Wood Stength vs Hardness

Oka

Casey
Corporate Member
This postings’ objective is not to get into the “weeds” of the engineering but rather, to help the new(er) woodworker understand and decipher some of the erroneous and misleading hype often thrown about when talking about wood and its hardness. That said, this will likely be longer than I envision for a normal posting.
I will only discuss two basic considerations in wood engineering, Hardness and the deflection resistance (bending). Another consideration is impact resistance, there are others forces engineers look at, but for this discussion, I kept it to just the basics two.

As we know, wood is harvested from something live. Because of this, there are many variables to each piece cut. Along the road of the woodworking, each piece you cut can and will provide you insights on how to improve your ability to know what to look for when buying wood and understand how to “piece it together” so it looks the best as a finished product.

Identifying wood type and matching need and appearance to a project:

Picking the end look is often how we all start. That said, the next thing to think about is how it will go together to be strong enough for the intended use. Weight is a consideration that come in to play as well.

Understanding the misnomer of wood hardness for strength is often a common statement with woodworkers. Strength of wood is more than surface hardness, which is the often touted “Janka” rating.

For those who are unfamiliar with the Janka system- The Janka hardness test measures how many sq ft lbs pressure is required to embed a .444-inch steel ball (roughly 11mm) to half its diameter in wood. The purpose of this test is to measure wood species ability to withstand denting and wear. It is also a good indicator of how hard or easy a species of wood is to saw or nail. E.G. Northern Red Oak, has a Janka hardness of about 1290. Brazilian Jatoba, has a rating of about 2350. If desired, you can look up more on Janka on the internet.

But with wood, that is only one consideration. Modulus of Elasticity (MOE) is another. Simply put, how much bending stress (deflection) can a piece of wood take before it fails. Failure can be described as warping due to the stress or cracking or breaking. All are failures if that is not what you want.

This is what engineers look at when a building is being constructed out of wood. As a wood worker, this is something we need to have a cursory understanding of. So, when we build anything, we are also considering how to reinforce something in the build to assure it does not fail.

An old adage one of my engineering instructors told me –“ anyone can make something strong and heavy, but the skill is making it strong, light and efficient”.

Back to wood. The things I look for when matching wood is grain and its direction. Be a keen observer of wood grain and really look at how the wood grain moves through the board. This is really important., epecially if the wood is being sold at a cheap price. Things to avoid would be when wood grain wanes (starts moving in a differing angles suddenly), of course, knots and “Y” forks, and other defects like ring or heart check, cat or dog facing (where the Wye starts but then is reabsorbed in the early growth of the tree), pest holes or pith or punk…….. Unless you want that. Sometime this can be a cool feature(s).

Of these, the subtle wane and things like that are the real scourge - often discovered only when you begin to plane. I run into this a lot with certain woods, Iroko, African Teak, some Mahoganies and some other Exotics. The thing is, when you look at it 1st glance, you really cannot see it. But, upon careful inspection you will see some signs, for me? about 80% the time I catch it. Great odds if it is a poker game, but when it happens after hours of work……… yeah, not so fun. Wane is a real potential structural failure point, particularly when it is near a joint. Keep that in mind.

THIS – is where understanding your tools and having them sharp will help.
Enough rambling- this is a big subject and better understanding and knowledge of this benefits all. Hope this helps enhance your discretion when acquiring materials and when thinking about how something goes together and how strong it will be when completed.
 
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tvrgeek

Scott
Corporate Member
I go about in a different way. Over engineer by a lot. Of course, surface hardness matters on desk-tops and floors. A Windsor chair or spindle rocker may need careful attention to grain. Fortunately ( for me) I prefer Oriental and "Stickley" styles where it is not very critical. :p

To make the subject even more complex is stability over time. I am not sure you can determine that much by looking at rough sawn at the supplier. I am envious of those of you with big lumber racks where your wood can sit for years before you use it. I have to buy and use. Not the best, except I guess it does mot make any difference is BB ply.
 

Oka

Casey
Corporate Member
Well, knowing the ambient moisture percentage where you live helps. Here on the big Island near hilo it rains alot but the ambient is about 12.2-12.7% In Honolulu where I lived got 1/2 the rain but the ambient was 12.-13.2% ...... go figure. My guess in NC the delta temp swing is so big that this is the real contributor of the wood moving. Here our delta-T is about 35 deg. I think NC is like 95-105 deg.
 

SteveHall

Steve
Corporate Member
Is hardnesss/Janka always measured on the face grain? Or are there different values for end grain, too? 99% of flooring is face grain, so hardness is usually critical there, but I have seen a few end grain floors (with massive expansion joints on the perimeter).

Wood is an amazing substance. It's interesting how many types of strength characteristics can be distinguished: deflection (bending without structural damage), failure (unrecoverable deflection), destructive/catastrophic failure (breakage), parallel to the grain versus perpendicular, torque/twisting, compression versus tension, shear versus bending, crushing, dry versus wet, etc.
 

mdbuntyn

Matt
Staff member
Corporate Member
Here are a few podcast episodes that go into some depth about the properties of wood:

 

Oka

Casey
Corporate Member
Matt is geeking out..... :D Those are good videos indeed

Here are a few podcast episodes that go into some depth about the properties of wood:

 

Dee2

Board of Directors, Vice President
Gene
Staff member
Corporate Member
OP, sounds like the makings of an article for the .com site. And, the comments provide more/references.
 

bob vaughan

Bob Vaughan
Senior User
Conspicuously missing here is the property known roughly as the "coefficient of linear expansion", or how much does a piece of wood move when the surrounding air's moisture content changes.
Redwood expands end to end. Don't butt the ends closely together when building a deck. The wood buckles. I've seen that happen when the contractor wasn't aware of this detail.
When building a dresser or other piece of solid wood casework, this factor can be a consideration when a wide variety of species are available for the project.
In today's climate controlled interior environments, this isn't as big a factor as it used to be, but the builder should do a little research to find out what wood species have the more extreme characteristics. It lowers the potential risk of failure due to wood movement.
 

Oka

Casey
Corporate Member
Got to give that idea some serious thought. The physics of wood and the differing characteristics of the varying species is so fascinating.

Bob, the redwood today pales in compassion to the old growth wood. The material reacts quite differently. If you can get your hands on a 1978 UBC (or before) and compare the tables to today's IBC on redwood you can see how the materiel is differing, at least strength wise.

OP, sounds like the makings of an article for the .com site. And, the comments provide more/references.
 
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bob vaughan

Bob Vaughan
Senior User
Some of the 1920s books I have talk about long leaf yellow pine being stronger than the oaks of the day. Yes, things have changed regarding wood properties, and not particularly for the better.

I've got some old growth redwood and Douglas fir. It just makes me happy to look at it. Using it would make me feel like I'm cooking a spotted owl for supper or something like that.

1    oldwood - 1.jpg
Redwood grade stamp
1    oldwood - 4.jpg
Annular ring stripes

1    oldwood - 2.jpg
Doug Fir grade stamp
1    oldwood - 3.jpg
Doug fir rings per inch.
 

Oka

Casey
Corporate Member
Actually a little known fact ........ spotted owl taste like a cross between a golden eagle and a bald eagle ....... :D

The Willamette stamp was a common stamp seen it alot when I was younger. My parents are a ranch in Northern Mendocino 20 min from the drive thru tree. Willamette and WCLB were the 2 grading stamps from that region.
If Y'all ever get to Jedediah Smith redwood park outside Crescent City Calif...... Got to see it ! THe rewoods are 20-30 Dia........ that is amazing, what is more amazing is seeing a 12 dia Doug fir with 6-8 inch thick bark.
 
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