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Author Topic: Putting living space over workshop in a TF  (Read 9170 times)

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Offline Dave Shepard

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Re: Putting living space over workshop in a TF
« Reply #20 on: December 19, 2007, 09:19:04 AM »
No rush, Don P. I like 12/12 as well. I wish I had some pics of the Tan House at Hancock Shaker Village. They are shut down for the year though. Hang on, I've got a key for that, I'll have to make a call and see if I can get permission to take a look. ;)


Dave
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Offline Don P

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Re: Putting living space over workshop in a TF
« Reply #21 on: December 21, 2007, 05:52:13 PM »
Some stuff from messing around;
I sketched a 30' queenpost truss and made a few quick assumptions. I put them 10' apart and put a total load of 16,000 lbs on each truss top chord from dead load and snow. I put 6000 lbs on the 14x10' floor area. All loads are in black, distributed on their corresponding panel points. The forces in the truss are in red and blue, add 2 zeros for pounds.


Below is a pic of a queenpost truss I slapped together this afternoon out of some sticker stock. The bottom chord is 3 pieces. There is a single pin in a predrilled hole at each panel point. There is an X of tie wire across the center box to stabilize it. The span is 6', I'm standing midspan on a 1x2. I'm about 175 lbs, there is no way a six foot 1x2 by itself could support me.











A laborer works with his hands
A craftsman uses his brain and his hands
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Offline Dave Shepard

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Re: Putting living space over workshop in a TF
« Reply #22 on: December 21, 2007, 06:33:27 PM »
Wow, that's awesome! I have to do a little studying in the engineering department do I can understand it better. What size beams, and of what species make up the truss? Thanks.


Dave
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Offline Don P

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Re: Putting living space over workshop in a TF
« Reply #23 on: December 21, 2007, 10:11:13 PM »
The truss is white pine and poplar 1x2's.

This is an analysis of the forces in the truss members.

 I've divided my weight and entered a 90 lb load on each queenpost bottom. Remembereing your trig or carpenters rules of thumb, to get the hypoteneuse of a 12/12 pitch you multiply the side by 1.414. Multiply the vertical load of 90 times 1.414 and you have the 127 lb compressive force on the top chord.

The top chord can be checked on the column calc for a compression member;
http://www.forestryforum.com/members/donp/columncalc.htm
length is 34"
EWP is good for 875 psi compression parallel to grain
E is 1.1 million
Since there is no bracing I checked the 1x2 as a 2x1 also, it could buckle either way.
 It passes fine.

The bottom chord has 90 lbs of tension on it. EWP can take 275 lbs in tension parallel to grain, per square inch. We have a 1x2, or 2 square inches so it can safely take 550 lbs in tension before risking pulling the wood apart. We pass that fine also.

Connections, I used a 10x3 common nail for my pins. In single shear through a 1x into a 1x. From the NDS tables I'm good for about 100 lbs. I've got 127 in the top chord so I'm in overload there.

One other point, my feet are bearing at midspan so I not only have 90 lbs tension in that member, I have 175 lbs in bending also. I won't subject you to that check now but notice the difference in stress on that member if my feet were spread to the panel points at the base of each queenpost. We'd have the same load but no bending.
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Offline Dave Shepard

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Re: Putting living space over workshop in a TF
« Reply #24 on: January 20, 2008, 10:12:46 PM »
I've been thinking about this a lot, again. I know that there is a fire risk, but there is that risk with building.:( Don, could you detail some of the joinery in this truss? I am guessing that having the bottom chord be a one piece timber would be paramount to success? Thanks.


Dave
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Offline Dave Shepard

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Re: Putting living space over workshop in a TF
« Reply #25 on: January 20, 2008, 10:40:48 PM »
Insulation is another consideration to, umm, consider. :) I want to avoid SIPS, for a couple of reasons. I would rather use materials that came off of my mill, not OSB shipped across the country, and I would rather the labor be my own. I am thinking of using a system similar to what Jack Sobon illustrates in his "house" book. A plank frame covered with some form of rigid insulation sheets and then board and batten or clapboards. Any input welcome.


