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Author Topic: How much bow is acceptable?  (Read 4242 times)

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Offline oe_alex

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How much bow is acceptable?
« on: February 27, 2014, 10:10:00 am »
Hello everyone, I'm a landscape contractor located in Iowa. Timber framing has always piqued my interest, this forum seems like a good group of helpful guys.

We are going to be building an 18x18 Western Red Cedar pavilion this spring. Plans are drawn, I will attach a photo from SketchUp of what we're planning on doing. I just the other day got in the 8x12x24's for the headers, plates and ridge beams. I pulled a string along the edge of one and noticed a little bow in it, maybe a 1/4" in the center. Is this acceptable? There is also one with a bit of crown in it. I'm thinking the best place for that would be the ridge, and the two that have the slight bow in it for the headers. Planing will probably not be an option, so any advice you may have would be greatly appreciated! I also must admit that this won't be an actual timber frame, we're planning on using 3/4" lags with malleable iron washers.

  

 

Offline Raider Bill

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Re: How much bow is acceptable?
« Reply #1 on: February 27, 2014, 10:18:07 am »
Welcome to the FF!

The way I build a 1/4" off it in 24' is dang accurate :D
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Offline Dave Shepard

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Re: How much bow is acceptable?
« Reply #2 on: February 27, 2014, 12:28:44 pm »
That shouldn't be a problem. Does the sag go away if you put the timber on saw horses with the horses right at the ends? I bet it does.
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Offline oe_alex

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Re: How much bow is acceptable?
« Reply #3 on: February 27, 2014, 01:06:01 pm »
I just went out and pulled the string on all five. Three are pretty true, one is 1/4" and the other 3/8". So for 24' I would also say that's pretty dang good. Aside from the middle one, crown is very minimal.

These will be sitting for a while before we cut them, should I have them sitting another way than how I have them in the picture? I am thinking of sitting the two with bowing on the face with the bow up to get them to settle a little. I also have the dunnage spaced about 4-5' currently. Trying to correct the crown on a 12" tall piece of timber seems impossible to me, will have to make sure to crown up when building and let the load take care of them.

Offline thecfarm

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Re: How much bow is acceptable?
« Reply #4 on: February 27, 2014, 01:18:27 pm »
oe_alex,welcome to the forum. That will look nice!!
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Re: How much bow is acceptable?
« Reply #5 on: February 27, 2014, 01:19:03 pm »
I'd place them all crown up, and move the dunnage out. Maybe ten, or even twelve, feet apart. I sawed some 30' white oak timbers last June. They had a little bit of crown to them, but the framers said that if they put them on horses and someone sat in the middle, it was enough to take the crown out.
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Offline Peter Drouin

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Re: How much bow is acceptable?
« Reply #6 on: February 27, 2014, 09:59:07 pm »
You won't see it after it's up. :)
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Offline Brucer

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Re: How much bow is acceptable?
« Reply #7 on: February 27, 2014, 11:36:55 pm »
WRC is notorious for moving as you saw it. That amount of bow is trivial.

Where possible try to have your beams a little higher in the centre of the span, rather than lower. The weight of the roof is going to try to straighten them in any case.
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Offline oe_alex

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Re: How much bow is acceptable?
« Reply #8 on: February 28, 2014, 10:14:50 am »
Thanks everyone. When I say "bow" though, I am talking on the face side (the 12" side). That's where my 3/8" and 1/4" inch are coming from. The main reason I was concerned about that was that 5/8's of an inch throwing something off. The roof is something I definitely don't want to mess up.

As I was turning them all crown side up yesterday I ran across this. I didn't see it at the yard in Colorado as they had them wrapped for me. This particular beam is going to be a plate on the side. Is there any way of remedy-ing (?) besides facing it outwards and trying to have the rafter overhang hide it? Unfortunately it's right smack in the middle of the beam, and driving 10 hours to replace I think would be a bit silly.


Offline dukndog

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Re: How much bow is acceptable?
« Reply #9 on: February 28, 2014, 11:17:03 am »
Chamfer the "bad" looking part out. Basically take a draw knife and chamfer it to a 45 on the edge. If you do a search on "timber frame beam chamfer", there are some pictures explaining it.

Good luck!

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Offline Jim_Rogers

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Re: How much bow is acceptable?
« Reply #10 on: February 28, 2014, 01:14:34 pm »
Patch.

If you bought 24' long timbers and the frame is 18' you've got an extra 6' of timber, cut a patch out of one of the extra pieces and put it in where this defect is.

like this:

 

 

It can be done and not show much.

Good luck with your project.

