Tractor Shed Design (Renamed)

[Raphael]

Raphael,
Do not know if this makes sense but the top plate, which will be joined to the posts will support part of the roof load.  I plan on having the top plate mounted high on the post and plane mill it to match the roof line.  Also, I have no overhangs whatsoever.  I plan on having a ridge beam, that will have a strut back to the king post.  (Most of these idea's come from Chappell's book)

I like that look of the king post to ridge beam brace...
I've got an idea what you plan with that top plate, I've got a variation on it in my frame.  In my case I milled triangular timbers that are locked into the top of the plates by the timber links that hold my roof panels in place.  You can just make them out in this picture.

[Max sawdust]

I see one in the upper left corner of the pic.  That is one reason I did not want to go with the English tying joint, and a continuous top plate.

I see your purlins are flush with the rafters.  Are the dovetails housed for strength?

(Again, that is a pretty frame)
Max

[Raphael]

It's housed dovetails on the gable ends and tusks at the center rafter pair.

I was tempted to start with ~8x13s for the top plate and mill the 6:12 pitch into the top surface so it would be a single timber...  But prices from the sawyer start climbing as timbers get larger and I'd pay $170/hr to have an engineer check timbers I milled myself.  As it stands the triangular timbers are fill rather than structural and don't need to be checked.

[Max sawdust]

Keeping your purlins flush with the top of the rafters adds a lot more to consider.
I can't go into it a lot right now as I have to leave to go home for the weekend.
But I'll discuss it further with you on Monday......
Jim Rogers

I played with the calculator over the weekend (with Raphael pointing out some errors on my part)

My conclusion is I can not use 6x8 red pine for the rafters and have flush purlins.  Allowing two inches for the depth of the dovetail and 1 inch for housing, effectively making the rafter a 5x6.

Running the calculator for #1 Northern Red Oak and it looks like it will work, also looks like  I can  maybe get rid of the struts too.  (keeping the queen posts)

Is this reasonable thinking?  Do you have any tricks up your sleeve where I could still use red pine :D
Max

[beenthere]

Can you fashion steel bracket hangers to hang the purlins on the rafters and thus use the size of purlin you need, but stay under the height restriction you impose? 
I've found the discussion informative and interesting, but admit I'm not keeping up with all the descriptive terms.  When you mentioned cutting into the rafters to make the purlins flush, I was concerned with decreasing the rafter cross-section, and reducing strength (meaning needing more rafter size).

Great to have Raphael and Jim Rogers on board, and interested builders like you too.   

[Max sawdust]

Can you fashion steel bracket hangers to hang the purlins on the rafters and thus use the size of purlin you need, but stay under the height restriction you impose? 

Yes I could, but steel does not grow in the woods :D   Though Don P seems to have some "high iron" #1 red pine :D Seriously though, I am "into" the traditional design and joinery, only the joint itself and a few wood pegs to hold things together ;) 

When you mentioned cutting into the rafters to make the purlins flush, I was concerned with decreasing the rafter cross-section, and reducing strength (meaning needing more rafter size).

Yes this is the problem, a 6x8 with cuts to receive the pulrins is effectively reduced to a 5x6, and that will not support my load, so far the only choice is to change species of wood for the primary rafters.

I am very appreciative of the "lessons" from people like Jim and Raphael, and for people like Don P who put lots of time and energy into nifty calculators for us all to use.  Heck, I enjoy everyone's view points and inputs FF is great ;)
max

Yes

[Raphael]

Do you have any tricks up your sleeve where I could still use red pine

  Well there is the obvious which is to increase timber size across the frame.  Using a square rule approach the 6x dimension of your timbers is actually a bit undersized for softwood joinery since the 'perfect timber' is only 5½" giving you 2" shoulder, 2" tennon and only 1½" of remaining timber.  Not a real problem when milling your own timbers as you control their quality and finished size.
  Using tusk tennons instead of dovetails is a way of preserving rafter strength, it leaves the timber unbroken across the top surface where compression stress is highest and removes the wood near the center of the rafter where stresses are at the lowest.  This does make assembly challenging as a lot of frame elements need to be supported and guided together simultaneously (got lots of friends?).  I've got a hair brained solution to that but I'll keep my mouth shut until I can think on it some more.

