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Staad Design - Tension Column - erroneous?
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mtamil
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PostPosted: Wed Apr 02, 2014 6:30 am    Post subject: Staad Design - Tension Column - erroneous? Reply with quote

Dear Sefians,

I tried to design a column using Staad Pro for DL+Siesmic Load combination.

Axial compression due to DL = 1000 kN
Axial Tension due to SL = 1025 kN

I got a small tension in the column = 25kN.

But Staad Pro designed the column for Tension of 1000+1025 = 2025kN.

I have used the "Design column" command in Staad with pertinant parameters.

Staad design gives me exorbitant design reinforcements.

Am I missing something here? Or this is a known error?

Has anyone come accross this situation?

Rrgards
Tamilarasan
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sakumar79
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PostPosted: Wed Apr 02, 2014 8:37 am    Post subject: Reply with quote

Dear Sir,

1. Please post the input file and indicate the column where you are facing the issue.

2. Please inform which version of STAAD you are using. Is it the latest (Select Series 5)?

3. Earthquake load is applied in opposite directions(for example, +X and -X). Hence, the column that would have axial tension in one direction may have compression in opposite direction. Would that be the cause of the deviation?

Yours sincerely
Arunkumar
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mtamil
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PostPosted: Wed Apr 02, 2014 9:52 am    Post subject: Reply with quote

Dear Er. Arunkumar,

Thanks for the reply.
I am using V8i (SELECT Series 4) - Build 20.07.09.31.

I am attaching the .std file. Please have a look.

I am having the problem at Memb No. 850. L/C 211 being repoted as critical case with huge Tension in the design reports (See .anl file). But not so with Member End Force reports.

Looking forward for your comments.

Thanks
Tamilarasan



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Staad Input File

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PostPosted: Sat Apr 05, 2014 10:06 am    Post subject: Reply with quote

Dear Sir,
    I have not used Response Spectrum analysis before, so I am not checking this in detail, but...

a. I note the following issues in the modelling

1. Not sure why some beams have been given zero weight
2. EQ definition says R=5 is adopted, but CODE INDIAN indicates IS13920 is not used. This is incorrect. Since Z is 0.16 indicating Zone III, Ductile detailing is must. Hence, R=5 is correct, but design should use CODE IS13920 instead of CODE INDIAN.
3. You can release torsion in all the beams and columns which will make the solution a bit more economical.
4. For 1.5 (DL+EQ), you have reduced EQ by a factor of 0.792. How did you get this factor?
5. In EQ definition, Structure type is probably 3 and not 1 (if you use brick walls). Anyway, you have given PX and PY so that will be used directly...

b. However, despite this, I feel there is some issue in the design

As per output table, critical load combination for column no. 850 is 207 (1.5 DL + 1.187 EQZ)

Member end forces of 850 in FX (Axial) direction is 723 kN/-671 kN
Member end forces of 850 in EQZ direction is 861 kN/-861 kN

This means, if we were to add as per load combination, we should get
Start node 1.5 x 723 + 1.187 x 861 = 2106
End node 1.5 x -671 + 1.187 x -861 = -2029

But it is reported in STAAD as 2106/15 instead of 2106/-2029. By cross checking, I see that at end node, it has taken 861 kN without side (1.5 x -671 + 1.187 x 861 = 15)

I remember reading that when using Response spectrum analysis STAAD ignores sign. However, even in that case, STAAD should have designed for 15 kN uplift only instead of 2106 kN uplift. I suggest that you raise this issue with Bentley for urgent clarification...

Yours sincerely
S Arunkumar
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PostPosted: Sat Apr 05, 2014 10:12 am    Post subject: Reply with quote

Dear Sir,
    In addition to my earlier mail, please find a copy of a staad model for response spectrum at the following link
http://communities.bentley.com/cfs-filesystemfile.ashx/__key/telligent-evolution-components-attachments/00-5932-01-00-00-26-80-10/Response-Spectrum-Verifications-Example_5F00_revised.std
It is based on IBC code, but it should be similar for IS code also.


This is for your reference.


EDIT: One more link for Response Spectrum analysis in STAAD is at

http://communities.bentley.com/products/structural/structural_analysis___design/w/structural_analysis_and_design__wiki/2044.aspx

Yours sincerely
S Arunkumar
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mtamil
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PostPosted: Mon Apr 07, 2014 4:11 am    Post subject: Reply with quote

Dear Sir,

Thanks for your detailed review. Below are my responses.
Quote:

a. I note the following issues in the modelling

1. Not sure why some beams have been given zero weight

[/Quote]
Must have some reason. I will look at it.


Quote:

2. EQ definition says R=5 is adopted, but CODE INDIAN indicates IS13920 is not used. This is incorrect. Since Z is 0.16 indicating Zone III, Ductile detailing is must. Hence, R=5 is correct, but design should use CODE IS13920 instead of CODE INDIAN.

Seperate design operations exercised,  for ductile detailing.


