How much load can a timber post actually carry?
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- Опубліковано 26 кві 2022
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In the video, we investigate timber posts and their carrying capacity. The video starts with an explanation of the general failure modes of columns. Further emphasis is cast on buckling and the Euler critical buckling formula. The buckling formula is investigated and explained with simple and intuitive examples. The video concludes with the calculation of the carrying capacity as per the design codes and compares that with the analytical capacity obtained for a perfect column.
References:
[1] J. M. Gere and B. J. Goodno, Mechanics of Materials, Cengage Learning, 2013.
[2] J. Dinwoodie, Timber: Its nature and behaviour, London: BRE, 2000.
[3] Canadian Wood Council, Wood Design Manual, Canadian Wood Council, 2017.
[4] J. Porteous and A. Kermani, Structural Timber Design to Eurocode 5, Blackwell Publishing, 2007. - Наука та технологія
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weak video. I can imagine the level of that course. Do you participate ?
WHY is providing full moment fixity difficult for wood beams?
I have UA-cam premium to avoid ads.
As a coalminer, I used to trust my life every day to plain, round, de-barked, timbers posts, [usually spruce] 6 to 10 feet long and 9 to 12" diameter ... and I'm still here. With good cap and foot blocks these posts would still be standing when the 6"x4" rolled steel joists they were supporting were buckling under the weight of rock above. In addition, timber 'talks to you'. As a 'weight' comes on and it is getting close to failure, timber gives you fair warning
Nice piece of knowledge.
very good point about the "timber talks to you", the cracking sound means RUN hahaha
Fair warning. I like it. Respect
You're absolutely right. there is a big difference between logs (de-barked timber) and wooden 8x8 beams. Logs are PERFECT! Processing timber creates new problems. We cut the grain for the shape, not function. The tree grows in order to survive. Every cell is tested ( daily!!!!) with that equation.
@@voya8480 Absolutely correct ! You only have to look at pre 20th century timber frame buildings or wooden ship hulls to appreciate how much more respect craftsmen had for the natural form of the wood and its use in cleft timber components.
I still remember having a 15min argument/discussion in my statics and then again in strength of materials class. My question to the prof was "who came up with 1.5 or 2X as being the right safety margin? What I thought they missed in all the eng classes was a discussion about COGS or the economics of design choices. As they say, "anyone can build a bridge that stands, but only an eng can build a bridge that barely stands"
My take: engineers only care about what’s practical. Theory is for scientists. Engineers do. Engineers only need enough theory to get the job done.
Even 2x is a very small safety margin in many contexts. The only reason such small margins are acceptable is that there tends to be many safety margins intersecting to produce a true safety margin that’s much higher in practice. The size of the margin should be proportional to the cost of being wrong. If you’re building a thing which could kill thousands of people due to a material defect or installation error or end-user modification, you tend to increase the safety margin!
1.5 or 2 is not always the correct safety margin. Safety margin is often determined by other factors such as cost, weight, risk, how accurate are the load profiles, etc. Very low safety margins of 1.1 or less usually require testing to prove that the design analysis matches real world performance. Larger safety margins account for variations in build, materials, and loads. Many times safety margins can be very large to protect life or even just keep there from being downtime cost for an entire plant. Codes and standards generally give large safety margins, though some will allow for analysis and testing if a design requires lower margins.
I've been in the building industry and I can tell you first hand that an engineer is not capable of building anything. And engineer is good at design and putting that design on paper in the form of a detailed plan to articulate his idea to the builders.
The builders read that diagram and translate it to the end product that you use and see.
Architects and engineers have zero practical skills to turn their thoughts into reality.
@@stevenstart8728 Isn't that how it's supposed to work? Builders don't worry about moments, equilibrium, and Newtons. Just like eng's don't have to know how to make forms, make a square cut, or make things physically square, plumb and level. But I always thought every structural/civil eng students should work on a job site for a summer.
@@stevenstart8728, *most. Boy do I have some stories.
Excellent! I have been studying this type of structural member and the effects of buckling so your segment was perfect. Well done with graphics and clarity easy to see. Thank you.
Thank you for the video! As a builder, it's great to get a perspective on the design side of things I construct every day!
Thanks for the comment Jordan! We are always glad when our viewers find our content interesting and/or informative.
Besides the Euler length curve, wouldn't it be true that the longer the post, the greater the chance of there being a defect in the wood thus raising the odds of a failure?
This is an excellent point. Size effects are very important in wood members and have to be accounted for. The code has a rather simple strength reduction factor based on the volume of wood used. Since, as you pointed out, the more material there is, the higher the chance of knots or other defects and higher chances of failure. There is also another factor that offsets this reduction and is related to the type of system, i.e., parallel or series system. This means that if the member is "single-handedly" supporting a substantial amount of the structure, then the strength should be assumed even lower. If the system is parallel (e.g., roof joists), and there would have to be multiple failures before a substential collapse, then the strength can be assumed higher because the chances of all parallel members having defects is reduced drastically. Thank you for pointing this out @Burntsider .
Yes, and that is probably why gluelam beams are so popular in larger wood buildings, they are easier to calculate on!
Also, doesn't wood grade come into that area of concern?
