How to do a timber beam deflection calculation

Here's how to calculate the amount of deflection in a timber beam. After you have calculated the strength of your timber beam, joist or rafter, you need to do a timber beam deflection calculation to check if it will deflect more than the allowable amount. Even if your beam is strong enough, it may still deflect too much! In this easy to follow tutorial I take you step by step through a worked example, explaining what all the formulas and values are, so that you can easily carry out your own timber beam deflection calculations.
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[@RobindeJongh]

✅ Download the tables used in the video here: geni.us/timbercalculation

[@martinsmith3831]

Well done Robin - very clear and helpful. It's been over 45 years since I used my civil engineering knowledge and just needed your great videos to get me going!

[@RobindeJongh]

Hi Martin - glad to have helped get you started again!

[@discostuart1981]

Great video....thanks i was one of the folks asking about this

[@danielmaloney9311]

Cheers robin, finding your videos extremely helpful for my current work after just graduating last year. Would love an example for pad stone sizing on beam bearings if possible !

[@RobindeJongh]

Thanks Daniel. Great suggestion!

[@davidreid2301]

Just what I’ve been looking for to refresh my engineering studies of 25 years ago! Thank you so much for sharing this!

[@RobindeJongh]

Hi David - you're very welcome!

[@davidreid2301]

@@RobindeJongh Hi Robin, can I ask, what is the load figure in Kn/M for a slate roof with an average of 430mm top chord centres, roughly? Do you have a download PDF for that? Sorry if that's a big ask!

[@davidreid2301]

Also, sorry, but I think you might have made a little mistake on either the video, or the PDF, for the 2nd Moment of Area. You say cubed in the video, but it's quartic in the PDF. Which should it be? Sorry to be a pain!

[@RobindeJongh]

@@davidreid2301 Hi David. Well spotted! You are correct - I wrote the units as mm3 instead of mm4 (it should be mm4, although this does not alter the calculation).

[@JS-zi5gt]

Hi Robin, great video, I'm trying to calculate which timber size & vertical spacings / centres I need to support an external vertical timber wall, wall weighs circa 10kg/m2, horizontal span / centres between hot rolled 533x210x101 universal beam is 5670mm.

[@travelzxc0984]

Hi Robin, if I have a 2 storey timber house, what is the step by step to design its columns and beams? Thank you😊

[@muneeraltiti9387]

Hi Robin I have Meranti timber how can know which strength class is it and the allowable stress and the other characteristics Thanks

[@jonathanharrison2240]

thanks a mill :-)

[@RobindeJongh]

You're welcome!

[@ankzuy2532]

thanks for the helpful video! just one question: 1:19 what is the 5 for? is this a constant number for every deflection calculation?

[@paulcurtis3888]

Thanks Robin, I've found your structural timber videos very interesting. I want to use a timber lintel/beam above some 3m bifold doors on a 5m x 3m garden room and was wondering if the 0.86 kN/m figure would be suitable to calculate any deflection due to the load from the roof, or would I need to use a load figure closer to 2, as in your recent ridge beam video? I could go up to 225x50 doubled up for the lintel/beam. The roof will be 150x50 joists @ 400c in an 18mm OSB/EPDM cold flat roof construction spanning 3m front to back, with a 600mm overhang at the front. Any advice would be greatly appreciated, thanks

[@RobindeJongh]

Hi Paul. You can calculate the load based on the method I show in the ridge beam video. However, for a bi-fold door you may need to limit deflection to a lot less than the 14mm specified in the british standard, especially if it is a top hung door. For a top hung door you will also need to take into account the weight of the bi-fold. For peace of mind you may wish to look into a steel beam, and (if you're quick) enrolment to my course is open for another day or so: geni.us/beamcalculations

[@salikhanan]

I have two timber horizontal in the wall with RawalBolt, Now I want to give it a suport with steel underneath the timber to give extra support so that I can build a ceiling, what would you suggest? should i give extra support with steel joist or double the amount of rawl bolts? WHat I am trying to do is, I am trying to attach two timber 47*150 mm to the wall opposite to each other and then hang 10 47*150mm joist on those two timber with the help of the hanger and then cover it with edge osb. but someone told me that either I should double the number of bolts or give a support with steel bar as explained above but my joiner thinks there is no need for it. any Sugessions? Thanks in advance

[@kentaylor9811]

Is there a video about construction of a girder truss ?