[Jim_Rogers]

Max:
You've said you want to use a dovetail joint for connecting the purlin to the rafter in your tractor shed, as an exercise in learning timber framing.

There are several things you have to understand about dove tail joints.

First of all they are going to shrink. And what is going to happen to this joint when the purlin does shrink?

Here is a top view of a purlin with a dovetail joint. It is also housed:




The shrinkage is going to happen across the distance shown here as "A." This will cause the purlin to be loose in the joint and can move in the direction of "B." To prevent this from happening, Steve Chappell installs wedges in the joint at point "C." These wedges are made of dry hardwood and are supposed to compress the wet green pine fibers and take up any space created later when these fibers dry out, and prevent the purlin from moving in direction "B." Most of Steve's work is with planed timbers and he seldom uses housings, therefore if the purlin does move it will show a gap between the purlin and the rafter.

Also looking at a side view of the same joint:




Not using a housing can cause the purlin to crack at the black arrow, if the size of the purlin's dovetail tenon is not big enough for the load it has to carry.

I do not recommend a dovetail joint without using a housing.....

The shelf of the housing will hold some of the weight and it will hide any gaps that could/will occur when the purlin shrinks.

Now to rules of thumb.

I learned these at the engineering course I took at Heartwood school for timber framing.

First rule for sizing. The dimension "h" has to be less then or equal to 5/6's of "D."
Second rule for sizing. The dimension "d" has to be less then or equal to 5/8 of "D."
Third rule for sizing. The dimension "d" has to be greater than or equal to ¾'s of "h."

What these means to you is that you can't have a 6x8 purlin and a 6x8 rafter. If you need a 6x8 purlin for the weight then the rafter has to be larger. If you can't make the rafters larger then you're going to have to reduce the load by changing the spacing, such as adding another bent or adding more purlins. If you can't add another bent because of doors or windows your design may have to be completely reviewed.

If you don't use a housing then you have to cut the bottom of the purlin with a scoop like this:



And there is a rule that for every inch you cut up into the purlin/joist you make the scoop four inches long. If, for example you reduce the purlin/joist from a 5x7 to 5x5 at the end then the scoop has to be 8" from the end.



If you don't cut the scoop the purlin will crack at the end of the tenon as shown above and it could lead to timber failure....

Instead of a dovetail and housing a tenon as shown above is better.
Here is a view of the mortise for such a tenon:



This joint would be pegged down from the top and it makes a stronger rafter as much more of the wood at the top surface of the rafter is left intact. A lot of timber framers have moved away from dovetail joints to these types of joints called "tusk" tenons as some code rules state that you can't cut the top surface of a rafter anywhere to make a joint, because it weakens the rafter.

Engineers look at the size of the rafter left after the dovetail is cut to see if what's left will be strong enough to support the load.

Also, you have to look at the amount of wood under the dovetail joint in the rafter to see if there is enough wood there to support the load.

This dimension is "D" subtract "d." And if this isn't enough to support the load your design will fail.

I've run the numbers through on of my spreadsheet worksheet and things don't look good in several areas.

I really don't want to get into a discussion of your grading your timbers to "select" as this isn't the proper thing we should be doing. Unless you completely understand the entire grade rules for Select, grade 1 and grade 2, you or anyone shouldn't be assigning values.
It takes some experience to understand these grade rules and to apply them to visually inspecting timbers. All the values I've used are for grade #2 as this is the lowest level of timber that can be used in timber framing.

If you buy higher grade timbers for this project then you can use their values in your equations and prove your design won't fail.

One area as mentioned is the actual size of the rafter. Holding up the weight, it has to hold without the strut and queen fails. So you'll really have to have them.

The next area of concern is the joint between the rafter and the purlin. If you want to use this dovetail joint you're going to have to create a housing for it and it has to be done right so that when things shrink the purlin will still be supported by the housing.

