Hi all! This is kind of a lot to ask of you guys, and we may get an engineer's help to finalize our plans, but I'm still in the design stage and would like to make sure I'm on the right track..
The plan in question is a timber frame for a 32'x48' cordwood home with a gambrel roof. Here's the tentative first floor plan:
(https://img.photobucket.com/albums/v667/AirsoftAndy/Weavers%20House/weavers-main.jpg)
Note the measurements at the top, indicating the bent spacing. That one space says 13', but it's been modified so none of the bents are distanced more than 12' apart. The big weird thing below the kitchen is a wood cook stove. The wall behind it is only 3'-4' tall, and their may also be a wood stove facing the living room, back-to-back with the cook stove.
Here's the loft:
(https://img.photobucket.com/albums/v667/AirsoftAndy/Weavers%20House/weavers-loft.jpg)
I'm mostly posting for advice regarding the timber frame, but comments or critiques about the floor plan are also not only welcome but appreciated!
Here's what the cross-section of one of the "bents" might look like. I say "bent", because we won't exactly be framing in typical "tip up the bent" fashion; more like one post and beam at a time. Also, not all of the joints will be complex works of art... Anything beyond a dovetailed loft joist and we will probably settle for metal plates and bolts to save time.
(https://img.photobucket.com/albums/v667/AirsoftAndy/Weavers%20House/side-elevation.jpg)
The rafters are standard 2x12's on 24'' centers. The main thing I'm concerned about is the offset 2nd floor post. However, I've seen this done in books many times, and it helps to shape the gambrel roof. I'm planning on 3''x6'' knee braces on all of the interior posts like the one pictured (I drew only one out of laziness), so that will help transfer the weight of the roof to the 1st floor posts. Also, the 8x8 cross beam is only notched 2'', so it should retain most of its shear strength. It seems strong enough to me, but the calculations for the shear+whatever strength the knee brace adds are beyond me.. The wood will most likely be spruce.
Moving right along, here are the 1st floor posts.
(https://img.photobucket.com/albums/v667/AirsoftAndy/Weavers%20House/posts.jpg)
Here are the beams and 3''x6'' loft joists.
(https://img.photobucket.com/albums/v667/AirsoftAndy/Weavers%20House/no-loft-posts.jpg)
Sorry about that first beam looking so goofy.. A SketchUp pro I am not! The beams connecting the interior posts are 8''x8''s.. The one center beam supporting the ends of the loft joists is 6''x8''. I have no knee braces pictured but am planning on them... The cordwood infill will brace the exterior framework, but I'm feeling that a house this big should probably have braces on the interior posts as well..
And finally..
(https://img.photobucket.com/albums/v667/AirsoftAndy/Weavers%20House/full.jpg)
With the rafters and some sort of sheathing I'm thinking I won't need to put knee braces on the upper posts and beams. Would that be okay?
Sorry for the length of this post and the complexity of the questions.. Any advice would be greatly appreciated! I can also give more info if I failed to be descriptive enough.. Our snow load is in the 70 psf range.. I'm probably forgetting some important info.. That's the problem with late-night posting!
Thanks a bunch,
Andrew
Not that I'm a expert but I think that summer beam needs to be a 8x12 instead of a 6x8. The frame should competly support itself the cord wood is just for filling. I would say you need knee braces in both derections on the upper posts and beams. If you are worred about the posts under the 2nd floor off set you could gun stock the top of them or use bigger posts. You need a engineer to look at this I think he is going to make you go bigger on a lot of your beams ???
Nice job on the design it is a step in the right direction [cool]
You can also get some answers and ideas at this web site might even be a engineer for timber framing that will give advise on the forum[noidea'
http://tfguild.org/
http://www.tfguild.org/forums/ubbthreads.php
Thanks for the info; I'll post my load calculations for the beam sizes later, but I just wanted to ask why you think knee braces are needed on the 2nd floor post and beams? It looks like the rafters would brace the posts in one direction if they were attached well, and the diaphragm bracing of the roof sheathing and/or metal roofing would brace the frame in the other direction. ???
Okay, check my math regarding the summer beam in question...
(I'll repost the picture for easy reference.)
(https://img.photobucket.com/albums/v667/AirsoftAndy/Weavers%20House/no-loft-posts.jpg)
It spans less than 12', but I'll round it up.. It supports 3.5' worth of loft. 12' x 3.5' x 47psf = 1974lbs total.
The beam is under a two point load.. At 12' with an allowable deflection of 1/360, my table shows an 8'' deep spruce timber at 386 lbs per inch. 1947lbs/386 = a beam width of at least 5.11''.
On the other end of the house, where the loft spans the full width, there is an extra post under the summer beam (next to the bedroom door) to aid with the extra load.
Whilst I'm at it, I'll present the calcs for the other beams as well...
The main 8x10 girders running both sides the length of the house..
They support 6' of roof and span no more than 12'. 6' x 12' x 92psf = 6624lbs. Because of the steep pitch of the roof, I can multiply that by a reducing factor of .45, which equals 2981lbs. The beam also supports 4' of loft, which is 2256 lbs. (In reality, the live load of the loft will be very minimal next to the eves, with this roof design.)
5237 lbs total. A 10'' deep timber under a uniform load and spanning 12' is capable of supporting 1028 lbs an inch, meaning a 5x10 is really all that is needed, but there will be a porch roof added on as well.
The 8x8 girders simply support 6.8'x12' worth of loft.. 7.5' x 12' x 47psf = 3835lbs. At 386lbs an inch for a 1/360 deflection, the beam is undersize, but by allowing a bit more deflection an 8x8 is fine. I think that allowing 47lbs for a loft load is generous, so I'm not too worried about that...
Anything incorrect with my logic? I could very easily be mistaken in my thinking here...
I'm not sure I'm understanding correctly. Can you highlight the beams in question with various colors and the text in your posts with the same colors?
I'm assuming "Eastern Spruce", that would be Black, Red ,or White Spruce?
If so, in #2 grade it has an Fb of 575 psi, E=1.0, Fv=135 psi.
Plugged into a two point load equation here;
http://www.windyhilllogworks.com/Calcs/2ptbeam.html
You asked about the upper posts on the floor offset from the posts below. They are close to the posts below. If you count on the braces check out the posts for combined compression with a side load. I'd be tempted to sum the upper post's point load and the uniform floor load together, move the load point to the right a proportionate distance and checking the beam as a single point loaded beam. Would you like a calc like above for that equation?
I would agree, if the roof diaphragm is providing the bracing the upper posts do not need kneebraces. If it's board sheathing then its a different matter.
