Is there anyone out there who can explain the difference between limit state & working stress design so that the issue of safety factors is understood? I keep trying to explain when it crops up but I am clearly not succeeding.
Partial Safety Factors
- Thread starter TG6
- Start date
If you're trying to explain elastic and plastic design methods to a non engineer, you could be fighting a losing battle but here goes:-
WSM - Working loads (no factors) are used to analyse the beam/structure/whatever. The material properties have a full safety factor applied and these properties are taken as the allowable maximums for the element.
LSM - Partial safety factors (to loads and materials) are used allowing the material to be loaded a lot closer to it's ultimate stresses than allowed for in WSM.
The main argument I hear is that LSM leads to a more economical design.
WSM - Working loads (no factors) are used to analyse the beam/structure/whatever. The material properties have a full safety factor applied and these properties are taken as the allowable maximums for the element.
LSM - Partial safety factors (to loads and materials) are used allowing the material to be loaded a lot closer to it's ultimate stresses than allowed for in WSM.
The main argument I hear is that LSM leads to a more economical design.
Thanks. Perhaps the key is in quoting that full safety factor has been applied to the material properties?
There seem to be few people here who are getting confused by safe material strengths being quoted alongside loads that have been factored - better that way than the other I suppose but it certainly won't lead to an economical design!!
There seem to be few people here who are getting confused by safe material strengths being quoted alongside loads that have been factored - better that way than the other I suppose but it certainly won't lead to an economical design!!
It's a tough one with scaffold component data sheets because you're always trying to guess how someone else has come up the figures. The fully factored properties is my usual quick explanation. Whilst being not entirely right, it's not entirely wrong either and it avoids having to get into lengthy discussions about design methods.
pride2iceman
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For example:
Apollo 1.5m X-Beam
Shear values for 11m span: Ultimate 146.18 kN
Allowable 97.45 kN
The Ultimate Shear resistance is 146.18 kN.
Allowable Shear resistance of 97.45 kN means that a Global safety factor of 1.5 was applied to the Ultimate value (146.18/1.5).
No is up to the designer to chose the design method(Plastic or Elastic): if considering Ultimate Shear resistance value, in this case 146.18 kN, a factor of safety(1.5) or partial factor of safety must apply to the load as per Limit State Design.
Otherwise if considering Allowable Shear resistance then is not necessary to apply any factor of safety to the load.
Apollo 1.5m X-Beam
Shear values for 11m span: Ultimate 146.18 kN
Allowable 97.45 kN
The Ultimate Shear resistance is 146.18 kN.
Allowable Shear resistance of 97.45 kN means that a Global safety factor of 1.5 was applied to the Ultimate value (146.18/1.5).
No is up to the designer to chose the design method(Plastic or Elastic): if considering Ultimate Shear resistance value, in this case 146.18 kN, a factor of safety(1.5) or partial factor of safety must apply to the load as per Limit State Design.
Otherwise if considering Allowable Shear resistance then is not necessary to apply any factor of safety to the load.
A shear capacity for a scaffold beam quoted to 2 decimal places is remarkably precise.
The partial safety factor you would apply to the loads depends upon what sort of loads they are and you need to consider the various combinations required by the Eurocode which applies to the design. In this case, I would suggest that simply applying a single factor of 1.5 is not carrying out a limit state design even though a quick look at the calculations might fool an observer. It is much more like the pseudo limit state of TG20.
Plastic design is a form of limit state design (but so is elastic design). The two methods use different yield stresses, plastic yield and elastic yield and shouldn't be used to describe the alternative approaches we are talking about here. The two stresses only appear on the stress strain curve of some materials and so I don't think that plastic design can be used on timber or concrete because they have the wrong shape curve. I think that working stress and limit state are probably the best way of describing what we are discussing.
The partial safety factor you would apply to the loads depends upon what sort of loads they are and you need to consider the various combinations required by the Eurocode which applies to the design. In this case, I would suggest that simply applying a single factor of 1.5 is not carrying out a limit state design even though a quick look at the calculations might fool an observer. It is much more like the pseudo limit state of TG20.
Plastic design is a form of limit state design (but so is elastic design). The two methods use different yield stresses, plastic yield and elastic yield and shouldn't be used to describe the alternative approaches we are talking about here. The two stresses only appear on the stress strain curve of some materials and so I don't think that plastic design can be used on timber or concrete because they have the wrong shape curve. I think that working stress and limit state are probably the best way of describing what we are discussing.
IDH
Well-known member
Permissible stress approach is described in BS 5975 - it is the amount of bollox you listen to on these forums before you feel the need to contribute some more bollox.
Limit State is when you have heard enough and sign off..
Limit State is when you have heard enough and sign off..
Stella Fella
Well-known member
Has BS 5975 been re-introduced to G.B. & BSEN scrapped thanks to Brexit?
Last edited:
philliosmaximus
Moderator
But don't ,
" British Standards " sound so much better
" British Standards " sound so much better