Wood species and grade are also crucial because they affect both the strength and stiffness of the member. The compression strength numbers reported in a code are usually in the 5th percentile which also provides a bit of a safety factor. This means that if they test 100 pieces of wood to failure then they would report the strength value of the 5th weakest sample. Some strength numbers are reported here: www.engineeringtoolbox.com/wood-beams-strength-d_1480.html
This helps deal with the uncertainties that are present on the load side. Loads are in general much more uncertain and hard to predict so these safety measures are absolutely necessary. Wood is also significantly affected by moisture. and load duration. This is captured by something known as the "Madison Curve" (can be seen here: www.researchgate.net/figure/Madison-Curve-with-ramp-loading-test-trends-and-constant-loading-test-trends-by-Wood_fig2_265275548.
Wood members become weaker over time, they could lose 30% after 3 years of service (this is species-dependent). So the 5th percentile strength is important to take.
That effect is limited by wood grading, placing limits on the kL/r, and calculating the wood column stability factor Cp.
I really appreciate that you took the time to include reference manuals, including for other parts of the world. Solid explanations and presentation. Thank you.
Glad it was helpful!
WOW! off-center consequence is phenomenal.
This all went right over my head, but it was really enjoyable to listen to. Thanks
Unfortunately, code assumes that the materials delivered meet the specification. Lately it seems if it looks like wood and is the correct size it passes. Cutting 4x4s out of a 6" diameter farm grown tree is nothing at all like a 4x4 quarter sawn out of a 30" log.
I tried to tell people this years ago...the difference between heart wood and old growth is alot
And what wood
Absolutely!
Agreed!
That's why engineers design everything with a factor of safety in it. Take the code minimum and then over-build it by whatever FoS value is standard in the industry. Mind you, the code itself has their own built in Factor of Safety, so you really don't need to overbuild it by much to get a good peace of mind as long as the wood isn't literal swiss cheese.
Thank you for that info, you did a great job presenting it!
Interesting, thankyou. I was recently talking to an Australian engineer who used to examine old trellis railway bridges for integrity. He was saying that if you had a 40x40mm column of the hardwood used in them and had a way of preventing them from buckling, then they were capable of supporting up to 70 tonne. Amazing. Of course you can't really prevent the buckling at that size, thus why the elements of the bridge are so large.
Subbed for the amount of effort you put in your comments. You are clearly very committed
Thank you so much, so flattering that you noticed that. Much appreciated!
IFearlessINinja You make an excellent point, so many videos are narrated by computer generated voices and they definitely don’t interact with viewers in the comments section!
Great analysis. Love the code vs theory comparison
Physicist: First we assume a perfect wood. Me: I’m using a 6 X 6. Excellent presentation, thanks.
It's nice to see the cost of lumber plummeting to affordable levels.👍🏻💲
Found your channel recently and love it
oh what an awesome channel cant wait till i have some time to watch more!
That’s why I buy construction heart rough redwood 4x4s, plentiful in NorCal. The actual size is closer to 3 3/4-3 7/8” depending on the mill. They are beefy and strong. I built a 12x12’ redwood tent platform out of them with GRK 5/16 x 5 1/8” RSS Structural screws, the best
I've seen them wrap concrete columns under the freeways here in California with steel and more concrete for seismic reasons. But now I can visualize the buckling and crushing forces they're trying to protect against.
Thanks for the metric units!
Great video my dude! 🙌
Thank you Engineer 🙏🙏
This is an engineering explanation of why I don't use 4x4s for second story decks. They are strong enough but tend to bow over time. 6x6s stay stable longer with the load of the deck, even though they are overkill structurely.
Hi Ryan! That's an excellent example. I suspect creep plays a role there as well since wood tends to undergo creep for its entire life. Though, when exposed to high stress, it deforms much more over time. The longer length of course makes it to bend more -> stress more -> bend more due to creep and so on in a circle for the rest of the structure's life. As you pointed out correctly, this is more of a serviceability failure than ultimate limit state (structural) failure.
Back home we'd often say "If a little bit's good, a lot's gotta be better." And 6x6s look better anyway.
Generally, most codes require 6x6’s for any deck over 8 feet above ground level. Generally it is good practice to use 6x6’s for all deck supports since the live load is rarely guessed correctly.
Wood is for the fireplace. Concrete pillar is far superior.
@@assassinlexx1993 If only concrete was as easy to work with and as inexpensive to buy/use....
cool analysis and nice animations!
Glad you think so!
I prefer to build-up my posts, using 2x material, using no smaller than 2x6s. 16d every 12" staggered on the faces & an appropriate adhesive between layers. This because WARPING is a reality, & using layers tends to offset that. I know, I know...that's slow & persnickety, but along adherence to Code, I build as if my own sweet Mamma was to live there :-) We Italianos have been building that way for centuries, for we love our women! Perhaps...do a vid comparing such posts with solid wood ones?!!
I do the same thing and construction workers hate my "overkill".
It's not.
It's always better, always safer.
Great to know. I am now off to cut the primary support beam in my house in two to increase its carrying capacity.
You need to be more...supportive!!
Great and interesting video!
