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Forum topic by Richard posted 04-20-2020 09:04 PM 326 views 0 times favorited 8 replies Add to Favorites Watch
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Richard

120 posts in 4185 days


04-20-2020 09:04 PM

Topic tags/keywords: question cherry design load

I searched around and could not find a direct answer. When designing a project how do you determine how much weight a given dimension of lumber can hold?  I am looking to use cherry.  I found the hardness value and the PSI that it can withstand.  I am not sure how to translate that into a dimension.  For example if I want to support a load of 100LBs then each of my four legs would need to be at least 2×2 or 4×4.  Any experience with this?  Any advice would be welcome.

Thanks


8 replies so far

View Think0075's profile

Think0075

36 posts in 495 days


#1 posted 04-20-2020 09:24 PM

View Woodknack's profile

Woodknack

13383 posts in 3150 days


#2 posted 04-20-2020 09:33 PM

+1 Sagulator is good for shelving. For normal furniture I never worry about it. There are ways to figure out things like structural loads but unless you’re doing something peculiar, good joinery and design will get you there.

-- Rick M, http://thewoodknack.blogspot.com/

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Madmark2

1362 posts in 1358 days


#3 posted 04-20-2020 09:38 PM

USDA “Wood Handbook: Wood as an Engineering Material” free .pdf available everywhere on the net. Every physical parameter of most every wood available commercially in the US.

-- The hump with the stump and the pump!

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CaptainKlutz

3156 posts in 2264 days


#4 posted 04-20-2020 09:44 PM

+1 Sagulator; good for table top sag too.

+1 Wood Handbook

Comments FWIW:

Issue with wood is the grain direction defines the strength parameters. This web article explains it pretty well:
http://workshopcompanion.com/KnowHow/Design/Nature_of_Wood/3_Wood_Strength/3_Wood_Strength.htm

If you have all the required data for species, grain direction, and leg design: the tables above will provide numbers for strength calculations. Engineering specific load carrying capabilities with wood is always a little nebulous, as leg strength varies piece by piece due differences in every board. The only way to overcome this issue; over designing.

Another issue with making a direct recommendation for XX size is difference between static/dynamic loading and all various types of joinery used to attach the leg?

From a experience stand point will say this: There are french dining chairs with skinny 1×1 tapered legs that easily hold 200lb person. Those same chairs creak, groan, and eventually break if 250lb person bounces up and down on them. To support a 400lb person; would suggest ~2×2 leg would be good starting point, assuming the right joinery is used, and any potential high stress zones are reinforced.

Best Luck!

-- If it wasn't for bad luck, I wouldn't have no luck at all, - Albert King - Born Under a Bad Sign released 1967

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Richard

120 posts in 4185 days


#5 posted 04-20-2020 10:08 PM

Thank you for the replies. Looks like I have some light reading for tonight.

View theart's profile

theart

192 posts in 1324 days


#6 posted 04-21-2020 01:44 PM

I would advise against trying to do any kind of engineering analysis. Without knowing what kind of load case or failure mode you’re trying to design for, it’s easy to get into trouble. For example, just looking at static compression and the axial yield strength of cherry would lead you to believe that you only need about an 1/8” square leg. Calculations for joinery, which is where failure usually happens, are much more complicated.

A better approach would be to take a step back, think about your general design and application, then go looking for examples.

View Nubsnstubs's profile

Nubsnstubs

1723 posts in 2500 days


#7 posted 04-21-2020 02:12 PM



I would advise against trying to do any kind of engineering analysis. Without knowing what kind of load case or failure mode you re trying to design for, it s easy to get into trouble. For example, just looking at static compression and the axial yield strength of cherry would lead you to believe that you only need about an 1/8” square leg. Calculations for joinery, which is where failure usually happens, are much more complicated.

A better approach would be to take a step back, think about your general design and application, then go looking for examples.

- theart


Agree 100%. We have been making stuff for thousands of years that doesn’t break under normal use. Improperly used glued piece oriented wrong will give someday, but legs from solid wood won’t. ......... Jerry (in Tucson)

-- Jerry (in Tucson) www.woodturnerstools.com

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MrRon

5910 posts in 4013 days


#8 posted 04-22-2020 11:10 PM

You design a piece of furniture by aesthetics or what looks pleasing to the eye. Chair legs are treated as columns and can take a much greater load than if it were used as a beam. There is a slenderness ratio which is expressed as L/D where L is the unsupported length of the column and D is the smallest cross sectional dimension. The ratio of the two should not exceed 50. Example: 36” long colimn that is 2” wide; 36/2 = 18 OK. 36” /.5” = 72 NG.

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