Impact of Knots on the Strength of Wood

The strength of wood beams and columns is an important parameter that governs the design of structures. In this video, we investigate how strong wood actually is and why we use lower values during design. As one of the main and most important construction materials, understanding wood is absolutely crucial. The video also touches on and explains size-effects as an important design consideration. Furthermore, a statistical explanation is provided for the strength value used in design manuals around the world.
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References:
[1] J. Dinwoodie, Timber: Its nature and behaviour, London: BRE, 2000.
[2] Canadian Wood Council, Wood Design Manual, Canadian Wood Council, 2017.
[3] J. Porteous and A. Kermani, Structural Timber Design to Eurocode 5, Blackwell Publishing, 2007.
[4] Z. Bazant, Scaling of Structural Strength, London: Elsevier, 2005.

[@TheEngineeringHub]

To further improve the watching experience, we did not include a sponsor in this video. If you enjoyed the video and you feel like we deserve your support, you can check out the link below. Alternatively, clicking like and subscribe or writing a comment also helps a lot. BUY ME A COFFEE LINK: If you enjoy our work, you can buy us a coffee on the link below: www.buymeacoffee.com/engineeringhub

[@artemaung5274]

Hey Engineering hub! You could improve visuals if you use wood material that clearly shows wood grain in your videos. Material is fairly straightforward to make in blender with wave texture. In essence it's just texture coordinate - > object mapping (type point) > wave texture (rings, Y, Sine or Saw) (something like scale 90, distortion 100, detail 15, detail scale 0.1, detail roughness 0.45) and color ramp to color the wood grains. This will project wood grain everywhere inside and outside object space so if put a "piece of timber" along the Y axis what where tree rings would appear, and That's basically all that's needed for simple visualization. It would already look much better than what you have here. If you don't know blender I could just send you textures or renders to show it in your visualization.4

[@jennyjohn704]

Good video but the thumbnail makes the bold claim "wood is actually stronger than concrete but ... ". This bizarre claim (what kind of wood, what kind of concrete?) is not mentioned in the video itself.

[@patty109109]

Thx. That seems baity and I know this channel can do better than bait cause the vids are good

[@marioxerxescastelancastro8019]

Also: What kind of strength? (tension/compression/shear, yield/ultimate/modulus).

[@leorbuis9024]

You've got a great channel and you're doing everything right so keep up the good work! I'm a builder, and I'm always interested in why things are what they are rather than just accepting what I've been taught so channels like yours are very helpful to people like me. Thanks!

[@pavanpatel4150]

Enjoyed your discussion on the distribution. I used normal distributions and Monte Carlo simulations to calibrate LRFD factors, so this was quite the throwback.

[@emiliodavid5668]

My mind just blow up it is a really sharpe way to aboard this problem I will wait for a second part of this topic. Thanks a really good explanation!!!

[@TheEngineeringHub]

Glad you liked it! More videos on wood properties will come, stay tuned!

[@sangomasmith]

This is a really cool video for me, as I occasionally make bows (which requires woods that perform at the limits of their tensile and compressive strengths). It's accordingly very important to find as flawless a piece of wood possible when making a wooden self bow. Tangentially, I should also mention that one of the most useful measures of performance in bow materials isn't maximum yield strength per se, but rather the material's energy per unit mass. This can be found by taking the modulus of resilience and dividing by density to provide a measurement of J/kg for the material at maximum (yield) strain. One insight that this provides is that the "classic" bow woods (yew, lemonwood, hickory, some bamboos, white ash etc) all have high values of between 800 and 900 J/kg. These woods became popular for bow making precisely because they can store a lot of energy. The same measurement also explains why horn and sinew can produce such powerful bows, as these materials provide far higher values (up to thousands of J/kg, competitive with modern linear-strand fibreglass composites).