Eu citizen here, 1 inch is 25.4 milimeters which would mean 4*4 is 101.6*101.6mm
You would think, right? But no, a 4 by 4 is 89 by 89 mm, corresponding to 3.5 by 3.5 inches. Check out the video on why the 2 by 4 is getting smaller and smaller for more details.
is this why balconies fail when 15 people walk out to see fireworks
The lesson to be learned here is that it's best not to try to improve on nature. If you want to do a very interesting comparison, do a few calculations on a whole log. A vertical log, without having been compromised by cutting the natural rings, will raise many eyebrows. Minimal shrinkage (1%), maximum strength, by evolution, and a better fire rating. If memory serves, a 200 mm diameter vertical log, 2400 mm tall, will carry 20 tons. With whole log joints, in log home building, a simple wooden wedge prevents any movement in the structure, and the extra stability is obvious.
thank you, what interesesting info!! Of course, you cut through the rings you have all this soft wood exposed, more rot, more fire hasard.....i never thought about that though the strength surplus was already obvious to me (when you cleave a wood beam vericaly across the grain the principle becomnes clear....) why o why did we stop using logs.....though i have an idea why.....time for the old craftmanships to return !!
Thank you. Subscribed
Would be interesting to see the difference in load handling when considering a 6x6 column. I'm not an engineer, but have generally had doubts regarding structural loads on 4x4s, most assuredly when length/height exceeds, say, 8 feet. Have replaced 4x4x10ft deck supports with pronounced bows, with 6x6s. No bows!
I believed 6x6 wood column has more than 30% compression capacity vs of that of 4x4..
@@BOUNTYEATER You'd think it would be at least double
@@BOUNTYEATER it has 246% the surface area (and volume) of a 4x4 so I imagine it’s a great deal more than 30%.
I just lifted the Supporting beam in my basement holding up 3 stories of my old Victorian house with a 4x4 post and a heavy jack. It made some strange noises but did lift the beam enough for me to make several telepost adjustments. It was a 20 ton jack and was probably near its limit.
That’s sketch, I would’ve used something a little heavier, any checks or cracks and that thing would explode like a claymore
@@rezzbuilds8343 I was prepared to go down with the ship
Lol
I have a beam I installed in my barn that uses triple stacked 2x8's for the posts and beam. I have yet to see it even deflect the slightest bit when I hook something up to the electric winch attached to it.
Please please make a similar video when the load is applied midcenter on a horizontal wooden beam. How things change when you create a slight angle, and also move the load from center to something like 1/3rd of the length. Thank you!
Standard homesteader M.O. is : when in doubt, over-compensate. Obviously, loads vary, but generally it becomes gut-feel, so trust your instincts. If your parameters are that your family's lives depend upon it, then give your subconscious credit to come to the plate.
I live here in the U.S., in Arkansas. At Home Depot, a 4"x 4" square treated post is, as has already been mentioned, really 3.5"x 3.5". But...if you get a 5"x 5" square treated post, it really is 5" x 5". I think that is interesting.
Thanks. For me as a European 4x4 means (4 x 2,54) x (4 x 2,54) or 11,6 x 11,6 cm. Not 0,89 x 0,89 cm.
So the Americans not only have a funny system of units, they also do whatever they want with it..
@@Roadkill3120 Yea, here in the USA, it does seem odd and not consistent, but hey...that's what the "Free Enterprise" system does...LOL...!!
Seriously though, the older houses from years ago; perhaps 50 yrs ago and earlier, used actual 2"x 4" lumber, and much of it came directly from the saw mills and they were still rough cut. These older houses were built so much better with better lumber and usually with good carpenters that knew the trade.
My own opinion about today's 2x4 that is actually 1.50 "x 2.50 " is that more 2x4's can be cut from a log by using the smaller dimensions, and that means more profit for the lumber owners. Yea, there is wood that is lost from the thickness of the big saw blades, but that is not why all of todays 2x4's are not true 2x4's anymore.
@@marbleman52 Wood is also far cheaper in the US than in Europe, so make of that what you will.
@@marbleman52 The actual vs. nominal size of softwood lumber in the USA has been consistent since about 1964. So when you see "2x" that means 1.5 inches. In fact all nominal sizes (for softwood lumber) over 1 inch and up to and including 6 inches are a half inch over the actual size. So a nominal 1x2 is 0.75" x 1.5", a nominal 2x2 is 1.5x1.5, up through a 6x6 being 5.5x5.5 - above 6" nominal the actual is .75 less, so a nominal 2x8 is actually 1.5x7.25, a nominal 4x10 is actually 3.5x9.25, etc.
Note that for hardwood the nominal vs. actual are a bit closer, a hardwood lumber piece that is nominally 2x2 inches is (if surfaced 4 sides) actually 1.75x1.75, and a nominal 1x4 is actually 13/16 x 3.75, etc.
See also:
www.harvarddesignmagazine.org/issues/45/nominal-versus-actual-a-history-of-the-2x4
www.archtoolbox.com/lumber-dimensions/
@@matisseenzer2383 Good information there, thanks..!! As I mentioned in my first comment, I know that Pine dimensional lumber has a smaller thickness in the actual than the nominal. But with treated Pine, a 5"x5" piece is actually 5'x5", but a 4"x4" treated is 31/2"x 31/2". Both are treated Pine, so why the difference? Do you have any knowledge about this?
Very interesting!
