Steel Design

[PEin2010]

Reference LRFD AISC 13th edition:

To find the critical combined stress ratio, an equation, [(Pu/2*phi*Pn)+(Mux/phi*Mux)] is used. I'm not able to understand where the denominator terms are coming from, basically where this whole equation comes from and what it means?!

I know Pu comes from given loading conditions and Mux and phi*Mux are given.

Thanks!!!!!!

 

[civilized_naah]

Reference LRFD AISC 13th edition:
To find the critical combined stress ratio, an equation, [(Pu/2*phi*Pn)+(Mux/phi*Mux)] is used. I'm not able to understand where the denominator terms are coming from, basically where this whole equation comes from and what it means?!

I know Pu comes from given loading conditions and Mux and phi*Mux are given.

Thanks!!!!!!

Actually, it is [(Pu/2*phi*Pn)+(Mux/phi*Mnx) +(Muy/phi*Myx)]

Pu = factored axial load

Mux = factored bending moment about strong axis

Muy = factored bending moment about weak axis

These will be given from analysis results

phi Pn is the axial load capacity of the member (Tension or compression) when considered as a purely axially loaded member.

phi Mnx and phi Mny are flexural capacities of the member when considered as a pure beam (bending about strong and weak axes respectively)

This expresses the fact that when axial and flexural loads coexist, this expression gives an approximate measure of the PASS/FAIL (expression less than or greater than 1.0) of the member as a result of combining those stresses. Equations of this type are called INTERACTION EQUATIONS.

 

[XOXOXO]

Reference LRFD AISC 13th edition:
To find the critical combined stress ratio, an equation, [(Pu/2*phi*Pn)+(Mux/phi*Mux)] is used. I'm not able to understand where the denominator terms are coming from, basically where this whole equation comes from and what it means?!

I know Pu comes from given loading conditions and Mux and phi*Mux are given.

Thanks!!!!!!

You really should break out your AISC CD (should have come with the 13th edition), and go through all of the example problems. They have them categorized (tension, bending, shear, combined, composite, connections, etc).

They give you every friggin example problem you could possibly face, and step by step on how to solve using ASD and LRFD.

Then you will have no more questions about when to use Z vs. S, or the omega factor for max values, or where the formulas are from or how they are used.

It took me quite some time to go through all the examples (try and solve them yourself, then go through their step by step) but it was well worth it.