I'll check in when I freeze again, busted a lift arm on Fergie :P
Quote from: Don_P on February 14, 2010, 12:36:49 PM
I'm not sure I'm understanding correctly. Can you highlight the beams in question with various colors and the text in your posts with the same colors?
I gave that a try.. Does it help?
Quote from: Don_P on February 14, 2010, 12:36:49 PM
I'm assuming "Eastern Spruce", that would be Black, Red ,or White Spruce?
You know, I'm not even sure.. I don't know my wood that well, and it'll be coming from a private party. Does it really make that much difference?
Quote from: Don_P on February 14, 2010, 12:36:49 PM
You asked about the upper posts on the floor offset from the posts below. They are close to the posts below. If you count on the braces check out the posts for combined compression with a side load. I'd be tempted to sum the upper post's point load and the uniform floor load together, move the load point to the right a proportionate distance and checking the beam as a single point loaded beam. Would you like a calc like above for that equation?
If it isn't too much trouble! You're the expert.. ;)
Quote from: Don_P on February 14, 2010, 12:36:49 PM
I would agree, if the roof diaphragm is providing the bracing the upper posts do not need kneebraces. If it's board sheathing then its a different matter.
What about a heavy metal roofing? Is that considered an adequate form of diaphragm bracing? I've noticed very large pole barn roofs that have no other form of bracing, but perhaps that's just bad practice. We don't have to worry about hurricanes or earthquakes though..
Quote from: Don_P on February 14, 2010, 12:36:49 PM
I'll check in when I freeze again, busted a lift arm on Fergie :P
Hmm.. Bummer. Sounds like a relaxing way to spend a Sunday. ;) Thanks for the help!
With timber framing the beams do a lot more that support a load the summer beam will tie the building together so it is in tension also not just compression.
(https://i455.photobucket.com/albums/qq278/Minermatt/Belton11.jpg)
Good luck,W
Awesome! Thanks [cool]
I put 1000 lbs on each of the points in this calc;
http://www.windyhilllogworks.com/Calcs/2ptbeam.html
At full dimension 6x8 it failed, at full 8x8 it passed. If there is joinery we're not out of the woods though.
I'm pretty sure these are your spruces, the range between species looks to be 25 psi in #2, not huge.
I'm trusting your assessment of the loads on the far end of the building.
I think it is reasonable to call the loading here uniformly distibuted; http://www.windyhilllogworks.com/Calcs/beamcalc.htm
I failed slightly in bending, again without joinery. If there are sheds then the slope reduction is unconservative, I don't take those but do bump the Fb 15% for snow typically... I didn't here since you had already told a whopper :).
At my design values I needed 1/3 more section modulus to pass, a 9x9 made it.
I am using current #2 design values which is pretty typical, I'm leery of calling it better unless you are trained to grade or have some really clear stock.
Yes the metal roofing is considered a diaphragm when attached according to spec.
You'll need to recalc the remaining section modulus after joinery.
I'll post it if I'm successful on the other calc, please go ahead and figure the heaviest area's post and floor loads.
The tension in this direction is minimal, you have resolved the infamous thrust issue with the purlins in the other direction. These are really bending members rather than tension or compression members. Nice building Whitlock.
It's not mine Don just showing that knee braces were used in this aplaction.
Quote from: Don_P on February 14, 2010, 08:18:51 PM
Awesome! Thanks [cool]
I put 1000 lbs on each of the points in this calc;
Wow, isn't 1000 lbs pretty high? The 3x6 joists are on 4' centers. They span 7' from the blue girder to the red summer beam. 3.5' x 4' x 47lbs = 658 lbs per point. (And I think considering 47 psf total load for a loft is pretty conservative..)
Quote from: Don_P on February 14, 2010, 08:18:51 PM
I think it is reasonable to call the loading here uniformly distibuted; http://www.windyhilllogworks.com/Calcs/beamcalc.htm
I failed slightly in bending, again without joinery. If there are sheds then the slope reduction is unconservative, I don't take those but do bump the Fb 15% for snow typically... I didn't here since you had already told a whopper :).
I agree about the porch roof somewhat negating the slope reduction factor; I hadn't thought of that, but I'm surprised that your calc is showing such different results than the tables I'm using, which are from several reliable timber framing books.. I'll have to come back to that one..
Quote from: Don_P on February 14, 2010, 08:18:51 PM
At my design values I needed 1/3 more section modulus to pass, a 9x9 made it.
Yeah, those should probably be beefed up a bit. However, most of the spans are closer to 10' than 12', and the beam will not be notched at all, except for the knee braces underneath, which will actually help support it.
Quote from: Don_P on February 14, 2010, 08:18:51 PM
I'll post it if I'm successful on the other calc, please go ahead and figure the heaviest area's post and floor loads.
Do you mean the weight of one of those 2nd floor posts? Using the same process as before I came up with 11788 lbs.
Thank you very much! Cool building there, Whitlock. Looks very stout.. Another example of the offset posts.
Quote from: Ernest T. Bass on February 15, 2010, 01:02:25 AM
Quote from: Don_P on February 14, 2010, 08:18:51 PM
Awesome! Thanks [cool]
I put 1000 lbs on each of the points in this calc;
Quote
Wow, isn't 1000 lbs pretty high? The 3x6 joists are on 4' centers. They span 7' from the blue girder to the red summer beam. 3.5' x 4' x 47lbs = 658 lbs per point. (And I think considering 47 psf total load for a loft is pretty conservative..)
I divided the 1974 lbs between the two joists, ~1000 lbs each. Yes you can figure the moments more accurately.
Quote from: Don_P on February 14, 2010, 08:18:51 PM
I think it is reasonable to call the loading here uniformly distibuted; http://www.windyhilllogworks.com/Calcs/beamcalc.htm
I failed slightly in bending, again without joinery. If there are sheds then the slope reduction is unconservative, I don't take those but do bump the Fb 15% for snow typically... I didn't here since you had already told a whopper :).
Quote
I agree about the porch roof somewhat negating the slope reduction factor; I hadn't thought of that, but I'm surprised that your calc is showing such different results than the tables I'm using, which are from several reliable timber framing books.. I'll have to come back to that one..
What design values are you using, or which books are the tables from? The upper roof will dump onto the steep roof. That will crash down to the shed roof transition and pile there. Next snow it will do it again and the pile will grow. I have that situation on my barn.
Quote from: Don_P on February 14, 2010, 08:18:51 PM
At my design values I needed 1/3 more section modulus to pass, a 9x9 made it.
