Load Factor for Gravity in Elastic Plastic Analysis

[NRP99]

One query troubling my mind-
How to consider the gravity(acceleration) in limit load or elastic plastic analysis? As ASME Sec VIII Div 2 Part 5-Table 5.4 and Table 5.5 suggests to use 1.5 and beta as the load factors on the dead weight/gravity load, which comes under the "D" category. How should we take care of the gravity acceleration in analysis?
My options as per understanding-

(1)If gravity is opposing the other loads-Apply the factored gravity(1.5g/beta*g) as per tables separately (and ramp it up till convergence is not achieved to calculate allowable limit/ultimate load). Apply other factored loads/load combinations without gravity to calculate allowable limit/ultimate load(conservative design)

(2)If gravity is in same direction of the loads- Include it in the load combination and apply the loads (and ramp it up till convergence is not achieved to calculate allowable limit/ultimate load).

But trouble comes when I consider the physics. Gravity always remains constant then why we should apply factored gravity? Then (1) becomes redundant and (2) is unnecessarily reducing the capacity as gravity forces will limit the actual load carrying capacity which will be lower than allowable load/load combinations when gravity is excluded.

 

[TGS4]

I know that this is going to be semantics, but the load factor is not on gravity, but on the dead load. Practically-speaking, this is accomplished by applying a factor to the acceleration due to gravity.

The manner in which you choose to apply it depends on how you are planning to comply with the requirement "Loads listed herein shall be considered to act in the combinations described above; whichever produces the most unfavorable effect in the component being considered. Effects of one or more loads not acting shall be considered." That is, in my opinion, engineer's choice.

Regarding your comment of "gravity always remains constant, then why should we factor gravity" - well, again the factor is on the dead load. And a design margin equal to β is necessary, just as it is for pressure or static pressure.

 

[NRP99]

Thank you TGS4. I agree with you regarding the design margin on dead loads which comprises lot of different other loads.

But when we anticipate these loading, gravity is none among them which is probabilistic and fixed in every case(except when we have seismic or dynamic loading but that is distinguished load than gravity/sustained case(without pressure)). All other loads are probabilistic and hence design margin on the same is required/justified. Are we doing thicker design or over conservative design in the case when we include the gravity(case (1) and (2)in earlier post)? Practically this load will be never experienced.

Just a thought! Are we expecting our designs to be withstanding some X galaxy's "X" planet's gravity also?

 

[TGS4]

The same could be said for static pressure.

If you think differently, then I encourage you to get involved and help change the Code.

 

[DriveMeNuts]

As per TGS4, a vessel which will experience the same static internal (or external) pressure for its entire life is the same as the equipment experiencing constant gravitational dead load.

Perhaps it will be helpful if you shift your focus from "the load" to the reason and need for a "Design Margin".

If you have a design margin of 1, then the vessel starts its life on the cusp of experiencing permanent deformation or even failure.

The design margin includes a series of various factors to protect against unwanted minor deformation or failure from happening.