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Floor Plate Bending
2

Floor Plate Bending

Floor Plate Bending

(OP)
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

RE: Floor Plate Bending

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.

RE: Floor Plate Bending

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

RE: Floor Plate Bending

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

RE: Floor Plate Bending

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.

RE: Floor Plate Bending

(OP)
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.  

RE: Floor Plate Bending

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

RE: Floor Plate Bending

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.

RE: Floor Plate Bending

(OP)
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.  

RE: Floor Plate Bending

Looks like to me you've lost 20%.

RE: Floor Plate Bending

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.

RE: Floor Plate Bending

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.

RE: Floor Plate Bending

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.

RE: Floor Plate Bending

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.   

RE: Floor Plate Bending

Quote (JoshPlum):

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.


Quote (JoshPlum):

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.

RE: Floor Plate Bending

(OP)
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.  

RE: Floor Plate Bending

(OP)
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.  

RE: Floor Plate Bending

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.   

RE: Floor Plate Bending

We have veered off topic, but since the original question has been answered, I don't guess there's anything wrong with that.

Quote:

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....

Quote:

The differences are not nearly as confusing as many engineers have made them out to be.
I agree with those statements, although the first one implies the wrong conclusion.

Quote:

You and ToadJones seem to show very little tolerance for engineers who cling to their old ASD manuals ...
I don't think I have been intolerant to those who want to use their old green books.  I simply object to those who claim that the world has been turned upside down, that the current way is nothing like the old way.

Quote:

... 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?
No, I strongly disagree with that.  Had AISC not abandoned the old ASD for 15 years, nobody would be complaining of the differences.  The specification evolved since 1989, and the current specification is the result of that process.  ASD would still look the same.  We just lost the benefit of the two interim ASD versions between 1989 and 2005.

RE: Floor Plate Bending

(OP)
How am I being intolerant of other engineers?
Quite a bizarre assertion....bordering on preposterous.

Multiply by 0.6 or divide by 1.67, I don't care how you do it.

For the most part I see it like this:
 1989 = 1/2005  

RE: Floor Plate Bending

(OP)
I dont agree the ASD is a modified version of LRFD.
Load and Resistance Factor Design.

ASD doesn't use Load Factors or Resistance Factors.  

RE: Floor Plate Bending

I fought the ASD fight for a long while but model codes don't give me much choice.  I tell the younger engineers to use the load combinations and safety factors that make sense to you but in the end your serviceability checks better match mine.  To me Chapter C of the 2005 spec is the biggest change that I still cannot fully accept.

RE: Floor Plate Bending

ToadJones -

The way you (or I) write a response on here is not how everyone else will read it.  The responses that I got (from you and Nutte) were surprising to me.  I said nothing that I seemed to be controversial (at least not to me).  Nutte responded rather strongly to that....  It may not have be a "flame mail" response. But, right or wrong, that's how I viewed it.

Saying that you have "Little tolerance" may not be the right wording.  Perhaps, it should have been something along the lines of "you two display little patience" for engineers who still cling to the green book.

FWIW, the diference between ASD 9th and LRFD has never been a big deal to me.  So, I agree with what you are saying there. Steel design is steel design.  Buckling, yielding, et cetera.  It's just different formulas to capture the same behavior.  However, I also recognize that many, many engineers skipped over LRFD completely.  They are now trying to transition directly from ASD 9th to ASD 13th. These folks frequently express annoyance and frustration with the switch. I choose to have patience with them.  

Nutte disagrees with me for some reason. To me, however, it is clear.  These Green Bookers would be better off viewing the ASD 13th NOT as an update of their beloved green book, but rather as an update of LRFD 3rd. Otherwise, they'll cling to the stress rather than looking at capacity.... which was the basic issue with the original question.      

RE: Floor Plate Bending

(OP)
I have to admit I am still a little lost...
I learned LRFD in school and still prefer the Green Book.
So I can't see how I am saying anything against guys who like the green book...I am one of them.
 
I think the frustration in some ways comes from that fact that those who used to using allowable stresses have developed a good feel for how stresses relate to design. A 340 kip-ft moment causing 40 ksi bending stress is a high bending stress for grade 50 steel.

340 kip-ft on its own doesn't give you much feel for anything.
Not sure if I am making sense here.

There are many instances where I still equate my "available strengths" in the black book to "allowable stresses"
 

RE: Floor Plate Bending

Toad Jones

If you are not using load factors with Allowable Strength Design, then you are not meeting the AISC Code.  The User Note of Article B2 requires load combinations from Section 2.4 of ASCE7.  Those combinations have Load Factors just like the ones for LRFD Design, shown in Section 2.3.  The factors are just different, and some of them are unity.