As shown in the drawing above (top view) the dovetail will shrink left to right, but the entire purlin will also shrink in height (dimension "h"). When this happens if the dovetail is cut to be flush with the bottom of its pocket then the bottom of the purlin will be lifted off the housing and the entire load will be on the dovetail tenon. If the dovetail tenon is not the right size then it will fail. So the dovetail has to be cut a little smaller to allow for shrinkage and this means this dovetail is only going to hold the two bents together until things shrink.
That means the housing has to hold all the weight until this shrinkage occurs. So now the housing has to be the right size to hold the entire load.

When you use a tusk tenon, these things don't happen. First of all the tenons will always be in contact with the bottom of the mortise. And the size of the purlin and the size of the tenon have to be large enough to support the load, as there is no housing to help support it.

The next area is the amount of wood left under the purlin dovetail pocket in the rafter.
This amount has to be large enough to hold the weight. Without some serious up sizing of the rafter this area is too small, and the joint will fail....

I understand how you want to learn timber framing, and how you want to use certain joints to create this tractor shed, but some of your design choices need careful review.

When I start a design project, I figure the loads first, and try and size the purlins (for example) to hold up that load. Then figure the rafter sizes to hold up the purlins. Then the posts size to hold up the rafters. Sometimes each size is bigger than the last due to added loads from other areas.

I personally don't like the modern bent style timber frame design. I like the more traditional timber frame design. In the traditional timber frame design there are beams that run from wall to wall to hold the building together at one elevation. Then at another elevation there are timbers that run from gable to gable to hold the building together, usually called plates. The plates are supported by the posts, and the plates support the rafters. In traditional timber framing the rafter never touches the post. The older style roofing systems almost always used some type of common roof rafter system. That is smaller rafters supported by plates. And sometimes these rafters were supported mid span by purlins that ran from gable to gable called principal purlin plates, which can be or were held up by queen posts. Sometimes these queen posts were plumb and sometimes they were leaned over to make the purlin's top surface flush with the bottom of the rafter. This would then be called a 'canted' queen post.

There are rafter systems where there were principal rafters, located at each bent, with purlins that ran between them. And the location of this joint would be usually supported by a queen post (canted or not). And then common rafters between the principal rafters with one set of commons going from plate to purlin and one set going from purlin to ridge.
As shown here:



Here you see the queen post connecting to the rafter just above the joint of the purlins to the principal rafter. And also you can see the common rafters that join the purlin and each other at the ridge with no ridge board.

This barn is the barn I worked on with the guild where another timber framer took my picture that I am using under my name on this forum. My job was to cut new common rafters by using the broken old common rafters as a pattern.

There are a lot of things to think about when designing a building.

Jim Rogers

[beenthere]

Jim
Thanks for the excellent, easy to read, understandable, and descriptive essay on those two joints.  I'm impressed (a bit jealous too) with your ability and command of words and writing. Good supportive info to back up your points too.

[Jim_Rogers]

Thanks beenthere....

I don't want to tell you how many hours I spent on that last post.......


Jim Rogers

[Max sawdust]

Wow Jim, That was a whole chapter!  I read it through three no four times so far and still have lots to absorb from it.  I find your writing style and illustration to be better than what is found in most  books.  Thank you very much.

First off, I hear what everyone is saying on grading, I will not grade timbers select for design purposes anymore.  I can think of a multitude of reasons why it is not a good idea. (period)

Secondly, I hear what you are saying on the dovetails.  I did plan on housing them and may still do so if the Purlins become oak like the principal rafters did.  (Will keep the struts and queens)  However I would like to keep the Purlins as Red Pine though, so I am liking the idea of tusk tenons very much.   (Never would have thought of that on my own.)
Thank you for the detailed info on the tusk tenon  I will study them further. (And have questions I am sure.)

The problem I see with the tusk tenon is what Raphael brings up about needing lots of help raising the frame.  I had hoped to raise it with one other person and a gin pole.   I guess having a bunch of friends over to help will not be a bad thing.