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Offline witterbound

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Re: How much bow is acceptable?
« Reply #11 on: February 28, 2014, 06:05:36 pm »
I'd chamfer it too, and put it outside and up.  The roof will be coming down there.  Nobody in their right mind should care ... But some clients think timbers should all be perfect like trim work.  It isn't trim ...

Offline oe_alex

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Re: How much bow is acceptable?
« Reply #12 on: February 28, 2014, 06:07:35 pm »
Patch.

If you bought 24' long timbers and the frame is 18' you've got an extra 6' of timber, cut a patch out of one of the extra pieces and put it in where this defect is.

like this:

 

 (Image hidden from quote, click to view.)

It can be done and not show much.

Good luck with your project.

Jim Rogers


Yes! Exactly what I was looking for. Five of the timbers will actually by 22' long for a two foot overhang on the gable ends, but I will still have enough to patch it up like that. Any other photos that explain the process?

Thanks

Offline Magicman

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Re: How much bow is acceptable?
« Reply #13 on: February 28, 2014, 08:24:08 pm »
Welcome to the Forestry Forum, oe_alex.   :)

The tear out, personally I would turn it up and never think any more about it.  No one will ever see it but once, and then only if you point it out to them.
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Offline VictorH

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Re: How much bow is acceptable?
« Reply #14 on: March 01, 2014, 01:10:56 am »
Welcome to the forum oe_alex.  Not sure what your time frame is on your project but Jim Rogers will be teaching a workshop on Basic Timber-framing in Southeast Nebraska in late May.  We have 2 slots still open if you would like to join us.

Victor
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Offline Peter Drouin

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Re: How much bow is acceptable?
« Reply #15 on: March 01, 2014, 07:23:01 am »
Yup the yard boy with the forklift, they knew they did it before they sent it.
2008 LT40 super,2008 edger, Cat telahandler, JD 5410 And can cut up to 45' long
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Offline oe_alex

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Re: How much bow is acceptable?
« Reply #16 on: March 01, 2014, 09:16:01 am »
Welcome to the forum oe_alex.  Not sure what your time frame is on your project but Jim Rogers will be teaching a workshop on Basic Timber-framing in Southeast Nebraska in late May.  We have 2 slots still open if you would like to join us.

Victor

We're hoping to have the everything wrapped up by Memorial Day, so there's plenty of season left for use yet this year. However, with 4+ feet of frost in the ground here, I'm starting to question that time frame as we will need a good 6 weeks to get everything done. The fireplace will be about 18 feet tall with 400 SF of thin natural veneer. 800 SF of concrete pavers, and the model image doesn't even begin to show the extensive landscape design with just about everything you can imagine included.

That said though, maybe you could provide me some more details? If I can make it work I'd love to come down. I have a brother that attends UNL in Lincoln.

Offline Brad_bb

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Re: How much bow is acceptable?
« Reply #17 on: March 01, 2014, 10:19:03 am »
Welcome!  It would actually be uncommon not to have some bow or crown.  This can be accounted for in which timbers you select for which location in a frame.  You can often orient to work with the crown.  Additionally, laying out your joinery with square rule method will help you compensate. If you have some very bad crown or bow, snap line square rule will allow you to use a wonky timber.  If you have a location where you are going to need a straight wall, you select posts that are going to help you with that.  If you have a free standing post, you can use a wonky timber for that.  If a timber is really too wonky, you can cull it (set it aside and not use it).  Maybe you can then re-cut it later or have that timber re-cut for brace stock or something else?  Even very wonky timbers can be used, provided they are structurally acceptable.  Much like round or natural logs are incorporated into a frame using scribe rule layout technique, so too can a very wonky timber be used that way.  But you should first learn basic square rule, then snap line square rule.  Once you're very comfortable using those, then you can learn scribe rule.  Your timbers sound relatively straight though, so I doubt you'd need scribe for this project. 
   You also need to learn how to identify defects that will affect strength, joinery etc., basically how to grade your timbers and cull as needed.  It's not that hard to learn.
   That corner damage is not a big deal.  As someone else said, draw knife it until it's relatively smooth (won't catch anything or stick anyone).  If that section will be exposed, maybe sand it a bit to round the edges and blend it in.  Basically make it look like natural wane.  It's then possible to still layout joinery in that area using square rule layout.  This is, provided there is enough wood left to layout the joinery that it will not weakened.  For example, if there's a tenon on the end, It may have no effect.  It's not all that uncommon to occasionally get some wane on a timber.
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Offline Jim_Rogers

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Re: How much bow is acceptable?
« Reply #18 on: March 01, 2014, 11:30:57 am »
Patch.

If you bought 24' long timbers and the frame is 18' you've got an extra 6' of timber, cut a patch out of one of the extra pieces and put it in where this defect is.

like this:

 

 (Image hidden from quote, click to view.)

It can be done and not show much.