Quote
Yeah, those should probably be beefed up a bit. However, most of the spans are closer to 10' than 12', and the beam will not be notched at all, except for the knee braces underneath, which will actually help support it.
The weak link is the one that fails, it is not typical to count on the braces in the load calcs. We can massage the data but in the end this is an attempt to model reality and apply a margin of safety. We aren't going to change reality, its mighty easy to unwittingly eat into our safeties. I stay very conservative and let the engineer earn his money shaving it down based on his experience.
Quote from: Don_P on February 14, 2010, 08:18:51 PM
I'll post it if I'm successful on the other calc, please go ahead and figure the heaviest area's post and floor loads.
Quote
Do you mean the weight of one of those 2nd floor posts? Using the same process as before I came up with 11788 lbs.
Thank you very much! Cool building there, Whitlock. Looks very stout.. Another example of the offset posts.
Thanks for the post load, I had it up and running but crashed when I tried to save it. I'll try again tonight.
I see what you're saying about erring on the side of caution... I'm just trying to keep the timbers at a manageable size, since there will only be a few guys working without heavy equipment. ;)
I've been using tables from 'The Craft of Modular Post and Beam' by James Mitchel, 'Building the Timber Frame House' by Tedd Benson and James Gruber, and 'A Timber Framer's Workshop' by Steve Chappell.
I have a table for basic stresses from the Forest Products Laboratory that lists dry spruce at 1600 psi in fiber bending, 120 psi in shear and 1.2 million psi for modulus of elasticity. I'm not arguing, just trying to figure out what's safe without going too overboard. ;)
I think there is a link to the NDS Supplement on one of the calcs I posted. These are the design values recognized by code and all engineers working in wood. Benson's engineer and head of operations, Ben Brungraber, was part of a roundtable after giving presentations along with the head engineer from AF&PA who publishes the NDS. A short article by him preceeds the NDS design values in the Guild's "TF Joinery & Design Workbook". I can see how the values were derived by Mitchell and Benson, they speak to an evolving understanding at the time. Don't use them, they are above Select Structural grade (clear) for a sawn heavy timber. Bensons description of working through the design process is excellent, just use current accepted design values. I suspect the FPL table is specifying those values for uniform loading of "dimensional" stock with several adjustment factors included. The FPL among others does testing of small clear samples (2x2's) and those numbers are the tables in chapter 4 of the Wood Handbook, that is their role in the process. The numbers there are nearly useless to the average person for anything more than comparison between species. The grading agencies using the methods described in ASTM D245 and working with the AF&PA then derive the allowable working stresses for each grade and publish them. They have backchecked these design values by doing "In Grade" testing. Breaking loads of graded lumber.
Notice the difference in design values in the Supplement between dimensional lumber and heavy timber. Heavy timber has not been tested in large scale. The shots I've seen from the lab when they do are mighty impressive. In one series the technician is diving behind the equipment as half a flying 8x12 ricochets into the ballistic glass. It is harder to grade boxed heart heavy timber and there are fewer alternate load paths if a timber fails. If a 2x joist fails, there are others close by. If a timber fails a large load gets thrown to its few neighbors. These are all reasons for caution.
I've been through the training to grade lumber. In lumber I can roll the board and visualize with confidence just about all the defects. Think about this. Branches grow from the heart, they grow radially from heart to bark. Many are pruned by one thing or another at some point in the growth of the tree and are then covered by new growth. Knots are one of the big factors in grading. In a piece of dimensional lumber if it doesn't include the heart and if I identify the grain orientation as I approach it, I can tell you everything about the knot I'm looking at. I know how much sound wood surrounds it and what it's grain orientation is like. In a boxed heart timber there can be more lurking under the surface that I have very few, or no, clues about. Most stress is carried on the surfaces of a beam (an I beam tells you how little is needed in the middle) so there's a grain of salt. Your eye is still capable of saying a whole lot about the load carrying abilities of a timber.
It is not the knot so much that weakens a timber, sure there is essentially missing wood, but it is the slope of grain surrounding it that plays a larger role. If the grain swirls substantially around a knot and then we saw across that grain in making the board, the short grain will break relatively easily. So here's the other end of the spectrum; a log is not manufactured, a saw has not cut across any of the grain. Even if it swirls around a knot it is a continuous strap of fiber. Unsawn round logs carry the highest design values, eastern spruce;Fb 1400, E 1.2, Fv 135. Saw up to 3/10 of the diameter off the top and Fb drops to 1150 in a #1. Saw any more and you just made a heavy timber.
Notice the difference between #2 eastern spruce beams and stringers and #1, big difference. I don't assign anything above a #2 unless it is a very sweet timber. I hope this explains some of the reasons behind the values and which to use. This is the biggest variable and requires the best judgement. We can do elegant calculations and in the end what I'm hoping is that the timber you choose has an ultimate strength at least double what the numbers say, (remember the one that broke above).
I don't consider you to be arguing, its part of developing a feel for how the design values relate to the stick in front of you. What I'm doing I hope is confirming where I can and directing you to the resources your engineer will be using. The supplement and those calcs are from the NDS. Good judgement allows you to adjust the variables and I don't mind if we disagree on those details. For me on residential work, a card carrying grader is going to mark on the timber the grade and thus the design values.
Seems to be working;
http://www.windyhilllogworks.com/Calcs/AnyPtLd.htm
Make sure the left and right sides sum up to the total span. I'd check the post at correct location and then drift it right a bit while watching moment, deflection and shear.
Following Steve Chappell's book and you will have as good of a idea of what to do and how to do it 8)
Cool web site Don [cool]
waay off topic but this got me thinking. On our higher elevation peaks around here we have some red spruce. The tree by all rights shouldn't be growing here. These are remnant colonies from a colder time left stranded by natural climate change. Quite a few are now dying, acid rain got some and the southern pine beetle got some others in the Nat'l Forest that I got to saw up. Nice trees, the really good ones are making music as the tops of guitars. If you read "Clapton's Guitar" those spruces have a walk on part. The local guitar maker got those, I sawed the other ones :).
Thanks for taking the time to explain all that to me! I tried to download the NDS Supplement, but it failed to open for me.. ??? I'll try and find it somewhere else.
I would prefer to work with round timber as much as possible; I feel that nature knows how to make a strong chunk of wood regardless of the knots in it, and sawing into the grain severely weakens it like you describe.. It's just a lot more work!
I'm surprised that the experienced framers who wrote my books would use inferior tables.. You would think they'd be aware of the stuff you're talking about.
Basically, you can publish anything. The numbers I've posted are the correct ones to use for design. I have checked the '82 NDS, the closest I could come to publication era, the numbers were the same. I've pm'd you a little more.