[@TheEngineeringHub]

Hi Thom, thanks for the great discussion on bows. As structural engineers we are mostly concerned with very fine displacements which is maybe why we don't look 'directly' into the ability of the material to store energy. I say directly because these principles are usually equivalent. For example, the amount of energy stored in a structural member (say a beam) is the product of the stress and strains integrated over the volume of the member. This is known as the elastic strain energy, see the link for more info: www.engineeringcorecourses.com/solidmechanics2/C6-energy-methods/C6.1-elasticstrainenergy-for-variousloadings/theory/ As seen here, the total energy in the member is closely related to the loads it is subjected to. However to be able to account for the entire stored energy, one needs to know all the forces that the member experiences and integrate over the whole volume which would produce something in Joules [J]. Dividing by the mass of the member should produce the material's energy per unit mass (which I believe should be the same parameter that you mentioned). In structural design, energy methods (i.e. the principle of virtual work) are often used for analysis but not often for design. I am not sure why but my guess is maybe because structural members are often exposed to complex load combinations which makes it easier to analyze each loading separately. Also, obtaining the maximum yield load is usually a fairly simple uniaxial test. I am not sure what the testing procedure is for obtaining the maximum energy per unit mass but maybe it's a more complicated testing procedure which further makes structural engineer not inclined to it (just a guess).

[@sequoyah59]

My late Father was a professor of Civil Engineering for a time and back in the 50s his office was full of these old and early improved studies such as the weakest link theory. These guys who did this kind of work were passionate, insatiable and inexhaustible in their work. Their graphs were exquisite for the most part and assumed little in filling in missing data. I am always amazed at the construction of thermodynamics graphs. Process, process, process and more process are the keys with rigid adherence to the process. Becoming an engineer was hard for me as I was not inclined to higher math but I stuck with it determined not to fail. Medicine might have been an easier choice as I did well in the biological sciences. Organic chemistry may have been my downfall and rote memorization in general. I have to understand process behind something, not simply that it is.

[@TheEngineeringHub]

Absolutely agree, the 20th century brought many geniuses and our understanding of wood improved significantly. As little as 150 years ago, builders knew very little about wood and its structure/properties though of course they were great at using it.

[@shaneprather6743]

Practically, the longer chain would fail first because it would have more mass, so there would me more stress on the first link of the longer chain compared to the shorter chain for any given hanging mass.

[@shaneprather6743]

If you said assuming massless chains, then it is an accurate comparison lol

[@morelanmn]

My son had a high school project of making a paper bridge that would span a 18" gap and no wider than 5 inches. The paper bridges would be tested by putting weight on them until the bridge buckled. No other rules. No problem. I was a paper maker. We used pine pulp. (The fibers are longer than hardwood fibers). Then orientated the fibers, pressed and steam dried the fibers into 30" x 45" pieces. The bed of the bridge was three triangles side by side. Will many details later we had a bridge. This broke the school record and still stands. I can see this natural fiber being used to make many things.

[@TheEngineeringHub]

Amazing Rider! It does sound like a fairly strong and stiff bridge!

[@EricPalmer_DaddyOh]

Very well explained. Thank you.

[@grantshishido3517]

I just discovered your videos. They are great! Thanks for the work you put in.

[@TheEngineeringHub]

Welcome! So glad to hear comments like this!

[@wayneroyce6870]

Great informative video. I appreciate the your chanel.

[@TheEngineeringHub]

So nice of you, thank you!

[@matthew8505]

Love the video as always! Would have liked a comparison between wood and typical hot rolled steel in both vertical and horizontal directions

[@TheEngineeringHub]

Great suggestion! Thank you Matthew!

[@4n2earth22]

Thus, the advent of engineered lumber products such as Glued Laminated Beams, Trusses, Joists, and Micro Lams. The grading requirements remove or significantly reduce flaws (knots, checks, voids, or divergent grain) from the component.

[@matheo4712]

I love your videos and the way you explain the content! 💪

[@TheEngineeringHub]

Thank you Matheo🙏, it's always nice to hear from our viewers.

[@ajazmahemoodsayyed4122]

Amazing sir

[@RonJohn63]

0:39 This is why it's easier to split wood than to cut it across the grain.

[@TheEngineeringHub]

Excellent point! I should have mentioned that in the video, too, very good intuitive example!

[@bdykes7316]

Great video. Small critique: The animation of the referenced source numbers in brackets appearing and disappearing seems unnecessary and distracts from the narration and other animations. I think a fade in/out or a cut in/out would be more appropriate. (See 4:25 in video)

[@techsyncengineeringgroup5491]

Very clear explanation. Initially I thought the additional weight caused the longer chain to fail, seems I was wrong

[@TheEngineeringHub]

Hi Techsync, that is also true, you were not wrong! Though, the additional weight will not be too significant since steel has a high strength-to-weight ratio.

