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Woodshop / garage

Started by Alain Vaillancourt, May 07, 2013, 12:05:25 PM

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Alain Vaillancourt

Hi, I wish to build a woodshop in my backyard (25' X 40') and I want to build it in timber framing style. I have a small experience with that style of construction. 19 years ago I built my cottage this way (if someone is interested I have a video showing it off at (youtube.com/watch?v=g_GprgPHJFM ). I have several questions and I  hope someone can help me.
First the woodshop will be on a concrete slab.The guy who came for digging tells me that just a slab is enough, I was thinking more of a 4 feet foundation with a footing and the slab on top of it. I live in Quebec south of Montreal, we have 3 to 4 feet of snow each winter. Which one is better?

Second, I was thinking of putting the 5 bents (10 feet apart) directly on the concrete, with some sort of barrier between the 8X8 post and the concrete, then I'll just bolt the 8X8 post on the concrete with some angle iron instead of putting a sill plate directly on the concrete.?

Third, I read some books and I'm still confused with the size of the rafters and the girts. At my cottage I used 6X8 for both of them but it's only 20 feet wide, I want a 25 feet wide woodshop with a 6:12 slope, so will 6X8 rafters and girt be enough?

Thanks a lot
Alain Vaillancourt

Jim_Rogers

Alain:
Welcome to the forestry forum and the timber framing section.

Lots of good questions.

As you know a timber frame puts all the roof load down the posts to the foundation. You'll need more that just the slab at these points or the load will break the slab and put it down a lot.

It is called "point load"... and you have to plan it out to be supported correctly.

Until you tell us the type of wood you want to use for your rafters. And the snow load for your area, we can't tell you much about rafter size.

Jim Rogers
Whatever you do, have fun doing it!
Woodmizer 1994 LT30HDG24 with 6' Bed Extension

Alain Vaillancourt

I'm thinking of using pine or hemlock, I'm not sure right now. I'm still waiting for my submission  at the lumber yard.

I found this web site for the snow load http://www.jabacus.com/engineering/load/snowload.php
And here are the results
Climatic Data
Location
Province: Quebec
Location: Montreal (dorval)
Importance factor
Is: Normal
Factors
Cb: 0.8
Cw: 1.0
Ca: 1.0

Roof Geometry
Pitch: 6/12
Slippery:  no
Specified Snow Load
S = Is[Ss(CbCwCsCa)+Sr] [4.1.6.2]
Factors
Location: Montreal (Dorval), Quebec
Ss = 2.4 kPa / Sr = 0.4 kPa
Importance Factor, ULS: Is = 1.0 / SLS: Is = 0.9
Roof slope = 26.6 degrees
Slope Factor
For non-slippery roof:
Slope <= 30 degrees.
Cs = 1
ULS:
S = 1.0[2.4(0.8*1.0*1*1.0)+0.4] = 2.32kPa
S = 2.32 kPa
S = 48.5 psf

SLS:
S = 0.9[2.4(0.8*1.0*1*1.0)+0.4] = 2.09kPa
S = 2.09 kPa
S = 43.6 psf

I hope you understand those numbers!
Thanks again,
Alain

Jim_Rogers

Alain:
You should have your design reviewed by a structural engineer licensed in your area and familiar with your snow loads.

But, I think you could use a white pine 6x8 if spaced no more than 16" on center. For Hemlock you maybe able to make them 24" on center.

I was rounding up and using 50 lbs for snow load and 15 lbs for roof dead load.

Again, have your finial design checked by an engineer.

Jim Rogers
Whatever you do, have fun doing it!
Woodmizer 1994 LT30HDG24 with 6' Bed Extension

Alain Vaillancourt

But my idea was to make the frame like this picture (my cottage is like that),


I built a roof on top of the rafters with 2X4. I was expecting to do something similar. So best to your knowledge before going to see a structural engineer do you think that 6X8 rafters and girt would be enough?
Thanks Alain

Jim_Rogers

My calculation for your rafters were for common rafters spaced 16" on center and/or 24" on center.
You design is for a principal rafter/purlin roof system. Those 6x8 rafters will not work in this situation, in my opinion.
You're asking them to hold up a lot more roof.

Also, your design shows the bottom of the plate at the top of the tie beam. There needs to be some distance between the two, I was recently reminded of. There should be at least 8" to a foot between them.

