The application of limit state approach to scaffolding (1 Viewer)

Ryan_Berry

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Hi everyone. I am bored, I know that is sad on a friday night, so thought I would see what everyones views are on the application of limit state approach to scaffolding.

What are everyones thoughts on the application of limit state approach to the design of scaffolding as this ties in with the Eurocodes system which we have adopted (albeit not fully)? The eurocode suite is based fully on limit state and cannot be adopted effectively without the use of EU 0.

I am just doing some designs (trials if you like) to see what the impact will be on the design of scaffolds using limit state when compared with permissible stress. The results seem to be going in favour of LS at the minute. "The permissible stress method is not used widely in most engineering disciplines due to overcomplication of the design process and also leads to more conservative solutions. In addition to this, the quality of material production has increased and safety margin decreased. The assumption that stress is proportional to strain became unjustifiable making it impossible to estimate the true factors of safety". As suggested by Arya C. ‘Design of Structural Elements: Concrete, Steelwork, Masonry and Timber Design to British Standards and Eurocodes’. 2nd Edition.

On the other hand, Limit state approach is a more comprehensive approach than permissible stress and load factor approaches. It is a combination of the two which takes account of both in appropriate ways. Most modern codes of practice require limit state approach, including BS EN12811-1. We do not engineer scaffolds to limit state as many published figures for scaffolding materials are given in permissible stress values giving a safety factor of 1.65 to 1, as opposed to BS EN 12811-1 that requires limit state approach. Limit state applies a partial safety factor of 1.5 to 1 for live loads and 1.1 for dead load giving a single factor of 1.5 x 1.1 = 1.65 similar to permissible stress.

The consensus of most seem to be against the idea but if adopting it can assist the industry as an whole then maybe it isn't such a bad thing - just a different look. I think it is long overdue as it is the only area of engineering which has not adopted it yet. Even timber, which is not homogeneous, uses the limit state approach. I personally think that limit state would bring us into the modern world of engineering and even save some kit for the scaffolders.

what are your thoughts on this?
 
Hi Ryan

You must be bored thinking about limit state, do you need some more work . LOL

The problem with any method of scaffold analysis is that we are not dealing with fully fixed or fully pinned jointed structures, so the analsysis of the structures as a whole cannot be easily modelled, unlike concrete and structural steels frames.

It may be more in like with system scaffolds which welded joints, but speak to any scaffold about Kwikstage and he will tell you that it is not very stiff even with welded connections

The full size tests done by the NASC which developed TG20:08 more closely reflect the scaffold in practise.

I prefer to use a more conservative method of design. Site conditions do not always allow for the idealised drawn design to be erected so there needs to be a further factor of safety.

Scaffold/ temporary works is not the same as permanent works which can be more closely controlled.

Also the physical lift heights, bay widths, etc are limited by the physical characteristc of the erectors and may only increase by a small percentage and would not be a large enough saving in most jobs

I expect we will have to go to limit state one day, but I hope to have retired by then.

All the best

Chris eng
I would be concer
 
Ryan / Chris / everyone...

Personally I can take it or leave it (limit state design that is)... permissible stress is easy to understand and therefore quick to compute, mainly because you deal with unfactored loads. Limit state I think takes longer (probably as I don't do it as often) but like you say, would bring us in line with other disciplines.

I use limit state for all steelwork design and timber design for formwork but thats it. That said, I still use BS449 when it suits (for stuff like steel road plate) because it straight forward, and I've never had anyone say "Im not accepting it".

There is one more slightly political reason that you haven't mentioned either... TG20! Can you honestly see the NASC saying OK go ahead and use limit state direct from BS12811 when they have just spend thousands on developing TG20, the permissible stress solution to a limit state code? I can't. I also can't see people wanting to move to a 'new' design method so soon after the BS5973 to TG20 switch... I can hear "if it ain't broke down fix it".

The practicalities of permissible stress are also quite good, and we probably take them for granted. Having unfactored loads (as mentioned before) and hence unfactored reactions is a lot easier for clients to deal with for example. If we were giving out factored reactions I think that would cause confusion, and, more likely, we would have a conversion back to unfactored reactions at the end of the calculation.

I also agree with Chris regarding how much gear it would actually save, maybe a beam here or there, but not a significant amount I don't think.

I am more... concerned (may be not the right word) about the introduction of 2nd order analysis. I like dealing with effective lengths (albeit TG20 is bemusing) and then using critical interaction equation for members under combined axial & bending... probably because it's easy. 2nd order analysis is OK, but can be long winded and more often than not will give a conservative answer due to the classification of nodes (pinned vs fixed) and their subsequent moment capacity in multiple directions. The reason I am "concerned" is because if we are forced to do that kind of thing, the cost of design will shoot up and scaffolding contractors just won't tolerate it.