For weather and rot resistance - is an end cap a good idea, or painting the end, or some other water barrier? And how often do these need to be inspected or replaced?
I'm a home improvement contractor along the south Jersey coast. I do alot of deck construction. I never put a cap on the bottom end of a post because over time it will collect moisture which in turn allows the end grain the absorb water through the end grain of the post/column. I usually seal the bottom end in a heavy coat of wax. The top of the post will usually be capped if the cap can roll over the edges. To water can't penetrate the end grain.
Excellent job. And for the Americans out there, 89mm is 3.5” and the author of this video correctly called out a 4x4 nominal dimension like we use in dimensional lumber
Where I'm from dimensional lumber (planer guaged) is 90x90mm.
98x98 mm is the dimension used in Europe.
This video was awesome. I am a farmer. This is great info for framers to know. Please make more videos that explain wood frame engineering.
Are you a farmer or a framer? Do farmers do framing work from time to time? Do framers sometimes farm? I need to know 😃
Would love to see a chart that continues the load capacity of column at 4, 5, 6, 8, 10 meters just to see how the curve changes
just chug thew equation into excel
New subscriber from Davenport Iowa 👍
Welcome aboard Charles, we are glad to have you!
Absolutely amazing 🥳🥳🥳
The number of people who don't know how structural lumber is sized is staggering.
Thanks so much
I' m just an electrician but find this vidio very interesting. Maybe 445 years ago eleven decks became popular in my area home owners never got a permit or inspection for deck they installed on their houses. Most only used two 4 by 4" post to hold up a 18' wide by up to a 16' deep deck. A few years later the city sent out inspectors and made them replace fluently undersized 4 by 4" post with 6 by 6" post which have about 250% more area. A lot of these decks were built over driveway that could easily be hit by cars parked under them.
I shudder when i see a balcony with supports on a footpath kerb.. all it takes is one Not careful driver
Was really hoping to see some posts explode in strength tests, or at least some images from failed posts in the field...
As an engineer myself, I can definitely relate to the sentiment of I love Engineering! Coding has been such an invaluable skill that I've been able to develop and see the world through a new set of eyes. Engineering is such an interesting and ever-changing field, and the sheer variety of opportunities that it offers is unparalleled. I'm truly inspired by the passion and enthusiasm I've encountered with my engineering peers. I'm sure the I Love Engineering channel serves as a great platform to showcase the limitless possibilities of engineering and it encourages others to pursue their dreams as well. Just awesome! Keep up the great work.
Excellent presentation!
The thumbnail demonstrates the load being distributed on the beams below - not just the timber post. Supporting the argument: In most situations, the timber post is just for support, and the load barring can mostly come from the material of the building.
Well the beams are attached to the columns so the load path goes through the column from all the members above it. Not sure what you mean by "just for support"
So what this tells me is that almost Every deck that've seen on a hillside may fail prematurely. I see 12 ft and even 16ft 4X4s regularly, all with no diagonal supports (because those are ugly and cost more). Look out below!
Girts n purlins, take a cue from timber framing. And from the bracing used with floor joists. Ugly? So is unexpected failure.
16ft 4x4 holding up a deck on a hillside? I'm no engineer, but holy crap, how can someone with any intuition look at that and think it's strong enough.
That chart says a 3m x 89mm x 89mm can take ~4.5t of load, and code load is 2.25t. Given that your typical deck has quite a few of these underneath, sounds like there is way more than enough load capacity. How heavy does anyone think a deck would be? 20 people standing on it would only add something around another 1.5t ie way less than the conservative Code rating for a single post.
This makes me wonder whether I could use the same volume of wood in a 4x4 post of given length to hold up more weight if it were split into multiple members even if joined by only wood interlocking joints; i.e. no additional fasteners or adhesives. I feel like I definitely could for longer 4x4s especially if I don’t have to subtract blade kerf and drilled holes, but just have the final volume of wood in the structure be equivalent.
I mean yes, but who has time to select wood members that have no blade kerf and drilled holes and interlock them to a long post xD. So you'd rather just take a 1cm larger post and be practical about it. But I like the thought experiment nontheless :D
Congrats on making a big stump. Yea, super stronk. Do it a few more times and you got yourself a tree. LETS BUILD HOUSES OUT OF TREES! Man, this wood is heavy! Maybe we should cut it.
If you have enough lateral bracing then you have reduced the unsupported vertical distance (between braces) then the load capacity does increase compared to no bracing because the length that is subjected to buckling is shorter.
And a lot more varibales. Just started the video. And non of the main points ive already considered were mentioned. Not sure if theu will be adressed later in the video or not.
But, they type of tree the timber post came from is on main factor. Not all woods are the same or have the same properties and fiber structures and grain types. There are a lot of very weak, medium, and strong veriations in the wood of trees. Alao it depends on which direction of the post beam or timber you are expecting to cary the load. Trees grow vertical and are used to carrying the virtical down force of their own weight . If you use a timber as a vertical post and ypu plant it with its natural base end up then it wpnt be as strong as if you put its natural base aa the base. That working with the natural properties and how that tree the post came from grew. Also sometimes there is a big gifference in strenght of a beam as to which side is up or down and where the loads its carrying are pla ed. In most construction with post beams and timbers. The size of them far and their robustness of streanth far exccwds the application they are being used for ao builders dont pay any attwntion to these fa tors. And there are plenty of othef fa tora ro consider also. This is just a few that sprang to mind in an instant to me. Lol hope its a vauable contribution. Thanks.
