Advanced Earthquake Engineering Analysis

Fundamental characteristics of soil behaviour during earthquakes are reviewed; field and laboratory evidences of non linearities and energy dissipation mechanisms are presented. Different kinds of soil constitutive models are discussed with special emphasis on the equivalent linear viscoelastic model commonly used in engineering practice.

Determination of soil characteristics is a key aspect in constitutive modelling. The most sophisticated constitutive models are useless if the input parameters are incorrect or not properly defined. The strain range of interest for earthquake engineering goes from very small strains up to 10⁻³ and requires the use of especially dedicated tests. A combination of field and laboratory tests is the most effective way to achieve a reliable definition of oil behaviour.

Usually in the seismic design of ordinary building, soil structure interaction is neglected and the dynamic response of the structure is evaluated under the assumption of a fixed based response. However during seismic loading the soil undergoes deformations which are imposed to the foundation, the question naturally arises of knowing if the motion in the vicinity of the structure is altered by the presence of the structure and how the structure response is modified by the compliance of the supporting soil. This interaction between the structure and the soil is named soil-structure interaction (SSI). The purpose of this chapter is to illustrate whether and under which conditions SSI is important and what are its consequences on the dynamic response of the structure.

Earthquake foundation design is a challenging task that requires analytical capabilities and extensive understanding of soil behaviour and soil structure interaction. The classical approach involves the determination of the forces applied to the foundation, the seismic demand, and the verification of the bearing capacity, the seismic capacity. However not all situations can be tackled with analyses. Seismic building codes and in particular their chapters on foundation detailing are fundamental to achieve a safe design.

It is widely recognized that nonlinear time-history analysis constitutes the most accurate way for simulating response of structures subjected to strong levels of seismic excitation. This analytical method is based on sound underlying principles and features the capability of reproducing the intrinsic inelastic dynamic behaviour of structures. Nonetheless, comparisons with experimental results from large-scale testing of structures are still needed, in order to ensure adequate levels of confidence in this numerical methodology. The fibre modelling approach employed in the current endeavour inherently accounts for geometric nonlinearities and material inelasticity, without a need for calibration of plastic hinges mechanisms, typical in concentrated plasticity models. The resulting combination of analysis accuracy and modelling simplicity, allows thus to overcome the perhaps not fully justifiable sense of complexity associated to nonlinear dynamic analysis. The fibre-based modelling approach is employed in the framework of a finite element program for seismic response analysis of framed structures, freely downloaded from the Internet. The reliability and the accuracy of the program are demonstrated by numerically reproducing pseudodynamic tests on full-scale structures. Modelling assumptions are discussed, together with their implications on numerical results of the nonlinear time-history analyses, which were found in good agreement with experimental results.

Estimating seismic demands on structures requires explicit consideration of the structural inelastic behaviour: to this end, the use of nonlinear static procedures, or pushover analyses, is inevitably going to be favoured over more complex nonlinear time-history analysis methods. Currently employed pushover methods are performed subjecting the structure to monotonically increasing lateral forces with invariant distribution until a target displacement is reached, basing both the force distribution and target displacement on the assumptions that the response is controlled by the fundamental mode, unchanged after the structure yields. However, these invariant force distributions cannot account for the contributions of higher modes to response, nor for the redistribution of inertia forces because of structural yielding and the associated changes in the vibration properties: in order to overcome drawbacks arising from conventional methods; an innovative displacement-based adaptive pushover technique for estimation of the seismic capacity of RC structures is illustrated. Analytical parametric studies show that, with respect to conventional pushover methods, the novel approach can lead to the attainment of significantly improved predictions, which match very closely results from dynamic nonlinear analysis.

A brief description of current force-based design is given. The historical basis for current seismic design philosophy is mentioned. Conceptual problems for force-based design using initial stiffness to represent structural response, which are often not recognized by designers are discussed.

This chapter presents the fundamentals of the new seismic design method known as direct displacement based design. It is a simple design approach where the multi-degree-of-freedom structure is characterized the secant stiffness and equivalent elastic damping of an equivalent single-degree-of-freedom structure. Design is based on achieving a specified displacement limit state — defined either by material strain limits or non-structural drift limits — under the design level seismic intensity, rather than using these limits merely as upper bounds of acceptable behaviour. The characterization of the structure by secant stiffness avoids the many problems inherent in force-based design where initial stiffness is used to determine an elastic period, and forces are distributed between members in proportion to elastic stiffness.

The most relevant improvement in the design methods for bridge structures traditionally followed new evindence in damage and collapse due to the major earthquakes. This logical process is critically review through the description of the most recent experience in bridge design and assessment, emphasizing its progressive evolution from acting force and strength, to ductility capacity and demand, to displacement — based approaches. Some controversial or often neglected aspects, related to seismic input, response and design issues are discussed.

This lecture presents the different seismic testing devices, their advantages and limits together with the last developments in this very active field.