Rank: Forum user 
|
Hi - Happy new year.
We have in-place at one of our factories, racking constructed of welded box steel & plywood. This racking has been in use for many years without any concerns. It is used to hold machine jigs etc.
My question is how would you go about establishing the safe working load to display on each rack?
Regards Paul.
|
|
|
|
|
|
Rank: Super forum user 
|
That could be quite difficult. Do you know if it was a standard off-the-shelf product or bespoke?
|
|
|
|
|
|
Rank: Forum user
|
No not off-the-shelf, its been made in-house some years (decades) ago & has been used for the purpose it was made for, without issue, ever since.
|
|
|
|
|
|
Rank: Super forum user
|
Paul
I think you are correct to question this. A number of years ago I was part of a team that investigation into an accident involving what sound to be similar racking as you describe. This was in a warehouse and it two had been used for years without problem, until the day it failed. Sadly when it did fail it killed someone.
There are many companies out there that would be able to offer you assistance, try putting racking assessment or similar into an Internet search. To post a company name here would be against the forum rules.
|
|
|
|
|
|
Rank: Forum user
|
I have been told that by self load testing (adding weights to the rack), photographing & documenting the result, you can then give that rack a SWL, beneath the weight applied.
The racking we have in use is less than 6ft high & is confirmed by the storeman for overall good condition each month.
|
|
|
|
|
|
Rank: Super forum user
|
Google 'rack load testing' and you'll find lots of companies offering this service.
|
|
|
|
|
|
Rank: Super forum user
|
Try this document
http://www.tecotested.co..._plywooddesigncapacities
Given the plywood will have a lower load bearing capacity than the box steel section, and is likely to be loaded as a simple beam. You will probably need to use the Engineers Bending Equation.
Measure the dimensions of the plywood in your racking.
You will also need to weigh the various items, jigs, tools etc that you store on the racking to determine a 'worst load' case.
You should then be able to come up with a reasonable estimation of the loading on the plywood and hence work out the stress.
You then put in a safety factor of maybe 2 or 3 etc to reduce your permissable load on the racking.
Any half decent mechanical engineer should be able to do the calculations for you.
Go on do it.... the maths isn't that difficult.
|
|
|
|
|
|
Rank: Super forum user
|
I wouldn't recommend a trail and error load test, as recommended by others on this thread.
PM if you want some assistance.
|
|
|
|
|
|
Rank: Super forum user
|
Paul
I am no expert in welding - the team I mentioned before contained one - but as understand welds they can give no indication of stress loading until they fail with catastrophic results. This was exactly what happened in the case I looked at. I may be teaching grandmother to suck eggs if so I apologise, but different materials have to be welded in different way I believe. The plywood may not be the week link.
|
|
|
|
|
|
Rank: Super forum user
|
As already stated by 'safety amateur' the welds and other fabricated elements are probably the weakest link so when you talk to a specialist, as has already been suggested, make sure that they know about fabrication as many do not
|
|
|
|
|
|
Rank: Super forum user
|
Accepted we don't know much about the quality of the welds.
But it is very, very unlikely that the weld joints will be weaker than the plywood.
Unless the racking is in a pretty cold environment, welds don't usually fail with out warning, they tend to crack first. Brittle failure is unlikely, which cold conditions contribute to.
As part of the racking inspection, I would recommend that any paint etc is stripped from the steel box section and the welded joints visually inspected.
You could also undertake dye penetrant inspection, NDT testing of the weld joints.
I would be more bothered about any impact damage to the steel work, caused be forktrucks etc, as this will reduce the load bearing capacity of the structural members. Eulers buckling equations anyone?
What would I know - a mere design safety engineer, having studied structural engineering/stress analysis to degree level. I Eng qualified
Forgot to add earlier, if it can be determined that the loading/stress is already sufficiently low for the dimensions of the plywood, it may not be necessary to add in an additional factor of safety.
|
|
|
|
|
|
Rank: Super forum user
|
Another simple way would be to go to a DIY store that sells construction plywood.
I would hazard a guess, that recommended loading limits are given in the sales data etc or would be available if you asked for it.
Or, take a look in Building Regulations, again there will be some guidance for wood used as flooring and acceptable load limits etc per sq metre etc.