[@techsyncengineeringgroup5491]

@@TheEngineeringHub Thank you for your response. Appreciated

[@TheEngineeringHub]

@@techsyncengineeringgroup5491 Thank you for the discussion! 🤗

[@emmabird9745]

The longer chain because it has a higher probability of containing one of the lower strength links. Only one link has to fail.

[@TheEngineeringHub]

Hi again Emma, that was spot on! Did not expect anything less from you (based on your knowledge from comments on other videos)! Cheers thanks for the comment!

[@emmabird9745]

@@TheEngineeringHub Thanks for that. I did as you said and commented before your reply. Incidentally quite a good vid, well done.

[@TheEngineeringHub]

Thank you, we appreciate your continued support🙏

[@misty_mountain]

Longer chain will fail first, because longer chain simply weights more!

[@TheEngineeringHub]

Yup that is also true

[@misty_mountain]

@@TheEngineeringHub your channel is gem! Wish you luck. Consider only one thing-upload shorts too! I’ve noticed the algorithm promotes better channels with shorts🤝

[@jennyone8829]

Thank you 📍🌻🎈🛰

[@TheEngineeringHub]

You are so welcome

[@zerohdilangalen4364]

Pls.make a video reguarding u shape stirrup compare to conventional square .....

[@TheEngineeringHub]

Noted! That would be an interesting discussion.

[@shaneomacgardner4034]

Chain L2 would fail first, as it carries the added weight of extra chain.

[@shaneomacgardner4034]

Well I just learnt something 😅 Really interesting info thankyou 👍

[@TheEngineeringHub]

That's great to hear ShaneoMac. Your answer is also correct though (based on the weight-to-strength ratio) might be less significant than the micro defects present in the material.

[@eliasloozen7948]

How big is the influence of a knot in wood, since many of the fibers go around it? In steel, it's different, the fiber lines (flattened crystals) are disrupted by impurities and micro cracks causing big increase in local stress.

[@TheEngineeringHub]

The knot still causes stress concentrations in the material that make it fail at a lower stress level. Based on the size, geometry, and location, the hole/knot may have significant or less significant effect. This is similar for all materials, here is an example of a steel plate with a hole: media.mscsoftware.com/cdn/farfuture/4VDI7yDR52ZAa8VwEom-3HHiumIf1gEKW4NggAJxHPk/mtime:1361829583/sites/default/files/p12s11.jpg

[@abdulrahmansaraiji2173]

the like button is not enough for your videos

[@TheEngineeringHub]

This is probably the best compliment we have ever received, thank you!

[@HeJurm]

Would the logic of weak links be true for an assembly of wood pieces? Is it so that as more pieces of wood, e.g. timber studs, are added within a structure like a single wall, so that the pieces are placed at smaller and smaller intervals, the design strength of the whole structure should tend to the average of the strength of the individual pieces? The weakest piece will matter less because it will be helped by stronger pieces close to it. So the design strength of a structure composed of many pieces of wood is higher than the design strength of one piece times the number of pieces in the structure! You have great videos about timber! Subbed!

[@kevpatguiriot]

Steel reinforcement in longer beams experiences a lot of tension or is it just longer and therefore more elastic? : )

[@alext8828]

A knot can't hide inside the wood. If you see one, grab another piece.

[@alexciocca4451]

If sugar is that strong why does it taste so good something ain’t right with that thought ?

[@videosight1]

Didn’t even mention the concrete link, wack

[@marialewis6779]

ρгό𝔪σŞｍ

[@boblordylordyhowie]

I once knew a guy who grew Willow trees. For the first 5 years growth he would remove base buds from the trunk so it grew flawless up to a certain height, then he would plant them next to where a tree had been cut down. It was for the cricket bat industry in England as they required logs with no flaws or knots in them.

[@kakashi_senpai042]

I'm an engineer myself, and I think I Love Engineering is amazing! They provide an invaluable service for upcoming engineers. They provide a platform for us to share new ideas, collaborate with one another, and leverage our technical skills. Through their content and outreach, they've sparked my passion for engineering even more. They help us build on our technical knowledge, and keep us informed on the ever-changing advances in the engineering field. Thanks to I Love Engineering, I'm well-equipped to continue learning and advancing my career.

[@TheEngineeringHub]

I haven't heard of that channel, share a link with us

[@jaredevildog6343]

Thank you for making this technical video not too complicated for my small brain. Please keep up the great work !

[@TheEngineeringHub]

The fact that you understood the video already proves that you are smarter than you think. Thanks for the comment Jare!