LRFD Wind and Seismic Loads, W and E. Load Factors-"Global" Factors of Safety

[structuralsteelhead]

Question Gents: Per ASCE the LRFD Load Combination have W and E loads included as factored Loads. Does this mean we can say that W abd E have inherent safety factor based upon it's load factor when using LRFD? And is the wind load factor equal to a demand FOS of 1/.6=1.67 and is the seismic load factor equal to a demand FOS of 1/.7=1.4285? So then as I've always understood, these load factors are where we get our factor of safety from with the additional measure of safety factor derived from the strength of material/loading condition reduction factor (whatever you prefer to call it) on the other side of the equation of course. Where does "global" safety factor come in, or is this term not used anymore? Actual plan check comment,..."what is the global factor of safety for the system"?" (I'm anchoring something,..with a number of deadman footings)

For instance: If were to compute a wind load demand using LRFD then and found a value, and designed a footing to resist this factored load with a .9 load factor for its dead weight-couldn't I immediately just respond to my client assuring he/she that they will have at least a 1.67 safety factor in the the foundation's uplift design. This then say better than a 1.5 factor of safety which may be required by the project specifications? I'm another 17% conservative in this case. Or do I have to multiply the utltimate/strength load again by 1.5 or my foundation weight by 1.5 in order to satisfy the safety factor requirement. Again, I don't want to increase my factor of safety another 50% if by using load factor design I already have it from the load factor.

Or am I confusing myself, again, which is common, and that the plan checker's use of 'global' factor of safety may be something that I should be breaking down to a service load value, work with straight up service load combinations and then multiple this force by 1.5 to achieve my 'global' factor of safety? And then of course leave my component and fastener design as is in LRFD. I'm in the process of asking the plan checker exactly what is meant by global factor of safety.

I'm frankly a little embarrassed to still be asking this question after so many years of using LRFD design. I'm thinking by just using it, I've got at least a 1.5 factor of safety especially in using load combinations with primarily wind loads. Thanks in advance for any comments anyone might have.

 

[Lomarandil]

You're not wrong to be confused here.

It's one of the major errors in how LRFD has lost its way. Because yes, the W and E (when derived at strength level, to be used in a combination with factor 1.0) do have inherent safety factors, and have diluted "real-world" meaning.

It works great within the bubble of structural engineers (who are keeping up on code updates). It doesn't work well with contractors who don't understand that 115mph now is the same as 90mph a few years ago, or manufacturers who provide product data following their own factors of safety, or when coordinating with other disciplines.

"Global stability" checks like sliding or overturning of deadmen can either be performed with ASD combinations (0.9DL factor + FOS = 1.5, or 0.6DL factor and FOS=1.0), or they can be performed with LRFD combinations and FOS =1.0. All three are roughly equivalent. But the precedent and many specifications still are based on the former, so it's often easiest to run the calculations in the old ASD manner.

 

[structuralsteelhead]

Thank you for your well formed and succinct response, Lomarandil! It's getting close to retirement for me, and at times like these, I feel I should already be. I will present my 'global' results in ASD as well for comparison for the plan checker's benefit, and for clarity to the owner (and myself!) Thank you again, Sir,....for the refresher! Much appreciated!