Designing for strength

[Weogo Reed]

Hi Folks,

I remember years ago reading that, generally, a beautiful boat will sail well, an ugly one won't.
Am guessing this holds true for furniture - good lucking furniture will stand the test of time.

There was an article in Fine Homebuilding about a furniture maker who was using both
wood strength tables and
physical testing:
) dropping chairs from a certain height,
) jumping up and down on tables,
) multiple joint options were tested to destruction, etc.,
to design furniture that was both strong and light.

I've read that Ikea does extensive furniture design analysis to
maximize their profit with a minimum of wood, machining, glue, etc.

Over the years, my design approach has been a bit on the over-built end.
Are there any book or online resources for design strength information like this?

Thanks and good health, Weogo

[Bruce King]

The most common item that develops problems would be built in shelves or any type of shelves that could get books stacked on them. You can Google wood shelf spans and find lots of information. Other items would be a piece of furniture that someone might sit or stand on during party antics. There are probably a million homes right now with kitchen island granite overhanging more than it should. Nothing will happen until someone over 150/200lbs decides to sit on it. Repair cost could include the whole kitchen granite if no matching material is available. The cabinet maker might get blamed even though it’s not his/her fault. As far as furniture goes it’s just common sense. Some modern designs are fragile. Probably located in a home without kids or large dogs.

[Brian Tymchak]

Along the same vein, I recently posted a question of how to calculate the load a piece of wood could support. I didn't come away with a solid answer to that question, but while doing a bit of research I came across this paper which seemed to possibly help. I didn't get an opportunity yet to read it thoroughly but while glancing thru the references it seemed there might be some other resources to look at.

[Stan Calow]

You're really talking about engineering, which takes into account the strength of materials in order to determine how to most efficiently carry the expected load.

I've read that Ikea does extensive furniture design analysis to
maximize their profit with a minimum of wood, machining, glue, etc.

Probably true of everything that gets built commercially. Thats the whole point of engineering, to solve a problem while being efficient with use of materials and cost effective. Its not about profit as much as keeping the price low enough for an Ikea consumer will buy it. A chair made with minimal safety factor is meeting the need it was made for (keeping you off the floor) instead of falling off a building or being a dance platform. You're just choosing a different benchmark.

[Malcolm McLeod]

There is an old saying that given sufficient resources, anyone can build a moon rocket. It takes an engineer to design one that is just strong enough to go to the moon. (...you can decide what you think this old saw is worth.)

As for books, go to your local (college) bookstore. Look at the textbook requirements for the Mechanical Engineering class in Statics, then find the one for Mechanics of Materials class and grab them for a little 'light' reading. These are fairly early in the curriculum for BSME, so the math should be manageable without loading up on Calculus book too.

Statics will give you a foundation in the simple concept of balancing forces (they MUST balance - or that chair will need a saddle!). The MoM book will explain the interplay of the various properties of your structure (aka 'desk'): from the cross-sectional area, to strength of materials.

What? ...TMI?? But it IS what you asked!

[Mark Hennebury]

You can read books to learn about wood.
But you will still have to learn to read the wood.

Reposted from the link

https://www.interactiongreen.com/5034-2/
“Wow, is that really what ancient people could do? How could this have been possible?”
This is our typical reaction when we see great architecture our ancestors have left us. We somehow automatically assume that modern people are more capable than people who lived hundreds or thousands of years ago.
But is that really true? Were ancient people less advanced, less knowledgeable and capable?

