Concrete Today May 2010 - the Irish Concrete Federation

concrete today - designing for earthquake - precast concrete
Designing for Earthquake – Precast Concrete
The Importance of the conceptual design of precast concrete structures in
seismic areas and lessons from past earthquakes – The SAFECAST Programme
By Georgia Kremmydia, National Technical University of Athens, Athens, Greece
Dipl. Civil Engineer, PhD Candidate
Introduction
In recent years, there has been a sharp
increase in the use of prefabricated, offsite
construction techniques, including
Precast concrete. Prefabricated elements,
such as architectural cladding panels,
Precast hollowcore and wideslab floors and
stair flights are also being introduced to
buildings which are primarily constructed
insitu. A shortage of site operatives, the
need to eliminate uncertainty in the
programme caused by inclement weather
conditions and the general requirement for
fast, reliable and economic construction
techniques are among the main drivers.
Joints and Connections
As is well known, the main difference
between traditional, monolithic castinsitu
R.C. structures and corresponding
prefabricated structures, is that the
latter are composed of several bearing
members cast in a factory rather than on
site; therefore the structure is composed
of a set of ‘elements’ which are joined by
‘connections’. Thus the main structural
issue in Precast construction relates to the
connections between Precast members
and to the extent to which connections
affect the response of the total structure
under seismic actions. In this regard, the
need for Precast structures to satisfy the
fundamental structural requirements of
‘non collapse’ and ‘damage limitation’ ,
L’Aquila earthquake central Italy 2009
caption
L’Aquila earthquake central Italy 2009
under design-earthquake conditions needs
to be carefully studied.
Experience based on the history of seismic
engineering and recent experience of the
seismic behaviour of R.C. structures, showed
that in spite of the enormous development
of computer simulation software, the
satisfaction of the fundamental requirements
under the design-earthquake cannot solely
be achieved by means of calculations and
that several basic design concepts proved to
be more important.
Structural simplicity and uniformity
Among the most important of these design
concepts is the concept of ‘structural
simplicity’ as characterised by ‘uniformity’,
‘symmetry’ and ‘regularity’ in the
configuration of the structural systems in
plan and/or, elevation. This concept should
be addressed in the first step of the design
process, namely ‘conceptual design’. That
is to say, every analysis has to be carried out
on a preconceived structural scheme.
In fact, by simplifying the structural
system, clear and direct paths (including
alternative paths) for the transmission of
the seismic loads should also be ensured.
In this way, the modelling, analysis,
dimensioning and detailing of the structure
is subject to less uncertainty and thus the
prediction of seismic behaviour is much
more reliable.
The ‘in plan regularity’ is primarily a
function of the geometrical configuration
of the building: the configuration should
be compact and clear. In plan set-backs
(re-entrant corners or edge recesses) or L,
n, E or - L shapes etc. should be avoided,
or otherwise limited and specially treated.
• The ratio of Lmax: Lmin. should not be
less than 4
• The distribution of the lateral stiffness
concrete today
5