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Author Topic: Post and Beam/ Timber frame Cabin Questions- New to timber framing  (Read 958 times)

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Offline TimberHawg1

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Hey guys. Long time reader, first time posting. Starting the process of building a cabin in Arkansas and am considering a timber frame/ post and beam design. I'm building this with my dad and we could "stick frame" with no problem, but like the idea of using timbers if we can figure it out. I know most on here are timber frame purists, which I can appreciate, but I don't think I have the time to learn and cut complicated joinery at the moment. Therefore we are likely to end up doing more post/beam with steel plates bolted for the connections. 

A few things I was hoping for some guidance on. I've been using the calculators in the tool box (very helpful), but what should I be putting in for the PSF load? My region has little snow load, but wasn't sure how to figure the weight of the other materials and factor in the wind, etc.

Secondly, as you can see from my drawings I've likely over engineered the design at this phase as I'm still trying to learn how to correctly size the posts and beams. Right now I have 12x12 posts, 12x12 beams, and 12x6 ridge beam. 6x8 rafters 4' OC. The overall size is 37'x41' (37'x31' living with a 10' porch). The beams overlap the post 6" and 2 beams would be bolted to the post with a steel "T" plate. Once section of the plan would be lofted for BR/BA, the other will be open over the living. I'd like to remove 2 of the post in the middle of the cabin if I can figure out how to span 37'. 

Any suggestions on the design, connections or how I can safely shrink the sizing of my timbers? One thought on the larger timbers was being able to notch slots for the wall and roof purlins so they are flush, without compromising the strength. Then also doing the same thing with the floor joists for the second floor. 

Attached are some pictures, rough sketches right now, that give a better visual of what I'm going for. THanks in advance and if we end up going this route I'll be sure to post pictures of our progress and what we've learned along the way. 

 

 

 

 

Offline Brad_bb

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Re: Post and Beam/ Timber frame Cabin Questions- New to timber framing
« Reply #1 on: February 07, 2019, 07:16:27 PM »
I don't think we are so much purists as we try to dispel incorrect assumptions.  We've seen the assumption a lot where someone thinks Post and Beam construction using steel plates and bolts or lags is either less work, less time, less money, or less learning needed.  I don't think that's the case at all when you factor in the fabrication time and effort needed for the plates, the careful measurement and cutting of the timbers, and the steps and techniques needed to do a post and beam well.  

I think most people on here respect both building types, but just want a fair comparison.  Here's just a few random points that come to mind:
Metal and wood don't get along so well, so if you choose post and beam, do something to prevent contact of steel and wood.  For fasteners/bolts/lags/timber screws, use either hot dipped galvanized or coated fasteners to prevent the contact.
A timberframed cabin can last a very long time and something to be proud of.  They can even be dissassembled and moved/sold.
Cutting the joinery is not difficult with some quick instruction.  I'm assuming you're not really versed in the use of framing chisels, mortising machine, or a hand saw.  There are specific techniques for using a hand saw to cut square.  Knowing those techniques and using a sharp saw will eliminate any past frustration and difficulty cutting with a handsaw and cutting square.
You can get detailed shop drawings of each piece for your timberframe cabin.  You could also get someone with experience to consult/oversee your cutting and raising.

I'm just saying you don't have to dismiss a timberframe to readily.  I know it can be intimidating when you don't understand the technique, but that might be as hard to rectify and pick up as you might think.

Your frame does look overbuilt in some places.  12x12 posts are way bigger than you need.  I don't think you said if it was softwood or hardwood?  You're rafters are longer than I'd like to see.  I'd rather see some intermediate support on the rafters and then you can use shorter rafter pieces which will cost less too.  Long rafters, and what I see there is over 20 ft, are more costly to get good clear pieces that long because you have to use longer higher quality logs.  The longer they are, the more problems you could have with twist and bow.  Using shorter sections is not only cheaper because you can use shorter logs, but twist and bow are not magnified.  I also don't like all the posts inside.  With a timberframe you could clearspan a couple of bents in order to vault the ceiling in the main room. IF you need all the loft space then you don't have to.  Alternatively, you could have a bent with a single center post if you want.

That is awfully big to call a cabin.  That's more house size.
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Offline rjwoelk

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Re: Post and Beam/ Timber frame Cabin Questions- New to timber framing
« Reply #2 on: February 07, 2019, 07:28:57 PM »
If you are building a place to live in and for bed and breakfast you should have it engineered.  Firetower engineering is who i used. I supplied the drawings and they redlined them . House may be a different ball of wax. But would be a safe way to go about it. Platting steel onto wood beams requires certain bolt placement etc.
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Offline TimberHawg1

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Re: Post and Beam/ Timber frame Cabin Questions- New to timber framing
« Reply #3 on: February 07, 2019, 08:19:40 PM »
Thanks for the feedback.
First- wood will either be SYP or White Oak. Those are my only options with mills in my region. From what Ive read they both have pluses and minuses. Open to suggestions. Im assuming the WO would allow for smaller timbers.

