max compressive due to bending vs bending stress

[bangerjoe]

HI

I am using an old steel design code (AS 3990) which desont use limit states.

it is an australian standard.

i am a bit confused. i will reference the clauses but explain if no on ehas the standard.

5.2 gives maximum permissible bending stress as 0.66 of yeild.
5.3 gives formulae for maximum permissible compressive stress due to bending. (depends on section and other properties)

Here is my (silly) question: aren't they the same thing???

This is my logic: consider a centre load on a beam simply supported. If i do f=M/Z, i get bending stress. At the outer fibres on the compression flange, this is the compressive stress.
doesn't this mean that it is the same?? so why have .66 yeild AND the formulae for compressive stress?

Please ask questions if you would like more explanation.

 

[KootK]

Could it be that 5.2 is the check for a beam fully braced against lateral torsional buckling and 5.3 is the check considering lateral torsional buckling under a particular unbraced length.

Also, I think that it needs to be M/Sx rather than M/Zx for allowable stress.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[desertfox]

hi
Yes you're right, the bending stress that's calculated doesn't mean its got to equal 0.66*yield but the way I see it, is the 0.66*yield is the maximum limit, so if you calculate the bending stress from the formula it's okay for it to be below the .66* yield but not in excess of it.

 

[JStephen]

You probably need to look at the wording more closely.
The first section may be the maximum tensile bending stress.
Or it may say "Use Section A, Section B, or Section C as appropriate".
In our old ASD, there is one section if beam is laterally braced, then following sections for beams not laterally braced, or noncompact sections, etc. Then another section for tubes, and beams bent the easy way, etc.

 

[bangerjoe]

Hi all,

thanks for your quick response.

KootK - is the f=My/I and Z=I/y therefore f=M/Z? Is S not he plastic section modulus which is for limit state design? (asking to learn, i'm a noob here).

When i look at the wording,

5.2 is called maximum bending stress. 'the maximum calculated stress due to bending in a member shall not exceed the maximum permissible stress Fb determined by ... (either .75 or .66 Fy)

5.3 is called maximum permissile compressive stress. the maximum calculated compressive stress due to bending in any part of a beam shall not exceed the maxium permissible stress Fbc, which will be greater of 0.6FY and... (list of formulae).

There's no guidance as to whether to consider bracing, etc. etc.

Saftest bet would be to go thru the forumlae and choose the lowest value?

I suppose what might the reason for is if it was an unsymmetric section (for instance a T) where the stress is different in compressive vs tensile bendingg stresses?

Thanks again for everyone replyies.

 

[KootK]

It's S = I/y. Z implies a fully yielded section which would be inconsistent with an allowable stress of 0.66 Fy.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[BAretired]

KootK, the terms S and Z do not have the same meaning in different parts of the world. Here, S is section modulus and Z is plastic modulus. In Australia, it is the other way around. Confusing but true.

BA

 

[BAretired]

5.2 gives maximum permissible bending stress as 0.66 of yeild.
5.3 gives formulae for maximum permissible compressive stress due to bending. (depends on section and other properties)

This could have been stated:
Maximum permissible bending stress shall be in accordance with these formulae but shall not exceed 0.66 times yield stress.

BA

 

[KootK]

Seriously? Okay, I take it all back then. I'm shamefully ethnocentric. Thanks for the clarification and sorry for the confusion.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[bangerjoe]

Thanks everyone, especially KootK and BARetired