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sizing beams while accounting for joist-pocket removal

Started by mrbrb, May 24, 2014, 10:19:20 PM

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mrbrb

Hello all,

My first post in this section of the forum. Cutting wood for a 24 x 36 barn/workshop. The 3 main interior tie beams (EWP) will span 22', and have been sized accordingly at 15x17" (live floor load of 30 psf as well as 30 psf roof snow load coming down through queen post setup.)

My initial plan was to simply run my 4 x 6 floor joists right over the tie beams, (8' span) as I'm not really that concerned with preserving head room on the second floor. BUT I have gotten curious about how to size these tie beams IF I were going to cut pockets to drop my joists into. Can anyone point me to a relevant article in the TF quarterly, or some other such resource, that covers how to account for this type of material removal when calculating bending stresses etc.

(I have taken a timber engineering course, as well as consulted with an engineer for this project, just not on the question of dropping in my joists)

(And a couple images of sawing 15 x 17s, using a hybrid egyptian/yankee technology for moving the logs around)



  

 



mrbrb

also thought I'd piggyback on this topic a few images of a project that I consulted you all on a several months ago-   harvesting knots. Heres the finished product- 1,058 inlays. Hope it doesn't give anyone a heart attack.



  

 

maineframer

Welcome to the timber framing portion of the forum.
What is your post size?
What is your rafter size?

I think that if I were to use a 15 X 17" EWP tie beam my floor joists would be around 10 x 12.

You could use a drop in joist or you could use a tusk tenon with housing.
Running them over the top is not the best option as they tend to want to twist.
Serious wood calls for serious I joinery.

I look forward to seeing your progress.

David

mrbrb

Hello Maineframer,

Thanks for the reply.
Joists are 4x6 ewp, 8' span, spaced 30"
Rafter setup is common rafters, 3 x 7, over continuous 8x8 purlin plates that run the full 36' length.

Im still a bit unsure about post size. Seems a little ridiculous to have 10 x 15 posts (frost walls are 10" wide), but Im not entirely sure how these 15 x 17s would look joined into 10x10 posts.

These huge tie beams are necessitated by the 22' span, and according to the math, 15 x 17s will do the job, and are also possible to cut out of the logs I have access to.

When I was doing initial research, I came across many pictures of 24 x 36 frames, with 24' clear spans, with relatively manageable sized tie-beams (nothing as huge as a 15x17.) Don't know how they were getting away with it. Maybe they are using Select Structural material..  BUT i feel I have to trust the math, which, for No. 2, ungraded material, calls for some massive tie beams.

Thanks for the input.. all my best.

shinnlinger

I have never seen a frame with timbers that big for a house.  My 24x50 has 8x8 posts and 8x10 girts with let in pockets for 4x8 joists like you want to do.  All EWP also.  A post splits the span at 12 ft  with 3ft wind braces.

As an aside that knot thing is quite impressive.  My only other comment is it seams your wasting some wood cutting slabs so thick.  Make boards cutting down to your timber if for no other reason to make it much easier to offbear.
Dave
Shinnlinger
Woodshop teacher, pasture raised chicken farmer
34 horse kubota L-2850, Turner Band Mill, '84 F-600,
living in self-built/milled timberframe home

witterbound

I've never seen a tie that is wider than its post.  My guess is that the pockets don't factor in too much as they are at the top of the beam and in this massive a tie, the removed wood will be such a small percentage.  But that's my guess, I, no expert.  Can't wait to find out them real answer.

Remle

mrbrb
I recommend A Timber Framers  Workshop, by Steve Chappell. It has a section called Bent Framing and Joinery Design that deals with your question of floor joist pocket design relative to beam and pocket size. A second book, Build a Classic Timber Frame House, by Jack Sobon is also a very good reference as to the design of the floor joist / tie beams and how to scale the joist connections to fit within a limited pocket size.

maineframer

Mrbrb,

In a typical 24 x 36' frame that we craft in our shop we would use 8 X 8 posts & 8 x 12 tie beams. 6x6 floor joists spaced 30" ---32"  o.c. and dropped in to 1" housings is standard.

We use 4x6 knee braces  and join the ties with through tenons and a diminished haunch.
This design works well for a clear span . If we have a center post we will use an 8x10 tie.

Perhaps you could re check your calculations or have an engineer that specializes in timber framing review your plan.

David

mrbrb

Thanks for the specifics Maineframer. I spent some more time today studying these 24 x 36 clear span barns that I see images of when googling. Maybe their second floor spaces are non-functional? Or they are measuring their spans between braces, not between posts?


I have gone over the math many times, using several different manual formulas, which I have then cross referenced with online calculators. Basically I always come up with a required Section Modulus (Sreqd) of 675.
This is for a simple beam, No. 2 (ungraded) eastern white pine, uniformly loaded, spanning 264", with a tributary of 22' x 8', cumulative load of 67 lb/sqft.   