Dave
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Offline moonhill

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Re: Putting living space over workshop in a TF
« Reply #26 on: January 21, 2008, 07:52:47 AM »
Hi Dave, I was checking the forum out this AM and saw your post on planking and foam.  I have done this a few times, for the same reasons you mentioned.  I'm very pleased with the system.  Heavy planks first, 3"-6" foam with multiple layers to cut out infultration of air (vent with open windows if needed), seal all outer joints with spray foam just before the straping is applied, and still wet, if it hardens up it will displace the foam sheets which is lightly tacked on with a couple of long screws and large washers.  I use SIP screws  through a variable thickness strap 2'OC, counter sink the screw. The straping thickness varies depending on the thickness of foam and thickness of the wall planks and the length of the screw.  I use a window bucket at all openings.  The bucket is screwed into the planks and comes flush to the inside and flush to the foam outside.   Around the window I use a 8"-10" board the same thickness as the straping for installation of the window, trim and siding.  I use clapboards for siding sawn on my mill 1/2"x5" with a 4" exposure, they aren't tapered, and I screw them on with a stainless screw, no paint.  Where the roof and wall meet, I leave a gap and seal with spray foam.  The roof is straped as well and sheet metal applied.  Trim details are alway different and made up on site, we do what we have to to make it work. 

Its a bit chilly this morning and I had a bit of extra time before heading outside so I thought I would pass my thoughts on.  This is only my second post but have been watching for some time.   The latest project we are working of is a plank house, with 2-1/2" planks, with splined and peged edges, the pegs are for shear strength, splines for the black hole/gap and to keep the cluster flies and lady bugs at bay.  The structure its all attached to is a light timber frame.  Hope everyone stays warm.  Tim   
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Offline thecfarm

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Re: Putting living space over workshop in a TF
« Reply #27 on: January 21, 2008, 07:57:50 AM »
moonhill,welcome to the forum.That's the way to do it.Help out another FF member.You have all ready added somthing to this great forum.
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Offline Don P

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Re: Putting living space over workshop in a TF
« Reply #28 on: January 21, 2008, 09:14:49 AM »
Tim, post pics of the plank house if you can, there's 2 guys talking about something similar  8).
Same here colder than a well digger's ... I've got a frozen supply somewhere we're trying to find before heading out  ::)
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Re: Putting living space over workshop in a TF
« Reply #29 on: January 21, 2008, 01:23:44 PM »
There is one drawing in the Historic Timber framing series that I can find that shows plank siding on a frame:



This shows that the tie beam was cut to accept the plank siding and then the upper siding over hung the tie beam so that shorter pieces could be used above the eave line.

Jim Rogers
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Offline Don P

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Re: Putting living space over workshop in a TF
« Reply #30 on: January 21, 2008, 11:49:13 PM »
I'm hoping one of the TF'ers will take on joinery design ideas, I'll give you bolts and plates if its left to me  :). Also review the TF Guild's pdf that Bob C posted on page 1 of this thread for some good info. These are serious forces way up in the air with no redundancy, one joint fails it all comes down.
I'll back up and try to rough size the timbers. The highest compression load I came up with was around 12,400 lbs on a top chord (see graphic in post above with my smiling face) that would span around 135"
I'm coming up with an 8x10 in hemlock or an 8x12 in ewp using this calc;
http://www.forestryforum.com/members/donp/44axbend.htm
Entries are as follows;
12,400 lb axial load
4,000lbs uniform load on span
135" span
10" deep
8" wide
#2 hemlock b+s

So that would be my minimum top chord dimension choice, think about joinery damage though.

Tension with bending math, for Jim and others with the book, this is NDS 3.9.1
The bottom chord tension is 8500 lbs, notice I did load the floor in the graphic (30, or 3000 lbs per node) basically I'm going to figure putting 6,000 lbs total on the 14' wide x 10 span section of center floor deck. If that'll take it I think we're safe. This is not stacks of lumber up there.
I've not made a calc for this so we'll go at it longhand, there are 2 checks, we must pass both.
ft/Ft' + fb/Fb*<=1.0
and
fb-ft/Fb<=1.0

talking through it might help, equation 1;
actual tension stress divided by allowable tension stress plus actual bending stress divided by allowable bending stress is less than or equal to 1 beam

equation 2;
actual bending stress minus actual tension stress divided by allowable bending stress is less than or equal to 1 beam

Kind of the same thing we've talked about before, if a member is undergoing forces from 2 directions the check is to make sure it can handle both simultaneously not one and then the other, hope that makes sense.