It is also not customary to take Mitchell's roof slope snow load reductions, ASCE-7 does allow reductions with caveats. Use good judgement there, I take full ground load. I've been called when a client had 3' of consolidated snow stuck on his 12/12 metal roof... never would have believed it. Happily the engineer had specced rafters on 12" centers, I quit laughing at him about then d*.
These are from the '01 NDS Supplement. There were adjustments to the shear values that have changed with the '05 version so use the new values I posted earlier. I have a pdf of the '05 Supplement I could email. PM me your email if you want to try that.
http://www.windyhilllogworks.com/Calcs/Fblist.htm
QuoteI feel that nature knows how to make a strong chunk of wood regardless of the knots in it, and sawing into the grain severely weakens it like you describe..
The FPL has been trying to work on that. We have an overstock of small diameter roundwood nationally. It needs thinning but the timber has alot of weak juvenile wood. If you can saw just 2 faces and leave the others round the resulting timber is about 50% stronger than if it were squared.
Quote from: Don_P on February 15, 2010, 09:08:16 PM
Seems to be working;
http://www.windyhilllogworks.com/Calcs/AnyPtLd.htm
Make sure the left and right sides sum up to the total span. I'd check the post at correct location and then drift it right a bit while watching moment, deflection and shear.
How exactly did you work this calc? I tried the point load @ 10'' from the support and an 88'' 8x8 failed. I imagine figuring out exactly how much additional support the brace adds would be complex and would depend on the size and type of joint, but if the figures came close without the brace it would be pretty safe, right?
Even though the brace will support some weight that is not it's job. Don't minimumly engineer this building :-\ Over build it Over build it Over build it.
Sorry to be slow, been doing a big takeoff, hard to concentrate with bare feet in the winter ;D
Whitlock nailed it, I was just being verbose, but its writ so my .02
Braces are normally not factored into the beam sizing, here's my take on it. The post will shrink some in service, this will move its bearing surface away from the brace's end a little. The brace will shrink a bit in width, the 45 degree angle you cut will become more acute, pointier.
This brace was cut by a computer controlled machine in green oak. ALL the tolerances were within a very few thousandths when assembled. It has now dried for about 3 years. Yes you should house them but it is good to see what is hidden in many housings. Dry brace stock cuts that scenario at least in half.
(https://i180.photobucket.com/albums/x109/windyhilll/MVC-007F.jpg)
If asked to do work the point will move over till it contacts the post and then the point will have to drive into the post until it crushes a wide enough bearing surface to support the load. "Load goes to stiffness", where is the load? The ends of the beam are supported on the vertical grain of the post top (note to self, check beam end on post for crushing), the posts' length really won't change appreciably so there are our stiff supports. The load will definitely be entirely upon them initially. If the beam deflects enough, near it's end, it will begin to bear on the brace to some degree. At the point where all this "hooks up" have we bent the beam enough to begin weakening it? Your shears are bad near the loaded end, if that begins to shear the beam will seperate into horizontal planes and bend like the pages of a phone book. This may happen before the brace takes the load, you are beyond me there. Braces are ideally just that, they resist instant racking loads like wind (and they don't do real a good job of that).
A post with a loaded brace is considered to be also side loaded as opposed to just axially loaded. Instead of the load being applied to the top of the post and travelling straight down its vertical axis, the post has a vertical load as well as a bending load from the side. Take a thin stick and put it between thumb and forefinger and squeeze. That is an axial load. Now push in on the side of the stick while squeezing the ends, combined loading. It should have been much easier to buckle the stick in the second experiment under the same axial load. There is a complex interrelationship equation to account for combined loading but it is modelling this simple fact. The post can do a certain amount of work, the more axial load it takes, the less bending load it can handle and vice versa.
I've tried to make a calc out of the modern combined loading equations and haven't succeeded yet. I might scan them in just so you can get a laugh, complex doesn't begin :D. This WWII era math for the interrelationship is cruder but better than nothing and might help get a feel for the concept;
http://www.windyhilllogworks.com/Calcs/44axbend.htm
The point loaded beam calc is only figuring for a point load. We have a point load from the post and a uniform floor load on the beam. This affects the location of the maximum bending moment... the effective bearing point of our point load, that's why I said to drift right a little, out into the span. I've been lazy and thrown the entire load onto the post so we just have a point load. The actual maximum bending moment is going to be a short distance to the right of the post. As I've played with it a bit this is more critical than I thought if you are trying for the minimum beam size. There is a balance between shear and bending failure as the post moves that looks pretty fine. We need to read more on figuring the true point of maximum bending, it will be where shear passes through zero so approaching it from either of those directions should get us there. Please remember you're talking to a carpenter, I like to err big and brutally basic. This is all above my pay grade, do get it checked if you are pushing around the edges or if any of this isn't adding up for you. A dairy barn up the road collapsed yesterday, another old TF gable barn is twisting and listing, it won't take much more snow and is a deadfall trap waiting to be thumped. You sure don't want that call :).
Lifting big beams is just a matter of outsmarting them. Cathedrals were built without modern cranes.
Okay, I've got to ask one more question before I can let this thread die.. :)
First of all, I'm rethinking the frame a little bit. By adding two posts to the middle of the living room for 2nd floor support, I'd like to scoot the two inner rows of post out to sit directly under the 2nd floor posts (no offset worries).
My question is about knee braces and if they are necessary, considering the cordwood infill that will completely brace the perimeter of the building. My thought was that they wouldn't hurt, but I'm having trouble designing the frame (in a straightforward way), to incorporate large beams that intersect over interior posts for brace attachment.
Conventional houses are only braced by their exterior sheathing though, so could the knee braces be safely omitted for simplicity's sake?
If the answer is no, than what is the minimum size for a floor joist to accept a let-in knee brace from below? It's a ridiculously simple question that I haven't found an answer to.. I know that the braces themselves can be as small as a 3x6, but would notching into a 3x6 joist weaken it too much?
A thousand thank-you's. ;D
I'm enjoying the conversation myself.
The cordwood infill could probably provide some level of in plane lateral resistance, I don't know what it would be. Out of that wall plane the floor or ceiling diaphragm can be bracing the building. As long as the whole is braced I don't feel the bracing has to be timber knee braces, they aren't much anyway.
As far as how much can you remove from a joist, the correct way would be to check the bending moment at the location of the mortise and then figure the joist dimension there as the mortised section. Check it as a simple beam. Often the sizes of timbers revolves around the size of timber remaining after joinery.
