Time-history analysis with recorded accelerograms with SAP2000

Time-history seismic analysis with SAP2000A step-by-step guide for BEng/MEng/MSc students familiarizing with this piece of software

List of ContentDownload 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 DatabaseThe 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 DatabaseThe database in entirely searchable

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

As an example, we want to download the accelerogram recorded during the Imperial Valley 1940 earthquakeOnce the event has been selected in the top drop-down list, we must specify to DISPLAY the results In Table rather than On MapThen we can SEARCH

Find the accelerogramBy 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 eventIf we scroll down, the links to the three components of the accelerogram (180, 270 and vertical) appear

Download the time history of the ground accelerationBy 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 ValleyDate: 19th May 1940Time: 4:39amStation: El Centro Array #9Direction: Horizontal, 180Units of acceleration: g= 9.81 m/s2 (acceleration of gravity)Number of time instants: 4,000Sampling time: t= 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

Download the time history of the ground acceleration

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)Download the time history of the ground acceleration

Upload accelerograms to SAP2000

Step #2

Define the accelerogram in SAP2000An 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...

Define the accelerogram in SAP2000The accelerogram must be uploaded to SAP2000 as FUNCTION FROM FILE, an option which can be selected from the drop-down list

Define the accelerogram in SAP2000We 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)

Define the accelerogram in SAP2000By clicking on DISPLAY GRAPH we can visually check the waveform

Define the accelerogram in SAP2000In 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...

Define the accelerogram in SAP2000

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

Define the accelerogram in SAP2000The appropriate analysis in presence of accelerograms is a TIME HISTORY case, which can be selected from the drop-down list

Define the accelerogram in SAP2000This leads to a more complicated windows, in which few parameters must be input

Define the accelerogram in SAP2000In 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 SAP2000The 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 analysisOnce 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 analysisOnce the analyses are complete, you can read the pertinent results from tables and diagrams

Display the resultsIn 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 stepsAs engineers, we are generally interested in both minimum and maximum value, i.e. by the envelope of these responses

Display the resultsWe 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 resultsIn 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 resultsIn the window with the graph, min and max values of the selected time history are also reported