Area of compression flange.

[LicensedToPEe]

I have a few double angle braces at various orientations that will require to have a plate welded along one of the sets of the legs. I have calculated all the combined sections properties (double angles plus plate) no problem. When it comes to the point of calculating of the allowable bending stresses using eqs. AISC ASD F1-6 thru F1-8 I need to determine the area of the compression flange. How does one go about that if the section is non-symmetrical, meaning there could be 2 flanges about x-axis and two flanges about y-axis, depending how the sections is oriented and where compression/tension occurs? In some cases, I only have two angle legs (webs) above the neutral axis, so does that mean there is zero compression flange?
Any help appreciated.

 

[TPNY]

I believe that determining the area of the compression flange is for local flange buckling. Since you have no flange in compression, you need only to worry about localized web buckling, correct?

 

[UcfSE]

You should try the LRFD spec, F1-15 and F1-16 is specifically for tees and double angles, something you don't have in the ASD.

 

[LicensedToPEe]

Zo40,

That is true but I need it to find rt and ld/Af quantities to be used in the equations cited above. See AISC ASD
Chapter F1-3. Below is the section in guestion. If bending occurs along the horizontal axis, one can see the "flange" (that being one of the vertical legs of the angle) is very close to the neutral axis and is engaged after the vertical angle leg and half the added plate. So, I am not sure as to how to define the compression flange in this situation.

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[LicensedToPEe]

I should add the sections are L3.5 x 2.5 x 5/16" with a 8"x3.8" A36 plate welded as shown.

 

[LicensedToPEe]

Sorry for the typo, it is a 8" by 3/8" plate (not 3.8").

 

[SlideRuleEra]

AdamU - The equations you reference are for "compact or noncompact sections as defined in Sect. B5.1"
For the above quote, see the first sentence in Sect. F1.3 (page 5-46 of my copy of 9th Ed AISC).

Sect. B5.1 (page 5-35) says "steel sections are classified as compact, noncompact and slender element sections".

Some of your examples sound like slender element sections, covered in Sect. B5.2.

Sect. B5.2 refers you to Appendix B5 (page 5-98).

AISC has really hidden the information - must be a good thing that the manual will soon be replaced.

http://www.SlideRuleEra.net

 

[UcfSE]

With your angles turned up like that you don't have a flange obviously, so you cannot use equations that require a flange dimension or a property based on a flange, such as rT or Af.

 

[UcfSE]

Depending on your judgment you might be able to define some dimensions to use for rT, since rT is based in part on 1/3 of the stem in compression.

 

[LicensedToPEe]

That's what I figured but since I am pretty much guessing (with engr. judg.) the parts of the dimensions to be used to define the above mentioned properties, I don't want to end up with allowables smaller by an order of magnitude from 0.60*Fy. The thing is my member is a brace (compression/tension only) and adding a plate will induce a bending which becomes criticcal since the axial stress ratio is around 0.8 and the brace is 38 ft (unsupported) long.

 

[JAE]

AdamU - Equations F1-6 through F1-8 are not applicable to your situation - Read the second paragraph of Chapter F - the whole of chapter F is for "singly or doubly symmetric beams including hybrid beams and girders loaded in the plane of symmetry.

You are not loaded in the plane of symmetry...thus, Chapter F here does not apply. This is a similar condition to WT shapes with web sides in compression.

ASD here is silent on what specifically to do. Some engineers use a very conservative value for Fb such as 0.4Fy or less in these cases. But using the Chapter F equations for a leg in bending-compression isn't in line with the spec.

 

[LicensedToPEe]

Thank you all for your comments. I actually ended up using a conservative aproach for Fb = 0.45 x Fy.

Each time I use this forum I can't be thankful enough that it exists!