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.