I don't know how you could possibly determine exactly what amount of bracing the infill will provide, especially since we will be using a cob mortar. You wouldn't think a massive 18'' thick wall would have any problems, though. In order for a wall to rack, the infill would have to totally smush..
I'm just a tad nervous because of the size and height of the place.. It'll pick up a lot of wind.
Can you not integrate some big diagonal X's of something across the frame in the cordwood as you build? If not timber in compression, Hi tensile steel in guying tension x's, think about the end of a fence?
Might be kind of a pain to cordwood around, but certainly a possibility. (The infill will basically wrap around the outside of the timbers with shorter log ends.) Perhaps each corner could have a pair of diagonal beams running down to the nearest post in either direction. Could look kind of cool, but there would be glass in the way of a couple of them..
Just like a fenceline, every bay does not have to be braced, every plane needs to be adequately braced. The rest of the bays in that plane collect and have their lateral load resisted by this rigid panel. If every exterior wall has a braced bay and if the roof is a rigid diaphragm, the forces will be resisted by the stiff bays, as long as they are up to the task.l
Off topic; It's no different for piers for that matter, if folks would brace corners or bays they would have a building that might survive a wind or seismic event.
Ken Kern published old USDA info that conservatively rated mud such as cob at 100 psi dry for around 14400 lbs per square foot. The wood is generally good for about a thousand psi, so it should make some pretty substantial bracing.
I recall him stating that a 12" cob wall weighed around 2000 lbs per lineal foot at 8 feet high, leaving 12400 or so for supporting the roof, live load etc.
Not a direct engineering study but some good rule of thumb info anyway.
Interesting, I've not seen any design values for cob. That would be the compressive strength of a mud wall. The strength you gave for wood was in bending, the strength we were wondering about with a cob wall is it's shear, or racking strength.
I'm not sure what to think of that 100psi number , the allowable compressive stress for a rough, uncoursed, rubble stone masonry wall with type N mortar is 100 psi. For wood it is around 300 psi (in perp. to grain compression, a log wall). A wall with a 14,400 lb load per foot is going to take one healthy footing.
Further aside; I know the design has moved on but I did find a neat program, Beamboy 2.2 is a free download and can handle the combined loading of a 10" offset post sitting on a uniformly loaded floor. It still failed, but hey we have confirmation. With that combination of loads the point of maximum bending moment had drifted a very little right, but still directly under the post, 8 thousandths of an inch
That was not the actual load - just what Ken Kern said was available per foot for supporting other loads after it's own roughly 2000 lb dead load.
I find that Portland cement added to mud decreases the strength of it. I does well with sand though.
Cob made of 30% clay and 70% sand including the aggregates in the clay, is really hard - about equivalent to sandstone. I realize we are dealing with a variable material here also. Good cob /clay is very strong. An improperly chosen or silty clay would likely not be that strong.
Ken was also a bit miffed that the USDA/gov sold out to big business and quit serving the people. He published some of their old earth building information upon which they had done some pretty extensive testing. Properly made cob is pretty well monolithic if done right and is much stronger than adobe, due to the reinforcment from the straw throughout .. much like fibermesh in concrete. It has been tested in Canada and survived an earthquake shaker table test - seems it was up to a 7 Richter scale test.
I have had my crane outrigger on this clay - same as used in my cob, and when dry or slightly damp it will support the cranes 60000 foot lb loading on an 8 foot setting - or even a 4 foot setting - I'm saying loaded to tipping. This is on a 6" round disk on the bottom of the outrigger, so If I figure it right, that is around 15000 lbs on 1/4 of a foot or 60000 lbs per foot. Nothing that will be accepted for building and not while wet, but just an observation. The undisturbed clay here will support it pretty well as water will not soak into it. A squishy part of an inch or so and it is solid. I admit that this clay is much tougher than the damp pliable clay most places have.
Some clays were rated at more than the 100 lbs -seems to 300, so the USDA was conservative in Ken's opinion.
These numbers I am referencing will likely not be allowed anywhere in codes. Just bringing up info that the trades do not allow to be taught any more.
Thanks for the info on Beamboy 2.2, Don - I'll check it out.
Did Kern give any other numbers for cob? Do you have titles for any of the old USDA pubs?
Glenn, I ran into it out west years ago, do your concrete guys have to send in samples to the lab from each pour? If so see what it would cost to run some cob samples. I do remember a comment from a fellow in Afghanistan that their mud walls could absorb a tremendous amount of small arms fire.
Default position when building though, if you can't quantify it as a load path find one you can quantify and make sure it is up to snuff. ... build it stout with something you know about :).
Woohoo, just got the truck up here for the first time in a month. If you don't hear from me it was not as fun going back down ;D
The range of compressive strengths of common masonry items in the codebook range from about 50psi for things like low strength block up to 720psi for coursed granite and type M or S mortar.
This is some of what beamboy can do, it was interesting to me anyway..
Here is Andrew's drawing of the situation the beam we were wondering about is the 8'4" one with the gambrel post landing on it... offset 10" from the post below.
(https://img.photobucket.com/albums/v667/AirsoftAndy/Weavers%20House/side-elevation.jpg)
The floor has a 40psf uniform load, it's bedrooms I assumed. The post has an 11,788 lb point load coming down. This is what I entered into beamboy;
(https://i180.photobucket.com/albums/x109/windyhilll/bb2.jpg)
This is the output;
(https://i180.photobucket.com/albums/x109/windyhilll/bb3.jpg)
Maximum bending moment is directly under the post, if you look at the bottom graph the maximum deflection is to the right of the post. I entered the information for the 8x8 beam and the middle graph is giving the maximum bending stress in the beam as 1240 psi. That's what we would check against the allowable "Fb" for the species and grade of timber. Depending on quality, red spruce allowable stress is in the 575-900psi range. So that beam was too small. Not trying to belabor what we've moved beyond but it is a neat tool.
Quote from: Don_P on March 07, 2010, 08:48:33 AM
Did Kern give any other numbers for cob? Do you have titles for any of the old USDA pubs?
Glenn, I ran into it out west years ago, do your concrete guys have to send in samples to the lab from each pour? If so see what it would cost to run some cob samples. I do remember a comment from a fellow in Afghanistan that their mud walls could absorb a tremendous amount of small arms fire.
Default position when building though, if you can't quantify it as a load path find one you can quantify and make sure it is up to snuff. ... build it stout with something you know about :).
I mentioned to Whitlock the other day that bullets would not go through the earthen walls. Just joking around but pretty well true. I tried a hammer and chisel for about 20 minutes to chop through a cob wall Sassy built for a stove pipe out of the underground complex. A small hole was all I had before I went to get the rotohammer and finish it in another 10 or 15 minutes of chiseling.
