Design factors in codes and a question on lifting

[ThomasH]

Hi all

As you all know the codes have design factors or partial coefficients. For dead load BS gives 1.4 while Eurocode gives 1.35. (I'm not sure about the exact values for UBC, IBC, ... etc).

Does anyone know what these factors are based on? Note: I don't want to question the factors only know the background.

Part of it is for safety, that's obvious since the allowed stress is the yield stress so the safety is on the load.

My guess is that there is also an allowance for added load in the future but how much?

My guestion is based on that I'm working on a design where we know the weight "exactly", we measure/weigh it. Also, is a very short duration so no allowance for the future is required.

Do you think I can reduce the design factor based on this?

Second question:

When dealing with the lifting of structural elements, what code/codes is applicable? I know of codes for the lifting device and codes for the element when it's installed at the final destination. But what code, if any applies, when it hangs in the crane?

Any input is greatly appreciated.

Thomas

 

[msajjadh_imported]

Thomas,
For lifting device, you can follow ASME B30.20. Generally for lifting devices FOS is taken as 3.
What do you mean by short very shot duration. Is it some type of Blast Load etc. If this is the case, the situation is different. Otherwise follow the recommended factors.
See some text books, you will find the logic of these factors.
Regards,

 

[rowe]

The factors are based upon safety and variances in material properties (i.e. strength, geometry, etc.).

Future loadings are not a consideration in the load factors.

With regard to the dead load, you could use your own judgement as to an adequate factor of safety for temporary loadings, however, you should be aware that cranes and discrete lifting equipment (chains, booms, etc.) may be suspect with regard to previous loadings that may have caused slight overstresses - so beware.

Also, a "dead load" lift is actually a dynamic load so you may need to consider using a higher load factor to account for the dynamic effects.

 

[letrab]

As far as I know the load factors in the British based codes are based on statistical methods to derive the partial safety factors. It is an attempt to ensure the loading will only be exceeded very infrequently. The partial material safety factors are used to ensure the structural performance exceed the loading applied by a sufficient margin.
There is no allowance for future increases in load.
Consequently all of it is for safety.
However, should your structure be concrete BS8110 part 2 provides reduced partial safety factors for worst credible loads, which is the case in your instance.
For the lifting bits, I would remain with the normal Mechanical safety factors for slings, hooks etc. since they are derived through expensive experience.

 

[ThomasH]

First, when I said "short duration" I meant something like an hour. It's not impact loading it's just very temporary.

Let me put the question in another manner. Let's say that you design a building in for example Britain. BS applies and thats it. Let's say that you want to lift the building to a new location, does BS still apply or is there a better alternative?

Loadfactor 1.4 does not normally include the dynamics due to the lifting. But what is included, is it all for safety?

I have found codes for the lifting devices so thats not a problem. But is there a code fore the lifted object or is this open for my judgement. The structure is well defined and made of steel.

 

[msajjadh_imported]

ThomasH,
Now your question is more clear. Your concern is not lifting device design. You are concerned with the structure lifted. Generally building are not designed for such cases. But if client wants such design, it can be done. One can define lifting points and design lifting lugs. Then he should design the structure under lifted condition as well. It does not make any difference, whether the structure is designed according to BS or American Codes. The point is the destination. If the authorities there accept the design code, there is no problem otherwise you have to prove the adequacy of your design according to the law of the land.
Regards,

 

[ThomasH]

Exactly, my concern is the lifted object.

My question is if there are any codes for this situation. So far we have used BS, Eurocode or US codes depending on the destination. But are there any specific "lifting" codes or any applicable loadcase in the mentioned codes.

Today we work with "common sense" and it results in a lot of discussions and opinions.

Any references would be greatly appreciated.

 

[trac]

The Offshore code of practice - Recommended Practice for Planning, Designing and Constructing API RP2A-WSD
Fixed Offshore Platforms – Working Stress Design, refers to a load factor of 2 when designing a structure for a lift condition.

regards
Trac