Golden Hall and Five-storied Pagoda of Hōryū-ji are Japan’s National Treasures. Hōryū-ji is a Buddhist temple in Ikaruga, Nara prefecture, Japan. It was registered as part of the UNESCO World Heritage Site “Buddhist Monuments in the Hōryū-ji Area”.
By 663highland (663highland) [GFDL (http://www.gnu.org/copyleft/fdl.html), CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0/) or CC BY 2.5 (http://creativecommons.org/licenses/by/2.5)], via Wikimedia Commons

Golden Hall and Five-storied Pagoda of Hōryū-ji, a Buddhist temple in Ikaruga, Nara prefecture, Japan. Oct 2006 by 663highland

Take a look at this. This is Horyuji, an ancient Japanese temple, built in 607 AD. It is the world’s oldest surviving wooden structure. Horyuji was constructed from Japanese cypress that were roughly 2,000 years old. It has been 1,300 years since the cypress were cut down, and the wood still stands firm. “And they will be fine for the next 700 years,” said late-great chief Miya-daiku, Tsunekazu Nishioka. Miya-daiku is a specially trained carpenter who builds/maintains ancient temples and shrines. Their technical skills are usually passed down from father to son, through many years of hard, and skill developing, on-the-job training. According to Nishioka, “2000 year-old Japanese cypress is so robust yet resilient that it can maintain its great quality for another 2000 years, even after it’s cut down,” and that “people who initially build Horyuji in the 7th century knew about it. They knew that the building would last for another thousands of years, so they built it accordingly.”

It is mind-boggling. Even today, people are not that ambitious to try to build something that could last for a thousand years. (Well, today, we’d rather want to make things that will eventually fall apart, to create another business opportunity.) But ancient people had such a strong, and even desperate yearning for something that would last almost eternally, that could somehow defy their own fragility. That strong will made something seemingly impossible, possible. In addition to Horyuji, there are a number of great architectural structures in the world today, still standing strong thousands of years after initial construction.


But of course, the will or ambition itself cannot build something as extraordinary as Horyuji. It had to be accompanied by solid skills. In the case of Horyuji, one of the extraordinary skills the initial builders had was the capacity to “read” the wood, Nishioka says. Wood is considerably uneven and an unpredictable material for building. It’s organic. In addition to the fact that it comes in various lengths, diameter and age, it also expands or shrinks, depending on the moisture content (which changes even after it’s in use). It bends toward a certain direction, depending on the angle at which the tree grew, relative to the Sun. Some grow faster and some grow more slowly, affecting the sturdiness. The location where the lumber was to be used determined how susceptible it was to moisture and the potential to rot.

By 663highland (663highland) [GFDL (http://www.gnu.org/copyleft/fdl.html), CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0/) or CC BY 2.5 (http://creativecommons.org/licenses/by/2.5)], via Wikimedia Commons

Horyuji pillars made of Japanese cypress. Ancient woodworkers only had simple planes to finish them, yet they achieved beautiful entasis. It continues to impress visitors 1,300 years after the construction. By 663highland, CC-BY-SA-3.0 or CC BY 2.5, via Wikimedia Commons



If that was not enough, there were no sawmills available for Horyuji builders. There were no tools that would cut open the wood in a straight line. The only tools they had were something like a froe or shake axe to cleave wood, by splitting or riving it along the grain. Because of this “crudeness,” people had to be extensively knowledgeable about the wood they would cut.

All those factors had to be taken into consideration BEFORE each piece of lumber was to be used: no mistakes were tolerated because they could cause the building to collapse. It is an enormous amount of information to process at once. Plus, since there were no effective transportation methods at that time, people had to live with what they had.

Their extensive skills to be able to read the history and condition of each and every tree helped them to build something that would survive 1,300 years.
According to Nishioka, we’ve lost these great skills of tree “reading” over time. Not only that, it is said that there are only less than one hundred Miya-daiku’s in Japan. While technology continues to thrive and change the impossibles to the possible, we are losing our own skills that made so many impossibles possible. Although we sort of assume that all the skills can be reproduced using technology, it’s not that certain. Techniques that emerge from people’s own skills are beautiful, but machines and artificial technologies can only guarantee functionality, not the beauty.


Reference： Tsunekazu Nishioka (1978), 法隆寺を支えた木（The wood that supports Horyuji, NHK Books, Tokyo, Japan

[John K Jordan]

...
Are there any book or online resources for design strength information like this?
...

