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Author Topic: Timber Frame Column Calculations  (Read 828 times)

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Offline fromrfarm

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Timber Frame Column Calculations
« on: December 03, 2018, 07:49:58 PM »
Good evening all.

  What a great site and information. I have been going through the calculators playing around with numbers for a few projects and am scratching my head a bit on the column loads.

   Example: 16' x 30' structure; two storied(barn).
  • 3 bays 10' each with center posts dividing the 16' into 2 sections of 8' each.
  • Roof 15x32(one side) with 12/12 pitch equates to a live load of 21 for my area and pitch. Doubling for dead load since too lazy to calculate that yet ;) So lets say 40 for ease. So 15*32 = 480 * 40 = 19200 lbs.
  • Given there is the top plate and ridge, divide the 19,200 lbs by 2 so 9600 lbs on the ridge and top plate. 
  •  3 Bays equals 4 posts/columns. These will be 14' tall with bracing at 10' for second floor.
  •  That would mean the 9,600 lbs is spread over the 4 posts/columns so 2,400 lbs per column. (Yes I realize I do not have second floor in there yet).
  • Using Eastern Hemlock with a MoE of .9 million and ACParG of 6.1

    Hope I have not lost you yet. So if I enter all of this info into the calc, I get the following:


 

       Question 1: Have I properly thought through the basics and entered the information properly.
       Question 2: If no issues with Question 1, a Max column load of 610 seems low. I have several local barns I have been able to look at and most are 10x10 hemlock posts that are 16'-18'. Of course with knee bracing and such. One of those barns is 40' high without any signs of post stress. I suspect I have done something wrong and looking to be edumacated.

Thanks in advance for the feedback!
M.
   

Offline Don P

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Re: Timber Frame Column Calculations
« Reply #1 on: December 03, 2018, 10:01:54 PM »
Hey fromrfarm, I think I see the problem. I'm not trying to go through your load calcs here just reviewing the inputs.

The allowable compression parallel to grain (Fc) for #2 eastern hemlock 5x5 and larger posts and timbers is 400 psi (table 4D, NDS Supplement). I imagine you are sheathed and braced along the wall but in and out, out of plane, is probably unbraced. The second floor would provide bracing at I'm assuming the 10' height.  With that line of thought your unbraced length would be 120" in the out of plane direction.
Using those inputs I'm coming up with 38,061 lbs allowable column load.

Hmm, I see a glitch in the other column calc for that species, need to visit that.
I'm curious though, where did the 6.1 come from in your inputs? If it is from something I have provided I need to correct that.

Edit; I think I fixed the other calc;
http://forestryforum.com/members/donp/Simple_column.htm
(I had missed a "/" in the hemlock P+T inputs, should be good now)

Offline fromrfarm

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Re: Timber Frame Column Calculations
« Reply #2 on: December 04, 2018, 06:16:19 AM »
Hi Don. Thank you for reviewing this.

 I got the compression from the following link http://cwc.ca/wp-content/uploads/Timber.pdf which I now see is Mpa and I should have converted to psi. Interesting that I get 884 psi when converted; almost double yours. Maybe that above resource link is incorrect?.

Offline Don P

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Re: Timber Frame Column Calculations
« Reply #3 on: December 04, 2018, 07:55:23 AM »
I see it, will need to review that more, they should be the same... #1 in the NDS is 875psi with an E of 1.2.
Off to rent a concrete saw and crawl into a crawlspace, oh joy :D

Offline fromrfarm

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Re: Timber Frame Column Calculations
« Reply #4 on: December 04, 2018, 08:31:34 AM »
LOL now that requires some pictures!
I am off to finish some cedar shakes and then strip some concrete forms. Then I can start getting a drawing together for that barn rebuild.

Do you know anyone that has strengthened an old barn? Mine on my property is 30x60ish. Needs new floors on both levels and was originally only 2/3rds that size. Was added onto by bolting to existing frame etc. I plan on stripping it back a bit and adding in more supports/framing. Trying to do it without building an entire timberframe inside the existing..

Don't have too much fun today!

and I will stick with your 400psi..