The old time carpenters didn't have the math skills to calculate that, but the had the necessary knowledge to know exactly the load bearing capabilities of the wood they used.
.... proven by centuries of safe use
The length of a TIMBER in use should never be over thirty times its minimum thickness. Thus, a 4-inch x 4-inch shore can be ten feet long, a 6-inch x 6-inch shore fifteen feet long, and a 4-inch x 6-inch shore ten feet long. If the proportion of length to thickness is greater than thirty to one, the shore will buckle ...
For curiosity sake ? and not for something over 6-8ft tall, will using a metal "collar" or brace, midway on a 4 by 4 or even reinforcing with a wood brace help it support more ? & possibly help "distort" the bowing ? I have worked in the trades mostly with metal But also built MANY things,out buildings from reclaimed materials! The smartest thing I discovered by accident was using rail road & sign materials were already tested to stand up to weight and wind pressures:) thanks makes me want to still be building stuff:) Rick
Hi Rick! Bracing the post at the midpoint will help significantly. The post will behave like two individual posts with half the length of the original post. As we saw in the video, shorter columns are much better at resisting buckling. Similarly, if you see some old railroad wood bridges, they are braced regularly with cross bracing, which helps handle lateral loads better but also prevents buckling of the long wooden columns exposed to the heavy steam engines. See image: previews.123rf.com/images/rgbspace/rgbspace1807/rgbspace180700003/106438106-vista-del-puente-ferroviario-de-madera-de-caballete-kinsol-en-la-isla-de-vancouver-bc-canad%C3%A1-.jpg
Yes, if you can find the Euler formula again, the presence of braces is accounted for in the modifier "k" that is multiplied by the total length "L".
any beam you can increase its load capacity using boards nails the sides at a angle like this / on both sides of the beam.. My pole barn was built like this and so was my deck which uses 15 foot 4x4 posts
6:17 where can I find same table for cnp steel. what I want exacly like that. ex with cnp 7x3cm 1.6mm 1m length can handle load X kg
I was hoping to see an actual loaded column exploding under the pressure.
Me too.
Why not explain the difference between nominal and actual dimensions, seeing as 89mm = 3.5"
Hi tre bushett, yes in hindsight, we should have said a couple of sentences about that as well. Sometimes we tend to take things for granted when they are not as obvious. With most dimensional lumber, the actual sizes are 1/2 inch smaller than the nominal sizes (some 3/4 inch smaller). The short answer is because of the planing process. A slightly longer answer is available here: www.popsci.com/two-by-four-lumber-measurements-explained/ Thanks for bringing this up!
The nominal vs actual situation is just ridiculous. It's not a massive inefficiency, but it's definitely an annoyance.
@@TheEngineeringHub When you have a sawmill and a forest: We don't got no "nominal" crap. We don't need no stinking lyin' lumber dimensions!
Didn't I read somewhere that wooden roller coasters are safer then steel ones.
I noticed the CISC steel design guide in the shots of your desk... hence I think you are likely a canadian engineer
That's a very good observation drblitzzz!
The first illustration incorrectly identifies the post as a 4inx4in while a nearby notation identifies the post as an 89 x89.
The post in fact is 3 1/2” x 3 1/2” and also 89mm x 89 mm. The post is commonly referred to as a 4x4. Please correct.
Was there an explanation why a 4” x 4” post was equivalent to 89mm x 89mm? Are these “freedom” inches by any chance?
Hi Citizen, for most dimensional lumber, the actual sizes are 1/2 inch smaller than the nominal sizes (some 3/4 inch smaller). The short answer is because of the planing process. A slightly longer answer is available here: www.popsci.com/two-by-four-lumber-measurements-explained/ Thanks for bringing this up!
A 4x4 post measures 3-1/2 inches just like a 2x4 measures 3-1/2 inches by 1-1/2 inches thick. That equals about 89mm. The video is correct.
@@TheEngineeringHub My father who sold lumber and built homes for a good part of his life always considered nominal dimensions to be akin to fraud, sort of like what has been obscuring inflation for a long time in consumer goods: smaller quantities and sizes for goods, instead of maintaining sizes, etc, and raising prices. Clorox used to come in 128 oz (1 gal) jugs. Now, 121 oz. Tuna fish cans are probably the oldest story of near fraud changes that the packers hoped no one would notice (or for this group "wood" notice?).
Dimension lumber is originally sawn to the nominal dimension and then "dressed" to the smooth shape you buy at the lumber yard.
Below 2" they shave off 1/4" in the dressing stage; 1/2" when the wood is nominally for 2 to less than 8 inches; and 3/4" for dimension lumber eight inches and greater.
At some size, the name goes from "lumber" to "timber". Timbers are the largest sections. But today it is more economical to make large cross sections as gluelams instead of harvesting giant trees.
