STAAD.Pro Time History Analysis [FAQ]

1. Is there any facility in STAAD to design buildings for blast loading?
2. For one of our projects we are designing a platform sitting on steel framing. On the platform we have 5 Blowers. I was wondering if STAAD can take into account the vibration loads coming from Blowers. I could not find in Technical Reference as well as online manual.
3. In our research, We want to use a TMD(tuned mass damper) installed on the floor of a building to reduce the response of structure under earthquake. The TMD include a mass(m), a spring(k) and a damper(c). How can I put these three things on the building in STAAD? Thank you very much.
4. Is it possible to view the values of the displacements and forces at each step of a time history analysis?
5. How do I apply two harmonic loads PSinwt and QCoswt at the same loading point simultaneouly?
6. In a time history analysis, how do I get the Amplitude of Vibration of the beam?
7. I did a time history analysis and the combined file sizes for all my model files is more than 8GB. Why is that and how can I reduce the size without deleting my  analysis results ? 
8. HowcanIgettheabsoluteresponsesfromagroundmotiontimehistoryanalysis?

1. Is there any facility in STAAD to design buildings for blast loading?

The analysis for blast loading is done using the facilities of time history analysis.

The blast load has to be defined as discrete time-force pairs, with the force changing from a very small value to a large value, and then back to a small value over a very small time interval. Please refer to example 16 for help in specifying an arbitrary time-force function.

STAAD can perform design for the maximum forces resulting from the time history analysis. The examples manual contains examples on steel and concrete design illustrating the commands and parameters required to do this.

2. For one of our projects we are designing a platform sitting on steel framing. On the platform we have 5 Blowers. I was wondering if STAAD can take into account the vibration loads coming from Blowers. I could not find in Technical Reference as well as online manual.

In order to analyse a structure for dynamic loading which does not fall under the category of ground motion (earthquakes), no matter what the origins of the dynamic loading, it will have to be reduced to one of two types that STAAD can recognize :

a) Harmonic loading : This requires that the load be described in the form of a Sine or Cosine function. Input data in the form of the peak amplitude, operating frequency of the equipment, number of cycles, phase angle, etc. will have to be obtained by the user, and specified as input to STAAD.

b) Random vibration which can be described in the form of discrete time-force pairs. The force values and the time at which they act have to be specified in this case.

Details on both types on available in the following places :

Sections 1.18.3, 5.31.4 and 5.32.10.2 of the STAAD.Pro Technical Reference Manual
Example problem 22 of the STAAD.Pro Examples Manual for type (a)
Example problem 16 of the STAAD.Pro Examples Manual for type (b)

Question and answers on other related topics on time history is available at the following address :

http://www.reiworld.com/support/pro/faqs.asp

Look for issue numbers 1528, 1533, 1548, 1680 and 1746

3. In our research, We want to use a TMD(tuned mass damper) installed on the floor of a building to reduce the response of structure under earthquake. The TMD include a mass(m), a spring(k) and a damper(c). How can I put these three things on the building in STAAD? Thank you very much.

Mass is entered the way you have entered it, using joint forces to represent weight in calculating modes.
Springs to ground are entered through the Support command.
Staad does not have dampers, just modal damping. However, if the TMD has a first frequency much lower than the rest of the structure, then that first mode can be given the damping that you want and the rest of the structure (all of the remaining modes) given a different damping.

STAAD requires mass to be defined in each direction of motion not just in the Y direction.
You may need more modes than the default 6.

If the El Centro data is every 0.01 seconds then there is a 26.88 second integration time span. You may want to stop at a lesser time.
The integration step size, DT, should not be as large as you have entered. Use 0.0016 or less.

