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SEISMIC ANALYSIS PROBLEM IN STAAD

 
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ARUP_MUNSI
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Joined: 21 Sep 2010
Posts: 14

PostPosted: Wed Oct 08, 2014 5:21 am    Post subject: SEISMIC ANALYSIS PROBLEM IN STAAD Reply with quote

Dear all
While carrying out seismic analysis by equivalent static method we calculate the forces manually at each floor level and apply them at each beam column junction as horizontal nodal force in both direction as required. After that we carry out the analysis and obtain the result.

To avoid this manual calculation STAADPRO has introduced a reference mass loading which can be given as primary load case and the software calculates the masses at each floor level. Accordingly base shear is also calculated as per equivalent static method and nodal forces are generated at each member node irrespective of its loaction whether it is a beam column junction or it is the ends of a secondary beam. I have checked the masses considered for equivalent static method for both the cases are same and seismic base shear coefficient is also same.

My query is whether the 2nd approach is a correct approach? I feel that if a mass is present at a node irrespective of it's position, seismic force shall be generated as the mass will be subjected to a acceleration due to the ground motion. Let us consider the case of a bin supported on four beams at mid point through bracket. In this case the entire load is transferd to the structure through the mid point of the supporting beam and inturn it goes to the column as beam reaction. Definitely the beam will be subjected to horrizontal forces due to seismic shock.
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Regards
Arup Munsi
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RAHULGEHLOT2009
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Joined: 19 Sep 2008
Posts: 28
Location: Mumbai, Ratlam, Ujjain, Kurukshetra, Kota, Jodhpur, Jilling Orissa, Delhi, Noida

PostPosted: Fri Oct 31, 2014 9:32 am    Post subject: new features of STAAD pro V8i Reply with quote

Dear ARUP_MUNSI,
                               STAAD have changed a lot. I have used reference load method along with FLOOR DIAPHRAGM.
I find results of both manual and STAAD almost same. Few points you must remember:
1. Sa/g will be calculated by STAAD so you have to provide factor for Ah value as (Z/2*I/R) only Sa/g will be calculated and final Ah will be provided by the Staad.
2. floor diaphragm will be defined by the user,  mass will be lumped accordingly by STAAD.
If you have latest version of STAAD pro V8i, I can send you a reference input file.
regards,

Rahul Gehlot


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ARUP_MUNSI
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Joined: 21 Sep 2010
Posts: 14

PostPosted: Mon Nov 03, 2014 4:58 am    Post subject: SEISMIC ANALYSIS PROBLEM IN STAAD Reply with quote

Dear Rahul
Thanks for the reply. We have done extensive research on this and found the same thing. In fact the logic is wherever there is a mass it will contribute to the seismic loading in other words as the mass is subjected to acceleration it will in turn generate loading. I asked this question in the forum to get  a second opinion. I have also checked with global consultants and they have also confirmed the same.
Regards
Arup Munsi
Deputy Chief Engineer
Saipem India Projects
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es_jayakumar
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Joined: 24 Nov 2011
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Location: Cochin

PostPosted: Tue Nov 04, 2014 3:20 am    Post subject: Re: SEISMIC ANALYSIS PROBLEM IN STAAD Reply with quote

ARUP_MUNSI wrote:
Dear all
While carrying out seismic analysis by equivalent static method we calculate the forces manually at each floor level and apply them at each beam column junction as horizontal nodal force in both direction as required. After that we carry out the analysis and obtain the result.

To avoid this manual calculation STAADPRO has introduced a reference mass loading which can be given as primary load case and the software calculates the masses at each floor level. Accordingly base shear is also calculated as per equivalent static method and nodal forces are generated at each member node irrespective of its loaction whether it is a beam column junction or it is the ends of a secondary beam. I have checked the masses considered for equivalent static method for both the cases are same and seismic base shear coefficient is also same.