You will obviously have to ensure that any data that you extrapolate is applicable to your circumstances.
|
|
|
|
|
|
Rank: Super forum user
|
JJ Prendergast wrote:Another simple way would be to go to a DIY store that sells construction plywood.
I would hazard a guess, that recommended loading limits are given in the sales data etc or would be available if you asked for it.
Having worked in the DIY sector for a very long time I would say you are being very optamistic at best.
|
|
|
|
|
|
Rank: Super forum user
|
Safety Smurf
Having worked in the DIY sector for a very long time I would say you are being very optamistic at best.
Fair enough, but it was only a guess/suggestion. I still think if you asked, the information would be forth coming, or if you could establish who actually manufactured the plywood, contacting the manufacturer for a data sheet about the plywood.
I would guess that construction plywood is manufactured to a standard (Britsh Standard or otherwise), and probably has a recommended loading limit etc.
You just have to ask and do a bit of research, I think.
Bob
Are you suggesting that steel/welded joints are weaker than plywood.... a strange suggestion
Provided the original welding was done to even a half competent standard, then the plywood will be the weakest element.
Some dodgy structural advice been given on this thread.
|
|
|
|
|
|
Rank: Forum user
|
As the racks have for many years carried the load they would be required to hold, would it be safe to assume this as the SWL & just apply 20% (?) more weight when testing & documenting?
|
|
|
|
|
|
Rank: Super forum user
|
paul-ps wrote:As the racks have for many years carried the load they would be required to hold, would it be safe to assume this as the SWL & just apply 20% (?) more weight when testing & documenting?
While it is a fair observation that the racks have been in use for many years and appear to be ok.
It would NOT be safe to assume that adding 20% extra weight when testing is ok.
How are the racks loaded - point loaded or with a uniformly distributed load?
The additional 20% may make all the difference to the failure of the plywood. You are effectively potentially testing to destruction, which is not the point of the exercise.
It is a relatively simple stress/load calculation if you know what you are doing.
One of your saving points is that the plywood is unlikely to suddenly fail if loaded gradually.
It will tend to yield first (bend). Giving you chance to remove the load, which I would assume you would increase in small amounts.
You could of course load as you suggest, but just make sure everyone is standing clear and there is no other vulnerable equipment in the immediate area. But this is not the engineering approach I would recommend.
After all, if you were to risk assess the task of loading the racking for your SWL test, would not one of your risk assessment control actions be to make an initial calculation of the loading capacity of the racking.
Is it reasonable practicable to make the simple calculation.... I think so.
|
|
|
|
|
|
Rank: Super forum user
|
jj - provided the original welding was done to even a half competent standard- I could not agree with that statement more - but assuming that it is from the information provided?
sorry but as I say had to provide evidence to the corner where this was not the case makes me very wary of diy jobs like this without knowing a LOT more about the people doing the work.
|
|
|
|
|
|
Rank: Forum user
|
The racks were made to a suitable standard in a high skill engineering workshop.
Plywood has lasted so far, but could be replaced & the welds have not failed in the last ?? years.
I am just after a practical method of assessment. The racks wont be replaced anytime soon & bringing in a company to test each rack is very expensive & wont be top of the list anytime soon.
|
|
|
|
|
|
Rank: Super forum user
|
Paul
I would say look up what the Building Regs say about wooden floor loadings.
Building Regs are free to download.
Then extrapolate from that.
Or as previous, find some data from a manufacturer of plywood about loading values etc
Brian
Its easy to cast dispersion and doubt. However showing you have the necessary understanding of the technical problem is important and the ability to know how to solve the problem and the underlying theory is also important. You have already said that you are not a welding expert.
I see no reasoned engineering argument for your view point. If you are qualified to comment on a structural issue, please do so. Or do we go back to Victorian engineering and just build something bigger/thicker because we don't know any different.
Paul is looking for advice on how to solve his problem, not just a h&s geek to shrug his shoulders and say 'Oh don't like the look of that' without justification and rational argument.
|
|
|
|
|
|
Rank: Super forum user
|
Paul.
If YOU are happy with the welds then I would recommend the following document.
