Advances in Seismic Design Methodologies - Earthquake Engineering for Structural Design - page 439

Advances in Seismic Design Methodologies 439

The incremental dynamic analysis is used for evaluating the behaviour of the q-
factor (Mazzolani and Piluso, 1996). The use of this factor allows performing a simple
elastic analysis which is very easy for design purposes, instead of very complex non-
linear inelastic analyses. For a given structure under a specific acceleration (Fig. 9.35),
according to the Ballio-Setti method, a series of computations of non-linear dynamic
response is performed by scaling step by step the initial ground acceleration shape
(Ballio, 1985). The q -factor is defined as the ratio between the ground acceleration
leading to collapse and the one corresponding to the first plastic hinge formed in the
structure. The maximum value which can be assigned to the q -factor is given by the
intersection of the behavior curve coming from the incremental dynamic analysis and
the bisectrix of the orthogonal axes, which represents the elastic behavior of the
structure. This point of intersection between the two forms of behavior, elastic and
inelastic, allows a direct link between the linear and non-linear computations.
The main criticisms referring to the incremental dynamic analysis are based on the
observation that a correlation exists between the earthquake magnitude and natural
vibration periods. The reduced magnitudes are associated with the reduction of natural
periods and these periods significantly increase with the amplification of acceleration.
In the incremental dynamic analysis these periods remain constant, and, therefore, the
results are affected by this shortcoming of the IPA procedure. The challenge for
improving this methodology is to increase the natural periods simultaneously with the
scaling of the initial acceleration.

9.5.7 Estimation of ProgressinSeismic Analysis

Despite the fact that the accuracy and efficiency of the computational tools have been
substantially increased, there are still many reservations, mainly about their complexity
and suitability for practical design applications. Moreover, the calculated inelastic
dynamic response is quite sensitive to the characteristics of the input motion, thus the
selection of a suitable set of representative acceleration time histories is mandatory. In
addition, there are many difficulties in adequately representing the cyclic load-
deformation characteristics of all the important structural elements (foundations,
columns, beams, connections, infilled elements, etc.). The computational effort
significantly increases to consider all these aspects in a correct way, raising doubts on
its suitability in the everyday practice (Mwafy and Elnashai, 2001).