@@TheEngineeringHub most... meaning new 4x4's you buy in lumber stores.. but in reality 4x4 is 4x4 and 6x6 is 6x6 if you get lumber from real saw mills or have very old structures... for example my pole barn built 50 years ago the 6x6 beams are actually 6x6 and the 8x8 are 8x8.. but the replacement 6x6 is like 5 1/2x 5 1/2... kind dumb,, but oh well
What safety factor are the manuals using? I read where the ultimate strength of a SYP 2 x 4 was in the 8 ton range which makes this fir calc seem low.
The safety factor is distributed over many parameters on the resistance side and additional safety factors on the loading side (increase of the expected load). The safety factor is slightly different for every scenario, depending on the size of the member, location in the structure, environmental conditions, loading type etc. On top of this, the reported strength is the 5th percentile strength which means that if 100 specimen are tested in the lab then the strength of the 5th weakest will be reported. In other words, 95% of the specimen will be stronger than the reported strength. All these conservative assumptions sum up to a conservative design. My guess would be that the SF ranges between 1.5 to 2.0 but again it is situation specific. The values reported in this video are for ''default'' conditions. Lastly, as a disclaimer, the high variability of wood makes it important to have large safety factors that could account in a case of an ''unlucky'' selection of a bad piece. Do not ever rely on the hidden conservative assumptions and design an overloaded the members!
@@TheEngineeringHub OK, I guess with an SF of 2 this would make what I heard about right with a an ultimate compressive yield of a 4 x 4 to be in the 16 ton range. Obviously, it would all depend on potential buckling of course but is a good place to know where your maximums could be.
@@kenbaustin4533 That sounds reasonable to me. I would highly recommend this reference: www.taylorfrancis.com/books/mono/10.4324/9780203477878/timber-dinwoodie
for more info on many properties of wood. For example, on page 155 (Table 7.1) you can find the compressive strengths for small clear samples of different species. Based on this, small, knot-free samples achieve much higher average strengths. But of course, we can't use the mean value since that would mean we over-estimate the strength half of the time and we assume no knots. Another example is Figure 4.2 which shows the relationship of strength to moisture which is another modification factor in the manual. This book is an absolute gem! If you are interested but don't have access to the book send us an email!
@@TheEngineeringHub 10/4...... appreciate your time and input!
There is no way I am going to understand all of this but I do have a question. If I was to sandwich plywood ( glued & screwed ) between 2. 4x2 to make one post would that be stronger than just a 4x4 post.????
The plywood would be negligible in strength but the extra space between the 2xs would make it a little stronger in the case of buckling
so smart calculated for 4x4 instead of 3.5x3.5
Im planning to make a 20x23 ft bungalow house..6x6inches wood column 12feet in height, 10feet post to post distance in the perimeter..no post in the middle, almost a long span truss design..hope it works..
Good luck
You may have to hide an i beam in there for safety
That depends on the wood. Spruce, Hemlock, Fir, Oak, Maple, they all have different load capacity.
Yes absolutely! As mentioned in the video, the assumption is that the member is SPF lumber of a standard grade. That is the wood compression capacity parallel to grain is 10.8 MPa. This is, the most common type of dimensional lumber in North America.
Some Australian Hardwoods select grade can be F34. For the record, nails bend! Have to pre-drill holes for nails removing the drill bit about 5 times to clean it for 3.5mm dia 80mm long hole.
WD40 on the nail to hammer it in.
@@tonydoggett7627 Oh, you guys cheat. Shame on you. Soap works much better. Just kidding. Oil soaks into the wood over time, stops rust too. Just sayin, should be an oversize nail, better yet a screw. Now the soap comes into play. G'Day Mate.
So I should be ok with a 2m square playhouse built on 6 x 89x89 posts 1.5m off the floor then?
The 4x4's I buy at the lumberyard measure 3 1/2 x 3 1/2
A 4x4 (3.5”x3.5”) It’s not called a column it’s a post when it’s nailed in vertically and a beam when it’s nailed in horizontally! When it’s framed in above a door or a window it can span up to 3Ft by Code over 3 Ft you need to use a 4x6 or sister two 2x6’s together up to 6Ft. Over 6Ft - 10 feet you need two or three 2x8’s sistered together.
The term "column" in this context is an engineering term: it refers to a structural element that supports vertical loads without lateral bracing. So if you have a 12 foot 4x4 post but it is restrained in the middle then from an engineering point of view you can consider it as two 6 foot columns stacked one atop the other.
Depends on length size of knotts
If we assume complete lateral bracing, isn't a longer post stronger than a shorter post?
In that case, the column would be governed by material failure which should be equal for both cases.
@@TheEngineeringHub In a longer post, isn't there more wood being compressed? Like, a thick concrete slab can bear more weight than a thin slab. A big rock can bear more weight than a small rock, all else equal. Amount of material counts for nothing?
@@Heraclitean In case of slabs, they are loaded in bending which means that the thickness would increase the load capacity. In case of a column, the situation is different since they are loaded in compression. The governing factor for a laterally braced column under compression is the cross-sectional area (and not the volume). In other words, the material at the bottom of the column does not help the material above it. In fact, longer columns would be slightly weaker because the bottom of the column has to carry the applied load + the weight of the column above it. This is not critical for wood because wood is a light material but in theory it is the case that longer columns would be weaker even if properly braced. If you are still confused, send us an email and I would be happy to send you a better explanation including diagrams since I can't post pictures here.