My query is whether the 2nd approach is a correct approach? I feel that if a mass is present at a node irrespective of it's position, seismic force shall be generated as the mass will be subjected to a acceleration due to the ground motion. Let us consider the case of a bin supported on four beams at mid point through bracket. In this case the entire load is transferd to the structure through the mid point of the supporting beam and inturn it goes to the column as beam reaction. Definitely the beam will be subjected to horrizontal forces due to seismic shock.
Request expert comments from the community
Regards
Arup Munsi


Sir,
Is there any step-by-step procedure for the bold-underlined process above, with you ? Or else, is there any provision for STAAD to calculate the same automatically ?
E S Jayakumar
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ARUP_MUNSI
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Joined: 21 Sep 2010
Posts: 14

PostPosted: Tue Nov 04, 2014 6:52 am    Post subject: SEISMIC ANALYSIS PROBLEM IN STAAD Reply with quote

Dear Jayakumar
you have to define the reference load in load definition of STAAD. After that add all the load items in simillar way you do for a load case, once this is done you can use the same for any load generation be it seismic or any load. Here is an example

Typical Staad Input for REFERENCE LOAD  


DEFINE REFERENCE LOADS

***********************************************************

LOAD R1 LOADTYPE Dead  TITLE REF LOAD CASE 1

SELFWEIGHT Y -1

ONEWAY LOAD

YRANGE 7 7 ONE -5 XRANGE 0 25  ZRANGE 0 24  GY

YRANGE 14 14 ONE -5 XRANGE 0 25  ZRANGE 0 24  GY

MEMBER LOAD

3 5 7 9 11 TO 16 19 28 52 61 85 94 118 127 135 137 139 141 143 TO 148 UNI GY -5

29 TO 33 62 TO 66 95 TO 99 128 TO 132 CON GY -2.5

***********************************************************

LOAD R2 LOADTYPE Live  TITLE REF LOAD CASE 2

ONEWAY LOAD

YRANGE 7 7 ONE -2.5 XRANGE 0 25  ZRANGE 0 24  GY

YRANGE 14 14 ONE -2.5 XRANGE 0 25  ZRANGE 0 24  GY

**************************************

END DEFINE REFERENCE LOADS

DEFINE 1893 LOAD PART4

ZONE 0.075 RF 1 I 1 SS 1 ST 1 DM 0.05

REFERENCE LOAD Y

R1 1.0 R2 1.0

***********************************************************************

LOAD 1 LOADTYPE Seismic  TITLE SEISMIC +X

1893 LOAD X 1 ACC 1

LOAD 2 LOADTYPE Seismic  TITLE SEISMIC -X

1893 LOAD X -1 ACC 1

LOAD 3 LOADTYPE Seismic  TITLE SEISMIC +Z

1893 LOAD Z 1 ACC 1

LOAD 4 LOADTYPE Seismic  TITLE SEISMIC -Z

1893 LOAD Z -1 ACC 1

****************************************************

Hope this helps. One issue is there, you can not use this for two type of load generation of seismic load. Like we use two different condition for seismic load generation  1. operating condition 2. empty condition. In these cases you can only define seismic load for one single condition. Even STAADPRO technical team has also raise their hand and told that they will be developing this option in nextrelease.

Regards
Arup Munsi
Deputy Chief Engineer
Saipem India Projects



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RAHULGEHLOT2009
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Location: Mumbai, Ratlam, Ujjain, Kurukshetra, Kota, Jodhpur, Jilling Orissa, Delhi, Noida

PostPosted: Thu Nov 20, 2014 9:23 am    Post subject: Reply with quote

Dear ARUP_MUNSI,
                 As per manual these new features, added to staad can be used in various ways.

Their are different methods for defining reference load, we can define it as gravity load (as you have explained)

or we can define it as mass (which I prefer) as explained below

reference load is defined here as mass, in each direction, live load contribution as per table 8 of IS:1893.2002 is also added as mass. Floor Diaphragm is defined


