Analysis and modelling of the seismic behaviour of high ... - Ingegneria

4. SEISMIC RESPONSE OF PARTIAL-STRENGTH COMPOSITE JOINTS
4.9.1 Pushover vs. incremental dynamic analysis results
Pushover analysis
As said in Chapter 2 Subsection 4, the purpose of the pushover analysis is to
evaluate the expected performance of a structural system by estimating its strength
and deformation demands in design earthquakes by means of a static inelastic
analysis, and comparing these demands to available capacities at the performance
levels of interest (Krawinkler and Seneviratna, 1998). By means of the numerical
model above described, which implements the non-linear behaviour of the
structural members of the prototype structure, the NSP analysis based on the
Capacity Spectrum Method (CSM) and on the Performance Point Method
proposed in the Eurocode 8 (2002) and FEMA-273 (1997) were performed. As
explained in Chapter 2, the pushover analysis based on the CSM obtains the
performance point in the ADRS space (standard pseudo-acceleration Sa vs.
deformation spectrum Sd). Differently from this, the response curve based on the
EC8 and FEMA assumptions is determined by nonlinear static analysis of the
structure subjected to lateral forces with invariant distribution over the height but
gradually increasing values until a target value of roof displacement is reached
(Target Displacement). The floor displacements, storey drifts, joint rotations, plastic
hinge rotations, etc., computed at the target displacement represent the
earthquake induced demands on the structure. Three distributions of lateral forces
are specified in FEMA-273 (1997):
(a) uniform distribution: s *
j = mj (where j = 1, 2, …N is the floor number);
(b) equivalent lateral force (ELF) distribution: s *
j = mjh k
j, where hj is the height of
the j-th floor above the base, and the exponent k varying linearly from the value
1 for fundament period T1 < 0.5 sec and the value 2 for T1 > 2.5 sec;
(c) SRSS distribution: s * is defined by the lateral forces back-calculated from the
storey shears determined by response spectrum analysis of the structure,
assumed to be linearly elastic. This last distribution is not present in the
Eurocode 8 (2002) recommendations.
The lateral force profiles in static pushover analyses influence the structural
response. The first distribution represents the lateral forces that are proportional to
the vertical distribution of the mass at various levels. The use of the uniform load
shape may be justified in the light of a possible soft storey mechanism of irregular
buildings. If this mechanism occurs, the response will be controlled by a large drift
in the first storey. Therefore, this load distribution may give better predictions of the
overall response. On the other hand, the code lateral load shape represents the
forces obtained from the predominant mode of vibration. The inverted triangular
(code) and the rectangular (uniform) load shapes also represent the extreme cases
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