Nutte-

Granted, the 9th Edition modified some allowable stresses in recognition of plastic reserve strength.  But at the end of the day, the code check was in terms of allowable stresses.

If you are basing your current designs on allowable stresses, then you are not meeting the AISC Code.  Article B3 requires design to be based on Strength, not Stress.  In some cases there may be a simple relationship between the two, but I don't believe that's always so.

It's hard to comment on speculation of what might have happened had AISC issued updates to the 9th Edition.   Certainly, if ASD still stood for Allowable Stress Design, I think things would look quite different.  Note. I didn't write better, I wrote different.

If you don't see a difference between Allowable Stress Design and Allowable Strength Design, then I guess we'll have to agree to disagree.

What bothers me is that AISC apparently chose to confuse the issue, because LRFD needed a jump start after 20 years of neglect.  When AISC changed the basis of design from stress to strength, they should have created a new acronym for it.  Instead, they used the identical acronym ASD, and changed what the acronym stood for.  Why would they do that, unless they wanted to confuse the issue?

RE: Floor Plate Bending

In Canada, we had a period of time where both ASD (allowable stress design) and LSD (limit states design) were acceptable by code.  I don't remember the date, but a number of years ago, the National Building Code of Canada cut off ASD in favor of LSD.  Now, the use of ASD is not permissible by code and an engineer using it could be chastised by his professional association in the event of a complaint about professional competence.

At first, I was not in favor of this change because, as has been pointed out by a number of contributors to this forum, there is no significant difference between the two methods.

There was considerable resistance on the part of some engineers to this change, but now, thank goodness we have only one method of designing structures in Canada which is recognized by the code.

It seems to me that, whichever method you decide to adopt in the USA, it would be much better to recognize one and only one method.  Having two methods will lead to misinterpretations and errors along the way which could spell disaster in some cases.

BA

RE: Floor Plate Bending

(OP)
meicz-
That's a bit of a reach...as I am sure you know what I was saying.  

RE: Floor Plate Bending

Toad Jones-

Quote (Toad Jones):

I dont agree the ASD is a modified version of LRFD....ASD doesn't use Load Factors...

Honestly, I thought you were saying that Allowable Strength Design doesn't use Load Factors.

The load coefficients of Section 2.4 of ASCE7 look like load factors to me, even if a lot of them are unity.

What am I missing here?

 

RE: Floor Plate Bending

(OP)
I was talking about a general approach to each design method. If you want to hang me on saying a factor of 1.0 is not a factor, go right ahead. I don't care enough to get in a pissing match although it would appears as though I am mid-stream right now.

Read this DIRECTLY out of MSC, an AISC publication, oddly enough the term "allowable stress" keeps popping up:

Floor Plate Design

Do you know of any resources or design aids for the design of solid plate floor decking such as "diamond plate"? We tend to do quick designs using ASD and 0.75Fy for bending; then check deflections - but this seems too conservative.

Question sent to AISC's Steel Solutions Center
Answer

Strength and deflection of floor plates is addressed in the 13th edition Manual. The Manual tables are based on an allowable flexural stress of 16 ksi for ASD and 24 ksi for LRFD. These values are used because the yield strength of floor plate material is often undefined. The deflection-controlled tables are based upon a maximum deflection of L/100.

From a strength perspective, the 2005 AISC specification requires Mn = FyZ ≤ 1.6FyS for a plate in weak-axis bending. Since Z/S = 1.5 for rectangular elements and the factor of safety for ASD is 1.67:

Ma = Fy(1.5S)/1.67 = 0.90FyS

Thus, the 2005 AISC specification permits the use of Fb = 0.90Fy, which is larger than the value you have been using. However, depending upon the deflection criteria you choose, you may find that most floor plate designs are controlled by deflection.

Sergio Zoruba, Ph.D., P.E.
American Institute of Steel Construction

RE: Floor Plate Bending

If I speak for all "green book" ers (and I find I seldom speak for more than just myself), we don't have a problem with load factors.  We do it all the time with concrete design.  My personal feeling is that AISC tried to tackle too much in the first LRFD code.  

The first LRFD would have been a significant change by itself.  However, some of our familiar formulas were gone or significantly changed.  That publication was more than merely an incremental step like most of the editions before it.  Now 25 years or so later, we have ignored it enough and are now told that we must use it.  It is a shock to the system.  

What really insults me is that AISC thinks that changing the factors to give us "allowable" back is what we really wanted.  I can deal with factored loads.  I just feel like the code is printed in a different language because, yes, I am now 25 years behind.  

IMHO, if AISC really cared about more than just the code writers, they would have given us a transition code from green to black books.  My suggestion for back in the 80's would have been a code for allowable STRESS with all the new fancy formulas that save us up to 5% steel.  Then, the next version would be a true combined code.  AISC is still mishandling this and won't man up. Unfortunately, AISC is the only game in town.

