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Timber frame structural calculation  RSS feed

 
John Benham
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Does anyone have a good reference or resource for figuring out what loads different timber based setups could support? I know there are plenty of timber frame books out there, but so far I haven't found a lot of info for calculating what loads they will take. Ultimately I will consult with a structural engineer for final review of my plans, but I am the kind of person that really likes to know what is going on with everything I am involved with so I can speak with the engineer intelligently about it.
 
Terry Ruth
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John,

Does anyone have a good reference or resource for figuring out what loads different timber based setups could support? I know there are plenty of timber frame books out there, but so far I haven't found a lot of info for calculating what loads they will take.


In an effort to help some readers learn how to better communicate with a PE (Licensed Structures Engineer or Stress Engineer) they don’t typically calculate “loads” as in dead/live/static: Seismic/wind/dynamic: etc…..Alot of folks don’t understand the difference between a load type and a stress type…that is the first step getting the terminology correct. Loads are developed by labs, testing, instrumentation like stress/strain gages.

Code or your local Building and Safety Office (BSO) has the loads to design to. A LOT goes into developing accurate loads, if they are incorrect the stress calculations will be all wrong. Large Corporate has "Load Engineers" that have load test cells, bench test, or for example climate towers for wind/snow loads, etc....

The biggest stresses on building’s are,

Shear – as in shear wall (look up braced walls in International Code (IRC) study it and you will get a lot out of it, there are videos on line.

Compression, Tension, Deflection, Bending (moments of inertia), are the other basics to look up that confuse a lot of people. Calculations usually look at all these at a min, show the weakest good with a safety factor of at least 1.5.

The reason you will not find a lot of tabulated info like light wood has for timbers for basic timber “mechanical properties(mp)” needed for calculations (mp further defined as max or "limit load" shear, compression, tension, properties.... Limit Load meaning the mechanical (stress the materials starts to fail, "Ultimate Load" means it has failed, etc, properties, however, due to tree farming and uncontrollable variations in these properties making it difficult to document into like span tables, etc, like light lumber has. Light framing per code also requires a grade and/species verification for these reasons.

So a typical timber mechanical properties table would look like this, for example,

Shear Limit: XX PSF
Shear Ultimate: XX PSF
Shear Modulus: XX PSF

They are called "allowables" and are s/b denoted in code. Same for compression, tension, we design to limit and apply a safety factor there.

US Code does point to the American Wood or Pulp & Paper Association for the properties or design guide (tables, etc) but the path is broken there for the reasons stated above. They have some testing papers on mortise and tension, bolted joints, none I could find on traditional joinery.

That leaves the need to hire a PE. A good one will want the species or grade and if they do not have mechanical properties, info, otherwise they will apply a large safety factor like 2-3, over design, and pass the cost of unknown properties to the cost of wood.

Ultimately I will consult with a structural engineer for final review of my plans, but I am the kind of person that really likes to know what is going on with everything I am involved with so I can speak with the engineer intelligently about it.


Other terms you could look up are,

Creep – Permenant Deformation as a result of deflection
Modulus of Elasticity – Stress/Strain
Fatigue - often misunderstood and neglected due to lack of empirical data – A combination of stress over the live cycle of a building (also referred to as “Sustainable”)
Von Mises Stress – More Advanced Deformations

That is a start there is much more….

If one is not required by permit to hire a PE stamp there are DIY software programs called Finite Element Modeling” (FEM) such as,
http://www.iesweb.com/
http://www.structuralsoft.com/

If you stick to the wood species loaded in the software and validate it at the supplier, get the input loads right, FEM is accurate. You will learn that the same applied loads and stress are analyzed for light or heavy timber and the same mechanical properties are required to do the analysis.

Best to export a .dwg/dxf file from AutoCad or Chief Architect or similar CAD.

Hope that helps. Questions feel free to ask.




 
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