Weak axis bending of A36 plate 2

• Oct 22, 2015

• #2

bootlegend

Structural

Mar 1, 2005

289

The actual stress is still M/S. You just have a higher allowable stress for the weak axis bending.

 

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• Oct 22, 2015

1
• Thread starter
• #3

Deadblow

Structural

Jul 13, 2015

141

I am confused. Here are my numbers for a 2.5"x10"x14" A36 plate. As per AISC F1.1 Ω=1.67 therefore allowable bending stress is Fy/Ω=36,000psi/1.67=21,556psi. Now I look at AISC F11.1 and see that plate bent about the weak axis has a nominal moment capacity of Mn=Mp=FyZ=36,000psi(10")(2.5")(2.5")/4=562,500lb-in. Allowable moment, Mn/Ω = 562,500lb-in/1.67 = 336,826lb-in. Now if I divide the allowable moment by S, I get 336,826lb-in/[(10")(2.5")(2.5")/6]=32,335psi > 21,556psi. I don't understand how when I divide my allowable moment by S, the stress is larger than the allowable stress. If I divide the allowable moment by Z, 336,826lb-in/[10"x2.5"x2.5"/4]=21,556psi = 21,556psi. Please help me make sense of this.

 

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• Oct 22, 2015

• Thread starter
• #4

Deadblow

Structural

Jul 13, 2015

141

Or am I wrong by calling my allowable stress as 36,000psi/1.67?

 

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• Oct 22, 2015

• #5

sbisteel

Structural

Feb 24, 2011

178

The stress distribution changes when you get into the plastic region. Calculating stress from M/S when using allowable plastic moment will give you bad numbers.

 

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• Oct 22, 2015

• #6

bootlegend

Structural

Mar 1, 2005

289

Maybe my comment confused you. For a rectangle (weak axis bending) Z = 1.5*S so if you look at it that way it is an allowable stress increase, but it is based on the fact that you can develop the plastic moment in weak axis bending of a plate. Should be spelled out much more coherently in a steel design text.

 

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• Oct 22, 2015

• #7

DaveAtkins

Structural

Apr 15, 2002

2,894

In the 9th Edition of AISC, the allowable stress for weak axis bending of a rectangle was 0.75*Fy. This was compared to M/S. In the 14th Edition, stresses are no longer checked. Instead, the allowable moment (not stress) is Mn/Ω. Mn = Fy*Z <= 1.6*My. Interestingly, if you compare the two methods, the 14th Edition is less conservative.

DaveAtkins

 

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• Oct 22, 2015

1
• #8

Hokie93

Structural

Sep 9, 2007

375

If you convert Equation (F11-1) from AISC 360-10 to a stress-based format for weak-axis bending of a plate, the allowable bending stress is 0.90Fy. As DaveAtkins pointed out, this compares to 0.75Fy from the 9th edition AISC Steel manual. The 0.90 coefficient comes from 1.5Fy/1.67, where 1.5 is the shape factor for a rectangle and 1.67 is the factor of safety.

 

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• Oct 22, 2015

• Thread starter
• #9

Deadblow

Structural

Jul 13, 2015

141

Ok, thanks guys! Forgive me as this is my first year out of school. The engineer I work with always compares allowable stress to actual stress but he uses the 9th edition. So if I use the 14th edition I just design for the allowable moment and I never check stress?

This will be difficult for me when I have to check a flexural member that also experiences torsion. In the past, I would check the actual stress caused from the flexure added to the stress from the torsion (AISC Design Guide 9) and compare that to allowable stress. On a past job, I actually used M/Z to find my bending stress then added the torsional stress to it and compared it to Fy/1.67. Is this wrong?

 

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• Oct 22, 2015

• #10

canwesteng

Structural

May 12, 2014

1,701

Design guide 9 recommends conservatively not allowing yield to occur iirc so yes, using M/Z would be wrong. Although I would confirm that with the guide before taking my word as gospel.

 

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• Oct 22, 2015

• #11

sbisteel

Structural

Feb 24, 2011

178

The 14th edition is written to check moment capacities for flexure, no stress check is required. However, when you get into LTB or slender members you end up calculating an allowable stress (Fcr) as part of the design process.

Combined torsion & other stresses should be analyzed using an allowable stress method per H4.3 (on open shapes).

 

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• Oct 22, 2015

• Thread starter
• #12

Deadblow

Structural

Jul 13, 2015

141

Yes, you are correct I just checked it. Design guide 9 equation 4.5 states that elastic stress due to bending is M/S. It also states that the limit state of yielding under normal stress is <= 0.6Fy (eqn. 4.17). Thanks for catching my error canwesteng. Please note I am referencing Design guide 9 copyright 1997.

 

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• Oct 22, 2015

• #13

Lomarandil

Structural

Jun 10, 2014

1,917

It's not that you'll never check stresses in the 14th edition -- just that the primary checks are force/moment based rather than stress based. Your example of combined bending and torsion is one where the check is still stress based (except for HSS) per AISC H3.3

In your case, I'd be inclined to revert to determining bending stress using M/S for the combined bending and torsion check (only) -- but that may just be me trying to be conservative and a little skittish around torsion. I'm curious to hear what the others will say there.

(Typing too slow, beat to the punch)

 

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