PMRSA

Practice WorkbookThis workbook is designed for use in Live instructor-led training and for OnDemand selfstudy. The explanations and demonstrations are provided by the instructor in the classroom, or in the OnDemand eLectures of this course available on the Bentley LEARN Server (learn.bentley.com).

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Performing a Modal Response Spectrum Analysis

This workbook contains exercises to learn how to perform a modal response spectrum analysis in STAAD.Pro.

TRNC01238-1/0001

Copyright © 2014 Bentley Systems, Incorporated 2DO NOT DISTRIBUTE - Printing for student use is permitted

Description and Objectives

Course Description

This workbook contains exercises to learn how to perform a modal response spectrum analysis in STAAD.Pro.

Skills Taught

Defining Dynamic Seismic Loads

Specifying the Maximum Number of Mode Shapes

Generating Load Combinations

Performing a Dynamic Seismic Analysis

Reviewing the Results


Open a STAAD.Pro Model

In this exercise, you will learn how to open a STAAD.Pro model and review the loading information.

1. Launch STAAD.Pro.

2. In the menu bar, click File > Open...

3. In the Open dialog, navigate to the dataset file Dynamics_1.std that was supplied with this training. Then, click Open.


4. In the Page Control Area, click on the General tab and the Load & Definition sub-tab.

5. In the Load & Definition dialog, expand the Definitions section and the Load Cases Details section.

NOTE: The Seismic Load Definition and the Seismic Load Cases have already been generated in accordance with the IBC Equivalent Lateral Force Procedure. After the Dynamic Seismic Analysis is performed, we will compare the base shear of the ELFP load cases with those of the dynamic load cases to ensure that the scale factors are appropriate and in accordance with the requirements of ASCE 7, Section 12.9.

6. Keep this model open for the next exercise.


Create the Dynamic Seismic Load Cases

In this exercise, you will learn how to create new primary load cases to represent the dynamic seismic load acting in each direction.

1. Continue with the model from the previous exercise.

2. In the Load & Definition dialog, highlight the Load Cases Details section and then click Add...

3. In the Add New: Load Cases dialog, select the Primary item in the left pane. Then, enter the following information:

Number: 2001

Loading Type: Seismic

Title: DYNAMIC LOAD +X

Click Add.

4. In the Add New: Load Cases dialog, enter the following information:

Number: 2002


Title: DYNAMIC LOAD ‐X

Click Add.


Number: 2003


Title: DYNAMIC LOAD +Z

Click Add.



Number: 2004


Title: DYNAMIC LOAD ‐Z

Click Add.

7. In the Add New: Load Cases dialog, click Close.

8. In the menu bar, click File > Save.


Define the Dynamic Masses

In this exercise, you will learn how to define the masses that will contribute to the seismic loading. The masses are specified in terms of weights and in the directions in which they are dynamically active.

Self Weight: Acting in the global X, Y, and Z directions.

Uniform Member Loads: Acting in the global X, Y, and Z directions.

NOTE: This model will include four SPECTRUM commands. When more than one SPECTRUM command is used in a model, the mass is derived from the weight definition within the first spectrum load case only.


2. In the Load & Definition dialog, highlight the 2001: DYNAMIC LOAD +X load case and click on the Add... button.

3. In the Add New: Load Items dialog, select the Selfweight > Selfweight Load item in the left pane. Then, enter the following parameters:

Direction: X

Factor: 1

Click Add.


Direction: Y

Factor: 1

Click Add.


Direction: Z

Factor: 1

Click Add.


6. In the Add New: Load Items dialog, select the Member Load > Uniform Force item in the left pane. Then, enter the following parameters:

W1: 0.05 kip/ft

Direction: GX

Click Add.


W1: 0.05 kip/ft

Direction: GY

Click Add.


W1: 0.05 kip/ft

Direction: GZ

Click Add.

9. In the Add New: Load Items dialog, click Close.


10. In the Load & Definition dialog, highlight the SELFWEIGHT X 1 load item and select the Assign to View radio button. Then, click Assign.

11. In the STAAD.Pro dialog, click Yes to confirm the assignment.

12. Repeat this process to assign the following load items to the model:

SELFWEIGHT Y 1

SELFWEIGHT Z 1


13. In the Main Window, deselect any currently selected members. Then, click Select > Beams Parallel to > Z in th menu bar.

14. In the Load & Definition dialog, highlight the UNI GX 0.05 kip/ft load item and select the Assign To Selected Beams radio button. Then, click Assign.