@@TheEngineeringHub That's very clear. Thanks for the explanation.
4" is 101.6mm - 89mm is 3 1/2". Where I live 4" is not 3 1/2". A bit like the Imperial gallon compared to a US gallon termed a short gallon. In the case of the 4" post, 25% less material, could it be called a skinny post. Cheers
Almost all dimensional lumber is reduced by 1/2" (some 3/4 some 1/4) due to the planing process. Across most of North America, a 4 by 4 is 89 by 89 mm.
@@TheEngineeringHub This is true. I wonder where the OP lives. Here a 2X4 is actually 1.5 X 3.5 , and so on.
@@paulhopkins8148 Maybe he lives in 1924
I need to calculate the load of a 12 inch by 20 inch span of plywood 1 inch thick. However, I am really not sure how to mathematically calculate it with a formula.
You're looking for the weight? 1 inch thick plywood is generally 3 lbs per sq foot. In your case, that's 5 lbs.
If you're interested in the deflection, there are online calculators. For a span that small, you could just experimentally try it. Support it on both ends, and then add a known weight to the center. Measure the deflection, then add more.
@Shigg McDigg The online calculators for calculating the maximum load were all over the place. The most credible one to me was 1900 lb. Indeed, I finally found a video by a channel called SV Seeker (restores boats, which is my best guess) who had a similar span of regular and marine plywood. He put bars of lead in the center until it broke.
As I recall, making adjustments for the 2 inch difference between his setup and mine, my plywood would break just north of 1600 lb static load pushing on the center. This was roughly the same for marine or regular plywood.
I think we need a 10x overhead to get a working load. It is vague and depends on the distance that someone might jump onto the center, which is vague. ( I estimate 1000 lb would be the most force on a misused system).
My plywood actually should not need to support a center load (it is for a type 3 ladder that puts force closer to the edges, not not to be stood on). Nevertheless, I might add 2 or 3 supporting 2x3 on the underside to beef up the load to 3000 lb, incase someone 300 lb decides to jump a meter or more down onto the center of the 20 inch plywood span, which should easily hold 12 times my weight, even if I misuse my platform.
I could add metal channels to the side, but think adding 3 little support boards to the underside would do the job and be easier and cheaper.
The similarities outside of the math, to retention walls, is interesting. A shorter retention wall, is better, even if you have to stair stack them. (tier). whereas the load behind a wall, is kicking out the middle of the wall.
Short version: a standard SPF 4x4 can hold aproximately 2 tons, rounded down for saftey.
I used to work at a glulam place called structurelam so I know about timber frame things.
A timber post CAN be used as door and window lintels as our house demonstrates
Rot at the base of a wooden support column is a disaster waiting to happen. Water absorption from a concrete base can cause both wet and dry rot. Check your patio support columns soon.
Load bearing capability depends on the type of wood used, obviously hard red woods will have better load bearing than white wood such as pine.
color means nothing
As a carpenter for 47 years, loads are influenced by clear wood, knots,and defects. Also old growth or today's garbage. And of course species of wood. Hardwood, softwood. Also where said tree grew,hillside or flat land. And twist in trunk or straight growth in trunk. And lastly cut of the post.
My house, built in 1836 uses 3x4 studs and 3x9 joist. But of course it's timber frame with huge sills, corners and plates.
I always see those houses perched on 4x4's and just wonder how in the world that got approved. They're stronger than I thought, but long 4x4s still make me very nervous.
a 4x4 that can't buckle will hold up many tons if kept dry and insect free
It all depends what species of timber you use, and how it was grown.
In my country (New Zealand) the nominal size for 4x4 planer gauged construction timber is 90mm not 89mm. I guess we build them stronger here. ;)
@William Smith I guess it went over your head, but the 6” is larger too.
@William Smith Hey it’s not your fault metrics confuse you.
@William Smith I thought skating was more a Canadian thing. But up there they’re so confused they can’t even decide which measurement system to use for what.
@William Smith Plenty of construction work here and still lots of money being thrown at developments. It’s been an absolute boom last few years which has magnified any materials shortages caused by relatively short covid lockdowns. Lots of blame laid in different places. We have an acute shortage of drywall which is manufactured here. There was a knee jerk reaction by some huge construction players who over-ordered for jobs & developments a whole year ahead which wreaked havoc on manufacturing production forecasts.
if you put 1/8" grooves in the side it will make it much stronger
I liked you video on this, however, there is one thing I would like to point out. It might seem like semantics, but columns and posts are not interchangeable. Although you do somewhat make a distinction, the video is rather vague about it. A post is a structural element that weighs less than 300 us pounds and is either axially loaded only or is laterally restrained be a member above. A column is a vertical member that does not coincide with those parameters. The connection requirements are significant in difference, for example, a steel post requires only two bolts to the foundation wall, while a steel column requires 4 bolts to a foundation wall. There are more differences not listed in this comment, but that’s the gist of it. Our company had to change our wording from column to post on behest of our insurance company, so it does make a difference in liability.
Hi Bbdest,
Thanks for your comment. We agree generally that there is a nomenclature distinction between post and column. What jurisdiction do you work in that provides this specific limit of 300 pounds etc. ?