LOAD R1 LOADTYPE Mass TITLE REF LOAD CASE 1

******************IN X DIRECTION****************
SELFWEIGHT X 1
******************dead load************************
FLOOR LOAD
YRANGE 72.65 72.65 FLOAD 4.5 GX
YRANGE 76.25 76.25 FLOAD 4.5 GX
MEMBER LOAD
522 523 540 541 585 680 681 698 699 743 833 843 848 861 866871 TO 874 877 -
878 880 887 890 891 899 904 911 915 TO 917 920 922 923 9251131 1133 1149 -
1150 1153 1156 1157 1159 TO 1162 UNI GX 12
***************live load (0.5 times as per 1893)********************
FLOOR LOAD
YRANGE 72.65 72.65 FLOAD 2.5 GX
YRANGE 76.25 76.25 FLOAD 2.5 GX
************************ IN Y DIRECTION ***************************
SELFWEIGHT Y 1
FLOOR LOAD
YRANGE 72.65 72.65 FLOAD 4.5 GY
YRANGE 76.25 76.25 FLOAD 4.5 GY
MEMBER LOAD
522 523 540 541 585 680 681 698 699 743 833 843 848 861 866871 TO 874 877 -
878 880 887 890 891 899 904 911 915 TO 917 920 922 923 9251131 1133 1149 -
1150 1153 1156 1157 1159 TO 1162 UNI GY 12
***************live load (0.5 times as per 1893)********************
FLOOR LOAD
YRANGE 72.65 72.65 FLOAD 2.5 GY
YRANGE 76.25 76.25 FLOAD 2.5 GY
************************** IN Z DIRECTION *******************************
SELFWEIGHT Z 1
FLOOR LOAD
YRANGE 72.65 72.65 FLOAD 4.5 GZ
YRANGE 76.25 76.25 FLOAD 4.5 GZ
MEMBER LOAD
522 523 540 541 585 680 681 698 699 743 833 843 848 861 866871 TO 874 877 -
878 880 887 890 891 899 904 911 915 TO 917 920 922 923 9251131 1133 1149 -
1150 1153 1156 1157 1159 TO 1162 UNI GZ 12
***************live load (0.5 times as per 1893)********************
FLOOR LOAD
YRANGE 72.65 72.65 FLOAD 2.5 GZ
YRANGE 76.25 76.25 FLOAD 2.5 GZ
*******************************
END DEFINE REFERENCE LOADS
************************************

FLOOR DIAPHRAGM
DIA 1 TYPE RIG HEI 83.45
DIA 2 TYPE RIG HEI 87.05
DIA 3 TYPE RIG HEI 90.65
DIA 4 TYPE RIG HEI 94.25
DIA 5 TYPE RIG HEI 97.85

CUT OFF MODE SHAPE 25

DEFINE 1893 LOAD
ZONE 0.36 RF 5 I 1 SS 2 ST 1 DM 0.05 PX 0.3527 PZ 0.27816 DT2.5

LOAD 1 LOADTYPE Seismic TITLE LOAD CASE EQX
1893 LOAD X 1
*PERFORM ANALYSIS
*CHANGE

LOAD 2 LOADTYPE Seismic TITLE LOAD CASE EQZ
1893 LOAD Z 1
*PERFORM ANALYSIS
*CHANGE
*CHECK SOFT STOREY
******************************

LOAD 3 LOADTYPE Seismic TITLE LOAD CASE 1D
SPECTRUM CQC 1893 X 0.036 ACC SCALE 1.9791 DAMP 0.05 MIS 0
SOIL TYPE 2
LOAD 4 LOADTYPE Seismic TITLE LOAD CASE 2D
SPECTRUM CQC 1893 Z 0.036 ACC SCALE 2.0243 DAMP 0.05 MIS 0
SOIL TYPE 2
****************************************************

From the output we can get load distributed by the program on each floor as :


  ************************************************************************************

   FLOOR DIAPHRAGM     UNIT - KN   METE
   ---------------     ----------------

    NO.  TYPE        FL. LEVEL      FL. WT     CENTRE   OF   MASS    MASTER JOINT NO.
                                                                    X                Z

     1   RIGID          83.450     13209.32      14.057     19.325              521
     2   RIGID          87.050     12155.73      14.889     19.518              522
     3   RIGID          90.650     11838.03      14.947     19.524              523
     4   RIGID          94.250     8547.50        15.498     19.208              524
     5   RIGID          97.850     1129.41        22.721     18.404              525

   ************************************************************************************


also the other parameters from the output file such as peak story shear can be compared manually.

Regards,

Rahul Gehlot
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