15. In the STAAD.Pro dialog, click Yes to confirm the assignment.

16. Repeat this process to assign the following load items to the members parallel to the global Z axis:

UNI GY 0.05 kip/ft

UNI GZ 0.05 kip/ft



Define the Response Spectra Load Item

In this exercise, you will learn how to create the Response Spectra Load item. Within the load item, you can specify various code parameters and the following information:

Combination Method: Used to specify how to combine the contribution from the individual modes.

Direction: Used to specify the global direction in which the spectrum is to be applied.

Signed Response Spectrum Results Options: Used to select one of the methods available to add mathematical signs to the spectrum response output.


2. In the Load & Definition dialog, highlight the 2001: DYNAMIC LOAD +X load case and click on the Add... button.

3. In the Add New: Load Items dialog, select the Response Spectra > Response Spectrum item in the left pane. Then, enter the following parameters:

Code: IBC 2006

Combination Method: CQC

Long Period (TL): 8 seconds

Fa: 1.0

Fv: 1.3

Site Class (SCL): C

Zip: 92887

Spectrum Type: Acceleration

Interpolation Type: Linear

Damping Type: Damping 0.05


Direction:

X: (checked)

Factor: 1.0

Signed Response Spectrum Results Options:

Dominant: (checked)

Signed: (checked)

Then, click Add.


4. In the Add New: Load Items dialog, click Close.

5. In the Load & Definition dialog, highlight the 2002: DYNAMIC LOAD ‐X load case and click on the Add... button.


Code: IBC 2006



Fa: 1.0

Fv: 1.3

Site Class (SCL): C

Zip: 92887




Direction:

X: (checked)

Factor: ‐1.0


Dominant: (checked)

Signed: (checked)

Then, click Add and click Close.


7. In the Load & Definition dialog, highlight the 2003: DYNAMIC LOAD +Z load case and click on the Add... button.


Code: IBC 2006



Fa: 1.0

Fv: 1.3

Site Class (SCL): C

Zip: 92887




Direction:

Z: (checked)

Factor: 1.0


Dominant: (checked)

Signed: (checked)



9. In the Load & Definition dialog, highlight the 2003: DYNAMIC LOAD ‐Z load case and click on the Add... button.


Code: IBC 2006



Fa: 1.0

Fv: 1.3

Site Class (SCL): C

Zip: 92887




Direction:

Z: (checked)

Factor: ‐1.0


Dominant: (checked)

Signed: (checked)




Generate the Load Combinations

In this exercise, you will learn how to generate load combinations in STAAD.Pro. A load combination is a set of load results which are combined algebraically to produce a superimposed set of results for post-processing.


2. In the Load & Definition dialog, highlight the Load Cases Details section and then click Add...

3. In the Add New: Load Cases dialog, select the Auto Load Combination item in the left pane. Then, enter the following information:

Select Load Combination Code: AISC

Select Load Combination Category: GENERAL

Click on the Generate Loads button.

4. In the Add New: Load Cases dialog, click Add and then click Close.



Specify the Maximum Number of Mode Shapes

In this exercise, you will learn how to use the CUT OFF MODE SHAPE command. This command is used to instruct the program to extract the specified number of modes during a modal calculation, response spectrum analysis, and time history analysis.


2. In the menu bar, click Commands > Miscellaneous > Cut Off Mode Shape...

3. In the Cut Off Mode Shape dialog, enter the following parameter:

Maximum Number of Mode Shapes: 20

Then, click OK.



Analyze the Model

In this exercise, you will learn how to specify the analysis command and perform the analysis.


2. In the Page Control Area, click on the Analysis/Print tab.

3. In the Analysis/Print Command dialog, select the Perform Analysis tab and select the Mode Shapes radio button. Then, click Add and click Close.

4. In the menu bar, click Analyze > Run Analysis...

5. In the Warning dialog, click Save.

6. In the STAAD Analysis and Design dialog, select the View Output File radio button and then click Done.

7. Keep the Output File open for the next exercise.


Review the Mass Participation Results

In this exercise, you will learn how to review the mass participation results in the Output file to ensure that the dynamic analysis is in accordance with ASCE 7, Section 12.9.

NOTE: Section 12.9.1 of the ASCE 7 specifies that “the analysis shall include a sufficient number of modes to obtain a combined modal mass participation of at least 90 percent of the actual mass in each of the orthogonal horizontal directions of response considered by the model.”

