Allowable stress in Ultra high strength steels

[TimSchrader2]

Hello Again

Note that most all Mack Trucks use 120KSI steel in the Truck Frames. Automakers use Ultra HSS quite a bit. Since this use is seen in eveyday use, like with buildings, you would think the Steel Manual would address a allowable stress for it. 80KSI ASTM A-656 is used in steel crane booms and trailers. 80ksi may be in the latest Steel manual. The dynamic loads in cars is usually higher in everyday use than buildings. Unless there is high winds.

I am surprised by the lack of attention given to this class of steels by the codes.

See previous thread below

Note: AISC stand for American Institute of Steel construction. Buildings and briges are its main concern, but the society was intended for all steel construction.


REference thread172-447312

 

[Guest090822]

Actually, it makes perfect sense that AISC doesn't address ultra HSS. The code is focused on building design, not automotive or equipment. I've used HSS in buildings and bridges, but in the bridge world especially, deflections usually become an issue. 36 ksi, 50 ksi, 100 ksi, it's all the same E = 29,000 ksi and deflections normally have "E" in the equation.

 

[TimSchrader2]

Hello TheRick109

Thanks for the response.

Did not think local codes would allow the use of Ultra high strengths steels above 70-80KSI yield in buildings, maybe Bridges but its not specifically called out in the AISC.

I stay under .6 x Fy, even .5 x Fy (for bending) as its Yield/TS ratio is so high. What do you use (in general) for above 80KSI. The SAAB MFG says that the ultra high strength steels redistribute the stress under local buckling and so can handle more local buckling load then the given formulas based by E alone. As E is about same for Ultra and regular steel. There is plenty of tech data on thier site. They even say the Strenx 700 welding does not need preheating when welding. As other high alloy steels do.

Its not easy to find in stock unless your buying over 5000LBs min or large coils

 

[JoshPlumSE]

There is a an Ad-Hoc committee at AISC to address these high strength steels. Though they define this as anything with an Fy > 60 ksi.

Note: there are a number of prominent projects (buildings and bridges) that were built with 70 Ksi steel. Or, at least used it for certain critical members.

I've not been involved in the committee, but I have seen some of their meeting notes. There is likely to be a number of issues that get addressed in the next code cycle (or two).
1) Try to get upper Limits on Fy specified for most of the ASTMS
2) Better listing of availability of high strength steels for structural shapes (especially HSS).
3) Path towards using this via AISC.... Either getting new ASTM's for these materials, or allowing the use of other standards (euro, etc) for these types of steels.

 

[Guest090822]

I've used 70 ksi steel in bridge designs that were actually built and constructed. I did look into using 100 ksi steel years back on a project for a long simple span bridge around 300 feet. The capacity was there, but the deflection was around 18 inches so we scrapped that idea since it was well above the allowable, and the concrete deck would have crushed. I've never used anything stronger than 70 ksi for structural steel and 75 ksi for rebar. AASHTO generally has commentary and notes for higher grade steel. I'm not so sure about AISC - I've never had a reason to use anything stronger than 50 ksi with that code.

 

[Agent666]

Higher strength steels usually come hand in hand with lower levels of ductile material behaviour, which limit there usefulness in buildings. My own code only allows up to 50ksi (350MPa) in ductile regions for structural steel. The only area for example where it allows higher strength materials to be used is for cover plates in splices. I'd hazard a guess that difficulties in welding higher strength materials also factors into their lack of applications in building structures.

I'm not a mechanical engineer, but I'd hazard a guess that a truck chassis is not built with hinging regions for obvious reasons that you don't want it bending in half. They are designed primarily in the elastic realm unless I'm mistaken. This is the complete opposite of what we are sometime deliberately achieving in building structures.

 

[JoshPlumSE]

Agent66 -

Take a look at the image below:

This is an example of variation of material in a car frame. Each color represents different steel. Some are design with high yield, some are design with high elongation / ductility.

The red color for example, is ultra high Fy so that side impact doesn't crush the driver or passengers. Purple is also high strength (65 or 70ksi, I belief), but it also has a long elongation... to provide a crumple zone for front impact.

The point being that for cars, the material properties are varied widely depending on the components.

 

[JoshPlumSE]

One thing I forgot to add to my last post was a "vision" of how a company (say corebrace, durafuse or sideplate) could design members and connections to have better energy dissipation if they took this same approach with steel building.

Not saying that would be economically feasible right now. I'm just speculating on future possibilities if some of these more advanced steel materials were more permissible in our industry.

Obviously, I think it makes sense for AISC to be looking into this. Though I have no idea what will become of it, or how many years it might take.

 

[tmschrader]

Thanks for the input. The auto and mobile vehicles seems to have the most experience with these +80 to 130KSI Yield materials. Not sure if they reduce the safety factors used for 50-70KSI materials or not. I guess I will continue to do so till I see otherwise. Using .5 to maybe .6Fy for bending. And doing non linear Buckling analysis (Inventor) when local buckling is a concern.

With competion between auto makers being so prevalent, this info is probally not readily available or shared.