Thank you again.

Offline Don P

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Re: Timber Frame Column Calculations
« Reply #5 on: December 04, 2018, 05:21:05 PM »
That was fun, didn't get quite finished but the sawing is done, been riding the Hilti since lunch, the crawlspace entrance was a hatch in the floor before. I got sketches from the architect last night that determined where I could saw an entrance through the foundation wall.

I've done several barn repairs of various sorts as have others here I'm sure. The best thing to do is sketch up the existing so when you have questions we have something to refer to.

Just to give an idea, this sketch was for a friend who is underway with some repairs now. The basic frame was a good idea but whoever built it didn't know just how much they didn't know, it needed some help. I went over and we took some measurements. I came home and sketched it then drew in some repair details, we checked beam sizes, he sawed up some oaks and is probably done or close to it... I need to check in and get my laser back :D



Offline fromrfarm

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Re: Timber Frame Column Calculations
« Reply #6 on: December 04, 2018, 05:32:33 PM »
Sounds like a plan. Time to learn sketchup lol


Going to be a tricky thing to get measurements of in some areas given there is no floor in one spot on two levels. All because the previous owners didn't bother fixing a 12" by 8" hole in the steel roof. Oh well..


Offline Don P

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Re: Timber Frame Column Calculations
« Reply #7 on: December 04, 2018, 07:06:03 PM »
Sounds like the old saying "The easiest way to tear down a barn is to remove a foot of roofing".
One thought came to mind, Hem-Fir refers to western hemlock rather than eastern. Looking up the design values in the NDS Hem-Fir is still only 575 psi and 1.1E so I still don't understand how they are getting to 884 psi. The numbers I quoted for Eastern are when NeLma grades it, when NLGA grades as the species combination Eastern Hemlock-Tamarack(north) it is 600psi 1.1E so northern timber appears to be part of it.

Offline Don P

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Re: Timber Frame Column Calculations
« Reply #8 on: December 05, 2018, 07:36:30 AM »
Here we go, leafing through an old CWC span table book, (your link above is from them). Eastern hemlock falls into the northern species group, 4.1 Mpa in the link table.
4.1x ~115=471psi, that's sounding closer. 

Offline Heartwood

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Re: Timber Frame Column Calculations
« Reply #9 on: December 05, 2018, 05:22:17 PM »
Don,
Shouldn't the multiplier for MPa to psi be ~145, not ~115?

Offline Don P

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Re: Timber Frame Column Calculations
« Reply #10 on: December 05, 2018, 05:47:43 PM »
Yup, I have a great memory, its just short. Thanks Will. Hmm back to the same question, I wonder why those design values are so different ??? US and Canada are supposed to run the same numbers from my understanding of grading agreements.

Have you had any luck finding timber design values for Scots pine and Norway Spr?

Offline fromrfarm

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Re: Timber Frame Column Calculations
« Reply #11 on: December 06, 2018, 05:44:43 AM »
Not yet but I was working on the shakes all day and then was dealing with some cupboards in the evening lol

I am digging out some of my books today to try and track that down.

Offline Heartwood

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Re: Timber Frame Column Calculations
« Reply #12 on: December 06, 2018, 08:26:41 AM »
Don,
The values may be different because (I think) Canada uses a Limit States Design (LSD) philosophy and thus calibrate their values to that, where in the US we use either LFRD or ASD. This is getting beyond my knowledge since I'm not an engineer with international design experience, but I've found some resources in Sweden to help with my project there. Everything is geared towards Scots Pine and Norway Spruce since that's mostly what they use.
Thanks for the confirmation on MPa conversion.
fromrfarm: if you find anything to add, please do!

Offline fromrfarm

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Re: Timber Frame Column Calculations
« Reply #13 on: December 07, 2018, 07:19:52 AM »
Interesting.. having a very hard time finding eastern hemlock numbers.
This site : 
http://cwc.ca/wood-products/lumber/solid-sawn-heavy-timber/

Led me to this document which is the same one I quoted out of initially.  It has the US design numbers in it as well:

http://cwc.ca/wp-content/uploads/Timber.pdf

Unfortunately, it does not have Western or Eastern Hemlock listed for US values..