1. Continue with the model from the previous exercise. (The Output File should still be open.)

2. In the Results bar, click on the first PARTICIPATION FACTORS item.

3. In the Output file, you will notice that STAAD.Pro has calculated the mass participation of the structure for 6 modes. According to the results, the mass participation in the Z direction is less than 90%.

4. In the Output file, click File > Exit.

5. In the menu bar, click Commands > Miscellaneous > Cut Off Mode Shape...

6. In the Cut Off Mode Shape dialog, enter the following parameter:

Maximum Number of Mode Shapes: 55

Then, click OK.




10. In the Results bar, click on the first PARTICIPATION FACTORS item.

11. In the Output file, you will notice that STAAD.Pro has calculated the mass participation of the structure for 10 modes. According to the results, the mass participation in the X and Z directions is greater than 90% and now satisfies ASCE 7, Section 12.9.1.

12. Keep the Output File open for the next exercise.


Review the Base Shear Results

In this exercise, you will learn how to review the base shear results in the Output File to ensure that the dynamic analysis is in accordance with ASCE 7, Section 12.9.


2. In the Output File, review the results of the IBC Seismic Load in the global X and Z Directions that were calculated using the Equivalent Lateral Force Procedure:


3. In the Output File, review the results of the Seismic Load in the global X and Z Directions that were calculated using the Response Spectra Analysis Procedure:

X Direction:


Z Direction:


NOTE: Section 12.9.2 of the ASCE 7 specifies that “the value for each force-related design parameter of interest, including story drifts, support forces, and individual member forces for each mode of response shall be computed using the properties of each mode and the response spectra defined in either Section 11.4.5 or 21.2 divided by the quantity R/I.”

NOTE: Section 12.9.4 of the ASCE 7 species that “the combined response for the modal base shear (Vt) is less than 85 percent of the calculated base shear (V) using the equivalent lateral force procedure, the forces, but not the drifts shall be multiplied by 0.85 (V/Vt).”

4. After reviewing the Base Shear results, calculate the appropriate scale factor that should be applied to the Response Spectra load cases in each direction (assume an Importance Factor of 1.0 and a Response Modification Factor of 3.0):

5. In the Output file, click File > Exit.

6. In the Page Control Area, click on the General tab and the Load & Definition sub-tab.

7. In the Load & Definition dialog, expand the Load Cases Details group and the 2001: DYNAMIC LOAD +X load case. Highlight the SPECTRUM CQC IBC X 1 load item and click on the Edit... button.

SFxIR---- 0.85 V

Vt-----×× 1.0

3.0------- 0.85 17.5kips

81.29kips------------------------×× 0.023= = =

SFzIR---- 0.85 V

Vt-----×× 1.0

3.0------- 0.85 23.34kips

46.83kips------------------------×× 0.071= = =


8. In the Edit... dialog, modify the following parameter:

X: (checked)

Factor: 0.023

Then, click Change.

9. In the STAAD.Pro dialog, click Yes to confirm the change.

10. In the Edit... dialog, click Close.


11. In the Load & Definition dialog, expand the 2002: DYNAMIC LOAD ‐X load case. Highlight the SPECTRUM CQC IBC X ‐1 load item and click on the Edit... button.


X: (checked)

Factor: ‐0.023

Then, click Change and click Close.

13. In the Load & Definition dialog, expand the 2003: DYNAMIC LOAD +Z load case. Highlight the SPECTRUM CQC IBC Z 1 load item and click on the Edit... button.


Z: (checked)

Factor: 0.071


15. In the Load & Definition dialog, expand the 2004: DYNAMIC LOAD ‐Z load case. Highlight the SPECTRUM CQC IBC Z ‐1 load item and click on the Edit... button.


Z: (checked)

Factor: ‐0.071





20. In the Output File, review the base shear results for the Response Spectrum load cases.

21. In the Output File, click File > Exit.


Review the Member Force Results

In this exercise, you will learn how to review the member force results in the Post Processor.


2. In the menu bar, click Mode > Post Processing.

3. In the Results Setup dialog, select the Result View Options tab and enter the following parameters:

Enable Automatic Scaling: (checked)

Mode Shape: (checked)

Beam Results: (check all options)

Then, click OK.

4. In the Page Control Area, click on the Beam tab and the Forces sub-tab.



Review the Mode Shapes

In this exercise, you will learn how to view the mode shapes in the Post-Processor.


2. In the Page Control Area, click on the Dynamics tab and the Modes sub-tab.


3. In the Post-Processor, the mode shape can be viewed on screen for each dynamic load case and each mode by selecting the options in the toolbar.


Select a Dynamic Load Case

Select the Mode

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