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Question about high post cape design #29943 12/17/12 06:42 PM
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ironsheik Offline OP
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Hello all,
I've been searching since yesterday and can't find an answer to two of my questions. I'm designing a small house, 18x26 footprint, and I want to build it as efficiently and simply as possible. My idea now is a 3 bent (18' wide with ~13' bays) high posted cape with 40" knee walls on the 2nd floor which I would love to house 2 rooms. For now I plan on a principle rafter system with purlins and a summer beam down the center of the 2nd floor. This would use queen posts much like this frame posted by Daiku in an old thread:
x

My question is how far down the rafters can the queen posts go in order to minimize the intrusion of the braces into the 2nd floor space? In the middle bent, stairs will rise parallel to the bent framing, from outside (north) to inside, and reach the top about 18" from the center of the tie beam. Then on the other side of this bent will be a chimney so my goal is to make as much room for the chimney and an entrance way, on the far (south) side of the bent, to a small bedroom. In other words, in the picture below, the chimney would exit on the far side of the bent, fairly close to the center of the tie beam leaving little room to travel from bay to bay.

The other issue I have is can I then make the first floor of this bent a clear span if the queens are spaced far apart, using struts from the tie beam, directly below the queen, to the bottom third of the outer posts? Oh, and how high can I safely go with the knee walls? smile Thanks for all the help!

I'm excited to get started building this Spring as this is a first for me.

Josh

Re: Question about high post cape design [Re: ironsheik] #29944 12/17/12 07:15 PM
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ironsheik Offline OP
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I should have mentioned that the roof pitch I've got now is at 10.5/12. I'm factoring in 7x10" outer posts ala Steve Chappell's book A Timber Framer's Workshop.

Re: Question about high post cape design [Re: ironsheik] #29945 12/17/12 11:08 PM
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ironsheik Offline OP
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And now, since I'm so impatient and can't stop designing for even a second to eat (what's happening to me?? :)) I think I found my answer thanks again to another post by Daiku:


Re: Question about high post cape design [Re: ironsheik] #29955 12/18/12 07:12 PM
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daiku Offline
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The first frame you posted is overkill for 18' wide. The second one is more appropriate. CB.


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Re: Question about high post cape design [Re: daiku] #29958 12/19/12 12:13 AM
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ironsheik Offline OP
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Would the king post need to be 10" wide with a narrow bent like this? And how far up the rafters is safe for the brace-king post connection?

Also, since the king post and tie beam could both be considered to be under tension, how does that affect the choice to use a continuous tie versus a continuous king? I'd rather have an 18' tie, using a king post with wedged half dovetail and a 1st floor post, than a 20' or so king. Thanks!

Josh

Re: Question about high post cape design [Re: ironsheik] #29961 12/19/12 03:23 AM
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TIMBEAL Offline
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You would not need a wedged half dovetail if the king sits on the 18' tie with a post under it, at the first level. This configuration is not a true truss. The king is in compression, in this case not much thrust from the rafters on the post either. I would want through tenons and a heavy peg or two at the post/tie connection regardless of the lack of thrust from the rafters.

The width of the king may have importance due to the joinery at the peak where the rafters connect, you could flare it to give added wood at the peak and the lower half necked down to 7x7. Lay it out from a snapped line.

You wouldn't even need braces from the king to rafters,(the rafters are acting as bracing members if I am not mistaken) but they are fun, so why not. Just us a 45 degree angle from the king. I see the pitch is 10.5, perhaps square the brace from the bottom of the rafter the angle would not be a 45. Windows usually get in the way, so they may have to be on the shorter side.

Re: Question about high post cape design [Re: TIMBEAL] #29963 12/19/12 05:49 PM
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ironsheik Offline OP
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Last questions then. Would I be alright with a 4' knee wall in this configuration? And if I were to use queen posts in one of the end bents, as in the first pic, how far out/down the rafters could they be placed? This would be in order to easily add an addition later on.

CB: I've only read that purlins need to be 48" on center. Can you tell me why the purlins on your king post frame look to be much closer than that?

Thanks so much for the advice...

Josh

Re: Question about high post cape design [Re: ironsheik] #29964 12/19/12 09:08 PM
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bmike Offline
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if you've designed the frame with a ridge that brings loads down to the ground via a post, you can go with nearly any knee wall height you want. although, in some cases, going tall would have to be looked at in terms of wind loading.

if you are relying on the posts to take outward thrust, then the tie / post joint needs to be designed for this - and the post needs to be thick enough to not snap off above the tie when you load the roof.

for purlins, 4', 3', 2', 6', 4' 3 5/16" on center etc are all acceptable, really depends on what you plan on doing above - SIPs, conventional framing, just sheathing?

size the purlins based on the tributary area, then size the rafters based on their area, and take into account material removed to house / join the purlins, and size up accordingly.

you can start to chase your tail... but that is usually the process.


Mike Beganyi Design and Consulting, LLC.
www.mikebeganyi.com

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