It seem obvious after you say it that when designing think of the load first and work from the top down.  But somehow that escaped me with this project :

The barn you worked on is very attractive. I will have to take pictures of my parents TF barn and post them.  Unfortunatly it is failing (tie beam popped out) and has a leaky roof.

Thanks again. Hopefully someday I can repay the favor.
Max

[Max sawdust]

OK,
This is where I am at.  I would have to use 6x8 red oak for the principal rafters.  I can not seem to get around not fully housing the tusk tenon  I need the full 6x7 purlin in red pine to support the load.
So I would need a deeper principal rafter!?
(And I am trying to envision inserting 6+ 12' 6x7's with tusk tenons into the principal rafters as the frame is going up.)
SO,
I am considering changing the design away from purlins.  Leave the two bents as they are with primary rafters to post top.  But adding a hefty 8x10 or 8x12 intermittent oak top plates and a ridge beam between the king posts.  Then run common rafters (6x6) 2'oc that rest on the intermittent top plate and ridge beam or do a tung and fork at the peak.

Not asking for anyone to design my frame for fee.  (Already feel like I owe Jim my first born son or something  :'()

Just looking for a thumbs up or thumbs down on this idea.  (or a little more :)
Max

[Raphael]

OK,
This is where I am at.  I would have to use 6x8 red oak for the principal rafters.  I can not seem to get around not fully housing the tusk tenon  I need the full 6x7 purlin in red pine to support the load.
So I would need a deeper principal rafter!?

  If you want to house 6x7 purlins then a deeper rafter is a must and going an inch or two wider wouldn't hurt as either.  I'd try to engineer things so drop in purlins have enough bearing to be structural without killing the strength of the rafter, actually I'd got a bit beyond that so I had the option of adding tying joinery... This might be more rafter than you want visually but it definately solves assembly issues.  I haven't run any numbers but you should be able to get back to red pine rafters by doing this as well.
  The rafters in my office are 8x8 which is far more timber than I need for the expected load but without that full 8" of width my tusk tennons would be unpeggable which would leave me with nothing tying my roof but stress skin panels.  Often it's the case that you'll need more timber for joinery than you will for strength of the overall timber.

  The principle & common rafter approach works well, lot's of options, just keep in mind assembly when designing the joinery.

[Jim_Rogers]

Now, Max, don't run off thinking you'll have to scrap your whole design.....

You didn't ask "how do I raise such a frame?"

There are methods to do this, with tusk tenons that can be done with just one or two people.....

First of all you stand up one bent, and secure it plumb. In your case probably the one against whatever wall has the overhang that you're trying to be under.
And then stand up the other bent. This bent will have to be moved out a bit at the top in order to put in the tusk tenons on that side. So, what is usually done is that the timber framer uses straps to connect around the frame and come-a-longs to hold it. When you need to lean it out you loosen the come-a-long up a couple of snaps/clicks. When you have all tenons nearly in place you then pull it back together making this second bent plumb.

Now that we understand how to move one bent out and in safely, how do we put in all the purlins?

Well, first of all you will put one end into the mortise on the plumb bent. That's easy, you just lift them up by hand/gin pole or whatever and slip it in, and you might drive a peg part way in to hold it. But don't drive the peg all the way in, as you might need some "play" to line up the other end. Now you need to hold it up there as the others are installed in a similar manner, and while the loose bent is leaned/pulled in.
This can be easily done using cleats.

Here is a drawing showing what I mean:



In the above drawing, which is a top view, cleat #1 is attached to the top side of the rafter and is aligned with the mortise/housing of the rafter where the purlin will connect.

Cleat #2, which I colored as glass so we could see through it, is attached to the purlin. These are usually attached with screws or timberlock screws shown as black dots here. And I have drawn them as 2"x6"x 2'.

Here is a side view:



In the above picture you can see how the cleat will hold the purlin up and in line with the mortise.

Each purlin should have cleats on both ends and each rafter should have a cleat where the each purlin will enter.
Here is a raising in progress showing what I mean:



Above you can see the straps around the bents and the come-a-longs hanging between the bents you can see the crane holding the bent up.
You can see the cleats on each rafter for the purlins to align with. These also make nice ladder rungs to get up the frame to disconnect the crane straps from the frame, once the bent is secure.

