2001 AISI Standard - What is Sf?

[JSA2]

Does anyone know what Sf of Eq. C3.1.2.1-14 is in the 2001 Edition of the AISI Standard? Is it merely the elastic section modulus about the neutral axis of the shape? The definition of Sf seems to imply that it is a modulus taken about some other place, other than the neutral axis.

I'm trying to design a cold-formed track for flexure which is only braced between the studs - no sheathing or other material to brace the track to resist flexure.

Thank you anyone for assistance with this.

 

[ash060]

It is the Section modulus relative to the compression fiber for the unreduced section.


Pretty much it is the section modulus using the compression fiber for the whole section not the effective section; regular old Sx

 

[SteelPE]

I have always had a question in reference to the AISI (which is a code I find very confusing).

In the same section how do you calculate Sc?

 

[ash060]

Sc is the effective section modulus at whatever critical stress the section is at; typically calculated at Fy

Most stud manufactures supply the effective properties of their section at Fy. Check the SSMA catalog.

The procedure for caculating Sc is in Chapter B of the Specification. There are examples in the AISI manual as well.

 

[JSA2]

Thank you ash.

 

[SteelPE]

Thank you ash060

I have the 2002 AISI manual and it gives the calculation for Se evaluated at Fy (which is very long iterative procedure). I mostly use the book to calculate the capacity of C and Z gird sections (mostly C10x3.5x.105). They give the value Se evaluated at Fy=55ksi for these sections. From what you are saying Sc value is actually Se in this instance? Then if my moment is less than Mn I actually end up with a conservative result?

 

[ash060]

Yes, if you do not have LTB the Sc will equal Se which is calculated at Fy. If you do not have a fully braced section and LTB is possible than using Se is conservative because your Fc will be lower than Fy.

If you have the AISI manual and you check the beam tables they give you Mweb and Mflange which tell you how close you are to the whole section being effective.

 

[SteelPE]

So basically I have two choices when it comes to LTB.

1) Calculate Sc (using the procedures to calc Se) at a stress equal to Fc (Fe) when LTB is to be considered. This will give me the most accurate measurement of the bending capacity.

2) Set Sc = Se evaluated at Fy=55ksi. This will be conservative as my limiting LTB stress will be lower than Fy. Therefore I am not taking advantage of my lower stress in the calculation of Sc yielding a lower moment capacity.

Method #1 is used to create the Nominal Flexural Strength charts located on pages II-21to II-68 of the ASIS manual.

The calculation of Sc/Se is a pain. I have never been able to get the correct answer. I have looked into writing a spreadsheet but since I use excel I can’t really iterate in excel (without writing macros) until I get the write answer.