Another note... the Arya design book is really good.. I have it too (1st edition tho) from my Uni days and I still refer to it often.

Ben.
 
Wow!!!!!!!!!!!!!

I got lost with the title lads:eek: Crack on boys, we are all in this together. However, please dont send me a invite to any seminars, unless you want me to make the Tea and carry the biccies out:D
 
Do not belittle yourself Paddy.

Google limit state and then google eurocode, spend an hour or two reading, and I guarantee you will have an different opinion on this thread.

Probably that it will never affect scaffolding.
 
Interesting discussion guys. I looked into the use of limit state theory when experimenting with finite element analysis and came to the same conclusion as BMB. The 1.65 factor of safety given in TG20:08 equates roughly to the safety factors applied in limit state theory, and I was unable to find any real-world situations that a design scaffold passed the permissible stress method of calculation yet failed the limit state method. The permissible stress method is far quicker and also easier for the client to relate to.

A quick question for you design engineers - what do you use to produce your calculations? Is it a software solution (ie Mathcad / Excel), or simply handwritten / typed with aid of a calculator?

Mark
 
Hi Mark,

Production of calculations.. well.. in a nutshell, we use Excel to formulate our calc package as I find it easiest to manipulate and format. The calculations for the main scaffold are written out as if you were writing them by hand, where we use analysis software for beam sections say, the results are imported into excel into the section of the calcs where is relevant. If I need more complicated analysis, say for a temp roof, facade shore or something then I will generally insert a summary of the analysis results in excel with the full set of results in an appendix.

I think this is generally the way most designers do it except some use excel and some use word, just depends what you are comfortable with I guess. That said, I know some people still hand write their calculations, when I first started that's how I was thought: used a load of tipex!

Hope that helps mate! What do you use for the calcs?

Ben.
 
Ben
Bit cheeky but do you have a drawing of a canterleaver to take 10 ton ?
Its on a well head tower boat landing is 1.8 wide and 9 metres long there will be 5 lifts then the beamed lift 6m cant and 6m wide no calcs needed it just to give ruff id as there is ment to be someone coming out for the beach but i dont think is a real engineer. There is plenty of steel work to tie it back to but like i said just a ruff look.
cheers
andy
 
what the fook was all that about Ryan :confused:

---------- Post added at 05:42 PM ---------- Previous post was at 05:42 PM ----------

get your arse down the pub lively mate
 
fcuk knows phil ive been smokin some sh?t tonight strong enough to knock an african elephant out, My arms are heavier than my fcukin feet, but not as heavy as this thread :nuts:
 
Hi Mark,

Production of calculations.. well.. in a nutshell, we use Excel to formulate our calc package as I find it easiest to manipulate and format. The calculations for the main scaffold are written out as if you were writing them by hand, where we use analysis software for beam sections say, the results are imported into excel into the section of the calcs where is relevant....

Hope that helps mate! What do you use for the calcs?

Ben.

Hi Ben,

I've always used excel although as mentioned I did look in to FEA as an option. Came in handy a couple of times but found it a bit cumbersome for showing the maximum BMs or Reactions when a more simplistic approach could be done in excel. I find that using permissible stress calcs such as Clapeyron's equations and continuous beam bending formulae are sufficient to check beams, and the tables in TG20 vol 2 for axial loading on effective length of standard means you can skip multi-story frame type moment analysis. At the end of the day scaffold design is a very uncomplicated branch of structural engineering, almost 'textbook', so I like the calculations to be easy enough to understand without an IstructE membership!

By the way, had a look at your company's site I've gotta say it's by far the nicest site design for a scaff design company I've seen to date! How many people have asked you what 48.3 means? Haha the numbers 24.15, 48.3 and 96.6 are etched in to my brain from drawing up scaffolding in 3D, along with all the tube lengths in mm (4876.8mm, 3962.4mm etc!)

Mark
 
Ben
Bit cheeky but do you have a drawing of a canterleaver to take 10 ton ?
Its on a well head tower boat landing is 1.8 wide and 9 metres long there will be 5 lifts then the beamed lift 6m cant and 6m wide no calcs needed it just to give ruff id as there is ment to be someone coming out for the beach but i dont think is a real engineer. There is plenty of steel work to tie it back to but like i said just a ruff look.
cheers
andy

Andy,

You are right about one thing, that's cheeky! Surprisingly I don't have one of those laying around the office.. its a bit too specific to have something similar!!

Sounds to me like that needs a design AND calculations to make sure it's all gona work, I know most of the guys on here could erect just about anything and be happy it's gona stay up OK but cantilevers that rely on the existing structure are tricky - you may know the scaffold is fine, but who's checking out the existing structure!? If you have someone coming out to look at it for you then that's good.. not sure how you'd get on with an 'unreal engineer' !?! :amuse:

Good luck.

B.

P.S. Is this the heaviest thread on here? Must be like 5 ton or something..
 
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