Direct Analysis Method AISC Example

[RWW0002]

I am in the process of verifying the use of the Direct Analysis Method (per AISC Appendix 7)within a software package using the AISC 14th ed. examples posted on their website. (

http://www.aisc.org/WorkArea/showcontent.aspx?id=29596)

The example problem given for the DAM (Example C.1A) is a fairly simple problem, but I cannot seem to arrive at the same answer working it with the software package, or by going through DAM step by step manually (still utilizing the same software for pdelta).

It is late in the day and I may be missing something extremely simple, but the first order analysis results given in the problem do not seem correct to me. Can anyone currently using DAM verify that the referenced example is worked correctly.

I once told my grandfather that I thought the aim was off on his rifle. He shot once then informed me that it was not the aim that was off, but the "aimer"... that is more than likely the case here too, but I thought I would check in before wasting more time.

Thanks in advance

 

[JAE]

What is the software?

Some programs include shear deformation in addition to flexural deformation. Sometimes this creates results that are different from traditional hand calcs that only include the flexural.

 

[SteelPE]

First time I ran through the DAM using STAAD I had a few problems but once I figured it out I ended up with the exact solution found in the design examples of appendix 7. So yes, the AISC's equations are correct.

 

[RWW0002]

I'm using STAAD. My results are about 20% low, so I don't think shear deformation is the issue, but thanks for the heads up.

SteelPe, any tips on effective use of DAM in STAAD? I feel fairly confidant in the correct application of notional loads, the stffness reduction, and according to the ref. manual both large and small pdelta are accopunted for in analysis.

 

[frv]

I don't use STAAD, but some programs require you to actually include intermediate nodes (some do it automatically).

Did you reduce the stiffness of the members?

 

[RWW0002]

frv

Acording to STAAD's technical information, both the stiffness reduction and the intermediate nodes (p small delta) are taken care of within the program if DAM is specified. I have verified this by running DAM manually. I think the differing results is with the first-order moments (which probably points to an error in my input..) I will ahve to comb through carefully when I get a free second.

SteelPE, I do not see an examples within Appendix 7 (V13 or V14), only in the AISC online design examples. Which example problem were you referring to in your earlier post?

 

[SteelPE]

I don't have my book here at the moment. I thought there were some simple cases to proved that the program was properly accounting for second order analysis. One was a free standing column with an axial load and a lateral load applied. The other was a beam with both ends restrained against movement laterally with axial loads and local bending applied to the beam. These examples might have been in the commentary.

Maybe I don't know what I am talking about.

 

[frv]

Yes, the problems are in the commentary and there are two of them. The intent is to determine whether your software program accounts for P-Big delta and P-little delta (you check these independently.

May not be a bad idea to run these problems just to check that you are selecting all the appropriate "flags" within the program to run the DAM.

 

[SteelPE]

I did those problems over 3 years ago to make sure everything was being accounted for properly. If I remember correctly, there was a problem with scaling, something to do with using stress loads when the program was using strength design in the analysis. I remember it was very simple once I figured out what the problem was.

 

[frv]

Well, then, it may me the "aimer", after all..

 

[RWW0002]

My aim has been known to be off every now and then.. but I havn't given up on this one yet.

I'll work through the examples in the commentary during lunch. The example that I am unlimately wanting to compare to is a portal frame example in the AISC Design Example publication available free online. Would someone familiar with DAM mind verifying the results?? It is a fairly simple frame. I have checked my work and have not found any glaring errors and I'm beginning to think I am going crazy.

Thanks for all the help so far.

 

[SteelPE]

Do you have a link to this example?

 

[RWW0002]

Example C.1A at the following address:

http://www.aisc.org/WorkArea/showcontent.aspx?id=29596

 

[RWW0002]

I have verified my DAM analysis procedure using the two beam-column examples in Appendix 7 and all looks OK... I'm still not sure about the portal frame example, but I feel pretty confident in STAAD's DAM and that I'm "selecting all the appropriate flags."

Thanks for the help.

 

[JoshPlumSE]

I don't believe the 1st order results in the example are correct. The vertical reactions are correct, but everything else looks wrong. Granted, I've done this pretty.

I get a base shear (for the LRFD loads) in the column of 5.7 and 6.2 kips. Then I get a moment at the tops of the column of 113.4 k-ft and 124.9 k-ft.

If your numbers match mine then I would feel pretty confident that the AISC example is wrong.

 

[RWW0002]

Thanks Josh - I checked my results with a free trial of RISA 3-D and arrived at the same answers. In both cases your moments are within 0.5% of my results for "first order moments" - (Including notional loads but not large and small pdelta)

My results for the full DAM yield 109 k' and 128 k' at top of columns (although I ran the frame in ASD and I re-factored the results by a "weighted load factor" of 1.5 to arrive at LRFD)

I feel pretty good about results now, but you would think that if AISC is going to publish benchmark examples for the purpose of verifying the use of given methodology and computer output they would have gone through them with a fine-toothed comb...

 

[ATSE]

AISC has stated the following about the Example C.1A:
"We have looked into it and have confirmed that the 1st order analysis in the design examples is incorrect." per Heath Mitchell.
I'm a big fan of AISC and think the Design Examples are very useful, but they drop bombs occasionally.

 

[RWW0002]

Thanks for the update ATSE. I agree that the design examples are very helpfull. I guess everyone drops the ball every once in a while.