The 300 pounds comes from the Code of Federal Regulations, subpart R (steel erection), 29 C.F.R. 1926.751. The definition of post is located in this section.
So this is part of the US government regulations. Of course, this means that outside of the US, this doesn’t apply
When you design a structure such as a Covered deck you have to consider uplift from wind loads not just compressive loading. It's called TENSION. And just using Pin connections as input will produce failures. Never assume or use Pin connections in Timber Framing analysis .
the tensil strength of most wood is insane. it will always fail in the connections or from side pressure excluding knots
Cool video. Seems like you have spammers in the comments.
Depends on the timber - a fast grown less dense wood will hold significantly less and for shorter time than a slow grown dense wood of the same species.
4 in = 101.6 mm, not 89 mm, or have I missed something in the conversion?
You are right Daniel 4in is 101.6mm but a 4in by 4in wood post is actually 3.5 in by 3.5 in and therefore it is 89 mm
@@TheEngineeringHub Thank you. Is this somehow considered in the load capacity of the post, e.g., by a reduction factor? In terms of area moment of inertia, it makes a big difference.
@@danielrodrigobarreto yes all of the section properties are calculated for an 89mm by 89mm post so everything is fine in that sense. It is just the name that is disconnected from the actual dimensions (super confusing I know). Back in the days, the mills produced dimensional lumber that was the actual dimension as the name suggest. But now due to the planing process and shrinking of wood, almost all dimensional lumber sections have been standardized to different dimensions than what the name suggests. Usually the actual dimension is 1/2 in smaller but in some larger sections it is 3/4 in or in smaller sections 1/4 in smaller.
Stimmt alles was du erzählst.👍👍👍🇩🇪
Nominal dimensions in inches, but actual dimensions in metric? First time I've seen that.
Hi John, it is fairly common to express the actual dimension in mm (meteic) when designing timber members. A summary of common dimensional lumber sizes in both imperial and metric can be found here: www.archtoolbox.com/lumber-dimensions/
Metric countries most likely use the actual finished measurement. Nobody is going to start calling 4x4s 3.5x3.5s. The current system has been perfectly fine for over 100 years.
@@actionjksn I removed some studs during renovation on a 1924 build in Richmond VA years ago and they were FULL dimension. My dad the retired lumber dealer was thrilled!
Although I'm certainly no engineer I think starting this out with different species of wood and different grades are going to be a huge factor in load capacity. Coming from the Northeast United States I can tell you a knot filled piece of white pine has no where near the strength of a clear piece of white oak.
That's already considered in the Young's modulus (E) of Euler's equation, stronger woods would have higher E.
6:00, for viewers inside of North America, 1 inch is equal to 25.4 mm, meaning 4 inches equals 101.6 mm
Yes, but a 4 by 4 timber post is still 89mm by 89mm.
@@TheEngineeringHub you're getting ripped off.
How would
It's something referred to as nominal vs. actual dimensions. Go to a hardware store and measure it you will see or check out the link www.archtoolbox.com/lumber-dimensions/
@@TheEngineeringHub then it's up to you to specify what you want. Otherwise calculations for 3 1/2 inch must be used.
If you owe me $4, $3,50 won't cut the mustard.
@@bodgiesteve8849 Calculations for 3 1/2 (89 mm) inch were used and are always used. That's why it was written 89 mm. That was us specifying it. All engineers, architects and designers know this. All of new construction in North America is done with planed dimensional lumber whose actual values are less than what the name suggests. Though the name stuck around and are still called the old names from many decades ago when a 4 by 4 was actually 4 inches.. but not any more and it hasn't been for a long time. Same goes for a 2 by 4 which is 1 1/2 by 3 1/2 inches in reality and so on. You can read here why: www.popsci.com/two-by-four-lumber-measurements-explained/
Is buckling really a 'sudden' failure? Granted it can be but in my experiecne it can also be a very slow defect that gives plenty of time to rectify before failure. This is the beauty of wood. Catestrophic failures aren't normal, unlike say cast iron.
Buckling is considered a sudden failure because once the column begins to buckle, the effect of adding more load to it is substantial. You're right, many wood columns do not suddenly fail in residential applications, but also designing them as if they can handle more load once buckled would also be a huge mistake
That is probably a bow, potentially a bow made worse by the load over time. If the post had really buckled, that would be a sudden failure mode. One caveat is that you may have seen a post that failed in buckling, but was somewhat redundant with other support systems such that as the post got shorter due to the buckling, it simply stopped supporting much weight. I would still say that post had "failed".
Thank you for the great explanation Jackson! I would just add that the load over time phenomenon is very important in wood structures because it is accompanied by creep that could cause slow bowing of the post. In my opinion, the failure mentioned in the original comment is due to creep and buckling has not set in yet. The redundancy explanation by Jackson is also very likely and could be the case. Thank you all for the great discussion!
4inchby four inch is 25.4mm per inch is more equivalent to 100mm x 100mm
The one thing every formula can not foresee.....TIME AND VARIABLES
You missed an important point with wood beams, the different woods have different strengths. The commonly used pine is not the strongest wood available.
The assumption was stated as an SPF specimen with a compression strength parallel to grain of 10.8 MPa. Of course, other species would have different strengths and, therefore, different capacities. We took the most common species found at a hardware store for our analysis.
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