time-history with sap2000

Time-history seismic analysis with SAP2000

A step-by-step guide for BEng/MEng/MSc students familiarizing with this piece of software

List of Content

• Download recorded accelerograms from the PEER database

• Upload a selected accelerogram to SAP2000

• Run the time-history analysis and display the results

DOWNLOAD ACCELEROGRAMS FROM THE PEER DATABASE

Step #1

PEER Strong Ground Motion Database

• The Pacific Earthquake Engineering Research (PEER), headquartered at the University of California at Berkeley, makes available online over 10,000 strong ground motion records from 173 different earthquakes

http://peer.berkeley.edu/

PEER Strong Ground Motion Database

• The database in entirely searchable

http://peer.berkeley.edu/nga/

• As an example, we want to download the accelerogram recorded during the Imperial Valley 1940 earthquake– Once the event has been selected in the top drop-down list, we

must specify to DISPLAY the results “In Table” rather than “On Map”

– Then we can SEARCH

Find the accelerogram

• By right-clicking on the appropriate record (NGA0006 in our case), a new page is shown with all the relevant pieces of information about this seismic event– If we scroll down, the links to the three components of the

accelerogram (180°, 270° and vertical) appear

Download the time history of the ground acceleration• By right-clicking on one of

these links (e.g. the first horizontal component, denote with IMPVALL/I-ELC180), a new textual page is opened with the corresponding time history of the ground acceleration

• The first four lines contain general pieces of information about the record, namely:– Location: “Imperial Valley”– Date: 19th May 1940– Time: 4:39am– Station: “El Centro Array #9”– Direction: Horizontal, 180°– Units of acceleration: g= 9.81 m/s2 (acceleration of gravity)– Number of time instants: 4,000– Sampling time: Δt= 0.01 s (f= 100 Hz)

Download the time history of the ground acceleration

• The other lines list the ordinates of the record (in units of g), which must be read from left to right, row by row


• Importantly, the web browser (Explorer, Firefox and etcetera) allows to save this page as a TXT file (you may need to manually change the extension)


UPLOAD ACCELEROGRAMS TO SAP2000

Step #2

Define the accelerogram in SAP2000

• An accelerogram is basically the time-history of the acceleration experienced by the ground in a given direction during a seismic event

• We need to input the accelerogram as a generic function defined in SAP2000 starting from a TXT file

• Once the mechanical model of the structural system under investigation has been created, SELECT from the menu bar:

DEFINE → FUNCTIONS → TIME HISTORIES...


• The accelerogram must be uploaded to SAP2000 as FUNCTION FROM FILE, an option which can be selected from the drop-down list


• We can select:

– Name of the function (e.g. EL_CENTRO_1940)

– Location of the file by using the button BROWSE...

– Number of lines to skip (4 for the PEER database)

– Number of points per line (5 for the PEER database)


• By clicking on DISPLAY GRAPH we can visually check the waveform


• In order to be used in a seismic simulation, the waveform previously defined must be properly assigned to an ANALYSIS CASE

• To do this, SELECT from the menu bar:

DEFINE → ANALYSIS CASES...


• The analysis case in presence of accelerogram is a TIME HISTORY, which can be selected from a drop-down list after clicking on the ADD NEW CASE... button


• The appropriate analysis in presence of accelerograms is a TIME HISTORY case, which can be selected from the drop-down list


• This leads to a more complicated windows, in which few parameters must be input


• In particular:– The load type is an ACCELeration of the ground (first drop-down list)– For a planar model, the ground shaking is generally assumed to happen in the X direction, i.e. Load name U1

(seconddrop-down list)– The records from the PEER website are given in units of g, so the scale factor would be 9.81 m/s2 or 981

cm/s2, depending on the units selected in the model of SAP2000– The number of time steps should be given by the duration of the accelerogram divided by the sampling time:

• In our example: 40 s / 0.01 s = 4,000 steps

RUN THE ANALYSIS AND DISPLAY THE RESULTS

Step #3

Run the dynamic analysis

• Once the accelerogram has been defined in SAP2000, it is possible to run the time-history analysis as any other type of analysis

Run the dynamic analysis

• Once the analyses are complete, you can read the pertinent results from tables and diagrams

Display the results

• In the next slides the results of the dynamic analysis are shown in terms of:– Axial Force N (left-had side graph)– Bending Moment M (right-hand side graph)

• It is worth mentioning that these blue diagrams are the ENVELOPE of the corresponding outputs:– Indeed, in a dynamic analysis we have a value of the structural

quantities of interest (e.g. N and M) for each time steps– As engineers, we are generally interested in both minimum and

maximum value, i.e. by the “envelope” of these responses

Display the results

• We can also display the same structural quantities in a specific time instant, e.g. at t= 10 s...– In this case, for instance, the red colour in the axial-force diagram

means compression, while the yellow colour stands for tentions

Display the results

• In order to show the time history of a given structural response, e.g. The horizontal displacement of the top floor, you mus SELECT from the menu bar:

DISPLAY → SHOW PLOT FUNCTIONS...

Display the results

• In our example, we want to display the displacement along the X direction of the joint no. 23, for the time range from 0 to 40 s

Display the results

• In the window with the graph, min and max values of the selected time history are also reported

time-history with sap2000

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