Guide to Industrial Shelving Using Chipboard
It is produced by SEMA - (storage equipment manufactures association)
and can be found on their web site
http://www.sema.org.uk/guides.asp
Mods I hope that as this is a trade body I have not breached the forum rules and I stress I have no link to the association.
|
|
|
|
|
|
Rank: Super forum user
|
JJ
I stated from the outset where my experience came from - dealing with the aftermath of ASSUMING a weld was a good weld. Paul has now posted further information and you will see I have proved a link to information on the subject which is not about floor boards but shelving which I believe is more relevant. You may see no reasoned engineering argument for my view point fine but have you ever dealt with a situation where it goes wrong? In his initial post Paul gave no details about where this information where the racking was produced I was simply offering a word of caution not just shrugging my shoulders and saying I don't like the look of that. We are all entitled to offer different views, my initial response was if you are unsure to seek help.
I do not want to get into a game of health and safety one up manship I respect your professional opinion and would ask that you respect mine. I find your tone offensive.
|
|
|
|
|
|
Rank: Super forum user
|
Try these links, some info here
http://www.mrroofer.co.n...anual_Mar_2008_final.pdf
http://oak.arch.utas.edu...article.asp?articleID=51
Brian
Its not about one upmanship, its about providing competent advice. If you freely admit you know nothing about a subject - this topic, or any other, why try to give advice?
In design/structural engineering many assumptions are made as a matter of course as regards loading of structures etc, it may surprise you what is designed on the basis of assumption. But they are fully justified assumptions, using established criteria and vailidated equations and known material/mechanical properties.
I agree, more information is needed. But it is perfectly possible to come up with a reasonable estimate of the loading on these racks, given as it appears to be a simple loading model. Nevetheless the calculation, if carried out, would have some basis in recognised practice and criteria/equations.
|
|
|
|
|
|
Rank: Super forum user
|
jj I simply offered a word of caution about assuming a weld was sound. I personally heave dealt with one case where that assumption cost a life. The cupboard used as the "shelf" was intact after the weld failed sadly the persons head it fell on was not!
|
|
|
|
|
|
Rank: Super forum user
|
Sorry that should be chipboard not cupboard
|
|
|
|
|
|
Rank: Super forum user
|
I would also dispute if you could use the guide you mentioned.
Chipboard structure is different than plywood.
Chipboard is usually made by compressing chips of wood in a glue/resin, to make the finished product. Hence the name. It tends to fail in 'bits' unlike plywood.
Also chipbard would be not as good in a wet environment, as it can swell and separate - clearly we don't know anything about the environment in this case.
Plywood, is as you are no doubt aware, is made from various plys of wood glued together to make up the laminate sections and the 'ply' of the wood.
Because the grain of each ply runs perpendicular to each other the load resistance / response of plywood is likely to be different than chipboard.
|
|
|
|
|
|
Rank: Super forum user
|
Paul
I have sent you a personal message, if you want further help
|
|
|
|
|
|
Rank: Super forum user 
|
Brian H,
It seems the link you provided does not cover the description of the racking in question as I see it.
It seems to me to be an all welded structure designed and manufactured in an engineering company workshop almost certainly by skilled tradesmen and designers.
The link you have given is to mass produced racking almost certainly manufactured by semi or unskilled operators.
I don't think the two add up really.
OP, I feel that you should go back to the designers of your racking i.e. your in house people and discuss with them their even basic rudimentary design calculations.
Going back to 1983 when we designed similar platforms and racking we used our gut feeling and experience then verified this with structural calculation values, and, had the equipment manufactured by skilled tradesmen, we never had issues as far as I know, and there should be no reason to have issues.
If the work is done correctly then the plywood will be the weak link.
I feel that one must be careful when using the term DIY when it comes to engineering companies because their DIY is your bought product. Which you are happy to accept.
|
|
|
|
|
|
Rank: Super forum user
|
Paul skyrme
I agree with you. But to be fair when I used the Day term there was no information about where or how the racking had been produced that only came later.
I could not agree more that items produced in skilled tradesmen and designers are best, but in 25 years of health and safety work I have seen many tragedies and deaths caused by semis skilled odd job men,gifted amateur's etc and employers not questioning this type of work. Hence my caution to the original poster. Its a very fine line between genius and stupidity.