There are other methods to holding up your plates, with one end in place, while your bent leans out for inserting the purlin tenons.
Sometimes this leaning is only a few inches and can be done every easily.

Just some more things to thing about, in regard to methods of assembling a frame....


Jim Rogers

[Max sawdust]

Often it's the case that you'll need more timber for joinery than you will for strength of the overall timber.

Raphael,
I am painfully learning this :

Jim,
Thanks for the great tutorial on raising.  The cleat idea makes sense  I can not wait to give it a try.  I enjoy lifting/moving/assembling the "impossible" with the mind (good engineering).  My experience has been the larger the group of people the less thinking and the more brute force is used.

Raphael, Jim,
I am not scraping my whole plan, just trying to avoid having to increase timber depth as much as possible, and with Purlins the principal rafters would need to go from 8 to at something like 10 inches deep (in oak) with common rafters I would only need the two top plates with increased depth.  I have boxed myself in a corner for several reasons:

First:
I have 20 or 30 6x8's in red pine.  (Had a couple of jobs to saw on halves and figured 6x8 would be good for the shed >:( : :-[ :'(  )  I have no more red pine to mill right now.

Secondly: Any new members of different depth need to be in oak because that is what I have.  Guess what Yes earlier in the week I did go fell a oak I had slated to go, and milled one log made some 2x material and guess what I made another 6x8.  I really really have a bad habit of cutting before thinking :-\

So you can now see why I have a "fixation" with the 6x8.

And yes I have several days of joinery cut into posts including the king post. So I have built several "boxes" around my self limiting my options.

If I can get away with the principal rafters in oak 6x8 or even 6x9 or 6x10 I may stay with the purlins.  If not I am thinking keeping the bents as designed and adding a hefty Oak 8x8 or more likey 8x10 top plate and running common rafters.

Respectfully,
Max

[Jim_Rogers]

Did you do the load calculations for common rafters?

What size will you need?
Maybe you can use 6x8 red pine?

Jim Rogers

[Max sawdust]

Here is what I did. 
Total Load on Beam(pounds)  1170
Dead Load on Beam (pounds)  1170
Span of Beam (inches)  114
Width of Beam  4
Depth of Beam  7
Select Species  #2 Red Pine
Pass Pass Pass

The load was determined as follows:108sq x 65LB=7020
Rafters 2'OC = 7 rafters for 12' span  end rafters (principal rafters) only carry 1/2 load each so I divided 7020 by 6 for the load per rafter number (1170)

Looks like I could get away with 4x7 #2red pine, but I think I might be able to find some 6X8 Red Pine that will work even better ;)

At the top plate I plan on cutting a birdsmouth with a peg through the top But I can not decide what would be better a ridge beam (Which I am not sure how the rafter connects to the ridge beam. )  or do a nice simple tongue and fork with a collar tie.

max

[Raphael]

Jim - I like that cleat solution...  Simple, effective and totally obvious in hindsight (all the best solutions are).

 I've got to start typing faster... you guys are way ahead of me.

Max - Since I've already typed this here are my notes on using purlins:

6x10 N.Red Oak rafters will work with a little care in the joinery design.

We may be able to come up with something that works with 6x9 depending on how much bearing the purlins need but pretty sure it will require all of the tying to be done by the ridge beam so we'll want to look closely at that.
 Or taking a step backwards, we might raise your purlins just 2" then you could peg the 2" lap over, you need to cut it like a dovetail in that the weight of the purlin must always rest in the joinery below the lap and not on the lap itself.

[beenthere]

Max_sawdust
Have you been to the big sawmill just east of Woodruff on Hwy 70, and possibly they are structurally grading there (and then maybe not). Pukall Lumber if I recall correctly.
My point being that finding out (learning more) what makes a #1 timber (purlin and rafter) might be to your advantage, and I'm sure you could sort some from your timbers for your structure. Knowing a bit about what kicks the timbers down to #2 and what keeps them in #1 grade will also be helpful in selecting which logs you want to saw into timbers and how to saw the logs.
Maybe a chat with a grader from that mill will be helpful too, if they are willing to chat.