Offline fromrfarm

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Re: Timber Frame Column Calculations
« Reply #14 on: December 07, 2018, 07:30:33 AM »
Eureka!! I believe..

https://www.nelma.org/wp-content/uploads/DesignTable_PostsTimbers.pdf

From Nelma(obviously) and shows Hemlock with a 400 PSI.

hat I find interesting is the No 2 Hemlock is not all that different from the oaks. 

Offline Don P

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Re: Timber Frame Column Calculations
« Reply #15 on: December 07, 2018, 10:53:55 PM »
I'll ramble a little bit, this might be kind of informative or just mumbo-jumbo, its full of governmental alphabet soup. Most of this doesn't matter in our daily lives, it's more of the behind the scenes stuff of how grading bureaucracy works.

The numbers from NeLma, one of the US grading agencies, and as a "rules writing agency" those are the design values they provide to the AWC, American Wood Council. They are the people who publish the design values in the Supplement of Wood Design Values to the NDS, The National Design Standard for Wood Construction. That is the wood engineer's reference. That was a long winded way of saying the NeLma design values are the NDS values. The Supplement of design values is a free download on the awc.org website.

CWC is your equivalent of the AWC and the CLSAB is the overseeing body over your grading agencies, in the US it is the ALSC who oversees the grading agencies and reviews their testing and determination of design values.

Heavy timber is the least full size tested dimension, most testing is done on either full sized dimensional material, 2x, or on small clear samples. Heavy timber design values have been derived from testing small clear wood samples and then very conservative adjustments are made for grade defects in developing those design values. Non commodity species in dimensional sizes are also derived from small clear wood testing. ASTM D245 is the publication that explains how this is done, somewhere here I have a copy but can't find it, I think it's on a crashed computer. It was just about clear as mud to me, deep end of the pool. Basically that doesn't matter at our level, the inspector recognizes the NDS design values. Where Will is searching for design values that are not yet published an inspector in the US might recognize an engineer's report using ASTM D245 or I spoke with one of the rules writing agencies that can develop those design values for a fee, rock and a hard place. There are a number of species in my backyard, some that I like to use that are in the same boat, black locust is one and I'd love to have allowable tension perp and horizontal shear numbers for black gum with its interlocked grain.

I went to the Canadian NLGA, National Lumber Grades Authority, website, nlga.org, looking for eastern hemlock post and timber numbers. Click on this link, their publications tab; http://nlga.org/en/publications-for-download/
Click on section 16, good explanations of some of the above, and scroll to pg 279, read the bottom of the table, remember eastern hemlock is also in the northern species group, there is the answer as to why you aren't finding US equivalent numbers in heavy timber in Canadian references. I think Will is right and there are different design methods going on that shouldn't be mixed.

Which leads back to the NDS Supplement and the 400psi number we've been getting from there in #2 eastern Hemlock.

The equations for the calcs  in the toolbox here are from the NDS and AWC and are based on ASD, Allowable Stress Design, which is the predominant way our engineers design in wood, so the NDS Supplement numbers work with the toolbox calcs.

Section 4 at the above link is where they describe the NGR, national grade rule, that graders in both countries use. Although written expressly for dimensional lumber those are the rules used for heavy timber as well. The NGR interpretations link third up from the bottom in that publications link is a good visual guide to grading. You can find basically the same links on the NeLma website as well as on several US grading agencies sites.

Hopefully that wasn't all babble and makes some sense.


Offline fromrfarm

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Re: Timber Frame Column Calculations
« Reply #16 on: December 09, 2018, 07:46:13 AM »
Thank you for this. Not a ramble at all. I agree with regards to the bigger timbers and different species. Be nice to have the values but too specialized. 

One thing that always gets me here in Canada, is that you are not supposed to build with Hemlock due to it's "issues". Funny, my 1870's house is all hemlock, including the original floors, and the barn is 80% hemlock. 

It does twist and have some splintering issues but otherwise...


Thanks again..

I am working on the drawings this week coming.


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