Also to refer to SEMA as "unskilled" I think is a little harsh. I think you will find the HSE LA Courts etc use their guidance in deciding what is reasonably practicable.
Unlike some I do not pretend I can offer an absolute answer based on the limited information in these posts. I freely admit where I know my knowledge is limited (is that not the true test of competency) and I hope that my post are of some help to people.
|
|
|
|
|
|
Rank: Super forum user
|
#27 Paul Skyme
Nice to see there are 2 minds thinking alike with this problem. A basic structural calculation is quite easy to do, once the information is available. I too thought the SEMA guidance is not suitable for this application. Also that the plywood is the weakest link and not the box steel/welds - how posters could be suggesting that is quite odd!!. Steel has a Youngs Modulus 'E' something like 15 times more than plywood (207MPa compared to about 13-14MPa for plywood). By that logic suspension bridges would be made of plywood etc!!
|
|
|
|
|
|
Rank: Super forum user
|
JJ Prendergast wrote:#27 Paul Skyme
Nice to see there are 2 minds thinking alike with this problem. A basic structural calculation is quite easy to do, once the information is available. I too thought the SEMA guidance is not suitable for this application. Also that the plywood is the weakest link and not the box steel/welds - how posters could be suggesting that is quite odd!!. Steel has a Youngs Modulus 'E' something like 15 times more than plywood (207MPa compared to about 13-14MPa for plywood). By that logic suspension bridges would be made of plywood etc!!
Just a little word of correction. Youngs modulus measures stiffness and not strength. Youngs modulus for steel is of order 200 GPa (not MPa). Wood is up to 20 GPa. You may be surprised to know that the Youngs modulus of aluminium and window glass are the same.
The tensile strengths of woods are of order 30-45 MPa, compared to 200 MPa and up for steel.
Using a material that has too low a Youngs modulus may lead to a structure being too floppy, or to buckling, but not to fracture.
|
|
|
|
|
|
Rank: Super forum user
|
Well said Jane, I got a bit carried away with my typing to check my units!!
Its been a few years since OU T357!
You are of course correct with respect to E being a measure of stiffness, but its commonly quoted as many people recognise its basic significance with respect to material properties.
Fracture is not the most likely mode of failure, don't really want to go into LEFM/EPFM theory and Ki factors again
|
|
|
|
|
|
Rank: Forum user
|
To be honest Paul,
If your going down the road of non destructive testing, the prep work involved, having calcs done, third party checking etc; it will be cheaper to go and buy a purpose built system with a cert. and SWL.
if during the NDT, you find that welds require attention (grinding out and re-welding by a welder working to a weld spec), you are into strong money
Regards
Phil
|
|
|
|
|
|
Rank: Super forum user
|
We simply do not have enough information to determine whether a weld will fail before the ply or v-v. Don't know what centres the ply is spanning, what thickness it is nor e.g whether the racking is braced.
|
|
|
|
|
|
Rank: New forum user
|
research and talk to CEMA for all your SWL racking loadings, or send me a private email and will happy to give name of reputable racking installers, A structural engineer would be happy to give calcs also.
|
|
|
|
|
|
Rank: Super forum user
|
Brian Hagyard wrote:Paul skyrme
I agree with you. But to be fair when I used the Day term there was no information about where or how the racking had been produced that only came later.
I could not agree more that items produced in skilled tradesmen and designers are best, but in 25 years of health and safety work I have seen many tragedies and deaths caused by semis skilled odd job men,gifted amateur's etc and employers not questioning this type of work. Hence my caution to the original poster. Its a very fine line between genius and stupidity.
Also to refer to SEMA as "unskilled" I think is a little harsh. I think you will find the HSE LA Courts etc use their guidance in deciding what is reasonably practicable.
Unlike some I do not pretend I can offer an absolute answer based on the limited information in these posts. I freely admit where I know my knowledge is limited (is that not the true test of competency) and I hope that my post are of some help to people.
Brian,
You misunderstood my use of the term unskilled.
It DID NOT refer to SEMA.
