etabss dynamic and static procedure check

[aqibusman]

dear fellows.
i m not used to of doing dynamic analysis.
i m doing a two story building problem and checking for both static and dynamic analysis.

i want to see the difference between the lateral story force by two type of analysis.

what i m getting is huge difference.
can any onle tell me where i m wrong.
i will be thank ful to u.
attached is the etab files

 

[zeemas]

When you define earth quake forces using static load cases menu still you need to perform dynamic analysis if you want ETABS to calculate time period of building based on original mass and stiffness. Dynamic analysis will be fully utilized when you define Response spectrum load cases. For Response spectrum you have to determine and use the scale factor in order to normalized your forces to static base shear.

You may be not assigning the proper scale factor in response spectrum cases. Please take care of the units for scale factor it will be always the unit of acceleration.

Let me know if you have problem about the scale factor.

Wish u all the best!!!

Zeeshan

 

[aqibusman]

i have applied the scale factor of 386 , hope u have seen my model

or can u help me by studying my earthquake load case based on ubc 1997 and my response function which is also based on ubc 1997 that what scale factor i should use.


thanks

 

[zeemas]

I checked your model and I have made correction to scale factor also.
1st thing according to code you have to consider atleast that much number of modes which can give you 90% of mass participation. For your model you atleast need 12 number of modes you can check by going to display tables and check building output.
2nd thing the value of g is used for initial model and after the first analysis you need to adjust the scale factor by the following

SF = Vstatic/Vdynamic x OLDSF where SF=Scale factor, OLDSF= Old scalefactor used in 1st run (in your case 386)

See the attach model hope it will help.

 

[stressed]

A RS scale factor of 386 in Kip-in is a full G. That's one reason why there is such a big difference between RS and static equivalent quake load. Typically you run a static quake along with dynamic RS using a full G (386 in Kip-in), then scale the RS factor down by a percentage to match static quake base shear per code. A RS scale factor of 96.5 would = .25 G, for example

 

[zeemas]

Stressed

One clarification that the RS scale factor is not in Kip-in unit, the unit of scale factor is same as for acceleration i.e.units of Length/seconds2 and that its value will change as you change the units in your model. Essentially ETABS assumes the response spectrum functions are unitless (normalized) and that the scale factor converts them into the appropriate units.

If you are scaling your response spectrum to match some static analysis results (e.g., base shear), you may want to include that in the scale factor specified for the response spectrum function in the input response spectra area. In that case you would input a scale factor equal to the product of the scale factor to convert the spectrum to the appropriate units and the scale factor to scale the response spectrum base shear to the appropriate level.

Please refer ETABS manual.

 

[aqibusman]

dear,
thanks for ur replies
one thing i can not understand as u told in ur previous posta that
" 1st thing according to code you have to consider atleast that much number of modes which can give you 90% of mass participation. For your model you atleast need 12 number of modes you can check by going to display tables and check building output."

From which out put of mass i get the understanding that 90% of mass participation.
i m pasting the output for ur review



ETABS v9.2.0 FileYNAMIC AND STATIC SEISMIC ANALYSIS CHECK Units:Kip-ft December 21, 2008 10:14 PAGE 12

M O D A L P A R T I C I P A T I N G M A S S R A T I O S

MODE X-TRANS Y-TRANS Z-TRANS RX-ROTN RY-ROTN RZ-ROTN
NUMBER %MASS <SUM> %MASS <SUM> %MASS <SUM> %MASS <SUM> %MASS <SUM> %MASS <SUM>

Mode 1 0.00 < 0> 62.54 < 63> 0.00 < 0> 98.07 < 98> 0.00 < 0> 0.00 < 0>
Mode 2 62.09 < 62> 0.00 < 63> 0.00 < 0> 0.00 < 98> 83.88 < 84> 0.00 < 0>
Mode 3 0.00 < 62> 0.00 < 63> 0.00 < 0> 0.00 < 98> 0.00 < 84> 59.31 < 59>
Mode 4 0.00 < 62> 25.10 < 88> 0.00 < 0> 1.29 < 99> 0.00 < 84> 0.00 < 59>
Mode 5 25.03 < 87> 0.00 < 88> 0.00 < 0> 0.00 < 99> 10.66 < 95> 0.00 < 59>
Mode 6 0.00 < 87> 0.00 < 88> 0.00 < 0> 0.00 < 99> 0.00 < 95> 6.41 < 66>
Mode 7 0.00 < 87> 0.00 < 88> 0.00 < 0> 0.00 < 99> 0.00 < 95> 5.21 < 71>
Mode 8 1.06 < 88> 0.00 < 88> 0.00 < 0> 0.00 < 99> 0.45 < 95> 0.00 < 71>
Mode 9 0.00 < 88> 1.27 < 89> 0.00 < 0> 0.07 < 99> 0.00 < 95> 0.00 < 71>
Mode 10 0.00 < 88> 0.00 < 89> 0.00 < 0> 0.00 < 99> 0.00 < 95> 10.38 < 81>
Mode 11 0.00 < 88> 11.09 <100> 0.00 < 0> 0.57 <100> 0.00 < 95> 0.00 < 81>
Mode 12 11.82 <100> 0.00 <100> 0.00 < 0> 0.00 <100> 5.02 <100> 0.00 < 81>
Mode 13 0.00 <100> 0.00 <100> 0.00 < 0> 0.00 <100> 0.00 <100> 18.68 <100>
Mode 14 0.00 <100> 0.00 <100> 0.00 < 0> 0.00 <100> 0.00 <100> 0.00 <100>
Mode 15 0.00 <100> 0.00 <100> 0.00 < 0> 0.00 <100> 0.00 <100> 0.00 <100>

 

[zeemas]

Aqibusman

I will advice you that before getting more in to Dynamic analysis of structure, first try to make a good understanding about the topic. Google the topic and you will find out some nice articles or you can get a book " Dynamic analysis of structure" by Anil K Chopra. This book is really simple and it will help you to understand the dynamic analysis theory.

Code is mentioning about the minimum number of modes based on mass to make sure that the forces are not underestimated. In dynamic analysis the froces are based on madal mass and these modes are combined by using SRSS,CQC or others.

To check whether you have enough number of modes go to Display>Show Tables>
check Modal Participating Mass Ratio table in Bulding Modal Information. For your convenience I have attached screenshots.

For your building you can see that for your one direction you are acheiving 90% of mass before mode 12 but for other direction 90% is achieved at mode 12.

Hope it will help you. There are many more things which are in my mind but I think if I tell you now it will confuse you, so we will discuss with time.

Best of Luck!!
Zeeshan

 

[aqibusman]

Thanks Zeeshan

i will further study this subject and i hope u will help me as the way did for this problem.