Combined Axial and bending Check 1

[niiarmahstruct]

Hi,

I have an issue with the Interactive Steel Design Module of Staad Pro. I noticed that when it design beams, although a beam may fail the Combined Axial and Bending Check, Staad Pro still passes such beams. I normally design to BS 5950 using Staad Pro 2007 Build 2.15.

My issue is that, does staad consider the Combined Axial and Bending Check not to be critical enough and hence passes such beams though they fail that test the loading conditions that would result in such a situation are remote (ridiculous, I know but i am at my wits end)

 

[patswfc]

Be very wary of the member design part on structural analysis packages. Software is great for working out the forces in a structural but generally not great for member design.

Normally i get the software to check cross section capacity only and then check for member stability (buckling, etc) by hand.

Is the software just doing a cross section or member stabiliy check too. It its doing both is the effective length or the member correct in the software.

 

[niiarmahstruct]

Thanks for your response.

I understand your concern about the accuracy of software packages in terms of member design.

I normally would do the design by hand myself.

Staad does a cross-section check as well as stability checks. the effective length is normally correct. In the few instances where they are not correct, you can actually change it to the correct value.

What I want to know is the ratioal behind staad passing beams that have failed.

When you have a huge structure to design, you probably will not have the time to design each member by hand. What I do is to use software to design, then manually check members that I feel are most critical.

Thanks for your concern and advice. I appreciate it.

 

[subbu1985]

Hi niiarmastruct !!

I've been trying to use the STAAD Pro steel designer recently and I'm facing some issues with the same (In both member design & connection design).

From ur post here I understand that you've been using the steel designer too.

I'm getting the following error: "Design of member (say) 1001 fails at 23 ".

I dont understand y??

Can u help me fix this issue??
Shall I create a new thread to discuss this issue specifically???
Using ...Steel Designer if would save a lot time

Regards,
Subbu.

 

[dozer]

It must be some problem when it uses the British code. I use the AISC 9th ed. code check all of the time and of course it considers combined axial and bending stress. As you say, to not to do so would be ridiculous. And we all know STAAD would never do anything ridiculous (tongue firmly in cheek).

Regarding the comments about code checking (design). I agree that you shouldn't just trust whatever a program spits out but where do you guys work where you can check members by hand? I've worked on models with over a thousand members and tens of load combinations. Without software to do the code checking you would be uncompetitive in today's market.

Oh, back to your original question. I just thought of something. Maybe you have the parameter "RATIO" set to some number greater than one. I think there's at least one other similar parameter that will also affect the pass/fail decision but I can't remember what it is.

 

[niiarmahstruct]

Hi Sabbu,

I have faced your problem before. It's been sometime since so I cant say I remember the exact circumstances that led to staad giving that error. What I do remember though is that it happened when I was doing member selection. What I'll suggest is that you do a code check. Afterwards, check out the interactions(ratios) for the maximum values. If all your ratios are greater than 1 or whatever value you specified, those beams will have failed those particular checks. what you should do is to use the steel tables to select a new structural section. You can do yourself a lot of favours by checking the slenderness yourself for the longest spans. Obviously, it will be better to even assign members based on that as the first criterion at the modeling stage.

Now if your members are passing the interactive check and you want to reduce the sizes, you should reduce them using the interaction ratios as a guide as to how much you can reduce the sections.

Dozer

Thanks for your response. I do not know much about the american codes so I stay clear of them when I design. Now I cross-checked whether I inadvertently changed the ratio. I hardly ever do, And if I did, it will be to reduce the ratio.

Lastly, I do not check all the members by hand. I'll simply check a few.

Thanks for the responses.

Nii Armah

 

[subbu1985]

Thanks Nii Armah !!!

Will surely try that..

I was just wondering whether the use of Double Step Method would save time, since it did something kind of P-DELTA analysis...

Anyways.. ll try ur suggestion out..

Moreover have u used the Connection Design module in Steel Designer??? I've faced some issues wit that too..

 

[niiarmahstruct]

Hi Sabbu,

I tried once to use the connection design but it didn't prove particularly responsive s I shelved it.

Also, i am not sure I understand the double step method you mentioned. Besides, no analysis is done in the code check or single selection sections of the Steel Designer's run. It simply uses analysis results from the initial staad run where you are supposed to do your p-delta analysis. Besides, P-delta analysis is usually for concrete members if I am not mistaken.

Bye.

Nii Armah