Basic HTML Version
814
IBRACON Structures and Materials Journal • 2012 • vol. 5 • nº 6
Comparative study of codes for the seismic design of structures
with the inverse of
T
). The region for periods superior to
T
D
is the
one displacement governed (accelerations varying with the inverse
of
T
2
), region of iso-displacements. The region between 0 (ZPA –
“zero period acceleration”) and
T
B
is the transition region between
the peak ground acceleration and the maximum spectral accelera-
tions. The values of
S
,
T
B
,
T
C
and
T
D
are defined as a function of
the type of subsoil in the two spectral types defined in the code,
Types 1 or 2, related respectively to higher and lower seismicity
regions, respectively.
It is to be noticed that all standards considers, for the definition of
the spectra, the nominal structural damping of 5%. The Eurocode
8 presents a numerical expression for defining the damping correc-
tion coefficient
η
for damping factors different from 5%.. The iso-
displacements region is considered by the Eurocode 8 and also
by the ASCE/SEI 7/10 that defines this region showing the period
T
D
through maps. The Colombian code provides also data for the
definition of the displacement governed region.
It is to be noted that the Chilean standard is the only one that de-
fine its spectrum through a single equation, varying the spectral
acceleration with an exponential function of the structural period
T
, being the exponent of the equation a function of the soil type.
3.4 Consideration of soil amplification, soil
liquefaction and soil-structure interaction
All the analyzed standards classify the ground conditions accord-
ing the shear wave propagation velocities (
v
s
) and/or the number
of blows in the Standard Penetration Test (
N
SPT
). For non-homo-
geneous sites, criteria for averaging these parameters in the more
superficial subsoil layers (typically in the first 30m) are proposed in
the standards. The number of soil classes varies between three to
five (e.g., in the Eurocode 8, between classes A to D), from very
stiff to soft deposits.
The seismic soil amplification in more or less stiff layers influence
the definition of the shape of the response spectra; in less stiff
deposits, the soil amplification is higher, leading to greater values
of the soil coefficients
S
.
All the analyzed standards define a separate class for liquefiable
soils (e.g., Class
S
2
in the Eurocode 8). Nevertheless, no specific
procedures are defined in them for analyzing these situations. A
notional definition of liquefiable soils is only found in the Chilean
standard.
Soil-structure interaction is considered in ASCE/SEI 7/10 (Chapter
19), in the Colombian standard (Chapter A-7 and Appendix A-2)
and in the Venezuelan standard (Item 8.8).
3.5 Classification of the structures in different
importance levels and partial safety factors
All the analyzed standards recognize the necessity of classifying
the structures in Importance Classes. This implies in a reliability
differentiation, according to the estimated risk and/or consequenc-
es of a failure. This reliability differentiation is simply defined in the
standards by the application of a multiplying factor
I
to the evalu-
ated seismic forces. Three or four Importance Classes are defined
in the standards. In all of them, the factor
I
= 1 is assigned to usual
structures, such as residential and commercial buildings. In some
standards, such as in the Venezuelan code, higher Importance
Classes can require higher ductility levels for the detailing.
single parameter defines the local seismicity: the ZPA (“Zero Pe-
riod Acceleration”) value of the reference peak ground acceleration
in rock ground (
a
g
). All the South American standards consider this
definition; their seismic zonation is accordingly presented in these
standards through maps. Santos and Souza Lima [11] presented a
tentative of roughly compatibilize these seismic zonations among
them in a map, in order to give a global view of the seismicity in
the South American continent. This map is reproduced in Fig.1. It
is to be noticed that the maximum value for the peak ground ac-
celeration in rock defined in these standards is 0.4g, excepting for
two small areas defined in the Colombian standards with design
accelerations of 0.45g and 0.5g.
In the standard ASCE/SEI 7/10, the seismic input is defined through
three basic parameters, i.e., the peak ground accelerations for the
spectral periods of 0.2s and 1.0s and the period T
D
that defines the
displacement governed region of the spectrum. These parameters
are defined in the standard through very detailed maps.
3.3 Definition of the shape of the horizontal elastic
response spectra
Is order to make possible the comparison among the horizontal
elastic response spectra defined in the several standards, Fig. 2
below reproduces the Fig. 3.1 of Eurocode 8, that establishes the
shape of its elastic response spectrum, including the several pa-
rameters that define it.
In the Eurocode 8 elastic response spectra, as well as in the elastic
spectra of all the other analyzed standards, the pseudo-acceler-
ations (
S
e
) are given as a function of the structural periods (
T
).
The spectra vary proportionally to the peak ground acceleration
(
a
g
), times a soil coefficient
S
, related to the soil amplification and
considering the parameter
η
, correction factor for damping values
different from 5%.
The region between reference periods
T
B
and
T
C
is the one accel-
eration controlled (constant acceleration). The region between pe-
riods
T
C
and
T
D
is the one velocity controlled (accelerations varying
Figure 2 – Shape of the elastic response