Floor Plate Bending 2

[ToadJones]

Anyone know why AISC 360-05 is now using the plastic section modulus for calculating bending in plates?
This seems to be a pretty significant change from 1989
This came up in comparing the floor plate capcity tables of 89 vs 05

 

[AVak]

I think the table in question is primarily deflection based. AISC has probably updated to using the plastic section modulus for plate bending to keep consistent with the provisions for structural shapes.

Adam Vakiener, P.E.

 

[ToadJones]

AVak-
there are two tables....one deflection based, one based on flexural strength.

 

[BAretired]

Toad. I don't have the tables, but the one that is deflection based would use elastic section modulus. The one that is strength based would be a factored moment based on plastic section modulus.

BA

 

[spats]

ToadJones is correct. Table 3-18b on page 3-155 of the 13th Edition Steel Construction Manual uses plastic section modulus for ASD design... doesn't seem right. Definitely larger capacities than the old 1989 Table on page 2-145, but deflection will normally control.

 

[ToadJones]

Spats-
That's exactly what I was getting at.
I have run into this in other parts of the 2005 spec where the plastic section modulus was used in ASD design...just doesn't jive in my head.
Page 3-29 in the code states that the flexural capacity of the plates for Table 3-18b are based on 16 ksi for ASD.
But, the calculate the bending stress using M/Z, not M/S like the old ASD.

 

[BAretired]

That sounds strange. I agree that for ASD, S (elastic section modulus) should be used, not Z (plastic modulus). On the other hand, 16 ksi sounds unusually low for steel plate material.

BA

 

[spats]

I don't know where the 16 ksi comes from. It's never explained, but used in both versions. I was under the general impression that allowable through-thickness plate bending was 0.75 Fy.

 

[ToadJones]

Spats-
The if you work the tables backwards you'll find they use fb= M/Z and fb = 16 ksi.
The 1989 code used 16 ksi as well, but it that was from M/S.

 

[ToadJones]

http://www.modernsteel.com/steelinterchange_details.php?id=61

 

[nutte]

This question is also covered in the current issue of Modern Steel Construction. Old ASD gave you M = 0.75 Fy S. Current ASD gives you Mn/Omega = 0.6 Fy Z = 0.6 Fy (1.5 S) = 0.9 Fy S. We've gained 20%.

http://www.modernsteel.com/Uploads/Issues/February_2011/022011_Feb11_si_web.pdf

 

[hokie66]

Looks like to me you've lost 20%.

 

[miecz]

Keep in mind that AISC changed the meaning of ASD when they issued the 13th edition. No longer does it have anything to do with elastic stresses. After failing for 20 years to sell LRFD to the engineering community, looks to me like they devised a trick to confuse the issue (Now you can have ASD!) and it worked. From my point of view, it was the low road. Anyway, there's no reason tables in the 9th Ed. should match those in the 13th. The 13th ASD is really just re-fractured LRFD.

 

[nutte]

Miecz, I disagree. The old ASD didn't rely solely on elastic stresses, either. That's why the allowable stress factor for the case in question was 0.75 and not 0.6. It's also why the allowable stress ratio for fully braced wide flanges in bending was 0.66 instead of 0.6. 0.66*Fy*S (old ASD) is about equal to 0.6*Fy*Z (current ASD).

Hokie, I'm also not sure why you say we lost 20%. Current 0.9 > Old 0.75.

 

[hokie66]

For a given plate, the new code sounds like it gives a higher loading, which comes at the cost of the reserve which you have lost.

 

[JoshPlumSE]

I mostly agree with Miecz. The new code is not an Allowable STRESS Desgin, but rather an Allowable STRENGTH Design. It uses LRFD type capacities reduced by a safety factor to bring it down to an allowable strength that can be used in conjunction with unfactored loads. As such, it is almost completely unrelated to our old 9th edition code checks.

Now, it is certainly worthwhile for us to go back and compare the current allowable loads to the allowable loads we used to get. But, we should do so more for our own edification. While the numbers should be in the same ball park a 20% increase in capacity will not be all that unusual.... especially when you keep in mind that deflection (which has changed at all) will probably control the design of this plate.

 

[nutte]

The new code is not an Allowable STRESS Desgin, but rather an Allowable STRENGTH Design. It uses LRFD type capacities reduced by a safety factor to bring it down to an allowable strength that can be used in conjunction with unfactored loads.

That's what we've always done: find the capacity and reduce it by a factor of safety. That's no different between ASD and LRFD, whatever you want the "S" to stand for.

As such, it is almost completely unrelated to our old 9th edition code checks.

Almost completely unrelated? It's a simple algebraic transformation to get from Fb to Mn.

 

[ToadJones]

If you think it is not still ASD, please reference equation H2-1 in the 2005 spec, then read the defined variables below the equation. It is a great lesson in semantics.

 

[ToadJones]

It is quite telling really when you read the user note of section H1.1 telling the user that H2 is allowed to be used in lieu of H1.1.
Most of the steel design I do boils down to Chapter H and therefore H2-1 which looks one hell of a lot like 1989 ASD.

 

[JoshPlumSE]

Nutte -

Okay? But, you still agree that the new Allowable Strength code is much, much to the last few LRFD codes than it was to the old ASD 9th edition, right? That's what I was saying....

You and ToadJones seem to show very little tolerance for engineers who cling to their old ASD manuals (to whom I was addressing my response). I guess I can understand that. The differences are not nearly as confusing as many engineers have made them out to be. And, the engineering communities despising of LRFD can be frustrating when viewed from that stand point.

However, for those who DO find the ASD/LRFD difference confusing or frustrating, it would be probably be helpful for them to think of the new ASD code as a modified version of LRFD rather than an updated version of their old ASD 9th edition. Can we at least agree upon that? That was the point that I was trying to make.

For goodness sakes, I hope your responses don't turn this thread into an LRFD vs ASD vent session.