I agree I dont like the rafter lengths. Not sure I can even get timbers milled that length. How could I go about reducing the length? If I did a queen post could I create a connection there and eliminate the middle posts on the tie beams?

Offline Don P

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Re: Post and Beam/ Timber frame Cabin Questions- New to timber framing
« Reply #4 on: February 07, 2019, 10:57:54 PM »
Just some quick stuff to help with inputs, hit the drop down heavy timber calc, southern pine has higher design values than white oak.

Your ridgebeam support post is point loading the center of the beam under it.

Snow load maps are in chapter 3 of the building code;
https://codes.iccsafe.org/content/VRC2012/chapter-3-building-planning
never use less than 20 psf live load whether snow or if snow is less than that, wind controls at 20 psf.

If you are intending to clearspan 37' with a truss, I'm playing with @Sedgehammer his truss is kind of similar, the bottom chord tension and heeljoint thrust is looking like over 12,000lbs, nothing to sneeze at.
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Offline Brad_bb

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Re: Post and Beam/ Timber frame Cabin Questions- New to timber framing
« Reply #5 on: February 09, 2019, 12:13:05 AM »
There are numerous possible timberframe designs that could work.  Here is a queen post with ridge design and common rafters.  
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Here is a very typical midwest barn frame design, I think they are called canted purlins.  You could break the rafter up over those too.  These are just two possibilities.  


 

Once you figure a design that you like, do your best to lay it out with as good a drawings as you can, plan view, and elevations, with some preliminary sizing which you might be able to get from some existing frames.  Then once you have good drawings, work with Firetower to engineer the frame for your loading configuration.  It's money will spent as they will do a load analysis of all the frame members and joints looking for areas that need to be beefed up with larger beam or metal.  If metal is needed somewhere to satisfy the loading, I would try to redesign that area so I can stick with all wood and no metal.  Wood and steel don't really get along all that well over a long time.  I don't think you said if you were planning a second floor/loft space? Or if you will have all the living at ground level?
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Offline TimberHawg1

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Re: Post and Beam/ Timber frame Cabin Questions- New to timber framing
« Reply #6 on: February 11, 2019, 10:33:46 PM »
Thanks for posting those pictures. I like that queen post design. To answer your other question, yes at least part of the cabin will be lofted for a second floor. My goal there is to go up an extra 3-4ft on the sides before starting the roof to increase the usable space upstairs. I'm working on some new sketches I may post for some feedback.

One other question. I saw the following barn from the Firetower engineers site you recommended. I like the look of it, seems like a pretty basic 24x36. Any ideas on how the roof beams/ rafters work in this picture? I don't see any posts supporting the what I'm assuming is a ridge beam at the top


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Offline Brad_bb

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Re: Post and Beam/ Timber frame Cabin Questions- New to timber framing
« Reply #7 on: February 15, 2019, 12:21:29 AM »
In that first pic I posted, the rear half of that frame has a second floor with a 3 ft knee wall.  The second floor is 3 bents, 2 bays.  The center bent has large struts because the tie beam in that bent is 36 foot clear span on the ground level (no posts in the middle).  Good space in the 2nd floor loft.  You'll get the best space on second floor with a steep pitch roof.  In this case it's a 12/12 pitch. Hopefully you can get a sense with these pics.



 

 
Anything someone can design, I can sure figure out how to fix!
If I say it\\\\\\\'s going to take so long, multiply that by at least 3!

Offline D L Bahler

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Re: Post and Beam/ Timber frame Cabin Questions- New to timber framing
« Reply #8 on: February 15, 2019, 02:01:04 AM »
For applications where a clear loft space is desired, but the tying beam is supported intermittently underneath, nothing beats the Liegender Stuhl. Essentially it's the exact opposite of the Midwest canted purlin post design. 
It's not a truss, so it doesn't pull up on the tie beam to create clear spans. There MUST be load-bearing walls underneath the tying beams, and the canted supports need well-designed footings because they will be exerting a great deal of outward thrust on the tie beam.

Here's a rendering


 
And here's one (slightly different design) in situ 
 

 

Best solution if you can't get full-length rafters is to have intermediate purlins, like Brad mentions. Then you only need rafters long enough to stretch between purlins, and they can be smaller since they don't have to span as far. 
There are of course more complicated solutions, but I doubt you'e interested in those. 

Regarding wood and steel not mixing well over time, well I sort of have to disagree depending on circumstances. Properly done steel and wood connections can lost a LONG time. I've seen steel connections in wooden framing built in the Middle Ages. 
With white oak, however, it's out of the question. The tannins in oak will destroy steel quite rapidly, and galvanizing is only a temporary measure to slow down this process. 

Brad, your last frame pics -are those large struts there to carry the roof loads out to the ends of tie beam? It's late, and my mind is quite processing everything going on here.


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