Advances in Seismic Design Methodologies - Earthquake Engineering for Structural Design - page 426

426 Earthquake Engineering for Structural Design

- The method neglects the duration effects and cumulative energy dissipation, by
assuming that damage is function only of lateral deflection of the structure.
- The push-over analysis is a static analysis which neglects the dynamic effects as
kinetic and viscous damping energies.
- The method is developed only as 2-D procedure, the incorporating of 3-D
(including torsional effects) being very difficult.
- The procedure considers only the lateral earthquake loading, the vertical
earthquake components being ignored. Identification of the strength discontinuities
in plan or elevation will lead to changes in the dynamic characteristics in the
inelastic range.
- Estimate of inter-story drifts which account for strength or stiffness discontinuities.
- Evaluation of the consequences of strength deterioration of individual elements on
the behavior of the structural system.
- Evaluation of the realistic force demands on potentially brittle elements, as
columns, beams, braces, connections, non-structural elements.
- Estimate of the ductility demands for elements which have to deform inelastically
in order to dissipate the induced seismic energy.
- Consideration of the completeness and adequateness of load path, considering all
the elements of the structural system and non-structural elements, which
significantly contribute to the lateral load distribution.
In spite of these performances, the push-over method suffers from several
fundamental deficiencies (Kim and D'Amore, 1999):
- The method implies that there is a separation between the structural capacity and

earthquake demand. Non-linear structural behavior is load path dependent and it is
not possible to separate the loading input from the structural response. So, at each
loading step, the lateral load distribution must be changed.

- The push-over procedure is overly simplifying, considering that the structure

behavior can be characterized only by two parameters, base shear and top level
displacement. It is difficult to capture all possible structural responses in the
vertical plan direction using only two parameters.

- The traditional push-over procedure does not account for the progressive changes

in modal properties due to the influence of superior vibration modes and,
consequently, the shear distribution on the structure height, which take place in a
structure as it experiences cyclic non-linear yielding during an earthquake. Figure
9.26 show the modification in lateral load distribution due to the changing of
structure properties as result of plastic hinges formation.

- Even if the top level displacement can be determined with some accepted

approximation using a constant load pattern (triangular or uniform), the inter-story
drifts estimation can be highly inaccurate, if higher mode effects are of importance
(Fig. 9.27). In the case presented in this figure, for the triangular distribution, the
error is about 51% and for uniform distribution, 92% (Pinho et al, 2005).