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





RE: LRFD (or LSD in canada) versus Allowable STRESS design.
RE: LRFD (or LSD in canada) versus Allowable STRESS design.
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RE: LRFD (or LSD in canada) versus Allowable STRESS design.
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.
RE: LRFD (or LSD in canada) versus Allowable STRESS design.
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.
RE: LRFD (or LSD in canada) versus Allowable STRESS design.
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.
RE: LRFD (or LSD in canada) versus Allowable STRESS design.
Mike McCann, PE, SE (WA)
RE: LRFD (or LSD in canada) versus Allowable STRESS design.
RE: LRFD (or LSD in canada) versus Allowable STRESS design.
RE: LRFD (or LSD in canada) versus Allowable STRESS design.