Jim Rogers
Whatever you do, have fun doing it!
Woodmizer 1994 LT30HDG24 with 6' Bed Extension

Alain Vaillancourt

Thanks a lot Jim especially for the tie beam. I'll modify my plan. This is why I plan in advance.
I dug to find some information for the rafter and I want your opinion on my reasoning.
If I have a 14 feet rafter spaced  10 feet apart, the snow load calculator for a slippery roof plus the dead load gives approximately 50 psf so the load on one rafter should be 14 X 10 X 50 = 7000 lbs. Then I found on this website ( http://www.woodworkweb.com/woodwork-topics/wood/146-wood-strengths.html ) for the max shear strength of most wood species. If I look for pine or hemlock they're approximately 1400 psi and on the American Wood Council web page I found some table at   http://www.awc.org/pdf/wsdd/c2c.pdf  saying that an 8 X 10 beam spanning 14 feet with a shear strength of 1400 psi can sustain a load of 7520 lbs, so I guess that 8 X 10 rafter should be enough for my woodshop?
I have another question. I bought "Timber Frame Engineering in Limit States Design", this book contains a lot of load calculation (I didn't understand most of it) and in one chapter it says, and I quote " A cross-beam strictly resist tension parallel to the grain, just like a spline. " and he uses a 6 X 6 for his tie beam, so can I assume that a 6 X 8 should be sufficient in my design?
Thanks again,
Alain

Jim_Rogers

Alain;
When you figure the load on a rafter you measure it by the span/run not the actual length. The actual length is used when you figure the dead load for the weight of the roofing materials.

So your math should be 12.5'x 10'=125 x 50 = 6250 lbs per rafter. Next you have purlins taking away some wood of the rafter. These pockets have to be taken into consideration. This is not an easy thing to do. Some people consider what is left only. If the pocket is 1 1/2" deep on both sides then this 8x10 rafter is now a 5x10 not an 8x10. This is why we hire engineers to do the math and make sure everything is right.

About your tie beam size. Again that all depends. Are you going to have a loft above the tie beam?
I can understand if you say "no" but what about future owners will they want a loft up there?

Just something to consider when you're building something that may last for decades after you are gone.

Jim Rogers

Maybe you should read this story https://forestryforum.com/board/index.php/topic,50714.0.html
Whatever you do, have fun doing it!
Woodmizer 1994 LT30HDG24 with 6' Bed Extension

Jay C. White Cloud

Hi Alain,

I have just been following along on this, as I knew more information would get shared in time.  I would stress Jim's advice about having a PE look at this design when you are done.

QuoteAlso, your design shows the bottom of the plate at the top of the tie beam. There needs to be some distance between the two, I was recently reminded of. There should be at least 8" to a foot between them.
I could be missing something here, but the design of the joint was of more concern than the location.  In traditional "Truss assemblies" the post,rafter plate, and tie beam occurs at the same location in almost all examples (ie. English Tying Joint.)  The move away from this seems to be a contemporary alteration. 
"To posses an open mind, is to hold a key to many doors, and the ability to created doors where there were none before."

"When it is all said and done, they will have said they did it themselves."-teams response under a good leader.

Jim_Rogers

Quote from: Jay C. White Cloud on May 08, 2013, 07:15:11 PM
The move away from this seems to be a contemporary alteration.

If the tie is below the plate then that's one thing. If the tie is at the plate that's another. The way the tie shown above is below the plate and is causes the problem/risk that I was reminded of recently:



 

This could be a major problem if not correctly modified.

Jim Rogers
Whatever you do, have fun doing it!
Woodmizer 1994 LT30HDG24 with 6' Bed Extension

D L Bahler

This is a situation, however, where outward thrust is minimalized.
A king post supports the ridge directly, bearing its weight straight down. In the case of the king truss, directly into the tie beam.

Outward thrust is caused by the downward pressure at the ridge, causing the angles f the rafters to change. This in turn causes the bottom leg of the triangle to elongate, i.e. thrust outward. When you directly support the ridge, it is not able to fall downward -this force is taken away by the post.

I can show you many examples of buildings that have directly supported ridges with no tie beams at all, some that have been there for centuries.

However, in a king post you also can have the situation also where the post wants to deflect the lower cord. This in turn would cause the ridge to fall. Since rafter length is constant, the rafter ends want to spread out and will thrust at the plate.

D L Bahler

OR here is another mind boggling concept.

