Davit OSHA Stability Factor

[RPMG]

OSHA states, "Stability factor means the ratio of the stabilizing moment to the overturning moment." and "1910.66(f)(3)(iii)(A)
Every davit installation, fixed or transportable, rotatable or non-rotatable shall be designed and installed to insure that it has a stability factor against overturning of not less than four." For the pipe base per the vendor, [Ma = 8] <= [Mn/F = 24 k-ft], where Mn = 40 k-ft. Is the stability factor for this 3.0 or 5.0?

I am designing the steel support, not the adequacy of the pipe base, but I was trying to figure out if the stability factor is a load factor or the factor of safety.

 

[r13]

For davit,

M_OT = lift load x arm length
M_ST = Design moment capacity at the support base ≥ M_OTxSF ≥ 4M_OT

 

[RPMG]

So you would say it is a load factor, [4.0 * 8 k-ft = 32 k-ft] > [Mn/F = 24 k-ft]. So N.G. The davit designer has undersized the pedestal. I should note that the davit designer notified us of this requirement.

 

[r13]

No. In my calculation M_OT, and M_ST, both are service load. The AASHTO stability factor, SF = M_ST/M_OT ≥ 4.0. When design using LRFD, I don't know if AASHTO requires another load factor or not. But I guess probably not, otherwise seems excessive. So your minimum design force should be 32 k-ft, if Ma = applied force = 8 k-ft.

 

[canwesteng]

In my experience OSHA and similar regs refer to safety factor not load factor.

 

[RPMG]

Nominal Moment Capacity = 40 k-ft
Service (ASD) Moment = 8 k-ft

Factor of Safety:
Mn/4 = 10 k-ft.
8 k-ft < 10 k-ft OK.
--OR--
40 k-ft / 8 k-ft = 5.0 > 4.0 OK.

Load Factor:
Ma = 4 x 8 k-ft = 32 k-ft
Mn/F = 40 k-ft / 1.67 = 24 k-ft
32 k-ft > 24 k-ft. N.G.

 

[RPMG]

The available or allowable moment, ASD, Mn/Ω = 24 k-ft.

 

[r13]

Be careful, ASD and LRFD can't be directly compared, as design assumptions and material properties are completely different. I'll say when code indicates stability factor, or safety factor, it has service load design in mind; when code calls for load factors, and strength reduction factors, it has strength design in mind, thus use LRFD is justified. For your case, I think the former is more like than latter, thus, the base was under designed.

 

[canwesteng]

retired - OP is asking about a stability factor in OSHA, not in a design code. Material properties won't factor into it. Usually these are applied to nominal loads and resistances to determine factor of safety.

 

[r13]

canwesteng,

Yes, I consider OSHA standard is the code here. The stability factor is a service criteria, similar to safety factor for retaining wall designs.

 

[RPMG]

In retaining wall design, designers solve for Ω, but the soil pressure is pre-factored by geotechnical. This argument can be used for either interpretation. If this were a foundation, I would know exactly how to interpret this bit of code.

In anchor design, the values are based on tests, and Ω is 4. This is a clear argument for the use of nominal capacity, Mn, which is derived based on the minimum yield stress. This is also more aligned with the davit crane designer.

The crane is also tested for a proof load of 2x. I'm just going to design for the proof load with 20% dynamic and standard factors of safety. [2 Proof]*[1.2 Dynamic]*[1.67 Ω] = [4.0 OSHA]. This is OSHA, and if they were going to dictate requirements for structural engineering, they could have hired a structural engineer to write them.

 

[phamENG]

if they were going to dictate requirements for structural engineering, they could have hired a structural engineer to write them.

They probably did. But remember, federal code updates make glaciers look fast. I've dug through some current HUD documents that were based on reports, testing, and model building codes that were as much as 50 years old despite several more recent advances in the topic areas. It worked when they wrote it, so they won't rewrite it - regardless of whether or not the language is in keeping with current design practice/terminology.