LRFD (or LSD in canada) versus Allowable STRESS design. 1

[Alexey881]

Hello, I'm having a problem converting myself between methodologies. Let me explain:

I'm a mechanical engineer who recently finished my studies, which were all in Allowable Stress Design (everything based on yield strength and safety factors). For different parts we would resolve force components into stresses and then apply some a criteria of Von Mises depending on what is required. (+ some safety factor). Now I'm working in seismic restraint design industry for equipements (such as chillers, exchangers, pumps, etc.) and the seismic force is LRFD or LSD force. My question is, can I take the LRFD/LSD force, use my university methodology to combine stresses (ex. T+V+Moment stresses) and then instead of using Yield Strength, use Ultimate strength, but Apply the reduction factor Phi listed in the code for a given type of the system?

So, am I doomed to only use all new formulas from CSA-S16 or AISC steel manual or there is some reasonably easy way of converting between the two methodologies? And also when I get to catalogs of small/thin channels or angles (12GA cantrusses, or 1/8" to 1/4") Do I simply take their maximum allowable buckling/shear/tension load and compare it with my LRFD/LSD force? Cause most of catalogs seem to specify allowable loads (or allowable design loads). It seems like im missing some step or conversion.

Thanks for your help!

 

[ajh1]

Using the AISC Manual it is straight-forward in that all of the equations for capacities are given in a nominal (or ultimate) capacity mode. Simply adjust by either the safety factor (ASD) or the resistance factor (LRFD) to get the actual capacity for the methodology you are using. LSD is effectively the same way although you will see the resistance factor in the body of the equation since there is only one choice. In general combined interaction equations are effectively unchanged regardless of methodology. You just need to be sure that the applied load and the capacity are expressed in the same methodology, i.e., don't use ASD load combinations to calculate applied load and LRFD level capacities. It used to be easy because you simply though of "without load factors" for ASD and "with load factors" for LRFD/LSD. Now that in the US both seismic and wind loads are defined as ultimate and you need a load factor to get back to ASD both methodologies use load factors in some fashion. Be sure you work with the proper sets of equations. While AISC equations are very similar to S16 equations there will be subtle differences on some. Your EOR or code official will be looking for the equations they expect to find, not necessarily the ones that you use the most frequently.

 

[JAE]

I think if you take ajh1's suggestions above, but translate AISC's Allowable Strength Design to stresses, you might have a rationale basis for design.


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[Alexey881]

Thanks.
So, just to confirm; Say I'm using LRFD or LSD and my factored load = phi(factored resistance). Does it mean that at this point my member is in the state of plastic deformation? Or it is in the elastic region? I find different answers, on some site someone said its in plastic, on other sites they say factors bring you back into elastic.

 

[ajh1]

I would say, No, you are not at plastic levels. The effect of ASD (without load factors) and LRFD/LSD (with load factors) should give the same basic stress levels in reality in your members. ASD uses a factor of safety, say 1.67, to cover all contingencies over working loads. This is generally viewed as something like 1.50 for excessive load application with the remaining 0.17 covering things like design errors, fabrication errors, and erection errors. LRFD takes an approach where the load factors in the combinations are blended based upon the likelihood of excessive loading (1.2 DL vs. 1.6 LL/SL) to give values similar to the 1.50 portion of the ASD safety factor. The remaining portion is the equivalent to the resistance factor and covers the error types noted above. The net result to the physical structure is exactly the same.
If in fact all of the loads actually existed on the structure including all of the overloads implied by the load factors, then you would be in the plastic area, but there is no more likelihood that those overloads would exist than that the 1.50 portion of the ASD safety factor would be taken up by overloading.

 

[Gumpmaster]

Alexey,

The LRFD (and AISC's ASD) methods take quite a bit more into account than just von Mises stresses. Von Mises is just the failure criterion for yielding. There are also buckling (local & global), localized rupture, and other failure modes to be concerned about, especially in the seismic realm where you have to consider low cycle fatigue. Many of the AISC seismic requirements for slenderness ratios & plastic loads are there to minimize local stresses to prevent low cycle fatigue and to create plastic fuses.

As far as cold formed framing (if that's what you're using for light channels & angles) there's a completely different code for that in the USA. I'm not sure if Canada has a different code or not. The cold formed code considers more localized effects.

So, yes, I think you're doomed to a whole new set of equations.

 

[msquared48]

As far as I am concerned, you must be on LSD to use LRFD. But that has already been discussed before...

Mike McCann, PE, SE (WA)

 

[ajh1]

Canada uses the same cold-formed steel design specification as the US, AISI / CSA S136, with slightly different resistance factors. The specification cites safety factors for ASD and resistance factors individually for LRFD and LSD. The only option in Canada is LSD design, and yes, I chuckle at that same joke every time I work with the Canadian side of things.

 

[ajh1]

Canada uses the same cold-formed steel design specification as the US. In the US it is referred to as AISI, in Canada S136. The specification is written to cover ASD, LRFD, and LSD and provides safety factors for ASD and separate sets of resistance factors for LRFD and LSD. There are a few country specific requirements but for the most part it is exactly the same for both in terms of requirements.

 

[ajh1]

Excuse the duplicates. When I was typing this in this morning it didn't seem to be registering, so I had tried again, then gave up.