Why not just forget about attaching your rafters at the bottom altogether, and let them move freely?

Nonsense, you say? It would never work, you say?

Well, they've been doing it in the Berner Oberland for thousands of years, and still do it today.
The rafters are fastened to a ridge beam at the top, and slide freely on the plate. The result of this is actually a slight inward force.

Jay C. White Cloud

Hi Jim,

Thanks for the clarification, I thought that is what you had been driving at.  The diagram is helpful.  I was concerned more with the joint design, but you took it all into context, with us both arriving at the same conclusion.

David,

QuoteThis is a situation, however, where outward thrust is minimalized.
A king post supports the ridge directly, bearing its weight straight down. In the case of the king truss, directly into the tie beam.
I don't think that is truly accurate.  You are describing this as though the king post is in compression and transfer the moment load down to the tie beam, that is not the case in a "king post truss," the post is in tension.

QuoteI can show you many examples of buildings that have directly supported ridges with no tie beams at all, some that have been there for centuries
You could be misreading the load dynamic in these cases.  Yes they are old and work, but you must really understand what is in compression and what is in tension.  As before, the "king post" would seem to be in compression, but it is not.

QuoteWell, they've been doing it in the Berner Oberland for thousands of years, and still do it today.
The rafters are fastened to a ridge beam at the top, and slide freely on the plate. The result of this is actually a slight inward force.
You are correct, but once again, do not take this out of context.  "Hung Roofs" are common in all styles of folk architecture around the globe and can only be applied in limited context to the supporting structures below.
"To posses an open mind, is to hold a key to many doors, and the ability to created doors where there were none before."

"When it is all said and done, they will have said they did it themselves."-teams response under a good leader.

Jim_Rogers

If we look at the drawing posted in post #4 we see that the king post in his frame is not hung from between the two rafters. It is below the two rafters which butt against each other.

I thought that normally in a truss that the two rafters have to be holding up the king post.

In this case it's more like a supported ridge. However the ridge beam seems to also butt against the rafters and due to the size of the photo/drawing it's hard to tell if the ridge is continuous or interrupted.

Jim Rogers
Whatever you do, have fun doing it!
Woodmizer 1994 LT30HDG24 with 6' Bed Extension

Jay C. White Cloud

Hey Jim,

You got it buddy, that is why I was silent on this one for so long because I didn't think I was seeing things straight.  I knew you would jump in, and I could catch up. I think the following:

If he wants a true "king post truss" it's time to go back to the drawing board.  The drawing in post #4 is not a king post truss at all in anyway, and the problem with many folks trying to build them is they often end up looking like this.  Of all the catastrophic fails I have seen in my career, a lion's share of them have been with "King and Queen truss" assemblies built by new timberwrights.  I know you have stressed this, I am going to again.

I am a timberwright with over 30 years of experience, I will not build a "truss system" for anyone, other than a small one for myself, without Professional Engineer approval of my design, and I believe Jim is of the same thinking.  It is not a safe game to play.

Hi Alain,

Please put "King post truss assembly" into a google search for both info and images then revisit your design, Jim or I will be here to see what you come up with.  When you have a sound design then get a PE to look it over.

Warm Regards,

jay
"To posses an open mind, is to hold a key to many doors, and the ability to created doors where there were none before."

"When it is all said and done, they will have said they did it themselves."-teams response under a good leader.

D L Bahler

IF

One would lower the ridge, keeping everything else more or less the same resulting in a construction where you have ridge posts (not king posts -my original thinking was of ridge posts, forgetting the unique situation present is a king truss assembly) then you can have an acceptable assembly, assuming members are proportioned accordingly.

Then, however, it is not a truss roof.

What I am thinking is this:
You have tie beams, much like in the photo, these tie beams support ridge posts. These posts, in turn, support a heavy ridge beam. The rafters in this situation would be hung from the ridge, and not part of a truss assembly. That does not necessarily mean they are common rafters though. I'll draw a picture and post it after a bit.

What I am describing is what my mind was geared toward in my original post, it is what we would call in German a Stehender Dachstuhl. It requires a heavy ridge beam and at heavy eaves purlins (many would call this a plate in English, but I get confused so I just stick with the German type terminology)

This is, again, not a truss assembly.

There are 2 ways to use it though, both are common:

Common rafters are hung from the ridge beam, transferring weight directly into the purlins
Principal rafters are hung from the ridge, large beams which in turn support common purlins.