You want to get technical? Includes wood properties, types of joints, and calculations applicable to things large and small.

Simplified Design of Wood Structures by James Ambrose
https://www.amazon.com/gp/product/0471179892

I paid $67 for the paperback in 2008, fifth edition. I see it's 1/2 that price now.

JKJ

[Weogo Reed]

Hi Bruce,

I've been working with woods, metals and some plastics for about fifty years, so have a feel for what will work.
"As far as furniture goes it’s just common sense." Agreed!
These days common sense appears to be in shorter supply with some folks, and
it would be great to have some references to send them to.
And I would like to challenge myself to do more with less.

Brian, I followed your shelf question thread.
I read the paper you noted and searched some of the references.
This one has some interesting notes on mortise and tenon construction,
and some might find the conclusions useful:
https://bioresources.cnr.ncsu.edu/re...niture-joints/
I'll try chasing down more of the resources.
Wish I had a deeper understanding of math, physics & engineering...

Stan and Malcolm, I've been called an (audio)engineer many times, as I do live audio mixing,
but am not an Engineer. (And no gigs for almost a year.)
What I would love to find is simple tables, similar to floor-joist or rafter span tables, where
the heavy math lifting has been done.

Mark, I agree on reading wood, and thanks for the link, that's a remarkable temple.
Years ago a German builder was showing me around his town, with many buildings more than three hundred years old, and
a few much older, going back to about 1400. I commented on how they built to last forever, and he replied,
"Oh no, our company only builds to last two hundred years. After that it is time for something new."
It would be cool if something I built lasted 200 years!

John, 'Simplified Design - - ' looks very useful. It's part of a series, about concrete and other building subjects.

I found some interesting testing here:
https://woodgears.ca/joint_strength/
Practical info:
https://www.tablelegs.com/resources-...tronger-table/
Including - don't slide a table on the floor.
Testing:
https://www.intertek.com/uploadedFil...re_Testing.pdf

First post read 'good lucking furniture' obviously should be 'good looking furniture' !

Thanks and good health, Weogo

[Mark Hennebury]

Hi Weogo,

Designing furniture, especially complex stuff like chairs, where the design can be far from rectilinear and have complex stresses involved. You need to have a good feel for structural properties of wood and how the stresses will be applied based on your design. There are so many variables involved that i don't think you will find them in a book. Wood of the same species is incredibly variable for piece to piece, grain direction is critical. Joinery is a delicate balance of proportions, grain direction, estimating the load and reaction to it. It requires a thorough understanding of the material and it's nature to find that perfect balance of how much wood to remove and how much to leave, where to remove it when designing a joint for a particular purpose. Wood is not uniform, it has directional strengths and weakness that must be considered and exploited. For example in metal the strength of a triangle brace may be quite simple to calculate, in wood, not so much; the strength is variable on the grain direction and the abutment joints. Change the angle of the adjoining pieces slightly and the joint can change from exploiting the best strengths of wood and creating a super strong joint, into a wedge splitting the wood apart as soon as any pressure is applied.

I always had Bruce Hoadley's book as a reference, There is a new book about wood Called "Cut and Dried" which is as i have heard a great reference.
I also had books on Japanese joinery.

But most of all experiment and observe. See what is not written.

I used to experiment with chair designs a lot, If i had an idea I would make a rough prototype just to see it in the flesh, to sit in it and see if the concept could work.

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[John K Jordan]

...
I always had Bruce Hoadley's book as a reference, There is a new book about wood Called "Cut and Dried" which is as i have heard a great reference.
...

I have that book, by Richard Jones(who also goes by Sgian Dubh) from Great Britain. It's available from Lost Art Press: https://lostartpress.com/collections.../cut-and-dried

It's a big book and over 300 pages, filled with a lot of information. Could be a good education on wood for someone without a library. There is a 30 page chapter on Wood Strength and Structures.

cut-and-dried.jpg


JKJ