2" bolts, run up into the trusses at a 45 degree angle and nail very well to bottom and top chords of the truss. The bigger those triangles are the stronger it'll be.

Offline Grizzly

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Re: Building stuff knowledge i don't seem to have
« Reply #19 on: December 20, 2017, 01:11:03 PM »
You're describing what i want to do. I just didn't do it very well. (the description)
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