Good luck with your project.

Jim Rogers


Yes! Exactly what I was looking for. Five of the timbers will actually by 22' long for a two foot overhang on the gable ends, but I will still have enough to patch it up like that. Any other photos that explain the process?

Thanks

The only other photo I have is of the entire piece with the two timber framers who did the repair:

 

 

They just cut away the rotten wood and shaped the ends to the angle. Made a repair piece the same size and slid it in.

Jim Rogers
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Offline Jim_Rogers

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Re: How much bow is acceptable?
« Reply #19 on: March 01, 2014, 11:34:38 am »
Back in 2009, I wrote up a paper on definitions from NeLMA's rule book. And included some pictures to show that these definitions meant.
I have attached it as a pdf so you can download it and keep it on hand to understand what they mean.
I hope you find it useful.

Jim Rogers
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Offline VictorH

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Re: How much bow is acceptable?
« Reply #20 on: March 01, 2014, 01:43:31 pm »
The workshop will be over Memorial weekend.  Here is the thread for it

http://www.forestryforum.com/board/index.php/topic,71929.0.html

Brad - he's planning on a post and beam so square rule won't really apply, although I'm trying to pull him over from the dark side  :D :D

Victor
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Offline oe_alex

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Re: How much bow is acceptable?
« Reply #21 on: March 01, 2014, 02:58:32 pm »
Back in 2009, I wrote up a paper on definitions from NeLMA's rule book. And included some pictures to show that these definitions meant.
I have attached it as a pdf so you can download it and keep it on hand to understand what they mean.
I hope you find it useful.

Jim Rogers

I should clarify myself then. The "bow" is very minimal, is the "crook" that I'm worried about. 1/4" in the center on one, 3/8"-1/2" on the other.

Offline PaAnkerbalken

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Re: How much bow is acceptable?
« Reply #22 on: March 02, 2014, 08:55:06 pm »
You might want to check with FF member Rooster.  I sorta remember him patching in pieces into beams that had rot, damage, etc.  There were pictures as I recall.
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Offline PaAnkerbalken

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Re: How much bow is acceptable?
« Reply #23 on: March 02, 2014, 09:10:20 pm »
 It's under re-facing a rotten beam by Rooster.....might give you an idea. smiley_thumbsup
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Offline oe_alex

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Re: How much bow is acceptable?
« Reply #24 on: March 22, 2014, 10:36:49 am »
So we have footings going in for this project sometime early next week.

My staff and I are still planning out the stages on how we're going to do this whole project. As we were discussing yesterday, the question came up about cutting the rafter seat cut. The two beams that have the 1/4 - 3/8" 'crook' to them are going to be the plates on which the rafters will sit. Will it be wise to get the beams up first and cut the rafters, individually measuring each one? The reason I ask is because the rafters are going to come together on top of the ridge with a 2" deep seat cut, my thinking that is if we cut them all the same that where the crook goes out on the plate beams, the rafters won't come together at the peak, leaving an open gap that will be seen. We also discussed maybe taking a strap and pulling them straight once in place, attaching the rafters, then letting it loose, but I would think that they will want to pull out, maybe taking the rafters with them.

Offline Jim_Rogers

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Re: How much bow is acceptable?
« Reply #25 on: March 22, 2014, 11:26:44 am »
Ok, well maybe we should go over this again.

Bow is a deviation from a flat plane of the wide face of a piece of lumber from end to end
Crook is a deviation from a flat plane of the narrow face of a piece of lumber from end to end.

You're telling us that you have a "crook" timber. That means the narrow face is higher on the ends then in the middle if crown is down or lower on the ends if crown is up.

If you want the rafters to sit on the timber all the same then you either cut all the rafters the same and shim the ones that sit on the low spots or cut the rafters to match the crown so that they all sit at the same elevation for a flat roof.

Now, you have a bow and the middle of the beam is either further away from the ridge or closer to it, then you may have to custom cut each rafter to fit.

What I have done, in the past, to test fit all my rafters to my plates before they are up in the air is to put the plate on the foundation/floor system in the exact spot where they will be except not up in the air. Basically remove the elevation of the posts. Check that they are true and square. And then sit the rafters onto the plates to ensure that the seat cuts are right and the rafters will sit correctly when they are up in the air.
I don't know if you can do this as you are using a ridge beam as well. You'd have to "mock up" the elevation of the ridge beam in between the plates to make sure it will all go together once up.

Good luck with your project.

Jim Rogers
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Offline oe_alex

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Re: How much bow is acceptable?
« Reply #26 on: March 22, 2014, 11:46:10 am »
Thanks for the further clarification Jim. As a matter of fact, I have a little of both on them. I think the best way to go about this is to cut each one individually like you said.

Thanks