Just thinking that you might be designing for #2 but have #1 or better.

[Jim_Rogers]

Max:
I thought you were going 18'x24' so wouldn't the rafter be a 9' run?
This shortens them and makes less weight.
I'm talking about common rafters between the two principals on each bent.
If you want a ridge beam, lower it. Put it be between the two king posts.
and place the rafters over the top of it with a birds mouth like this:



This shows a ridge beam with a fancy end on it but you'd just put a tenon on it and make a through mortise on the king.
Lowering the ridge beam relieves the top of the king post where the two principal rafter would go and you can run a brace from this lowered ridge beam to the bottom of the king as long as it doesn't run into the struts' mortises location.

Here is another tip:

One great learning tool is to make a scale model of the frame, before you
start cutting the joints.
Even experienced timber framers do this.
It helps you to visualize the frame and see where every part goes.
You don't cut joints in the model you just cut the timbers to scale and nail
or hot glue them together.
When you get to that point I can show you how....

Jim Rogers

[Max sawdust]

Lets see..
Raphael,
Thanks for the good ideas on Purlins!  (Good to know if not for this project then another.)

Beenthere,
Yes I know the saw mill well.  (My John Deere dealer is just across the street) So whenever I drive down that section of highway I am always drooling :D  Either over the huge pile of saw logs at Pukall's or at a new tractor on the other side of the street.
I was tempted by a flier from the Uof Minn. that offered a short course on grading.  I will check into it some day.  thanks

Jim,
The building is 18' deep by 12' wide (just the one 12' bay). This is why I used 9.5' (114 inches in rafter load calculation).
Thanks for the pic of the ridge beam and suggestions on it.  It seem like it would be less work than lowering the assembled tongue and fork rafter onto the structure.
My brother suggested the model idea too.  I rejected it because I have a pretty good imagination, and my first project was so "simple".  So I did not model my TF swing set and made a few glaring mistakes.  I know see the value in it as my structures become more complex.  I will heed your advice on modeling too. What scale is typically used?
Thanks again
Max

[Jim_Rogers]

I'll post some pictures and details tomorrow....

Jim Rogers

[Max sawdust]

No Pics yet, but am "rough" fitting the first bent to measure for braces and queen posts.  I must say Jim's friends spline for a two piece tie beam is real slick.  It is not too bad to cut and fit, and it sure brings things together in a real strong way.

I will post some pictures when I send pegs into the first bent, and also pictures while raising the frame. 

Since this is my first "real" size frame, I must admit it is quite a rush to come along and beetle the timbers together and have them lock tight

Another very addictive way to enjoy sawdust :D

max

[Jim_Rogers]

Here are some of the new shots from the Fox Maple school.

This is the king post at the tie beams:



And here is the back side of it:



There was a 7x10 beam entering the back side of this king post so that the top surface of the 10" tall beam was the same height as the top of the 12" tie beams. A tricky three way joint for sure.

This is the opposite king post as I'm standing under the other one.
You can see the new loft floor that has been added recently and the new hand railings around the loft area. Also new is the hanging light.
The wedges were cut off flush with the sides of the king post so the loft floor could be laid down right up to the king post.







This shot shows the rafter that connects into the king post above the struts but below the ridge. It also shows the struts.

Turning a bit to the left we see:



The principal rafter and the hip rafter with purlins.

I asked Steve about the king post to tie beam joint and again he said they are dovetailed and wedged.

Jim Rogers

[Max sawdust]

Well these are not quite like the library at fox maple :D
But here is the trial fit on the first bent, so I can measure for braces and the queen posts.
Here is the whole bent.


Here is a close up of the two piece tie beam with the cool looking spline.



Been having some troubles:
Not getting something with square rule yet.  Seem to be getting gaps on my reference face.  Reference faces are coming out flush, but with gaps, especially on the half dovetail to post connections and the birdsmouth to post connections.  The inside surface of the joints seem nice and tight.  (Which is the side you will see I suppose.)

Max