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IBRACON Structures and Materials Journal • 2012 • vol. 5 • nº 6
Comparative study of codes for the seismic design of structures
is supposed that the building is located in city of Reevesville,
South Carolina (ZIP code 29471), U.S. Considering a 475
years return period, the design ground acceleration, for rock
conditions, in this location can be taken as
a
g
= 0.15g. This
relatively small level of seismicity has been chosen is order to
make possible the comparison among all the analyzed stan-
dards, since this is the highest level of seismicity considered
in Brazilian standard.
Figure 5 shows the elastic spectra obtained according the sev-
eral standards. It is to be noticed that due the low seismicity
of the site, for the Eurocode 8 the Type 2 spectrum has been
selected. In this spectrum, the higher accelerations are con-
centrated in the 0.1s – 0.25s periods range; due to this, in
the range of the fundamental periods of the analyzed structure
(around 1.0s), the accelerations given by Eurocode 8 are much
smaller than the ones given by the other codes. It is to be no-
ticed that all the presented spectra considers the same seis-
micity (
a
g
= 0.15g) and the same type of soil (stiff soil ground).
4.2 Results of the analyses
Spectral analyses of the building have been performed using the
computer program SAP2000 [12], for the nine defined design
spectra. In order to make possible a direct comparison among
the standards, the analyses have been done using the elastic
spectra, without the consideration of the response modification
factors (reduction factors due to the non-linear behaviour).The
Modal Participation Mass Ratios obtained as results of the mod-
al extraction performed by the program SAP2000 are shown in
Table 1. The first mode appears in the direction X, longitudinal
to the building, and the second one in the transversal direction
Y. The 3
rd
mode is torsional. Up to the 7
th
mode, 95% of the total
mass is captured in both horizontal directions. Up to the 12
th
mode, there is not a characteristic vertical mode.
Obtained displacements in the top of the building are present-
ed in Figs. 6 and 7 for longitudinal and transversal directions
X and Y, respectively. These displacements are obtained in
spectral analyses using the CQC rule for the combination of
modal components.
As already commented, due to the consideration in the Euro-
code 8, of the Type 2 spectrum, the displacements obtained
with this code are dramatically inferior to the ones obtained
with the other codes.
Obtained total horizontal forces in the basis of the building
through spectral analyses are shown in Figs. 8 and 9 (in the
legend, “SAP2000”). The figures also show the obtained total
horizontal forces obtained through the static equivalent proce-
dures defined in the standards (in the legend, “Codes”).
It is to be noticed that in the analyzed standards, the total hori-
zontal forces obtained with the static equivalent methods are
always conservative (or practically equal) when compared with
the obtained in the spectral analyses.
Figure 3 – Plan view of a floor of the analyzed building