 

RE: Floor Plate Bending

Interjecting into the discussion:

It seems a main component of this discussion revolves around the use of the acronym "ASD"; which is my only problem with the code definitions. If you want to use LRFD, perfect, if you want to use allowable stress design, great; but why develop a mutiliated version and call it Allowable Strength Design? The issue as Toad points out is a fundamental philosophical difference between allowable stresses and allowable loads.

This sums it up:

"I think the frustration in some ways comes from that fact that those who used to using allowable stresses have developed a good feel for how stresses relate to design. A 340 kip-ft moment causing 40 ksi bending stress is a high bending stress for grade 50 steel."

My originaly educational background was mechanical engineering; where most all analysis still takes an allowable stress design approach. So in mechanical engineering given some state of stress we can make some quick decisions regarding applied stresses vs. allowable stresses.

With an LRFD approach I have similiary two peices of information; an allowable load and the applied load; I do not think you have the same insight into the behavior.

Either method is satisfactory and I do not care to argue the merits. I mainly feel that there is a strong difference between the underlying philosophy between the concept of an allowable stress and an allowable load. This to me is what has been lost in this tranistion period, only muddied further with the new concept of "allowable strength design".
 

RE: Floor Plate Bending

(OP)
Kudos LSPSCAT.
Well Stated.
I don't hate LRFD, I just like ASD a lot more.  

RE: Floor Plate Bending

(OP)
I'll try to make this the last time I chime in here (but after all, I am the OP and my thread was hi-jacked by me and others).

To me, it has always made a lot more sense to calculate the actual structure loading to the best of one's ability and apply the loads to the structure and determine stresses. It just makes sense.

Something seems arbitrary about assigning load factors. I was particularly confounded when ACI changed their load factors.  

RE: Floor Plate Bending

This subject has become the "third rail" of structural engineering, come even close and sparks fly and before you know it
a brush fire is raging.
One would logically assume that the basic goal of AISC is to promote the use of steel versus concrete.Then why on earth would
they produce such a document?...it has generated alot of ill-will in the engineering community. The purpose of a code is to aid
engineers in their already difficult task of providing safety to
the public and at the same time producing a cost effective design.
When I look at the ACI code I find, for the most part, ease-of-use,engineering logic,continuity, clarity and a clear connection to underlying theory with the obvious exception of Appendix "D".
In my opinion,it seems that there are elements in the AISC organization that are at cross-purposes to their original goal and causing irreputable damage  to the organization.
 

RE: Floor Plate Bending

Using the plastic section (Z) is more appropriate for strength and the elastic section (S) is more correct for deflection. I don't know their rational.  The difference between the two... Z/S = 1.5 makes for a huge difference since the shape always conforms to Class 1.

Dik

RE: Floor Plate Bending

But dik, you don't need either for deflection, just the moment of inertia.  I can't think of a situation where the plastic section modulus would be useful in floor plate design.

RE: Floor Plate Bending

My turn (this is fun!)

1.  I don't give a rat's belly button whether the "S" means strength or stress.  There's just waaaay more interesting things about engineering that I LOVE and this "difference" just doesn't get in the way.

2.  I once knew an engineer (much older than me) that I worked with who, at the time, designed concrete using equations and code provisions from 30 years earlier, many of which were no longer in the code, and some of which were actually discredited by research...but he had a feel for it, was comfortable with it, and just didn't want to change.  I swore I'd never get that adverse to change.  

3.  Some comments above seem to imply that LRFD hasn't "made it" or been accepted.  Almost every engineer I work with uses LRFD now.  Most of the "green-bookers" are retiring/gone or, like me, have changed to LRFD.  I know that is anectdotal but LRFD is here to stay. (I like both - see no problem with both).

 

RE: Floor Plate Bending

Every engineer I know pays his/her taxes.  Doesn't mean they like it.  Only means that they don't want to end up in jail.

Seems that whenever 13th vs. 9th is discussed, sooner or later the 9ers are accused of having some psychological problem, clinging for dear life to their green books, afraid of the dreaded 13th edition.

RE: Floor Plate Bending

I don't think it is fear of LRFD.  I think it is comfort with the green book.

RE: Floor Plate Bending

(OP)
What amazes me sometimes about this battle is the number of technical guides and specifications that also skipped over LRFD completely.
I know the ACOE has some specifications to this day that prefer the use of the Green Book.
AIST Technical Report 13 from 2003 still makes numerous code equation references to the Green Book.
From my experience, heavy industrial guys like ASD.
I am not very heavy, or industrial, but I do a lot of work in heavy industry.  

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