CUT OFF MODES 20
CUT OFF TIME 8

DEFINE TIME HISTORY DT 0.0013
TYPE 1 ACCELERATION
READ ELCENTRO.TXT
ARRIVAL TIME
0
DAMPING 0.05

LOAD 1 TIME HISTORY LOAD
SELFWEIGHT Y -1
JOINT LOAD
2 7 TO 10 12 14 16 18 20 FY -957
11 TO 20 FY -1040
JOINT LOAD
21 FY -590
SELFWEIGHT X -1
JOINT LOAD
2 7 TO 10 12 14 16 18 20 FX -957
11 TO 20 FX -1040
JOINT LOAD
21 FX -590
GROUND MOTION X 1 1
PERFORM ANALYSIS

4. Is it possible to view the values of the displacements and forces at each step of a time history analysis?

If you specify the SAVE option against the TYPE command, as in,

DEFINE TIME HISTORY
TYPE 1 FORCE SAVE

STAAD will create text files with the extension .tim and .frc containing those results. Additional information on this is available in Section 5.31.4 of the STAAD.Pro Technical Reference Manual.

After the analysis is completed, you can open those files using a text editor such as NotePad or WordPad. Please note that these files may be very large in size depending on the duration of the dynamic load and the time step value. So, the editor may take some time in opening that file.

To view the diagrams of the history of dispacements, go to the Post-processing mode. From the left side, select the Dynamics page. The sub-pages such as Time-Accel, Time-Velocity and Time-Displacements contain facilities for viewing the histories. If you click the right mouse button on those plots, you can view those diagrams in the Frequency domain also.

5. How do I apply two harmonic loads PSinwt and QCoswt at the same loading point simultaneouly?

The input is as follows. Please refer to example 22 in the STAAD.Pro examples manual for additional information.

DEFINE TIME HISTORY

TYPE 1 FORCE

FUNCTION SINE

AMPLITUDE 5.3 FREQUENCY 60 CYCLES 100

FUNCTION COSINE

AMPLITUDE 7.8 FREQUENCY 80 CYCLES 100

*

ARRIVAL TIMES

0.0 3.0

DAMPING 0.075

LOAD 2

SELFWEIGHT X 1.0

SELFWEIGHT Y 1.0

SELFWEIGHT Z 1.0

JOINT LOAD

9 25 FX 14.0

9 25 FY 14.0

9 25 FZ 14.0

TIME LOAD

9 25 FX 1 1

9 25 FX 2 2

6. In a time history analysis, how do I get the Amplitude of Vibration of the beam?

You get this from the joint displacement values. You can specify the command PRINT JOINT DISPLACEMENTS in your input file, in which case the values reported in the output file will be the absolute maximum values. These absolute maximum values can also be seen by looking at the node displacement table in the post-processing mode.

You can also get a history of the displacements of the nodes in the structure graphically. In the post-processing mode, select the Dynamics - Time-Dsp tab from the left side of the screen. Then click at any of the nodes in the model, and the history of X, Y and Z translations will be shown in a chart on the right hand side of the screen.

7. I did a time history analysis and the combined file sizes for all my model files is more than 8GB. Why is that and how can I reduce the size without deleting my  analysis results ?

This is most likely due to the fact that you have used the SAVE command as part of the time history definition as shown next

…

DEFINE TIME HISTORY

TYPE 1 FORCE SAVE

FUNCTION SINE

…

Normally when you do a time history analysis the software only saves the maximum responses as part of the file. However when the SAVE command is used, the software saves the responses at every instant of time in two files with extensions .frc ( member end forces and reactions ) and .tim ( nodal displacements ). Depending on the structure and the applied loading, the file sizes for these files can be really large. If you do not need to know the responses at every instant of time, you should not use the SAVE command. Alternately for backup purpose, you may delete the .frc and .tim files manually from outside the STAAD.Pro environment. Please ensure that the STAAD model is closed and delete the files using Windows Explorer. You will still be able to retain the post processing data.

8. How can I get the absolute responses from a ground motion time history analysis ?

For the time history responses to be reported as absolute values, you need to include the keyword ABSOLUTE as part of the time history load as shown next

GROUND MOTION ABSOLUTE
X 1 1 1.0

Also to get the plots of the absolute responses from within the Postprocessing > Dynamics page, you need to go to Results > and uncheck the View Relative Response option.

See Also

Product TechNotes and FAQs

Structural Product TechNotes And FAQs

External Links

Bentley Technical Support KnowledgeBase

Bentley LEARN Server

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