In the past, which system you used would have been decided by a combination of cultural preference and what type of roofing material is used.

Jay C. White Cloud

Hello David,

I don't want to detract from the OP, but some of this information is helpful for him to read.  I believe once again you may be taking things out of context. 

QuoteWhat I am describing is what my mind was geared toward in my original post, it is what we would call in German a Stehender Dachstuhl. It requires a heavy ridge beam and at heavy eaves purlins (many would call this a plate in English, but I get confused so I just stick with the German type terminology)
All "Stehender Dachstuhl" means is "standing roof."  This is a relatively common style of roofing in much of Europe outside of the British Isles.  You can find examples from France to Russia.  Most have extremely steep roofs, (53° and greater) and more than 80% are some form of a "compression truss" system.  With the lower pitches, such as the OP has designed, you would have to have a massive tie beam to support the loads of the roof and the ridge, far beyond what is functional or elegant in aesthetics for his proportion of architecture.

QuoteCommon rafters are hung from the ridge beam, transferring weight directly into the purlins
Principal rafters are hung from the ridge, large beams which in turn support common purlins.
Again, in most "Stehender Dachstuhl," this is a truss system very similar to a King or queen post truss, just with much steeper roofs and much more joinery.
"To posses an open mind, is to hold a key to many doors, and the ability to created doors where there were none before."

"When it is all said and done, they will have said they did it themselves."-teams response under a good leader.

D L Bahler

Jay, I must disagree

Unless you are taking this in the context of North German styles, Stehender dachstuhl in its true form is NOT a truss.

Normally, it is directly posted to the ground supporting a large ridge beam above. These are often steep, but not always. This, again, is dependent on roofing materials.

I have studied first hand Stehender Dachstuhl type roofs with very shallow roof pitches. Sometimes the posts bear directly into the ground, sometimes they don't

But in the context of Southern Germanic framing, they get around the notion of needing a huge tie beam by instead having a whole lot of these beams. They might have many tying beams spaced perhaps 3 feet apart with a beam running across them, which in turn supports the ridge posts.

And stehender dachstuhl does not mean standing roof. Denk dich nicht, das ich kann nicht richtiger Hochdeutsch sprechen. It means upright roof support. There is a difference. It is in general any roof system that relies on upright posts to support roof loads. The other classification is Liegender Dachstuhl, which is any system that uses reclining or angled members to bear roof loads. In the Southern Germanic tradition, trusses are almost unknown except in the case of special structures, such as Churches, bridges, and castles. If you are talking about a proper truss, you generally use another term. Usually it is Fachwerk. But this varies.

Most southern Germanic Stehender Dachstuhl is, again, NOT a truss system as we apply the term to roofs.  Think instead of Purlin framed barn roofs, supported by upright purlin posts.


Jay C. White Cloud

Hey David,

That's o.k., I don't mind if you disagree with me, but I feel uncomfortable taking away from the OP, so unless the OP is curious or has questions about our discourse, we should focus on his questions and concerns.  You and I both can "run away" on a tangent at times which is not fare to the OP, unless, as I said, they are of interest in the subject of the tangent. 

Regards,

jay
"To posses an open mind, is to hold a key to many doors, and the ability to created doors where there were none before."

"When it is all said and done, they will have said they did it themselves."-teams response under a good leader.

Alain Vaillancourt

Thanks a lot guys. I'm really glad I joined this community.
First of all thank you Jim. I checked the thread you recommended and it was exactly what I needed to start understanding the loads, and the book I had previously bought began to make sense.
Thanks to everybody. I checked for my king post and found those designs.
https://forestryforum.com/gallery/albums/userpics/32690/King_post.jpg
I guess this is a better design. But this means that the king post and the tie beam should be as wide as my rafter 8 inches?
Alain

Jim_Rogers

Quote from: Alain Vaillancourt on May 11, 2013, 11:13:30 AM
But this means that the king post and the tie beam should be as wide as my rafter 8 inches?

Most likely yes.

Jim Rogers
Whatever you do, have fun doing it!
Woodmizer 1994 LT30HDG24 with 6' Bed Extension

timberwrestler

What about the obvious solution of having the principal rafters bear on the tie beam?  It becomes a truss, it avoids awkward joinery, and has plenty of historic precedent.  The engineer can specify the amount of bearing surface area between the top and bottom chords.
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