factor of safety

[shnider32]

Hello every one;
I am a beginner in steel design ,i have a mezzanine joist as a simple beam with distrubuted load . if the stress ratio is 1.3 so it means it will collapse or it will be within the factor of safety and what is actually the value of factor of safety for this case (knowing that i am usin ASD method)

 

[DaveAtkins]

When you say "stress ratio," are you talking about the output from a software program? If so, the factor of safety is already built in, so anything >1.0 is acceptable.

I also recommend you design the joist by hand (without a computer) as a check of the computer output.

DaveAtkins

 

[andriver]

I think (or hope) Dave meant to say anything < 1.0 is acceptable. Which your situation as you have alluded to is not.

This should be a simple problem for anyone familiar with math should be able to figure out on their own. But first, you need to determine in what failure mode are you at 1.3 UC? For example, is this in bending, shear, compression etc? Then you find what the FOS is for that method. For example in AISC, bending has a 1.67 FOS in ASD. Your "equivalent" FOS would be 1.67/1.3 = 1.28. This should be intuitive for an engineer. I also would not accept a 1.3 UC, unless you could justify it with some underlying very conservative assumptions. Which if you were being conservative, I would recommend "sharpening" your pencil to shave off that conservatism to determine your actual UC.

I agree with Dave, that a simple beam should be done by hand calculations, especially for new engineers.

 

[rb1957]

depends on the program, maybe? I'd've thought a "stress ratio" =1 meant allowable = applied, and >1 means allowable > applied (like our Reserve Factor).

but not knowing any better, >1 could be good or bad ...

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

 

[DaveAtkins]

Sorry, I did mean <1.0.

But a hand check will reveal what the stress ratio really means...

DaveAtkins

 

[shnider32]

I am using staad pro . this ratio = actual stress / allowable stress.

 

[rapt]

You would appear to have answered your own question.

All you have to do is find out from the manual/software vendor what the program is calculating as allowable stress. This should have been your approach before you even designed anything. How can you use software without knowing what it is doing in its calculations and the meaning of the results it is giving to you.

And do a hand calculation to verify the values and your conclusions.

Then have a think about the question you asked and decide if anyone could have given a logical answer with the information you gave to us. Something akin to "how long is a piece of string"!

 

[racookpe1978]

OK. So your "simple beam" has a 1.3 Factor of Safety (meaning its assumed strength is only 1.3 times what your assumed maximum stress levels will be - if all of your assumptions are correct over the entire life of the beam and mezzanine.

How will you assure the future operators and users over the next 50 years that your assumptions about loading are correct will ALWAYS be followed exactly as you assume?

What Factor of Safety are your joints holding those beams in place into the wall/frame/columns?

 

[Lomarandil]

Hold on racook... if the ratio (actual stress/allowable stress)=1.3, then he does not have a factor of safety of 1.3. In this case it appears the beam is stressed beyond the allowable limit.

That point aside, the advice about performing separate hand calculations to verify the computer output are the real takeaway here.

 

[racookpe1978]

True, all too true.