Transcript Time-history analysis with recorded accelerograms 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
Step #1
DOWNLOAD ACCELEROGRAMS
FROM THE PEER DATABASE
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/IELC180), a new textual
page is opened with the
corresponding time
history of the ground
acceleration
Download the 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
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)
Step #2
UPLOAD ACCELEROGRAMS TO
SAP2000
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...
Define the accelerogram in SAP2000
• The 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 SAP2000
• 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)
Define the accelerogram in SAP2000
• By clicking on DISPLAY
GRAPH we can visually
check the waveform
Define the accelerogram in SAP2000
• 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...
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 SAP2000
• The appropriate
analysis in
presence of
accelerograms
is a TIME
HISTORY case,
which can be
selected from
the drop-down
list
Define the accelerogram in SAP2000
• This leads to a
more
complicated
windows, in
which few
parameters
must be input
Define the accelerogram in SAP2000
• 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
Step #3
RUN THE ANALYSIS AND DISPLAY
THE RESULTS
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