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Author Topic: 18 x 18 Timber Frame Design: Feedback Requested  (Read 651 times)

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Offline LimestoneWood

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18 x 18 Timber Frame Design: Feedback Requested
« on: July 27, 2017, 09:58:25 am »
Hello Everyone,

This is the 2nd iteration of my first timber frame design.  I received some feedback already about how to handle the mortise and tenons between the center post and the tie beams and removing an extra center plate.  After incorporating this feedback I ran into a few more questions:

1.  Do I need tie beams at the height of the outside plates to tie the bents together in the center?

2.  How large should I make the knee braces that would tie together the center posts and the ridge plate?  I know the general rule is about a third of the way up but is that too big in this case?

Thanks,

Andrew


Offline Don P

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Re: 18 x 18 Timber Frame Design: Feedback Requested
« Reply #1 on: July 27, 2017, 11:06:45 pm »
You actually don't need tie beams if the roof is hanging from a ridgebeam. If the ridge cannot drop the rafters have no outward thrust needing to be restrained by ties.

I'd just meet the braces at the ridge at a 45

That looks like a forest of posts, is there a reason not to truss it?

Offline Den-Den

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Re: 18 x 18 Timber Frame Design: Feedback Requested
« Reply #2 on: July 28, 2017, 09:01:50 am »
I don't have any input on the engineering but you might consider a king post truss to eliminate the center post, it might get in the way someday.
You may think that you can or may think you can't; either way, you are right.

Offline LimestoneWood

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Re: 18 x 18 Timber Frame Design: Feedback Requested
« Reply #3 on: July 28, 2017, 07:22:23 pm »
Thank you for the feedback Den-Den.  This is a big help. 

When you say that I don't need the tie beams can I also remove the tie beams from the left and right side to the very center post?

Regarding the king post truss, I like the utility of it but worry that it is beyond my skill level.  I agree that my design could be bothersome in the future though because of all the posts. 

I'd like to put up the rafters instead of a traditional timber frame roof structure if possible.  The more simple for me the better. 

Regards,

Andrew

Offline Den-Den

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Re: 18 x 18 Timber Frame Design: Feedback Requested
« Reply #4 on: July 28, 2017, 09:40:28 pm »
You do need the horizontal beams at the end walls, those and the diagonal braces from them to the posts are important to resist racking.  They are not needed as tie beams to resist rafter thrust.  You should be fine without the horizontal beams to the center post.
Is your lack of confidence on the truss a design issue or a construction issue?
You may think that you can or may think you can't; either way, you are right.

Offline LimestoneWood

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Re: 18 x 18 Timber Frame Design: Feedback Requested
« Reply #5 on: July 29, 2017, 03:51:50 am »
Again much thanks for your help!  My lack of confidence is a construction issue.  I am a wood worker and am confident in being able to do regular mortise and tenons but I'm a little nervous about putting up full bents with those king post trusses. 

I will make a new design without any tie beams connected to the center post and leave the tie beams on the front and back walls.

Should I put the 45 degree braces a third of the way down from the top of the ridge beam on all three posts holding the ridge beam?

Thanks,

Andrew

Offline Don P

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Re: 18 x 18 Timber Frame Design: Feedback Requested
« Reply #6 on: July 29, 2017, 09:06:47 am »
They aren't that difficult to cut and this is a small building, good learning opportunity.

I was just playing with some math to see if a crown posted arrangement would help, running an 18' beam across the width with a post on top to support the ridgebeam at the center of its' span without having to have a post to the floor. It should have been obvious but I got a chuckle. That beam would be point loaded by the post between the tie beam and the ridge. Since a ridgebeam is uniformly loaded and this is point loaded, which doubles bending moment compared to the uniformly loaded ridge, that tie beam would be exactly the same size as just running an 18' spanning ridgebeam... I'm easily amused  :D.