ASD to LRFD and Ultra high strength steels 1

[tmschrader]

Hello


According to several sources ASD and LRFD method will come up with eqivalent beams if DL= 3 x LL. If DL<3 x LL than LRFD will give larger strength sections. Does anyone know if this relation is effected by Ultra high strength ateel with a Yield of 100KSI and ultimate stress that is only slightly more? Like SSAB Strenx 700 (or 100KSI). Since LRFD theory is supposed to be related to the platic design region I would assume a low TS/ YIEld ratio should have a effect on the DL<3 x LL ratio. But do not find anything addressing this.

ref 2 gives a chart that is supposed to show how conservative LRFD is to ASD for different ratios of DL to LL.

Thanks for any insights



Ref 1

https://www.eng-tips.com/viewthread.cfm?qid=208740

REF 2

https://www.bgstructuralengineering.com/BGASCE7/BGASCE7002/BGASCE700203.htm#ASDvsLRFD

 

[phamENG]

The yield stress won't make a difference. Current AISC codes are no longer "Allowable Stress Design" but are, rather, "Allowable Strength Design." They've done this to sync up the theory between LRFD and ASD design approaches and make the designs more comparable. The difference is really only in the factor of safety and how it's applied.

The nominal strength will be the same for each - it's just a question of whether or not the you factor your loads to ultimate and take a small resistance penalty or just reduce the capacity by a large safety factor and compare to nominal loads.

EDIT: one thing to add, the AISC specification is only applicable to those materials listed in A3 MATERIAL|1. Structural Steel Materials, unless the engineer of record determines that it can be applied to other steels. So unless you're prepared to do the literature research to verify there are no properties in your high strength steel that would make any of the equations in the steel spec invalid, probably best to avoid it.

 

[tmschrader]

Hello phamENG

UHSS is becoming very common in mobile structures like cranes and Concrete handling Pump trucks. There is considerable weight savings when using it. SSAB is a major suppler of this steel in the US. It will be hard to avoid in the coming future on any machine that is limited by the weight the Truck can hold.

The AISC may not address this is it is more geared to buildings then machines. Although not totally so.

 

[phamENG]

Right - the spec also says it's specifically written for buildings and building-like structures.

Steel strengths have been trending up since we started tracking them closely. Metallurgists are an innovative bunch (I used to work at a mill, and those guys were great). I'm not sure they'll work their way into building design as quickly as they have, though. Until they crack the code on modulus of elasticity and can make the stronger beams stiffer while also maintaining ductility, we're pretty close to a ceiling for general structural items. More and more, most building designs are governed by serviceability, deflection, and vibration. Higher strength doesn't do anything to alleviate that.

Now, they may have a place in other fabricated components, like steel joists, where weight/material can be reduced without having a significant impact on stiffness.

 

[phamENG]

Also, LRFD has nothing to do with plastic design directly. You can use plastic design, but they are not the same. Both LRFD and modern ASD approaches allow for partial plastification of a section at ultimate loading, but that's not quite the same as designing around a plastic collapse mechanism.

If dik is around, he can explain the difference in quite a bit of detail. I learned plastic design in school, but was quickly "un-taught" it by my first design firm. dik has been using it in practice for decades.

 

[tmschrader]

Hello phamENG

This is the ref I was refering when mentioning Utimate strength and the plastic region which ASD does not address very much.

https://www.linkedin.com/pulse/what...se-article_more-articles_related-content-card

 

[phamENG]

Right. That's a largely irrelevant article. That article is a false comparison. Allowable Stress Design as described in that article has not been used in the design of steel buildings in over a decade except by some guys who have been around a long time and are really resistant to change, and is no longer part of the Specification for Structural Steel Buildings. Allowable Strength Design (confusingly also called ASD, probably to make it more palatable for those so resistant to change to adapt) is essentially the same as LRFD, but the factor of safety is fixed with variable statistical reliability while LRFD has variable factors of safety with a fixed statistical reliability.

"Utilization of...plastic...stages" is precisely what I said above. If there is partial plastification of a section, it hasn't failed. So at ultimate load (the load we don't actually expect to happen, but it could) the member can have partial plastification. The old allowable stress method was based on preventing plastification anywhere at any time.

Plastic design takes it a step further. You can actually have full plastification of the section, as long as you don't have a collapse mechanism. So a beam fixed at both ends can fully plastify at the supports. As long as you don't get a plastic hinge at midspan, it just turns into a simply supported beam for any loads above the plastic moment of the ends.

 

[dik]

Plastic design is another method of designing continuous steel beams. It's beneficial if they are the same or similar spans. It provides additional strength by relying on the redistribution of moments after initial failure. The classic example is a beam with two fixed ends, and a uniform load. Elastically this can be loaded to wl^2/12 when the end moment fails and the mid span moment is wl^2/24. Loading can further be applied until the mid span moment equals the end span moment at wl^2/16, where with three hinges the fixed end beam continues to fail without increasing load. Continuity is achieved by designing the cantilevered beam connections for both shear and moment. Some jurisdictions stipulate the minimum design moment, usually about 25% of the section capacity. It is excellent for large warehouse type construction.

