yield strength alum 6061 T6 vs ss steel 304

[Lucian09]

Hi guys,
I don't deal with this every day as an architect but can anyone please clear up my confusion regarding the yield strength of these 2 metals above? I'm getting 35ksi for the aluminum and about the same for the ss steel, which cannot be. And then do you take the working Fc=Ft=Fv all at 0.6 the yield strength? Thanks.

 

[r13]

Aluminum alloys are divided into the wrought and cast categories according to how they are produced. And the mechanical properties vary wildly. Suggest to check on AISI for information.

 

[phamENG]

Lucian,

There are, indeed, some strong aluminum alloys out there. And many stainless steels are a bit softer than your typical carbon steels. Neither of these materials are simple to design with, though. If you're talking standard structural shapes it's a bit easier, but they are still more prone to local buckling effects and other issues not usually considered in structural steel design. For complex extruded shapes (like window mullions), it gets pretty interesting. They are also affected by welding - it's called a Weld Affected or Heat Affected Zone. Much of the material strength comes from tempering, and melting it removes the tempering without a costly post-welding treatment. That can cut the strength in the WAZ down to half of the non welded strength in some cases.

To design with Aluminum, you'll need the Aluminum Design Manual. The most recent version is 2020, but check the building code in use in your jurisdiction; it'll give the appropriate publication year to use.

To design with Structural Steel, you'll need AISC Design Guide 27. I think they mentioned during the Steel Conference this year that a new Stainless Steel Spec is coming, but for now the design guide is all we have to use.

 

[rb1957]

it can be !

Annealled SS has ftu = 75ksi, fty = 30ksi

6061T6 has Ftu = 42ksi, and fty = 36ksi

this from our materials book (AR-MMPDS). You can find similar online as "Mil Handbook 5".

another day in paradise, or is paradise one day closer ?

 

[r13]

Get material certificate from the supplier.

 

[Aesur]

Be careful with Aluminum design, it's not as simple as applying a safety factor to the shape. The strength varies depending on the shape itself and how it buckles, I have seen some shapes with strength less than 10 ksi (much lower when welded), in fact I have seen few shapes have a stress higher than 18 ksi over the years of designing solar racking and canopies. As phamENG mentioned above, you would need the Aluminum Design Manual to properly design aluminum.

 

[Agent666]

The 0.2% proof stresses are probably similar. That's what you'd typically be using for design. This keeps the materials close to the elastic (recoverable) strain, since these materials do not have a defines yield plateau like structural steel, instead they follow a ramberg osgood relationship. The ultimate strength is likely much higher, but somewhat irrelevant for typical member design.

https://en.m.wikipedia.org/wiki/Ramberg–Osgood_relationship

 

[Lucian09]

Thanks! After years of being in love with 304 and other ss steels, I'm leaning more and more toward 6061 (T6). True, yield point is less well defined. and I shall further research the books for a safe figure to use. To simplify the original question to shear only though: say, a 1/4" thk alum plate or tube, drilled at one end and hanging from some support with a 1/2" dia through bolt. What sort of material shear strength would you apply to that?

 

[rb1957]

Fsu for 6061T651 is 28ksi (if you buy aerospace grade AMS4025,6,7).

what advantage does Al have you that good ol' SS doesn't ?

another day in paradise, or is paradise one day closer ?

 

[strucbells]

Good information provided above, just wanted to add- don't forget about stiffness. The elastic modulus of stainless steel is nearly 3x that of aluminum.

So if you're designing any sort of structure that is likely to be deflection-controlled, stainless steel should be more efficient.

 

[phamENG]

Thanks for adding that in strucbells. I was working on an aluminum truss this afternoon and that jumped out at me. This was my first chance to come back to this thread to throw it in. 6061 has a modulus of 10,100ksi, stainless and carbon steels are about 29,000ksi.

 

[Lucian09]

Yes, stiffness - that's why I was so incredulous at aluminum's "strength" in the first place. Ballpark I would always think about steel as generally 3x "stronger" than aluminum, but isn't it inappropriate to think in modulus and involve strain here? Strain in aluminum being higher than steel's, modulus would be lower, but isn't all that discussion moot because we apply safety factors and we never reach yield point?

 

[Lucian09]

rb1957 - thanks. (First of all I always avoid straight steel as I don't see the point of building with something that would start losing its strength and deteriorate / rust right away, or require high maintenance). My reasons to really like 6061 is its incredible relative strength, lightweight, and workability compared to ss steel. And price I guess. Have you ever tried to drill larger dia holes in ss steel, even with cobalt/titanium bits? But, as I said, I'm still skeptical about the true strength of alum.

 

[rb1957]

those are FAA/USAF allowables, good as gold ! … if you buy aerospace quality Al.

well, there's always a price to be paid for everything in a design … pick your poison !

good luck

another day in paradise, or is paradise one day closer ?

 

[strucbells]

Yes, stiffness - that's why I was so incredulous at aluminum's "strength" in the first place. Ballpark I would always think about steel as generally 3x "stronger" than aluminum, but isn't it inappropriate to think in modulus and involve strain here? Strain in aluminum being higher than steel's, modulus would be lower, but isn't all that discussion moot because we apply safety factors and we never reach yield point?

Think you are confusing some things here. My comment was re: serviceability-related deflection considerations, not strain-related strength considerations. Eg- if your aluminum floor beam is deflecting at L/80, the stress level may be totally fine but your occupants aren't necessarily going to be happy with that level of bounce.

 

[drawoh]

Lucian09,

Note also the difference between the ultimate strengths. High strength aluminiums are brittle.

--
JHG

 

[r13]

Aluminum corrodes too, especially in the salty environment.

Preventing Aluminum Corrosion
You can’t do much about aluminum oxidation, and unless appearance matters, it’s not a big problem. Aluminum corrosion could however be a serious issue. If there’s any possibility of it happening you have two options:

- Apply a protective coating.
- Minimize or mitigate the effect of galvanic corrosion.

You should also consider the grade or series of aluminum you’re using. Some of theses, notably 5052 and 3003, have better corrosion-resistance properties than others. More generally, 1xxx, 3xxx, 5xxx and 6xxx series alloys offer good corrosion resistance.} ]

 

[Aesur]

@retired13, you are correct, Aluminum does corrode, however this typically happens when the aluminum is first exposed to water or oxygen. This typically forms a layer of protection, preventing further corrosion below the surface. It has been years since I have dealt with aluminum in salt environments, however I don't recall anything extra being done for this environment. However, as you noted, galvanic corrosion is a major issues as it is common to use steel and aluminum together, including fasteners, which without proper protection will cause further corrosion over time. The typical architectural finishes for aluminum are Kynar paint, Anodized Aluminum or Baked Enamel, each having their benefits and disadvantages.

There are multiple mentions of Aerospace Aluminum, isn't the AMS designation just another specification that refers to specific alloys, which are designed per the ADM? For instance, AMS 4025 refers to 6061-0 and AMS4027 refers to 6061-T6 for sheets and 6061-T651 for plates, both of which only cover sheets or plates, but not extruded, etc.. If you wanted 6061-T6 drawn tube this would be AMS4082, unless it is hydraulic drawn tubing then its AMS4083, if you wanted 6061-T6 extrusions, then it's AMS4150. I don't believe Aerospace Aluminum is of any superior quality, just a different way of specifying the aluminum. Here is a good breakdown of the AMS designations versus the alloy to design the shape as: Link