Geoscience Reference
In-Depth Information
CHAPTER 2
LONG PERIOD STRONG GROUND MOTION AND ITS USE AS INPUT
TO DISPLACEMENT BASED DESIGN
E. Faccioli, C. Cauzzi, R. Paolucci, M. Vanini, M. Villani, and D. Finazzi
Structural Engineering Department, Technical University (Politecnico) of Milan, Italy
faccioli@stru.polimi.it
1. Introduction
Consistently with the use of seismic inertia forces for the verification of structural
members, buildings have been traditionally designed for earthquake resistance by rely-
ing on ground acceleration from strong earthquakes to describe seismic loading; in
most applications this is quantified by acceleration response spectra. However, structural
design is increasingly becoming
performance based
, whereby displacements and defor-
mations of structural and non-structural members, which directly control damage, are
broughttothefrontstagetoreplaceaccelerationsandinertiaforces.Thus,therelativedis-
placement of the structure caused by the imposed ground motion, quantified through the
displacement response spectrum (
DRS
), becomes the primary descriptor of the seismic
demand.Methodsareavailabletoreplacetheactualstructure(anon-linearmulti-degree-
of-freedomsystem)withanequivalentlinear1DOFsystem,inwhichenergydissipation
due to the non-linear response is accounted for through a large viscous damping factor.
A synthesis of the main steps involved in the (direct) displacement based approach is
provided in Figure 2.1. Since the design involves the response of the damaged structure,
which is “softer” than the undamaged one, the vibration period
T
at play is significantly
larger than the elastic, or initial, period. Hence, depending on the structure,
DRS
will
have to encompass a broad
T
range, e.g. up to 10s, well beyond the typical 0-4s range
of current norms (CEN, 2004).
With this background, a national research project called S5 was initiated in Italy in mid-
2005,aimedatproviding:(a)amodeloftheseismicactionintermsofarbitrarilydamped
DRS
, extending to long periods and (b) national hazard map(s) depicting the
DRS
values
needed for design. The project is sponsored (by the Department of Civil Defence) and
carriedoutjointlywithotherprojects,forwhichgeneralconsistencyofinputsandoutputs
is a key requirement. Thus, common seismic source models for the country are adopted,
and the hazard description to be provided by S5 in the 1-10s range should be consistent
withthe short period (0-2 s) hazard mapping provided by another project.
Highlighted in this article are some salient parts and results of the work undertaken in
S5 in the engineering seismology domain, including the empirical prediction of
DRS
at
arbitrarydampingforhorizontalandverticalmotionderivedfromanewlyassembledset