Got my "The Owner Built Home" book out to look up the info for you, Don.
Going from memory about 7 years ago I was off on details a little but not too bad.
What I was referring to apparently came from Chapter 17, Adobe Block starting at page 141. I wish I could put this material online - it is so good, but a few excerpts for education is fine.
"One effective lobby by the American Lumberman's Association to the US Department of Commerce succeeded in a complete shelving of the earth wall program."
"With unquestioning loyalty and unbelievable ignorance, building inspectors say "No " to earth wall construction in this country, yet, according to structural tests made at the School of Engineering, Christchurch, New Zealand, a foot length of soil cement wall, 8 inches thick will carry over 21 tons at failure. The weight of each lineal foot of wall 8 feet high is approximately one fourth ton. That leaves 20 3/4 tons for roof weight and safety. The Australian Commonwealth Experimental Building Station found that the compression strength of an adobe block is in excess of 25 tons to the square foot. Our own Farm Security Administration claims 33 tons compression strength for these blocks. This is actually about ten times the strength needed for conventional roofing weight."
He goes on to explain about bond beams etc.
Cob is monolithic -straw reinforced and much stronger than adobe.
He mentioned the report in the 1940 Bureau of Standards as pictured above.
A chart from Bureau of Standards - 1940 - not too good - sorry bout that.
(https://i778.photobucket.com/albums/yy62/the_troglodyte/Photo_00002.jpg)
The above Kern book is out of print but often available used.
Interesting note... our John Raabe - took the picture that is on the cover of the Scribner's version of "The Owner Built Home" and that book is the reason I am here. My son loaned me one of the original hand written copies and I wanted to find out more.
Interesting rammed earth article 1946 for educational purposes
(https://i778.photobucket.com/albums/yy62/the_troglodyte/rammedearth1.jpg)
(https://i778.photobucket.com/albums/yy62/the_troglodyte/rammedearth2.jpg)
(https://i778.photobucket.com/albums/yy62/the_troglodyte/rammedearth3.jpg)
(https://i778.photobucket.com/albums/yy62/the_troglodyte/rammedearth4.jpg)
[cool] article, thanks Glenn!
I'm really finding the whole cob thing really interesting...really getting the itch to try building something with it. ;D
My pleasure, Beavers. The hardest part is just getting started and doing it. Make mud - have fun - talk the gals into a bit of mud wrestling.
I use my Bobcat to mix the cob, but the traditional method is by your feet. http://www.weblife.org/cob/index.html (http://www.weblife.org/cob/index.html)
Don, I forgot - yes - we do testing here but Ken Kern gives full info on testing strengths of adobe - cob etc. in the same chapter. He details it out using a lever and tells how to do the math to calc out the strength. It would be a lot cheaper way to test various mixes.
Note - cob, rammed earth, adobe, compressed earth block, are all various methods of earth building that differ in methods but use soil, aggregate, stabilizers and straw or other reinforcement
Very cool stuff! I feel like the cob would probably provide plenty of bracing to the timber frame, but the cordwood aspect might ruin the monolithicness of it, plus we will be using chopped straw to make it more pliable. While I still think it'd be strong enough, a couple of corner braces would give complete peace of mind...
I'll post some pics of my second attempt at the frame after I get some sleep.. ;) That beamboy program looks cool, but probably not Mac-compatible..
I was wondering the same thing... whether the cordwood could somehow roll if the wall were racked. My gut says that contained withing the frame it would take quite a force but what that is I don't know. Working some other bracing in there couldn't hurt.
I did find one of the old publications googling, haven't read it yet but it sounds like others want to read more too so this is it's link;
http://www.aaronhauser.com/rammed-earth-books/farmers-bulletin-no-1500-rammed-earth-walls-for-buildings/
8x8, 8x12, joists are 4x8, 3' oc.
A frame doesn't get much more basic.. ::) I have the tension braces drawn as though they planed into the corner posts at the same height, but they would be staggered at different heights to retain the post's strength.
(https://img.photobucket.com/albums/v667/AirsoftAndy/Weavers%20House/full-1.jpg)
(https://img.photobucket.com/albums/v667/AirsoftAndy/Weavers%20House/side-ele.jpg)
(https://img.photobucket.com/albums/v667/AirsoftAndy/Weavers%20House/front-ele.jpg)
(https://img.photobucket.com/albums/v667/AirsoftAndy/Weavers%20House/full2.jpg)
Braces are normally considered to be working in compression, that's why they are in pairs, the one in tension is really riding and the one in compression is working. Wood shines in compression, steel in tension.
Your long braces reminded me of a German frame. Check these out;
http://trailridgetimberframestravel.com/germany1.htm
Call me boring, I like looking at the roof delivering load straight down the posts, very strong, no thrust, looks good.
I'd been thinking along another path, trying to clearspan the upper floor.
This is without the upper posts and purlins. The diagonal is just to make the software work, no load. The black vertical lines are loads not members. The loads are not in relation to your building this is all just a thinking exercise. There is the tie across the curb roof at the pitch break, and the upstairs walls just happen to be sloped steeply. Sloping the walls (the steep pitches of the roof) means there is a horizontal as well as a vertical reaction. This is thrusting so the upper floor is also a tie. Essentially you have drawn a pair of structural ridges and the other way is tied rafters. Just another thought.
(https://i180.photobucket.com/albums/x109/windyhilll/gambrel.jpg)
The floor system is all in one flat plane, that might be necessary. If not, stacking the 8x8's on the 8x12's then the joists on top of that avoids alot of joinery or hangers, and is stronger.
I like the idea of stacking the layers, but that creates a lot of time-consuming voids to fill, above interior walls and such. Also, might the timbers have a tendency to twist? I would like to make a simple version of this thing, (http://www.timbertools.com/Products/LignaTool.html), if possible for quickly hanging the joists.. The trick would be finding a large enough dovetail bit.
For some reason the timber framers call those corner braces "tension braces", so I was just throwing their lingo around... :)
So, if the 2nd floor system acts as a tie, the upper floor posts and beams might not even be necessary? It might just make the rafter layout easier to have them there anyway, but could we use smaller timbers, rather than hoisting an 8''x12''x12' 20' up in the air without a crane? ;)
Quote from: Don_P on March 08, 2010, 07:45:59 AM
I was wondering the same thing... whether the cordwood could somehow roll if the wall were racked. My gut says that contained withing the frame it would take quite a force but what that is I don't know. Working some other bracing in there couldn't hurt.
