danb3
Mechanical
- Jun 21, 2021
- 8
Hi all,
With the new edition of API 579-1/ASME FFS-1 (2021 edition) that has just been published, there are some changes to the load multiplier (beta factor, β) for the ASME B31.4, B31.8 and B31.12 piping codes, for when considering elastic-plastic analysis for a protection against plastic collapse assessment.
It is understood that β represents the design margin for the ultimate tensile strength for the specific construction code. For ASME Section VIII and other codes where the design margin is based on ultimate tensile strength, β is well defined in API 579-1. However for the piping codes such as ASME B31.4, where the design margin is based on yield strength, the load multiplier must be derived in a different manner. This manner has changed over the years, as summarized below for ASME B31.4 as an example:
API 579-1 2016: β = 2.4*F*RSFa
API 579-1 2016 (correction to 2016 document): β = (σ_UTS/(σ_YS*F))*RSFa
API 579-1 2021: β = (1/F) * RSFa
In my opinion, the API 579-1 2016 (correction) method makes the most sense, as it is a function of the B31.4 design margin, F, (which is based on yield strength), factored up by the ratio of UTS to Yield, so that β is then applicable to a UTS based failure criteria (i.e. plastic collapse). The current API 579-1 2021 method however only considers the design margin, F, which results in some very low β values, such as β=1.25 when using a design margin of 0.72. This is even lower than the load multiplier required for local failure (1.7*RSFa = 1.53) which seems a bit odd?
Any help in understanding the logic behind this would be greatly appreciated.
Thanks in advance,
Dan
(I originally posted this over in the 'Pipelines, Piping and Fluid Mechanics engineering' Forum a month ago, and didn't receive any replies, perhaps this is a more suited forum for it. I will delete the other post shortly.)
With the new edition of API 579-1/ASME FFS-1 (2021 edition) that has just been published, there are some changes to the load multiplier (beta factor, β) for the ASME B31.4, B31.8 and B31.12 piping codes, for when considering elastic-plastic analysis for a protection against plastic collapse assessment.
It is understood that β represents the design margin for the ultimate tensile strength for the specific construction code. For ASME Section VIII and other codes where the design margin is based on ultimate tensile strength, β is well defined in API 579-1. However for the piping codes such as ASME B31.4, where the design margin is based on yield strength, the load multiplier must be derived in a different manner. This manner has changed over the years, as summarized below for ASME B31.4 as an example:
API 579-1 2016: β = 2.4*F*RSFa
API 579-1 2016 (correction to 2016 document): β = (σ_UTS/(σ_YS*F))*RSFa
API 579-1 2021: β = (1/F) * RSFa
In my opinion, the API 579-1 2016 (correction) method makes the most sense, as it is a function of the B31.4 design margin, F, (which is based on yield strength), factored up by the ratio of UTS to Yield, so that β is then applicable to a UTS based failure criteria (i.e. plastic collapse). The current API 579-1 2021 method however only considers the design margin, F, which results in some very low β values, such as β=1.25 when using a design margin of 0.72. This is even lower than the load multiplier required for local failure (1.7*RSFa = 1.53) which seems a bit odd?
Any help in understanding the logic behind this would be greatly appreciated.
Thanks in advance,
Dan
(I originally posted this over in the 'Pipelines, Piping and Fluid Mechanics engineering' Forum a month ago, and didn't receive any replies, perhaps this is a more suited forum for it. I will delete the other post shortly.)