I used it to describe the operatives that would be utilised to manufacture mass produced components, such as may be found in mass produced racking.
Having worked in structural mass production (on the skilled side) it was common to train someone for a few weeks and then they would go and do repetitive tasks including welding.
Oh, and yes there was additional info which appeared after your post to be fair.
One would hope that in a skilled engineering company they have professionals that are competent and know their limitations.
Hopefully they will be registered professionals, and thus will comply with the requirements of their registration and not operate outside their areas of competence.
One reason you don't find me commenting on occupational asthma or skin issues for example, unless it is to pose a further question, or seek clarification of a comment, but not to give advise really, except perhaps from an outsiders point of view, because it is not my area!
|
|
|
|
|
|
Rank: Super forum user
|
JJ Prendergast wrote:Well said Jane, I got a bit carried away with my typing to check my units!!
Its been a few years since OU T357!
You are of course correct with respect to E being a measure of stiffness, but its commonly quoted as many people recognise its basic significance with respect to material properties.
Fracture is not the most likely mode of failure, don't really want to go into LEFM/EPFM theory and Ki factors again
Another T357 veteran "locks horns" ;) with a T357 tutor!
Been there JJ, I agreed to differ, though I hope Jane saw my point of view.
I did bow to her superior knowledge though when you get into the nitty gritty of a lot of this detailed materials stuff, the semantics, language and terminology is critical! ;)
|
|
|
|
|
|
Rank: Super forum user
|
JJ I don't agree with your assessment that the wood must be the weakest link in this case.
We simply do not know enough about the racking, and the way it was constructed.
It is perfectly possible for a shelf to be very sturdy and able to take a large load, but be attached to the uprights by a weld with insufficient cross sectional area. In this case, the shear forces at the weld might cause failure.
This, in addition to the bending of the beam, is a foreseeable mode of failure and needs to be addressed in the assessment.
|
|
|
|
|
|
Rank: Super forum user
|
Jane thank you for that.
Reading your post has refreshed my memory. That is exactly the cause the engineer gave for the failure in the system which resulted in the fatality I was involved in investigating where the wood shelf stayed intact. I was never suggesting that a properly constructed weld would be weaker than plywood but that you should not make that assumption that the weld is good without more information than was originally given. For any system must consider each individual part and their contribution as a whole. Your post make these points clear. You have managed to restore some of my faith in this forum.
|
|
|
|
|
|
Rank: Super forum user
|
Jane Blunt wrote:JJ I don't agree with your assessment that the wood must be the weakest link in this case.
We simply do not know enough about the racking, and the way it was constructed.
It is perfectly possible for a shelf to be very sturdy and able to take a large load, but be attached to the uprights by a weld with insufficient cross sectional area. In this case, the shear forces at the weld might cause failure.
This, in addition to the bending of the beam, is a foreseeable mode of failure and needs to be addressed in the assessment.
Jane
I don't disagree. We don't have enough information to draw and hard conclusions.
But it is probably unlikely, given that we have been told that the racks have been in use for many years.
Whats the saying 'never assume, check'
I would want the all relevant dimensions providing about the racking.
The point however stands, that normal engineering science bending equations can be used to make a reasoned estimate of the rack loading and hence SWL.
|
|
|
|
|
|
Rank: Super forum user
|
JJ Prendergast wrote:
The point however stands, that normal engineering science bending equations can be used to make a reasoned estimate of the rack loading and hence SWL.
Sorry, I do not agree that this is the way to estimate the SWL.
A proper assessment needs to be made of the shear forces at the ends of the shelves, so that the minimum cross sectional area required in the weld can be assessed in relation to the required SWL.
Also a proper assessment needs to be made of any possible buckling of the uprights.
As it has been in use for several years then, up till now, the adequacy of the racking in respect of all three of the above modes of failure has been proven. However, to set an SWL you need to make a careful study of the three principle modes of failure, and we do not have enough information to know which failure mode is the most imminent.
|
|
|
|
|
|
You cannot post new topics in this forum.
You cannot reply to topics in this forum.
You cannot delete your posts in this forum.
You cannot edit your posts in this forum.
You cannot create polls in this forum.
You cannot vote in polls in this forum.