I did find one of the old publications googling, haven't read it yet but it sounds like others want to read more too so this is it's link;
http://www.aaronhauser.com/rammed-earth-books/farmers-bulletin-no-1500-rammed-earth-walls-for-buildings/
Thanks Don, That's better than I did.
Note that I did a rammed earth wall on my RV garage lower level.
http://countryplans.com/smf/index.php?topic=1166.0
I found the zipped links to 3 books on the previous page - download them if you like.
http://www.aaronhauser.com/rammed-earth-books
Thanks for posting those Glenn, I'll check them out. We own the copyright to gov't pub's, copy and post 'em somewhere to keep them alive :). I've seen high compression numbers several other places.
Andrew, that's early concept, we can run the loads and see how it looks. What is your design snow load?
I've done drop in dovetails, never again. As the width of the joist shrinks narrower the tail gets narrower in the beam's mortise. The beam is also getting narrower and wanting to pull away from the joist end. It doesn't fail but the tail can withdraw enough to get your pinkie in the gap :o If you want to go that route the beam needs to be sized with the mortise depth removed. The stacking method requires lots of blocking, the twisting potential is about the same as with a shrinking dovetail. If there is slop twisted grain can unwind the timber as it dries. A tusk tennon is structurally probably the best joist/beam joint but would take a set of hands on every joist as the beams are brought in to slip each one into a pocket in the center, neutral axis, of beam height. It's mortise doesn't break the top or bottom "strap" of the beam but is in the center, low stress, area. If you can get a copy of Cecil Hewitt's "Historic English Carpentry" on interlibrary loan there is a good discussion and illustrations of this.
Quote from: Don_P on March 09, 2010, 07:05:32 AM
Andrew, that's early concept, we can run the loads and see how it looks. What is your design snow load?
Code says 70 psf. In reality, we get about half of what most of the U.P. gets, being just a couple miles from the lake..
Quote from: Don_P on March 09, 2010, 07:05:32 AM
I've done drop in dovetails, never again. As the width of the joist shrinks narrower the tail gets narrower in the beam's mortise. The beam is also getting narrower and wanting to pull away from the joist end. It doesn't fail but the tail can withdraw enough to get your pinkie in the gap :o
I was wondering if the tapered shape of the tenon that the router jig makes would help prevent that problem... As the joist narrows, wouldn't it simply follow gravity down into the mortise, maintaining a tighter fit? Tightly wedging the tenons from the top is supposed to prevent pullout as the timber drys, but that wouldn't be possible with the short 1.5'' tenon made by the router jig.
Quote from: Don_P on March 09, 2010, 07:05:32 AM
If you want to go that route the beam needs to be sized with the mortise depth removed.
If the joinery is fairly tight, and the mortises are on the upper (compression) section of the girder, wouldn't the beam retain most of its strength?
Quote from: Don_P on March 09, 2010, 07:05:32 AM
The stacking method requires lots of blocking, the twisting potential is about the same as with a shrinking dovetail.
Yet another possibility is metal connectors, as we used on our house, but I feel that good wood joinery would be stronger, cheaper and more aesthetically pleasing...
I agree, Don. It bugs me when I can't find the books and studies we paid for, and it bugs me more when it is hidden or done away with because of industry lobbying.
If we are the owners of those documents, did they hide them or did we lose our homework?
But yes I agree :)
QuoteI was wondering if the tapered shape of the tenon that the router jig makes would help prevent that problem... As the joist narrows, wouldn't it simply follow gravity down into the mortise, maintaining a tighter fit? Tightly wedging the tenons from the top is supposed to prevent pullout as the timber drys, but that wouldn't be possible with the short 1.5'' tenon made by the router jig.
The ones that withdrew were that shape, same house as the brace above, cut by a Hundegger, my understanding is that cutting station on the machine is like a universal router or end mill, doing the same thing as the template on your jig. Their template is computer code controlling the router but the result is the same. Looking at it that way your jig is a bargain ;D. The floor deck holds the joists up on the beam too so the joists don't really sink deeper is what I figured. The wedging would work if you can both hold the beams from drifting as it seasons AND have access to keep the wedges snug until they quit driving. The crush to fit joinery options don't work.
QuoteIf the joinery is fairly tight, and the mortises are on the upper (compression) section of the girder, wouldn't the beam retain most of its strength?
No, you will hear that over and over, it's wrong.
The bottom fiber is in overload long before the top "seats" and takes its share. The joist will shrink in width.
I had used the section modulus of the remaining cross section after the mortise had been made, an inverted T shape. Much more conservative than you. Dr Schmidt, a university PE, presented at that same TF Guild conference in Roanoke I spoke of earlier. He did destructive testing with full sized beams and found the most accurate way to look at it was to take the rectangular section between mortises and that is the actual beam. For example with any type of drop in joists or purlins where the mortise cuts through the top edge, the remaining width on the top projected down the depth of the beam is the beam to figure on. An 8x8" timber with 4x6 joists mortised in 2" from each side is effectively a 4x8 in the way it breaks. That removed a good bit more strength than what I had considered safe, I wasn't alone in the room :P.
Exception from Dr Schmidt;
Unless using a mortise that removes wood only in the neutral section. A soffit tennon fills that bill but the tenon can be reinforced by sloping the upper face, making a tusk tenon. The mortise slopes up to the top corner, it doesn't take a whole lot of fiber there in the beam to support the compression. Assembly takes a pair of hands on every joist as the beam comes in though. Doing that you can take full design value of the remaining section, the 8x8 less mortises.
Timber joinery is many things but even the old timers kept their options open. Old churches often have iron in high tension areas. The anti steel thing lives largely between some peoples ears, it's an option and shouldn't be dismissed automatically. Nothing wrong with trying to stay all wood first.
The purlin plate;
It looks like the upper roof is 18' wide X 12' bay spacingx 80 (70 live+10dead) psf=17,280 lbs
Half of that is resting on each beam, I'm coming up with an 8x12.
That was calling it a structural ridge.
If the load is travelling down the steep rafters though that purlin plate, it could be anything, or non existant, as long as the steep roof to upper roof connection is adequate. I think if the steep rafters had a birdsmouth under the bottom and up the outer face of the beam they would each act as a post, at that point if you jerked the vertical posts out, where would it go? The posts do form a triangle...bracing, sometimes you see purlin plate posts canted off plumb out to the roof plane as well.
This is with vertical posts, the black dots are "nodes", pivot points and also where I've hung the roof loads. Add a zero to all numbers and this should be close to correct forces for rafters at 2' spacing if it were bearing just on the outer walls, still thinking... It looks like you need to have about 1700 lbs worth of "strapping" across the floor in each 2' strip, fasten the flooring down good, stagger the joints.
