Failing Members!!! 1

[stallion0002]

So here is a question for any who whish to tackle...

Here is the scenerio...I am using the Select/Group commands to have STAAD design steel members for a conveyor system. After I run the program and see what sizes STAAD 'picks', I manually enter those sizes back in the member property section for thier respective memebrs, remove the Select/Group command, and re-run the file.

The problem...The exact same member that STAAD picked for the design which passed all design criteria under the Select/Group command now fails when the analysis is re-run...and not just by a little. Some memebers have ratios over 3.0!

I have tried asking STAAD support, but they are no help and just say "That's interesting. I've never seen that before." So much for Tech Support! (Side rant...I have service tickets with the tech support department that are approaching 1yr old. If I had a turn-around-rate on anything I do at work, I don't think that I'd be employed very long.)

Anyways, back to the issue at hand. Anyone else experience this 'phenomenon' and how did you correct the problem?

Thanks.

 

[edward1]

I have used this process and I use PROFILE W size BEAM for a beam number and SELECT MEMB beam number and run the analysis to get a size beam recommended. Then I put an * in front of these lines of input and run an analysis with CHECK CODE MEMB for these beam numbers. It works for me.

 

[alphaxy]

What version are you using? Is it pirated or original copy?

 

[stallion0002]

to edward1:

Yes, that is the same procedure that i use when using the select/group commands but when i manaully enter the sizes that it chooses and re-run the program w/o the select/group command, they now fail.

 

[stallion0002]

to alphaxy:

our office uses all original licenses of Pro.V8i edition.

 

[structuresguy]

Could your input file have been reorder, so that your code check command is before any unbraced length parameters? I have seen this where the code check runs before you tell it that the top flange is fully braced, so you get failure.

Have you looked at the allowable stresses in the beam design output (track 2) to make sure that the allowables are what you would expect them to be? This is the first place I look when troubleshooting failing members.

Maybe you could post up a screen shot of the beam design output for one of the failing members. Maybe we could help troubleshoot it more, if we could see the configuration, loading, and allowable stresses.

 

[JoshPlumSE]

I'm not really a STAAD user, but I am a tech support guy with another company (RISA) and have an idea about the cause of the issue.

Usually, when a program selects an alternate member size it is based on the assumption that the axial force, bending moment and torsion will remain the same in the new members.

But, once you replace the member and re-run the analysis we know that the forces will change. If they change signficantly then the actual code checks will not be very close to the code checks that were assumed when the program picked the original size.

Therefore, my suggestion is to take a look at whether or not the member forces have changed significantly between the two runs. If so, then that probably explains the difference in code checks.

 

[stallion0002]

To JoshPlum:

What would cause the forces to change in the program? The sizes are the same. The only difference is the first run it chooses and the second run I tell it the member size. I would think that regardless of how it 'gets' the memeber sizes, the forces would be the same.

 

[JoshPlumSE]

The first run uses the orignal member sizes when it formed the overall stiffness matrix. The program likely uses this stiffness when determining all member forces. The program likely then uses takes these forces and sees what effect those forces would have on an alternate member.

The 2nd run uses the new members when forming the stiffness matrix. Therefore, the stiffness of the structure has changed. When the analysis is re-solved the member forces can be different because of this change in stiffness.

Look at 3 cantilever columns of various stiffnesses that are connected together with a drag strut at the top to ensure load sharing between the columns. Apply a lateral load at the top. If I change 1 of the column sizes, then all the members will receive a different percentage of shear than they did in the original model. Does that make sense?

The same total load is applied to the model, it's just that the relative stiffness of the members isn't the same for the 2nd analysis. Normally, the forces in the members will be relatively close. That assumes that the member changes are incremental.... i.e. the you're changing from a W12x65 to a W14x90, not a W44x390. If you make big changes in the member sizes then you'll have big changes in the force results from the analysis.

 

[Sye123]

When STAAD does member selection, the design forces are taken from the analysis based on the initial member sizes. You had asked for a member selection followed by GROUPING but possibly did not perform another analysis after that. This is required because in a stiffness based analysis, member forces vary with the change in stiffness. So once members are selected by STAAD and those are grouped, it is important that the structure be re-analyzed to find out the changed forces. One should then perform a code check on the members to see if those selected sections still hold good or not.

The change in the force distribution may cause certain members to fail the final code check, specially the ones for which the utilization ratios were high during the initial member selection. A complete design cycle should include

SELECT
GROUP MEMB
PERFORM ANALYSIS
CHECK CODE

Check the results of the final code check and see whether all members have passed or not. If not, you may simply iterate the design cycle a few times as shown in the sample provided next

...

*First cycle
SELECT ALL
GROUP MEMB 1 3 4
GROUP MEMB 5 6 7
PERFORM ANALYSIS
CHECK CODE ALL
*
*Second Cycle
SELECT ALL
GROUP MEMB 1 3 4
GROUP MEMB 5 6 7
PERFORM ANALYSIS
CHECK CODE ALL

and eventually you would end up with a design where all members would be safe. Once you achieve that you may update the section properties, remove the SELECT/GROUP commands and just do a CODE CHECK and this time you will have a safe structure. I hope this helps.