19-10-2012, 04:32 PM
EVALUATION OF CURRENT APPROACHES FOR THE ANALYSIS AND
DESIGN OF MULTI-STOREY TORSIONALLY UNBALANCED FRAMES
EVALUATION OF CURRENT APPROACHES .pdf (Size: 641.87 KB / Downloads: 67)
SUMMARY
Plan-wise irregular buildings are quite common in many earthquake-prone areas of Europe and
worldwide, making up a remarkably important category of existing structures. Irregular structures exhibit
a complex behaviour under uni- or bi-directional seismic excitation because of torsional coupling effects
affecting the response; due to the inherent complexity of the problem only simplified models have been
developed and studied so far and a number of open issues still exist on the subject. Experimental activity
is therefore badly needed in order to validate analytical studies and to point out the way for their future
developments. In the framework of the research activity of the ELSA Laboratory of the Joint Research
Centre, bi-directional pseudo-dynamic testing of a real size plan-wise irregular 3-storey frame structure
was carried out in January 2004, as the core of a 3-year research project named SPEAR (Seismic
PErformance Assessment and Rehabilitation).
The SPEAR project, specifically targeted at existing buildings, pursues the aim of improving current
codified approaches to the assessment of older non-seismically designed structures, by means of a
balanced combination of numerical and experimental activity.
The data made available by the unique SPEAR experimental activity are very important in themselves,
given the scarcity or absence of test data on the behaviour of irregular multi-storey structures; in the
present paper, they have been compared to the predictions resulting from the application, to the same
structure, of current codified assessment approaches, thus allowing some conclusions to be drawn on the
effectiveness of the latter in dealing with torsionally unbalanced buildings.
INTRODUCTION
In the last decades, the progress of research has made it clear that regularity is a most desirable feature of
seismically designed structures, because it limits the likelihood of local and global misbehaviours during
the earthquake excitation.
Nevertheless, irregular buildings, both in plan and in elevation, do exist in earthquake-prone areas; the
first category is the largest and the object of the following study.
DESCRIPTION OF THE STRUCTURE
The SPEAR building model is a simplification of an actual three-storey building representative of older
constructions in southern European Countries, such as Greece, without specific provisions for earthquake
resistance. It was designed for gravity loads alone, using the concrete design code enforced in Greece
between 1954 and 1995, with the construction practice and materials typical of the early 70s; the
structural configuration shows the lack of consideration of the basic principles of earthquake resistant
design.
The materials used for the structures are also those typical of older practice: for concrete a nominal
strength fc= 25 MPa was assumed in design; smooth rebar steel was used; given the scarcity of the current
production, it was only possible to find bars with a characteristic yield strength larger than initially
requested (fy ≈ 450 MPa instead of fy= 250 MPa); the final hooks for the steel bars were designed
following the minimum requirements of old codes.
Introduction
The first part of the research activity of the SPEAR project consisted in a critical review of the current
code-format assessment procedures for existing RC buildings. To do so, they were applied to a number of
benchmark structures, among which the torsionally unbalanced SPEAR structure, whose results are
presented and discussed in the following.
The assessment procedures considered in the study were:
• FEMA 356 Guidelines (and FEMA 310 Guidelines) [4]
• Japanese Assessment Guidelines [5]
• EC8 Part 3 [6]
• New Zealand Building Authority Guidelines (Draft 1996 and 2002) [7]
Each of them has its own procedure for dealing with plan (and elevation) irregularity to take into account
the torsional coupling effects on the displacements and internal actions, resulting into different evaluation
of the so-called target displacement.
The preliminary calculations were conducted by enforcing the basic prescriptions suggested by each
procedure and on these grounds, when necessary, implementing a finite element model by means of a
commercially available programme.
In parallel, a number of numerical simulations were carried out, implementing models of the structure into
different research oriented 3D nonlinear dynamic analysis computer codes. The comparison between the
results so derived and the assessment outcomes allowed an initial evaluation of the latter to be made,
provided predictions for the test-set up, and led to draw some initial evaluation of the model-dependency
of the results of the numerical approaches.
Introduction
In the following a short preliminary description of the experimental activity carried out in the frame of the
SPEAR project is given; for a more detailed description see Negro, [3], Molina [9].
The first round of tests consisted of three PsD tests at different values of PGA: a small test at a PGA of
0.02g, a test at the PGA value of 0.15g and a higher level test at 0.2g PGA.
The bi-directionality of the PsD test, consisting in the simultaneous application of the longitudinal and
transverse component of the earthquake to the structure, introduced a higher degree of complexity, both
from the analytical and from the technical point of view, with respect to usual unidirectional PsD testing.
In fact, three degrees of freedom (DoFs) per floor needed to be taken into account: two translations and
one rotation along the vertical axis, as opposed to the single degree of freedom that is considered in
conventional unidirectional PsD testing.
Four actuators per storey were connected to the structure, three of which were strictly necessary. The
management of a redundant number of actuators thus required a more complex control strategy.
The PsD integration of the horizontal response of the structure was performed in terms of three
generalized DoFs at each floor, consisting in the in-plane displacements dX and dY and of the rotation
along the vertical axis dθ at the center of mass (CM) of the structure.