Allowable bending stress

[swerider]

Hi
I need some help with calculating the maximum allowable bending stress for a mild steel flat bar (fy = 248MPa). Is there a method for calculating this or do I just use 0.66*fy? Does it matter whether bending is about minor or major axis?
Thanks

 

[GregTirevold]

Well it depends on who you are designing this for. If they want it done to a certain spec then you should use whatever allowable is shown there. The one you mentioned is from AISC and is widely used but not exactly a hard rule if you are not required to design to AISC spec.

If this is a fatigue/alternating stress application then you'll need to check the loads against a Goodman or Soderberg diagram.

The direction you bend it will change your stress due to that load but not the allowable since steel is isotropic.

http://www.linkedin.com/in/gregtirevold

 

[jte]

Depends on the code. Could be 0.6*Fy, could be 0.66*Fy, could be 1.0*Fy. I've seen all of these in different codes for different equipment / structures.

Some codes will limit stress based on the idea of reaching yield at extreme fibers or yielding the entire section before buckling becomes a potential failure mode - so yes, it may matter which axis the bending is about.

jt

 

[hokie66]

In most codes, the allowable about the weak axis is greater than about the strong axis. .75fy is common about the weak axis.

 

[boo1]

first define the application and what country your are in

 

[JStephen]

The current AISC specification includes allowable bending for flat bars turned on edge (lower stress to allow for buckling). Applicable to structural steel.

 

[boo1]

Why would you use AISC (structural steel buildings) for a mechanical design (post is the mechanical engineering section)?

 

[GregTirevold]

AISC can be valuable for people doing steel fabrication of heavy equipment, like myself, because about 95% of the steel we use is structural shapes (channels, beams, angles, bar) and structural plate (ASTM A26, A572 Grade 50, A656 Grade 80) that is burned and formed. The AISC code is oriented for construction but all the materials are used in many other industries that lack their own spec/code book, so the numbers are a good starting point for a relatively cheap ($350) book.

I'm not sure what other specs to refer to regarding bolted/welded steel fabrication. The SAE, from what I have seen, has very little in the way to offer. I saw one link a member here posted for an out of print SAE handbook for off-highway equipment and I think they still have a magazine on the topic but that is all I'm familiar with. They may have more to offer but their price structure isn't really friendly for small companies to "browse".

What would you recommend he use? I'm always up for new reading material.

http://www.linkedin.com/in/gregtirevold

 

[hokie66]

Steel is steel. We structural engineers are happy to share. AISC is not just buildings, it is steel construction of all types.

 

[boo1]

My AISC code says: "Specification for Structural Steel Buildings, Allowable Stress Design and Plastic Design".

If you can apply it to your design or industry great use it (good reference). Some times I can use the AISC code, some times I can't. Different codes may govern the assembly. A different SF might be selected that applies to the uncertainty of materials, loading, calculations, material strengths, duty, or manufacture quality.

http://www.roymech.co.uk/Useful_Tables/ARM/Safety_Factors.html

Cheers

 

[JStephen]

I didn't say that AISC ought to be used here, just pointed out that it has allowable stresses for a buckling situation that is not well addressed elsewhere (or at least, I've never seen it anywhere else). The reference to 0.66Fy in the original post is typical AISC allowable stress, so the suggestion doesn't seem out of line. Reference to that method, or similar buckling criteria, will also answer the question of "Does it matter whether bending is about the major or minor axis?" If one of those British Standards also gives an allowable stress for flat bars turned on edge, that would be handy to know as well, but I've not used them to know.

The AISC code in question is a free download. The way the code is written, it is very convenient to adjust the factor of safety if the application requires it as well.

 

[swerider]

Thanks for the replies guys. I am in South Africa and the application is for a steel frame attachment for a motorcycle. I am using square tubes and flat bars for the construction, but am considering replacing the flat bars with small square tubes because of likelihood of twisting of the flat bars.
Thanks again

 

[MALK]

I am assuming this for your own personal use. Please consult any relevant codes if not.

One way of looking at this is as a 'safety' factor. 0.66fy without any additional load factor is the same as a safety factor of 1.5. So decide what safety factor you require and work to that, for all the stresses in your design.


I am doing a similar project (luggage rack) and will be looking for a load factor of 3 or 4 times the anticipated working load to account for my lack of fabrication skill and any other unknowns. If I were making a fairing bracket I might design against a utilisation factor of nearly 1 on yield depending on the consequences of any failure.

 

[Ron]

The key to your question, as others have alluded, is "allowable" stress. Allowable by whom? This is where you need to check codes, standards, and local practice to see what "allowable" references.

0.66Fy is reasonable and acceptable for most building applications in the US.

The stress level will depend on the bending direction, as the section modulus of the flat bar changes with direction.