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Timber Frame Structural Home Design Basics  RSS feed

 
Terry Ruth
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http://www.permies.com/t/50584/timber/wedge-size

I'm referencing the thread above since inaccurate information was provided I will attempt to straighten out for the safety of the individuals asking questions. To answer these questions properly requires a PE with structures education and experience. Trades that build Timber's for the most do not have that experience nor can they understand it due to lack of the same, some act outside their capacity beware!

I'll also be looking closely at this document: http://www.timberframeengineeringcouncil.org/images/pdf/miller_report.pdf A tradesman gave me thought applied and these joints therefore fail in shear which it shows is inaccurate for the most part.

After which I'll answer the questions in the thread more appropriately, and I think for those that follow, read the reference material, and ask questions, you will not have to rely on anyone to answer these questions perhaps in error anymore. At a minimum you will walk away with better basic knowledge and ability to communicate with a PE for hire, what to look for in threads. I started a new thread since I will go a little above and beyond answering the questions which I can do now.

I'll try real hard to keep it simple and I will not be going into math.

So lets get started by realizing that there is no such thing as "shear load"...Shear is a "stress" not a load. We'll see that in Miller's report as noted in test result tables, along with some other tested stresses (eg: tensile, compression or bearing, deflection, stiffness, bending and there are about half dozen more he did not test for).

We have the lateral loads or what is referred to as "out-of-plane" or "shear loading" as Miller does when we have the location of the build, or we use a "Load Cell" and hydraulic hammer like he did to get some mechanical properties(shear, tension, bearing, etc) of wood species or joints. In building design (not lab bench test),...The first question you will learn to ask is "where are you located"...that one simple question (if you know what to ask) determines what lateral loads are like from wind and seismic events since that information is in code and on the internet per climate zones. 2012 International Code Council Table R301.2(1) to be exact. We'll talk about code or "empirical data" vs opinionated lack of data soon). My loads are 15 PSF ground snow load, 90 mph-3 sec wind gust, SDC (Seismic Design Category: "A"(lowest, D highest), Weathering: Severe, Winter Design Temp: 97.5%-7, Ice barrier: none, Air Freeze Temp: - 400, Mean Air Temp: 55-60 deg, Heating Degree Days: 4, 620. You can ask your local inspection office for those loads or they can be found in code or online. More than we need to know for this basic online class, take note of the 15 PSF snow and 90 MPH wind load.

Below are the main load definitions and home load paths for a symmetrical multi-story. It never's that simple but, this gives a good basic idea of how those loads get distributed to joints. Also provided are loads for various materials or dead weights showing again we do have all the loads we need to answer the questions above (I'll get into safety factors later). Take note once again no "shear load". When those loads in unit pounds gets reacted at joints a stress pounds per unit area that causes a strain over a length (Miller measures with a potentiometer or "strain gage" to measure strain(stretching of the materials) are created. Depending on how the joint is loaded (dead, live, snow, seismic) will determine how it is reacted in shear, tensile, compression/bearing, bending, etc.... Miller takes a "static" one direction, not "dynamic (wind, seismic) live load such as from a floor and looks at shear, tensile, bearing, bending failures on the mortise and tenion.....more later since there are other ways to load a joint such as bending, compression, tension, etc, depending on where the joint is in the home.

I'll stop there read through the material, have questions ask....I'm not going to spend alot of time on loads, rather the capacity of mortise and tenon Joinery....

Usually in a hurry not the best speller or writer.

Here ya go: http://www.ct.gov/dcs/lib/dcs/office_of_education_and_data_management_files/sp_12_basic_structural_design_2012.pdf

Also, might help if you do not know the definition of shear, tensile, compression (same as bearing) look it up. Bending I'll explain later. "A" frames and trusses are great at taking out joint bending, especially in small homes.

"Load Resistant Elements " is what structures provide depending on if the loads are resisted by the structure and stresses.

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Elsie Smith
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Hi Terry belatedly thanks for this information. Best wishes
 
Terry Ruth
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Sorry been busy with other things, I'll get back to Millers test soon. I'll also look at Timber foundations/connections, and A-or W-Frames/Trusses.
 
Terry Ruth
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Elsie Smith wrote:Hi Terry belatedly thanks for this information. Best wishes


Your welcome! Since it is taking me longer than I thought to attend to this thread the quick answer to your question in the OP of the ref thread above is 1.5 D. It is no different than metal bolt and nut, rivet, designs only that it is wood and the geometry is different. Technically, the triangular areas shown below would be a projected bearing area onto the post if you want to be real accurate and do some trig, or just go with the end view dimension I have shown if you can obtain it without cracking your species of wood. So the linear dimensions show on your drawing's in the other thread are not critical, 1.5 D and obtaining bearing area is. Fasteners (or in this case pegs) see bearing stress primary where they fail, not shear, we will see more of in Millers report. The head of the fastener in this case in axial tension loading reacted in bearing, or diameter of the peg/dowel for vertical compression, lateral or out of plane loading often referred to as shear "loading"

I'll explain more what stresses this joint is seeing, FYI to get you going. In Structures design most everything has a relationship to something else called "relationship design".

The right is a typical hex head bolt or nut. Compare it to the left and one can the similarities. Nothing new just a different way to get there.

1.5D is standard design practice per ANSI/ASME. If one bonded the wedge in or had threads like a screw then it would also be used to take out the loads. In this case the wedge is only trying to "upset the head" just like a bucking bar on a rivet which just like aircraft or auto is inspected to 1.5D. That is based on centuries of empirical data. One could design to "actual loads" and size accordingly but the cost for Engineers is not cheap. More on "empirical data" and how to design to it coming.

Hopefully this will all make more sense as I slowly go through Millers report.

EDIT: I forgot to mention depending on how the joint below is loaded it can be used as primary or secondary structure.
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Terry Ruth
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I'm waiting for an email response from the Engineers at Forest & Paper Association for a code path/approved design guide for traditional(no metal) "Heavy Timber Framing Type 4"..They only have a group of 20 as soon as I get it I'll continue the thread. We can take to our jurisdiction framing inspectors and not have the expense of a PE.

I've been down this road before only to find a dead end, the explanation I got is none(removed) due to tree farmers and a lack of consistent properties of wood. UBC had it at one time and it was deleted needing a PE for those reasons according to Engineers and at the American Wood Council, but I thought I'd double check. It is the basis for Millers test. I also found some FP&PA tensile test we can go over.

International code does ref FP&PA for a code path.....I'm just not sure they have it, WFCM is light framing.....If not I have another path we can try.
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Terry Ruth
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Hi Terry,

We do not have a guide available that deals with timber joinery, I think the Timber Framer’s Guild (http://www.tfguild.org/) would probably be the best organization to contact for information on joinery techniques, since that is what they deal with specifically.

Lori Koch, PE
Project Engineer
AMERICAN WOOD COUNCIL
222 Catoctin Circle SE, Suite 201, Leesburg, VA 20175
Email: lkoch@awc.org

K, will try them next.
 
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