Temporary Propping - Factor of Safety for Loads

[Aytacoglu]

Hi Everyone,

Is there anyone from UK or Europe that has done temporary works design before? I am currently looking for a standard which would state that for a temporary prop and for a temporary loading (that we know the exact weight), can we reduce the Safety Factor? Typically safety factor for the live loads are taken as 1.5 and for dead loads taken as 1.35 as per Eurocode.

I know that the scaffold designers tend to not apply and factor of safety to the loads they apply on the scaffold platforms.

I have looked through the BS5975 which is the British Standards code for Temporary Works, but I couldn't find any definitive answer there regarding the use of factor of safety.

I am basically trying to justify a reduced safety factor on these loads since this is a temporary case or even not use a safety factor at all.

Any help would be appreciated!

Thanks.

 

[human909]

I don't know UK or European codes. However I would expect that the appropriate approach be reducing the design load not the LOAD FACTOR.

If you need 1.5/2.5/5kPa live loading for your temporary supported floor then you are stuck with that and factor (1.5x) appropriately. But if your finished floor requires 5kPa, there is not necessarily a reason that the TEMPORARY floor requires 5kPa.

I can't say I've directly dealt with works for live loads much. But I do it all the time for wind loads. My applicable code reduces the recurrence interval from 1 in 500 years to 1 in 100 years. As I've indicated reduction of the applied load is more suitable and academically consistent than reduction of the load factor.

 

[Denial]

I can vaguely remember being involved in a discussion on this general question, more than several decades ago. It concerned the appropriate load factor to be applied to the design of some short-term works that would provide access for repair workers to get to the places in need of repair. Most people involved in the discussion agreed that since the works would be in use for only a very short period of time, the required design load factor could be scaled back quite considerably. The repair company's representative in the meeting countered this view by saying something along the lines of: "You people might only be exposed to the risk for a short time, but my workers are exposed to it pretty much full time, job after job after job". It was the winning argument, and the full load factors were used.

 

[CJLCivilStruct]

BS5975 is a permissible stress based code, so the applied loading remains unfactored but you reduce the allowable stress in the prop to provide the factor of safety. Temporary works isn't my specialty but that's my understanding of it.

I don't believe you'll find the clause you are looking for in there - that would be a personal/company decision to make. However, I would always advise against knowingly lowering factors of safety.. something might change on site etc which would be out of your control.

 

[human909]

You really need to look carefully at the risk you are taking on and the control you have over the circumstances and whether that personal/company risk is worth the reward. In general the answers you have already received and will further receive is DO NOT DO IT.

However there are often circumstances that you can choose to play outside the 'rules' and get the job done. But nobody else here is in a position to advise you on taking on that risk, particularly with so little information given. Should an incident occur and you've played outside the rules well, things could turn pretty ugly for you.

 

[HTURKAK]

Regarding Eurocodes , in this case you may look to EN 1991-1-6 -Eurocode 1. Actions on structures section Actions during execution and EN 1990 basis of structural design.
You are expected to use recommended temporary loads ( e.g. for variable action the recommended value is 1,0 kN/m2 for construction crew ) rather than reducing the load factors .

..

 

[LittleInch]

Slightly different field, but I ban the use of the word "temporary" for exactly this reason - it encourages people to reduce factors and safety margins and somehow accept things you wouldn't if you were designing this to last 25 years or more. I don't think that is right. You can call it construction phase or initial phase or something without the word temporary.

Two things regularly concern me about this.

1) As a designer, you have no control over how long something is there for. There are many, many "temporary" facilities still in use 5+ years later
2) What has time got to do with anything? If you overload it / under design it, it will collapse. Doesn't matter whether this happens on day one or day a hundred does it?

So yes, you can design it to only take the construction loads or initial loading so long as it is fully marked with this reduced loading, but not the FoS. IMHO.

 

[rb1957]

as above there is a difference between safety factor and loads.

your safety factors are quite small and so shouldn't be a problem to anyone.

loads ... sure by code you may be looking at some very high loads, like 200 lbs per sq ft, which you may reasonably say really won't happen in practice. I think you have a case, but then you'll probably have to placard the scaffold "no more than 2 persons" or something like ... and that is likely to be a bigger problem to the installers (if they observe it).

my 2c ... not worth the hassle for what you might save. If someone else wants to do it, and it's not your responsibility then stay clear of it.

 

[phuduhudu]

CJLCivilStruct is right BS5975 is a permissible stress code so it works on working loads and then any temporary works e.g. props would also be rated for safe working load which would have an allowance in it for safety. However as the BS codes have been withdrawn for a while now it is expected that designers work to the Eurocodes which are limit state codes with factored design loads and design resistances. As others have said you can't reduce the load factors for temporary works but you may be able to justify lower loads e.g. for wind loads you don't need to take 1 in 50 year winds.​