[@waynepettipas895]

Robin; I’m designing a trussed beam to span 22’ using full size 2x4 lumber. The truss is a Howe truss with 18” panels, 18” deep. the loading is 10 lb for dead plus 40 lb for snow. Any thoughts on the analysis? The truss is for the front opening for a shed housing a small saw mill (the reason we need such a large front opening).

[@RobindeJongh]

Hi Wayne. My thoughts would be to pay particular attention to the connections - in timber design they can be what drives the chosen section sizes.

[@dickqwerty100]

Thanks for these videos Robin, I really appreciate the way you go through the calculation. I'm a little confused though - in your video 'The EASY way to do a timber beam calculation' at 5m58 you say that KNM = Nmmx10 to the power of 6 while in this video at 2m33 you say that KNM is the same figure as Nmm. The figures work in both calculations but I wanted to understand what I'm not getting!

[@RobindeJongh]

Hi. In this video it is kN/m, which is a load per m. In the other video it is kNm, which is a moment.

[@J-WHO312]

This is with load on top of timber, what if load hangs from timber is it the same formula? Thanks

[@RobindeJongh]

Yes, it's the same.

[@bsullivan1264]

Hi Robin, I must have missed it but is the 384 a constant. I'm not sure where that comes from. Great video by the way.

[@RobindeJongh]

Yes it's a constant.

[@LvPatriot1]

Can you neglect creep in timber beam deflection calculation?

[@RobindeJongh]

I believe in BS5268 it is taken into account already by using the "Long Term" loading condition. In Eurocode you would need to take creep into account.

[@DeclanLestat]

Hi Robin, bit confused wondering if you can clarify please. All your measurements are in mm to the 4th power, but your answer isn't. What happens to that 4th power and why? Thanks in advance.

[@xxwookey]

The units for the calculation is (N/mm x mm^4 ) divided by (Pa x mm^4), which resolves to Nmm^3/Pa.mm^4, or N/Pa.mm. Pa (pressure) can also be expressed as N/mm2 so resolving, we get to N.mm2/N.mm which is mm, which oddly enough is what the final answer is in. I agree it's not all that obvious :-)

[@liamodonnell9091]

Great video, doesn’t timber creep though? Doesn’t a modified E value need to be used for long term deflection?

[@RobindeJongh]

Hi Liam. With Eurocodes creep deflection is considered, but with BS5268 it's not specifically calculated. You should also consider shear deflection, but often this is insignificant, so I didn't cover it here.

[@liamodonnell9091]

@@RobindeJongh Many thanks, are you able to do videos using Eurocodes as this is now widely used/ being requested. A similar video looking at design using Eurocode would be great if you could do that.

[@RobindeJongh]

@@liamodonnell9091 Hi Liam. My view on Eurocodes is that they are generally over complicated and therefore more dangerous to use for newcomers and students. They appear to have been written for use in software programs. How do you find them?

[@liamodonnell9091]

@@RobindeJongh I agree I’m not a fan. I started my career with Eurocodes and had to go back to BS to understand the Eurocodes. Sometimes EC is impractical or leaves it open to interpretation. Thanks for the response.

[@ComeWhatThey]

If you're using multiple bits of timber bolted together to make up a beam, should you add together the second moment values in the table? Also, in the video you say I is x10^6mm^3, but in the table the units are shown as x10^6mm^4. Which is it as I imagine this would make quite a difference.

[@RobindeJongh]

Hi. Just a "typo" -- it should be mm^4 for value of second moment of area. For multiple pieces side by side, add them together.

[@pearse500]

@@RobindeJongh Is that another typo? as in you meant to type mm^3 above?

[@pearse500]

Looked up some other videos and it looks like mm^4 is correct and the typo was in the video and not the tables just to be clear

[@Yo_dream.]

What this 384 does?

[@RobindeJongh]

The 384 figure is just a constant.

[@waynepettipas895]

Sorry forgot to mention it is a shed roof.

[@johnomija1876]

Wrong units on I it's mm⁴