structural factors of safety (help a mechanical guy out)

[keyen]

Hi, I'm a mechanical engineer, but every now and then I'm tasked with something that's more structural in nature. I've trying to figure what kind of factors of safety are used in structural designs, and I'd like to double check what I've found with some of you. According to ANSI/AISC 360-10 paragraph F1, the factor of safety for members in flexure is only 1.67. This seems low to me (based on my pressure vessel experience). This random site indicates 4-6 is acceptable: [URL unfurl="true"]http://www.engineeringtoolbox.com/factors-safety-fos-d_1624.html[/url]. Am I missing something, or are steel structures built to a factor of safety (for flexure) as low as 1.67?

Please note, I'm not designing any buildings, just some simple structures for our fabrication yard.

thanks

 

[JAE]

I think your link is screwed up a bit on the building design aspect.
For buildings the general safety factors aren't quite that high.

For Allowable Strength Design the AISC Specification provides for safety using the Ωo factor for flexure at 1.67 as you indicated.

This means that the allowable stress in a steel member would be reduced by the factor 1.67 on its yield.
So the SF does = 1.67.

AISC also provides a separate load factor design method where safety factors are divided between the loads and the strength.
This has the advantage of responding to the variations in loads and the variations in strength.

The anticipated strength of the member is reduced by a φ factor (usually 0.9 to 0.75) to account for the variations and possible inadequacies of estimating the strength of the members.
The applied loads are factored up to account for the variability of the loads - 1.2 for "dead" loads and 1.6 for "live" loads as an example.

So if the average load factor is about 1.5, and the φ factor is 0.9, your overall safety factor is 1.67, very near the ASD method.

The higher factors you see on that link are perhaps there for more dynamic loads, or for more dangerous loads (pressure vessels exploding) where life safety risks are much higher?
I don't deal in pressure vessels so can't say for certain.

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[keyen]

JAE, thanks for the very quick reply. What you've said confirms what I've been reading. Thanks.

I have a follow up questions: if you were designing supports like the ones shown below or a large pipe rack like in the second pic, what would be a suitable design code? AISC 360 doesn't seem appropriate. Let's assume the pipe codes do not specify any of the structural needs.

 

[TehMightyEngineer]

Yep, 1.67 is the typical safety factor for flexure, tension yielding, and compression. Connections are usually higher at 1.75 to 2.0 (excepting shear yielding which is 1.5). Building design takes a lot of things right to edge and there is little room for engineering error.

For industrial design (which you probably do if you do pressure vessels) then you'll be happy to know that most people do not design industrial building structures to the minimum safety factor most of the time, though flexure usually is right to the limit even in industrial design. I'll often see simple structures with safety factors of 2 or 3 in industrial settings.

It's worth noting that monorails and cranes (above the hook items) are also given similar low safety factors. Not all 1.67 necessarily, you'll probably see a lot of 2.0 safety factors. It depends on the service class of the crane.

Maine EIT, Civil/Structural.

 

[TehMightyEngineer]

As far as pipe supports you can use AISC 360 easily enough. Steel is steel and it's not going to care what code you use, it will fail when it's capacity is exceeded.

For the pipe bridge you've shown I would absolutely use AISC 360 and it is included in AISC 360's scope. See A1 in the specification, you'll see that it covers not only buildings but structures of steel "designed, fabricated, and erected in a manner similar to buildings, with building-like vertical and lateral load resisting elements."

Maine EIT, Civil/Structural.

 

[dhengr]

Keyen:
Why not ask your boss what safety factors and standards you should be following for this work? Most industries and types of equipment have some stds. and norms that they follow for their specific design. In your case various ASME piping stds. would be a good place to start looking. Much of this type of design work takes a Mechanical Engineering tack on the problem. The break point or separation point/level is that the pipe carrying the fluid, the pressure vessel containing the fluid/process is designed under one code and to a higher std., and then almost everything else (outside of the outer skin) is designed under another code or code division and to a lower FoS. Contracts with clients/customers, construction documents and specs. usually call out the codes and stds. to be followed. As mentioned above AISC 360 doesn’t really cover your kind of design or product, but it is still a good guide to follow for steel design as regards the basic Engineering Mechanics of the various Structural Design aspects of your problem, if you can only wade through the complexities that have been introduced in the last few editions. Because of these newer added complexities it has become more difficult to use AISC 360 as a design guide for anything other than bldgs., you just can’t find the basic design concept or mechanical concepts for all the extraneous b.s. You might be as well off to go back to your Engineering Mechanics, Strength of Materials and Machine/Product Design textbooks for your basic guidance.