Tension this is to get ft
We've got 8500 lbs tension, lets assume an 8x12, so 96 square inches, 8500/96 or ft= 88.54 psi actual tension. NDS table says Ft is 275 psi allowable tension stress in ewp, 375 psi in hemlock. fine so far.
Bending this is to get fb
We've got a 6000 lb uniform load on a 14' span
Max moment is going to be M=WL/8
M=6000 x 14 / 8 or 10500 foot lbs, multiply x 12 gives us 126,000 inch lbs maximum bending moment in the beam. Draw a smiley, the bottom of the smile in midspan is that point, and we have the total force there.
Divide that by the section modulus bd2/6 or (8*144)/6 or 192"3
so, 126,000/192 or fb=656.25 so extreme fiberstress in bending is 656.25 lbs per square inch at the bottommost (or extreme) fibers in the middle of our beam.

we've got 656.25, the NDS allows Fb575 in ewp so we need to be bigger there in bending alone not to mention the combination with tension. Hemlock allows Fb 750 so we're still in the running there.

 I haven't taken any allowable adjustments to design values and its looking pretty tight to me for doing the combination check right now. But, I gotta hit the showers and go to bed. Feel free to work it tomorrow if anyone has time, or I'll pick up where we left off. You can see why I started making calcs, alot of this is try a size and run with it, then try another. You all haven't seen a couple of other attempts here that were my first guesses. gives me alot of respect for the old sliderule engineers.
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Offline moonhill

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Re: Putting living space over workshop in a TF
« Reply #31 on: January 22, 2008, 06:05:31 AM »
I am working on the picture thing and will post some soon.  I also wanted to add to my last post, the roof is boarded with 1" boards so the screws would show,  so I only screw into the roof members, rafters/purlins.  With purlins we run one course of straps top to bottom which holds the foam on,  and a second, lighter strap across that for the sheet metal.   The straps run long at the eves and gable ends for over hang, to screw trim onto.
 
All those numbers almost make my head spin.  Its like compound joinery, one has to jump right in with both feet and commit.  It will be interesting to see what you come up with, Dave.  I have often wondered if a computer programer could ,.put together a program that would consider all the factors that would go into such a problem, member size, speices, spacing, loads, region of country etc..  If they can do it with a chess game why not.  Tim
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Offline Don P

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Re: Putting living space over workshop in a TF
« Reply #32 on: January 22, 2008, 10:10:17 PM »
I imagine you could Tim. We are also only checking the frame under gravity load. The wind could be hitting one side, blowing snow to the other and that might be the controlling condition. We aren't engineer's and all of those variables and ones I don't even know about are reasons to just use this type of discussion for preliminary design. I think its a good idea to understand as much as possible about what makes the buildings we're working on tick, I also think its a good idea in most cases to have a pro review preliminary design.  Longhand does tend to make you think, and so hopefully better understand, what they call "rational design". I'm not sure relying too much on computers is a good idea in terms of understanding, it does make number crunching less mind numbing though.

I left off last night ready to make a run at the interaction math
sticking to hemlock from here out since the ewp already failed at this dimension
ft/Ft' + fb/Fb*<=1.0
88.54/375 + 656.25/750
.236 + .874 = 1.11 we have an 11% overload, keep running lets see how bad it is to get an idea for the next try

fb-ft/Fb<=1.0
(656.25-88.54)/750=.757 good there, we aren't too far off

Let me show you something because we got into it last week, I know Jim is very leery of doing this and for good reason. I just checked, there are no adjustments I could have made to bump the design numbers up on these timbers, our only options are to increase dimensions or improve grade. #1 Hemlock has an Fb of 1150 and an Ft of 775. Plugging those into equation #1;
88.54/775 +656.25/1150
.114 + .570= .684
An 8x12 bottom chord in #1 hemlock makes it easily.
I've also given you enough information that you can run a #2, 8x14... or any other size rectangle.


You asked about a one piece bottom chord. Here again is where an engineer is a good idea. For one you won't need a tiewire x brace with a continuous bottom chord. It will change the loading at the nodes somewhat, I think it will get better. A continuous top chord will load a bit different, I think about 1/8 of the load will vary. Part of what an engineer does is value engineering.

The vertical webs have a 3000 lb straight tension load on them, no bending so this is easy.
The net section area has to exceed the tension stress. we know the allowable tensile stress on #2 hemlock is 375 psi so 3000/375=8 square inches. You need a 1x8 or a 2x4 or better after joinery there.

The straining beam has no bending on it, assuming no ceiling or storage. So just a 6700 lb compression load along the long axis of the timber. It would be figured as a column, I'm thinking just like okie's post ;D
A laborer works with his hands
A craftsman uses his brain and his hands
An artist uses his brain, his hands, and his heart


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