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INTRO Essentials
Chapter 19
CivilFEM Seismic Design
19.1 Time or Frequency Domain?
Time
• All Types of Analysis
- Non Linear
- Any Damping
• Easy Concepts
- Few concepts differ from a
static analysis
• Difficult Calculation
- Calculation time is high
- Disc space needed is high
Frequency
Valid for Some Analyses
- Linear Analysis
- Simplified Damping
New Concepts
- Mode shapes
- Spectrum, Eigenvalues…
Easy Calculation
Better phenomena
comprehension
19.2 Frequency Domain
• Modal Analysis
This method allows the user to obtain the natural
frequencies and mode shapes of a structure. It
functions well as a first evaluation, but sometimes
is insufficient.
• Harmonic Analysis
Useful when actions are cyclic, have the same
frequency and act indefinitely.
• Spectral Analysis by Modal Superposition
The response of the structure is characterized by
the combination of some of the natural mode
shapes of the structure, multiplied by a coefficient
that comes from the spectrum.
19.3 What is a Spectrum?
• A spectrum is a function that shows the maximum response
of a system of simple oscillators with a specific damping
under a dynamic action.
• Usually, the X-axis contains frequencies or periods and the
Y-axis, displacements, velocities or accelerations. Normally,
a 5% damping is a good estimate.
Acceleration
Spectrum for a
certain location
19.4 Modal Superposition Analysis
• The solicitation is given in terms of spectra (accelerations).
• The analysis consists of evaluating the structure’s
behavior as the sum of the responses of each of its mode
shapes.
• The response of each one of these natural mode shapes is
obtained from the data given by the spectrum for the
specific frequency, multiplied by the modal participation
factor. This factor takes into account how much the
orientation of the solicitation excites every mode.
• Up to four different spectra with different damping
values can be defined for each direction (horizontal
and vertical).
• When analyzing the spectral value corresponding
to a mode, the damping for that mode will be
obtained and the spectral value will be the one
corresponding to the curve defined for that
damping (or interpolated).
• The damping of the mode is related to the part of
the structure affected by the modal shape and the
damping of the materials.
CivilFEM provides a set of tools that allow for a
simple analysis of forces and moments due to
seismic action in structures according to the
following codes:
Elements available:• All elements
19.5 Seismic Design
• Eurocode 8
• NCSE (Spanish code 94 and 02)
• IT 3274 (Italian code)
• The Greek code EAK 2000
• The French code PS 92
• Chinese code GB50011
• CALTRANS Seismic Design
Criteria
• AASHTO LRFD Bridge Design
Specifications
• Uniform Building Code (1997)
• Indian Standard 1893
The three aspects considered for seismic design are:
• Spectra definition
• Calculation of natural mode shapes
• Modes combination (SRSS, CQC, DSUM, GRP and
NRLSUM methods)
“Simply define the
spectra and select the
number of modes, and the
spectral analysis
will be performed”
CivilFEM builds and defines
the response spectrum from
a certain code with the
parameters that define it.
The data required to define
the response spectrum are
input into the CivilFEM
database with the
~DEFSPEC command.
Four spectra can be defined
for different damping values.
• It is also possible to define the spectra by points:
• First, the periods must be defined in ascending order.
• Then, for each of the damping values, different
accelerations can be provided to define the different
spectra.
The number of modes to be extracted can be defined
with the command ~MODLSOL. The default is 20
modes.
The number of mode shapes to be calculated is taken as
input data for the modal analysis of the structure that is
carried out in ANSYS.
Both the abscissas and ordinates of the spectrum can be
listed and plotted with the commands ~L_SPEC and
~P_SPEC.
Spectrum
component
to be drawn
Initial
period
Final
periodIncrements
The combination of
vibration modes can be
done with the command
~CMBMOD.
To use this command, the
spectrum data must be
defined and the vibration
modes to be combined
extracted.
*** CivilFEM DATASET ***
--------------------------------
LOADSTEP SUBSTEP CUMULATIVE
-------- ------- ----------
2 1 1
*** ALTERNATIVES IN .RCV FILE ***
---------------------------------
DATASET STATUS:
NUMBER OF ALTERNATIVES: 0
CURRENT ALTERNATIVE NUMBER: 0
CURRENT LOADSTEP: 0
CURRENT SUBSTEP: 0
• The result of the combination of modal responses, is stored in CivilFEM as Loadstep 2 (Modal analysis is stored in Loadstep 1). Therefore, to visualize results, use the ~CFSET command to point to this Loadstep.
19.6 Push-Over Analysis
The model of the structure surrenders
to a lateral load which increases
depending on a certain parameter .
Capacity Spectrum
Curve
Demand Spectrum Curve
Elastic Point
Inelastic
Point (IP)
Superposing the Capacity Spectrum curve with the
Demand Spectrum Curve in the (Sd, Sa) graph.
Performance
Point (PP)
Local phenomena of lamination that, cause an effect of weakening
< 2 Elastic.
=2, PC bar becomes plastic
= 4, the structure is weakened because AP and PC bars have
undergone yielding.
= 5 collapse.
General DataCoordinates
of nodeNumber
of node
Load state file
Vertical axis
Number of substeps
Load multiplier factor
~PUSHDEF
command
Modal Shape
Solve
Selected mode shape
(~PUSHMOD and
~PUSHSLV commands)
Plot
Performance
Point
Inelastic
Point
Elastic
Point
Curves
Lambda or
Sd Values (~RETROFT
Command)
Yielded
elements
~RETROFT, Lambda, VALUE
to view the yielding of the structure
for each substep.