(https://i180.photobucket.com/albums/x109/windyhilll/gambrel4.jpg)
Those are not our true support conditions :-\
OK, Viewing it as a clearspan truss with no posts, if the rafters are on 2' centers I looks good with 2x10's in #2 SPF or equivalent. There is about 2560 lbs vertical load under each rafter foot.
I've been playing with a program that let's me enter more support conditions. If the posts and beams take the load there is a 1440lb load from each rafter set on the purlin plate beam, carry that down the beam and that load theoretically travels down the internal posts. The tension across the floor drops way down. I don't think that will uniformly happen. At some point in the purlin plate's span and depending on its stiffness it will deflect enough that the rafters will deliver some portion of that load down them to the outer beam. So I would design the outer perimeter as if the posts and purlin plate were not load bearing. This is conservative but with that logic the outer 8x12's are overspanned for that load, and you've mentioned a possible later shed roof. I'd try to fit midspan posts and cut those spans in half.
If the upper purlin could handle a little deflection without failing, couldn't the lower beam as well? I suppose it wouldn't be that much more work to throw a few extra posts around the perimeter, but couldn't the roof easily be clearspanned in that case?
The upper beams would make it easier to lay the rafters out, but maybe temporary framing could be used for that and then removed after the connections were made? I imagine the upper rafters should probably be gusseted to the lower legs with a plywood sandwich or something, and the ties would have to be well attached, but do you think the truss would need any more framing than that? It just seems unconventional to have the lower "legs" sort of floating without anything to make a triangle down there..
There are also going to be a couple of shed dormers that would be simple to frame over an upper beam framework that might need a little more consideration in a clearspan design...
'Can't tell you how thrilled I am for all of your help and advice!
Well I'm glad I haven't driven you to distraction :).
I remembered, in the guild's free download, Sobon's "Historic American Timber Joinery" Chapter 3 fig 20 and the accompanying text are a good read on joist connections.
I have a gambrel barn done with just the plywood gussets at the pitch break and tie with no posts or purlins, 24' wide and 1/3 your snow load, 15 years and ok so far. I framed the pile of "trusses" on the upper loft floor and stood them up.
By the numbers the outer wall beam failed in bending not deflection. I guess if there is nothing under a beam, a fiber failure is going to immediately preceed a deflection failure ;D. But, if it deflects it will land and bear on the cordwood wall. Is that wall good for ~1250 lbs/lineal foot? How far below the beam will it shrink to? Will having a beam bouncing on it harm it? If it is capable of being called load bearing then a 2x6 on top of it as a plate is all that is needed. If it is nonstructural infill then the 8x12 beams supporting 6 rafter pairs @~2600 lbs per, are overloaded. We do already have posts at 12' spacing that will not change in height, if the cordwood does change height in my view a beam spanning that should not sag between posts. That's where I'm coming from asking for more posts, making the beam an independent, stable, known sill to launch from. I can understand other viewpoints.
Remember the posts are not under each rafter pair, but every sixth rafter or so. If there were a wall or very stiff purlin plate then I would be comfortable saying most of the upper roof load goes down to the purlin plate and travels down those posts. I just made the outer wall very stiff so it is definitely attracting load. If the purlin deflects much at all the entire load goes down the lower rafter to the outer wall. The rafter pairs on the purlins very near the posts will send a portion of their load to the purlin posts, how much is beyond me.
Where I'm going, if you use upper posts do keep them aligned over the lower ones. In other words if the posts and any type of potentially load delivering purlin is there, treat it like it will take that upper roof load. We haven't looked at what it does with a wind hitting the side. I can see in a dynamic situation one or the other posts driving down hard and delivering that load. I like the triangle in there for that reason but do not think it is necessary, it is bracing and can be a nice redundant load path, which is very good.
Thanks for the info, Don. I looked up a link to the D/L - maybe I missed it earlier as I can't keep up with everything... [ouch]
http://www.tfguild.org/joinery/joinery.html
That's the one Glenn, good stuff in there.
There I was studying in the "library" this morning, and saw something very similar to your graph on rammed earth in a book I had. The info is from the same source, just a different way of graphing it.
(https://i180.photobucket.com/albums/x109/windyhilll/ramearthstrengthopt.jpg)
Sorry about waiting so long to respond; got a little distracted and needed some time to get the gears rolling in this direction again...
I'm definitely thinking of running this by an engineer, but the closer I can get to a finished product that I feel good about the less work he'll have to do.. I also trust your judgment very much, Don, and if you are content with the design it would certainly give me greater peace of mind..
Just a couple other thoughts here..
As far as the joist connections go, I think connecting the 4x8's over the girders with steel plates and lag bolts would be ideal from a structural standpoint, as you have said.. I won't know exactly what option will be most practical until we get a little further into the project, but it looks to me like the 8x8's would be strong enough even if the joists were dovetailed in, as long as they weren't notched much deeper than an inch.
I have no problem adding additional posts to the perimeter to reduce the spans down to 6'.. In fact, according to the calc, couldn't the outer/lower beam be reduced to an 8x10, even if the roof was clearspanned?
What if I were to consider the roof as clearspanned (more exterior posts to support the beam), but still had the interior 2nd floor posts and purlins to make the roof and dormer framing simpler, as well as add bracing to the roof structure? Couldn't the upper purlin be reduced in size, as it isn't attracting roof load, only wind loads and over-dormer-roof-loads? (Hope that makes sense.. :) )
I was also wondering about the earlier plan of the load-bearing upper purlins; my biggest contention being the installation of the 8x12x12' beams 20' in the air... This might be irrelevant at this point, but IF we went this route, couldn't we just pair up two 4x12s with bolts? The seams could be staggered over the posts too, for a nice strong connection..
The mill called yesterday, my beams were out of the kiln. Just unloaded the trailer for my next project, a pair of greatroom trusses. 2 of the beams were dense #1 eastern white pine 8x10x20'. It's all KD to about 19%, green is about twice as heavy. I did get it all up on horses solo. One of the beams is heading off to the sign shop for some ecclesiastic carving before I begin notching it. The fun is about to start ;D. Did get some 4x10x20', no sweat, I could hold them up standing on a stepladder while you bolt.
Without going back through it, yes I think you are on the right track with everything but the joist notching...and I've led you to water there :). The engineer is working on part of my roof where I was optimistic. I made the numbers work but he wasn't thrilled, so don't assume that I am a tough audience.