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# Allowable design stress for Alum more then Stainless?

## Allowable design stress for Alum more then Stainless?

(OP)
Hello.

By the Alum Design Manual the allowable bend stress is (1.17x yield)/Ny= 1.17x35)/1.65=24.8KSI. Ref eqaution 6-2 and table 6-3. There are other equations but this governs bending.

For Stainless Steel 304L by the ASCE by ASD (Allowable stress Design) the allowable stress is:
Stainless Ult/yield= 80ksi/30ksi (away from welds) for easy to weld 304L is= yield/1.85 (per table 1). Or 30/1.85=16.2ksi.

So Alum allowable is 34.5% more then stainless. (24.8-16.2)/16.2. In situations where a catastophic falure would be worse then some local yielding, stainless would be safer by a factor of 80ksi/42ksi=1.9 times. and so would prevent injury or even death by that factor.

Part of the reason is that the ASCE in section 4 says you can use the 'nominal strength specified in design provisions' and most tables list min values or 'typical'. Typical should be the same as nominal.
The other reason is it is based on yield not ult. Another reason is the 1.85 factor compared to the 1.65 from two different code standards.

There is another body or code that lists allowable bend stresses for stainless. Does anybody have that value. I would have to dig thru my files to recall the name of that body or code.

The ASME below shows a fcator of 1.46 Yield/allow=205/138Mpa. But i believe that is geared towards boilers not structural design.

https://www.cis-inspector.com/asme-code-calculatio...

### RE: Allowable design stress for Alum more then Stainless?

Check out AISC Design Guide 27 "Structural Stainless Steel". Chapter 6 discusses the design of members for flexure. Early chapters might discuss the allowable bending stress but I am not sure; it has been a while since I looked at it.

### RE: Allowable design stress for Alum more then Stainless?

Per SEI/ASCE 8-02 (Cold formed Stainless Steel), section 3.3.1.1, I believe that you can use a 1.25 bump for allowable stress when bending in weak axis.

And if welding is an issue, the large reduction in capacity for weld affected areas in aluminum has to be considered.

### RE: Allowable design stress for Alum more then Stainless?

The right grade of Stainless for the local environment (salt, water, corrosive chemicals, pitting, stress corrosion, cyclical exposure or whatever) will (over time) retain its strength better than most generic or uncoated Al exposed to the same chemicals and localized stresses.

Galvanic corrosion (at fasteners or dissimilar metal contact) will affect both differently.

Your question seems to address only "middle of beam" calculated and Code allowable stress levels, not the rest of the world around the two potential metals.

### RE: Allowable design stress for Alum more then Stainless?

there is about 100 different typical alloys of aluminium, with various temperings, leading to various ultimate and yield strengths, varying from under 100MPa to over 400MPa.

all aluminiums are not created equal, it is important to know your alloy & temper.

### RE: Allowable design stress for Alum more then Stainless?

Hello thanks for the input. Yes I should have qualified the Alum as 6061 T6. One of the most common. With yield of 35 per Alum design.
Galvanic corrosion is another definite issue,and over time stainless will definitely look better.
Thanks for jogging my memory. Raptor77R. The AiSC uses 1.65 vs 1.85 of the ASCE. With several qualifications. The link and pdf of this ref 27 (I think) can be downloaded off dogpile. Could not find it on google. It is not as good as it used to be. Too many adds and it repeats hits. I am not at the same computer now. But I will post the link to a free download of the AISC document on stainless later, If anyone would like and request it.

Note that stainless does not have a definitive yield, like steel does.

Yes there is a big reduction in strength near welds for 6061T6. From like 40ksi to about 27. The reduction is a lot less for 5083.

### RE: Allowable design stress for Alum more then Stainless?

There are duplex stainless steels with similar chemical resistance and higher strength, if you need the strength.
The sum is: There's lots of different stainless steel alloys and lots of different aluminum alloys and tempers, and the ranges of mechanical properties overlap.

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