Quote:

3. You can release torsion in all the beams and columns which will make the solution a bit more economical.

I personally don't prefer this option.


Quote:

4. For 1.5 (DL+EQ), you have reduced EQ by a factor of 0.792. How did you get this factor?

To reduce the live load part of the seismic force.

Quote:

5. In EQ definition, Structure type is probably 3 and not 1 (if you use brick walls). Anyway, you have given PX and PY so that will be used directly...

Ok.

Quote:

b. However, despite this, I feel there is some issue in the design

As per output table, critical load combination for column no. 850 is 207 (1.5 DL + 1.187 EQZ)

Member end forces of 850 in FX (Axial) direction is 723 kN/-671 kN
Member end forces of 850 in EQZ direction is 861 kN/-861 kN

This means, if we were to add as per load combination, we should get
Start node 1.5 x 723 + 1.187 x 861 = 2106
End node 1.5 x -671 + 1.187 x -861 = -2029

But it is reported in STAAD as 2106/15 instead of 2106/-2029. By cross checking, I see that at end node, it has taken 861 kN without side (1.5 x -671 + 1.187 x 861 = 15)

Yes. This is the point I was looking for . It appears the Staad design of columns in tension are erroneous.


Quote:

I remember reading that when using Response spectrum analysis STAAD ignores sign. However, even in that case, STAAD should have designed for 15 kN uplift only instead of 2106 kN uplift. I suggest that you raise this issue with Bentley for urgent clarification...

Yours sincerely
S Arunkumar



Thanks once again.
Tamilarasan
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PostPosted: Mon Apr 07, 2014 4:48 am    Post subject: Reply with quote

Dear Sir,
    You have noted that you have reduced EQ component in 1.5(DL+EL) to reduce Live load component of EQ. Is there any clause based on which this is done? I do not think this is allowed by code.

Yours sincerely
Arunkumar
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mtamil
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PostPosted: Mon Apr 07, 2014 7:45 am    Post subject: Reply with quote

Dear Arunkumar

This particular combination of load is to consider "under construction / erection" stage. And to consider the adverse effect of lower gravity loads, especially wrt stability criteria.

Therefore it is logical to exclude the live load weights both in gravity load and in siesmic loads.

What's your view on this?

Regards
Tamilarasan
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PostPosted: Mon Apr 07, 2014 8:10 am    Post subject: Reply with quote

Dear Sir,
    Without code recommendation, it is difficult to justify the same. How is the factor arrived? I generally prefer to design 1.5 (DL+EL) with full EL only... If there is any legal issue at some point, the reviewers may not accept such reduction.

Yours sincerely
Arunkumar
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sanju_re
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PostPosted: Fri Apr 11, 2014 9:00 am    Post subject: Reply with quote

Reading your post, it is clear that there has been a confusion with the member end force results obtained from a load combination where Response Spectrum load case is a part of it. The major problem lies in the fact that the results of Response Spectrum analysis are absolute quantities as they arecomputed using modal combination methods like SRSS, CQC etc. The nature of themember forces cannot be determined. The member forces derived from a loadcombination involving a Response Spectrum load case may provide conservativeresults. This can also be inferred from this case also. Please have a look intothe following member end force results of the column no. 850 for the load case1, 4 and load combination 850 at the joint no. 135. Refer to the attached figure.

It is to be noted that Load combination 207 is comprised of (1.5 xLoad Case 1)+(1.187 x Load Case 4). Thus, the axial force that will for loadcombination 207 will  be computed as[(1.5 x 722.39)+(1.187 x 861.35)]= 2107.00 Kn. This is exactly what the programis reporting. The start node of the member is subjected to +ve axial forcewhich insists the program to consider this load combination

There has been a breakthrough on the way of using unsigned responsequantities computed in a Response Spectrum analysis into a signed response. Asimilar type of discussion can be found into BE Community forum with themethodology to be used in STAAD.Pro :

https://communities.bentley.com/products/structural/structural_analysis___design/f/5932/t/89739.aspx  

This situation can be avoided by using signed Response Spectrumresults. The mode having maximum mass participation is considered as dominantmode and all the response quantities are assigned the sign of dominant mode. Onusing the sign of the dominant mode, the members forces gets modified which isquite acceptable and can be used in design. In the present model, you can usethe following commands to use the sign of dominant mode in analysis.

LOAD 3
----------------------
----------------------
SPECTRUM CQC 1893 X 0.028 ACC SCALE 1 DAMP 0.05 DOMINANT 0
SOIL TYPE 1
LOAD 4SPECTRUM CQC 1893 Z 0.028 ACC SCALE 1 DAMP 0.05 DOMINANT 0
SOIL TYPE 1


The modified model has been attached where you can see that onusing the sign of dominant mode the load combination 213 produces compressionin the column. The column is designed for another load combination where thecolumn is subjected to tension. It is to be noted that the axial tension (-372.28 Kn) for which the column isdesigned using the sign of dominant mode is quite lower tensile force than theprevious case and the design forces are quite reasonable.

Regards,

SANJIB DAS



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