STAAD.Pro Direct Analysis Method Applying Notional Loads Correctly

Lauren Ruchti — Mon, 25 Jul 2016 16:43:24 GMT

Hello,

I am currently trying to compare the results in STAAD.Pro to an example provided by the AISC Guide 28 (Example 3.2). I am able to match the results of the axial load, but the moments in the beam-to-column and midspan of the beam are significantly different. I have noticed that the notional load has a significant effect on the moment and I am wondering if this may be the source of the error. Does anyone have any experience with this? Here is how I defined the Direct Analysis Method:

MATERIAL STEEL ALL
DEFINE DIRECT ANALYSIS
FLEX 1 ALL
FYLD 7200 ALL
NOTIONAL LOAD FACTOR 0.002
END

Then in each load combination I specified the notional load in the following way:

LOAD 103 COMB3 = 1.4(D) + NY
REPEAT LOAD
1 1.4
NOTIONAL LOAD
1 X 0.002

Finally, I gave the perform direct analysis command.

PERFORM DIRECT ANALYSIS LRFD ITERATION 15 PDITER 15

And I printed the results like so,

LOAD LIST 103
PRINT MAXFORCE ENVELOPE LIST 1 2
PRINT SECTION DISP LIST 1

Is this the correct way to read the results?

If someone could provide some type of guidance that would be much appreciated. Also, if you would like to take a look at my input code please let me know. Thank you!

Lauren

STAAD Direct Analysis Method FLEX and AXIAL Parameters

Lauren Ruchti — Mon, 27 Jun 2016 19:48:02 GMT

Which members are intended to be included in the parameters FLEX and AXIAL for a braced bay when using Direct Analysis Method? Does this include just the braced columns, beams and bracing or the entire frame (including gravity only beams/columns)? I'm a little confused by the definition provided by STAAD. The definition STAAD provides is as follows:

FLEX, identification of members whose flexural stiffness is considered to contribute to the lateral stability of the structure, along with the initial value of τb that should be used. Members listed with FLEX will have their EI factored by 0.80 times τb while performing the global solution. The final member forces and code check will be with 100% of the flexural stiffness.

AXIAL, identification of members whose axial stiffness is considered to contribute to the lateral stability of the structure. These members will have their EA factored by 0.80 while performing the global solution. The final member forces and code check will be with 100% of the axial stiffness.

I am currently trying to model Example 2.1 in AISC Guide 28 in STAAD and analyze it using direct analysis method and compare the results to those provided in the example. I think it may be worth noting that the example usese effective length method, where I am using direct analysis method. Currently, my results for the moment in the beams and axial loads in the columns match that of the example (or are within 0.50%) except for the axial loads in the columns for combinations 5, 6 and 7. The results for these combinations have been up to 30% off. These are the only combinations that include the wind load; which is why I think the error might be coming from the columns in the braced bay. Any help is appreciated, thanks!

Lauren

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STAAD.Pro Direct Analysis Method Applying Notional Loads Correctly

STAAD Direct Analysis Method FLEX and AXIAL Parameters