Even if I upgrade my materials to 'Select Structural EWP', I still cannot span this distance with a 10x12. To do this span with a 'Select Structural' 10x12, my floor load would have to be downgraded to roughly 40psf (combined dead, live, and snow loads (coming down through queenpost arrangement). This downgrading is not possible given that I anticipate using this second floor space for storage, and the local building code (northern Connecticut) requires a design snow load of 30 psf.

None of these calculations account for reduction in strength due to cutting of joist pockets, which, based on notes I received during a 'Joinery engineering' course at Heartwood, can reduce shear and bending strength significantly (I have attached an image, which still doesn't really provide a rule-of-thumb for strength reduction after beam notching)

I would be curious to know how a clear span 24 x 36 structure built with EWP is able to have a fully functional second floor (designed to, say, 30 psf ), and handle snow loads as well.
Thanks for the input.



  

timberwrestler

David,

I'm not sure where you came up with those timber sizes, but they are really inadequate.  An 8x12 can not span 23-24' with even a floor load only, never mind with a roof load as well.  An 8x10 will just barely eek by if there's a center post for floor loads only (certainly not with queen post roof loads right in the mid-span).  And 6x6s 30" OC spanning 12' does not work either.  While 1" deep joist housings appear to have become the modern standard, they give me the heebeegeebees, and I lean towards 1.5"

Mr brb,  I think that you're on the right track.  You're really trying to put a floor load on a truss, which would be pretty unusual historically.  And then you're skipping the trussing, and just going for monster sizes.  You may want to have a good timber engineer review your design, it may save you some timber, time, and money. 

The timber notching debate is going on now among the TF Engineering Council.  One prominent timber engineer (although he's really more of a theoretician) is pushing for all of the wood under the notches to be ignored in the calculations.  Hopefully, some reason will counter his proposal.  But yes, cutting big notches in a beam certainly reduces it's strength as shown in your pics above.
www.uncarvedblockinc.com
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maineframer

Timber wrestler,

Thank you for your input.
As a professional timber framer and designer we have raised scores of timber frame homes and barns. Many of these frames have been engineered and stamped by professional engineers.
You ask where I have come up with these timber sizes and I will have to tell you I come up with them through designing frames and working with engineers.

Hire an engineer,  get your plans stamped & then mill your timber  and cut your frame.

Happy timber framing.
David

shinnlinger

I'm sitting in my 8x8 posted and 8x10 beamed with joist pockets right now telling you it's a solid frame.   I took down an identical sized barn with even smaller timbers that was built 125 years ago.   
Part of the issue here might be your queen post.    Don't use one and it puts the snow load to the walls.

  Am I missing something?   15x17 on 10x10 posts is massive
Shinnlinger
Woodshop teacher, pasture raised chicken farmer
34 horse kubota L-2850, Turner Band Mill, '84 F-600,
living in self-built/milled timberframe home

maineframer

Mrbrb,

Do you have a drawing that you could post?
This would help a lot to see the frame as a whole.


David

timberwrestler

Mainetimberframer,

I'm a professional timber framer and designer as well.  I only point out the timber sizing because this a public forum, and without knowing all the details readers could base their timber sizing from what is written on here.  I agree on having a specialty timber engineer review the design, and then mill the timbers and cut the frame.

Shinnlinger,
You're not missing anything.  For simple, uniformly loaded beam design, with a post on either end, a 15x17 is about where he needs to be.  The math is the math.  Now that math depends on the loads, span, tributary area, species, and grade.  And yes, the code required loads are pretty unreasonable, and there are a lot of safety factors in the engineering values.  That's why he could very likely build with smaller timbers (as they often did historically) and get away with it.  Without seeing his design, we're all sort of speculating.  Although this thread was really about notching beams, and not sizing his timbers.  It's pretty likely that the roof load on his tie beam is not a uniform load, and that makes things more complicated. 
www.uncarvedblockinc.com
www.facebook.com/uncarvedblockinc

mrbrb

Thanks for the replies, Ill try to get around to posting the design in the next couple days.
Ill stick with the 15 x 17s, with calculations confirmed by a structural engineer. Shinnlinger, you are not missing anything, the main issue with the timber sizing is the clear span-  22'. In Chappel's book, he emphasizes many times that no simply supported, uniformly floor-loaded beam should span more than 16'. However, according to the math, this 22' span is possible if one has access to large enough timbers. (All of the information necessary to perform the calculations is in my previous post in this thread)


witterbound

Does the post need to be 15 inches wide to support the entire beam?  For example, I assume a one inch wide post wouldn't be adequate because it would leave 14 inches of the beam unsupported.  Or does that matter?

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