The plastic design approach takes advantage of this redistribution. I've generally found it to be more economical than elastic design. There are a couple of reasons for this. There is generally a reduction in weight of steel and the number of connections; and they are generally more simple. Alternate loading on adjacent spans is not normally an issue. This usually only increases the amount of the cantilevered moment design. Gerber beams take a huge 'hit' on this. Price is reduced because there are fewer sections to handle. For long continuous beams with multiple supports, only the end spans increase in size due to the reduced number of 'plastic hinges'. Beam lengths can generally be about 60', or whatever the maximum transportable length is. Statically indeterminate structures are not normally an issue... beams are elastic with fixed hinge strengths.

Because it relies on redistribution of moments, there are requirements that the sections be 'stockier'; this is why most jurisdictions require Class 1 sections. There are occasional times that a lighter Class 2 section has the moment resistance, but cannot be used. Bracing and lateral support are only slightly more restrictive, and columns are generally 'pin ended' with real cross bracing. Plastic design can be used for rigid frames, too.

On the Vista Cargo project, outside Toronto, I didn't do an estimate for the original design(I didn't know what was estimated), but the final design weight was had a savings of slightly more than 2 psf of steel. The engineering manager had to seal the drawings (a company policy) and I had to teach him the design methodology... It was the first plastic design he had done. It was about 400,000 sq.ft, and the first project I had worked on for that firm. He was aware of my design skills from the Cornwall Centre in Regina.

At university, our class was the first one to use limit states (not plastic design) for both concrete and steel. Working stress design was only briefly mentioned, for historical reasons... back in 1965.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[tmschrader]

Hello

Thanks for the detailed responses.

It's been awhile since doing a plastic design calc, but I do have it in my orginal structures book. It seems that FEA's do a simliar thing as high hot spots (plastic areas) are often ignored rather than spending countless hours on meshing. FEA's seems to be the best way to analyze possible buckling issues (linear or non-linear) which can be quite complex.

So, do you think that ultra high strength steels show the same ratios of Dl to LL when comparing strength of sections? If DL<3 x LL than LRFD will give larger strength sections.

The below file from Europe does address safty factors of Ultra high strength steels, to some extent. The LRFD they discuss seems different than what is used in the US.

It also seems to me that LRFD is mostly directed at buildings and not fully relevant to machines like cranes where live loads predominate.

 

[phamENG]

LRFD is a uniquely American creation, it seems. Other places, including Canada, refer to similar approaches as Limit States Design.

If you were to apply the AISC building spec exactly as written to those steels, then yes - as far as I'm aware it would be identical. And that's because the nominal strength of both ASD and LRFD are 100% the same. The variation comes from the difference between the safety factor for ASD (which is fixed and not steel-type-dependent) and the load factors and resistance factors for LRFD (also not steel-type-dependent).

I would say LRFD is inappropriate for machine design. Not only does live load dominate, but loads also tend to be more cyclical. Fatigue considerations revert even LRFD designs back to a sort of working stress design by either limiting stresses to below a fatigue threshold or determining number of allowable cycles based on the peak stresses.

 

[ATSE]

phamENG - I usually appreciate your contributions, but a slight disagreement here.
You say: "I would say LRFD is inappropriate for machine design." Untrue.
Machine design has many limit states and many loading conditions.
Hard to imagine LRFD as not an appropriate framework, unless it is impossible for demand > capacity in any plausible load condition. Quite the contrary, mechanical engineers usually miss out of the benefits of LRFD and use lumped factors of safety for acceptance criteria (combining the distinct material and loading variabilities into one amalgamated number).

Even when serviceability governs, a strength check for extreme loading - best accomplished by LRFD - is always required.
Also, I don't think LRFD is any more uniquely American than the other (unique and region-specific) term used for structural limit state methodologies across the globe.

 

[dik]

Plastic vs. Elastic design...



-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[JoshPlumSE]

If you're interested AISC has an ad-hoc committee related to the use of ultra high strength steels. Below is a link to their report on the subject. A lot of the discussion is related to the following:
a) Other fields which use these types of steels.
b) An ASTM type path (preferred) towards incorporating these high strength steels into the AISC specification.
c) An alternate path towards using these steels.
d) What other countries / specifications are doing about this subject.
e) Path forward / priorities

You'll notice that there appear to be some work to do before there is a clear path in AISC for using these types of materials. There are likely some good references outside of AISC (ASHTO, EuroCode or such) that may give you some guidance.

https://www.aisc.org/technical-reso...hoc-task-group-report-on-high-strength-steel/