Recent Advances in Earthquake Engineering in Europe

The analysis of structures is a fundamental part of seismic design and assessment. It began more than a hundred years ago, when static analysis with lateral loads of about 10% of the weight of the structure was adopted in seismic regulations. For a long time seismic loads of this size remained in the majority of seismic codes worldwide. In the course of time, more advanced analysis procedures were implemented, taking into account the dynamics and nonlinear response of structures. In the future, methods with explicit probabilistic considerations may be adopted as an option. In this paper, the development of seismic provisions as related to analysis is summarized, the present state is discussed, and possible further developments are envisaged.

Definition of design earthquake characteristics, more specifically uniform hazard acceleration response spectrum, on the ground surface is the primary component for performance based design of structures and assessment of seismic vulnerabilities in urban environments. The adopted approach for this purpose requires a probabilistic local seismic hazard assessment, definition of representative site profiles down to the engineering bedrock, and 1D or 2D equivalent or nonlinear, total or effective stress site response analyses depending on the complexity and importance of the structures to be built. Thus, a site-specific response analysis starts with the probabilistic estimation of regional seismicity and earthquake source characteristics, soil stratification, engineering properties of encountered soil layers in the soil profile. The local seismic hazard analysis would yield probabilistic uniform hazard acceleration response spectrum on the bedrock outcrop. Thus, site specific response analyses also need to produce a probabilistic uniform hazard acceleration response spectrum on the ground surface. A general review will be presented based on the previous studies conducted by the author and his co-workers in comparison to major observations and methodologies to demonstrate the implications of site-specific response analysis.

For several decades, seismologists and engineers have struggled to perfect the shape of design spectra, analyzing recorded signals and speculating on probabilities. In this process, several solutions have been proposed, including considering more than one period to define a spectral shape, or proposing different spectral shapes as a function of the return period of the design ground motion.

However, the basic assumption of adopting essentially three fundamental criteria, i.e.: constant acceleration at low periods, constant displacement at long periods, constant velocity in an intermediate period range, has never been thoroughly questioned.

In this contribution, the grounds of a constant velocity assumption is discussed and shown to be disputable and not physically based. Spectral shapes based on different logics are shown to be consistent with the experimental evidence of several hundred recorded ground motions and to lead to significant differences in terms of displacement and acceleration demand.

The main parameters considered to define the seismic input are magnitude and epicenter distance. The possible influence of other parameters – such as focal depth and fault distance, duration and number of significant cycles, local amplification – will the subject of future studies.

Novel forms of ground motion prediction equations and of hazard maps may result from this approach.

Specific points of interest include the generation and adaptation of acceleration and displacement time histories for design, the possibility of including the effects of energy dissipation on the side of capacity rather than on that of demand, the consistent generation of floor spectra for design and assessment of non-structural elements.

Earthquakes impart to structures energy and produce displacements, both of which depend on the structure’s pre-yielding natural period but not on its strength. The resulting seismic force is normally equal to the structure’s lateral resistance. Nevertheless, seismic design is still carried out for empirically specified lateral forces, proportional to the ground motion intensity. Displacement-based seismic design (DBD) requires realistic estimation of seismic deformation demands and of the corresponding deformation capacities. A comprehensive and seamless portfolio of models for the secant-to-yield-point stiffness (which is essential for the calculation of displacements and deformations by linear or nonlinear analysis) and the ultimate deformation under cyclic loading has been developed, covering all types of concrete members, with continuous or lap-spliced bars, ribbed or smooth. The effect of wrapping the member in Fiber Reinforced Polymers is also considered. DBD is now making an entry into European standards, sidelining the earlier, more promising idea of energy-based seismic design, although energy lends itself better than displacements as a basis for seismic design: (a) being a scalar, it relates best to the 3D seismic response and damage; (b) it has a solid basis: energy balance; (c) its evolution during the computed response flags numerical problems. The initial enthusiasm for seismic energy 25 years ago led to a boom in activity on energy demand, but ran out of steam without touching on the more challenging issue of energy capacity of components. This is a fertile field for seismic engineering research.

The use of nonlinear static (pushover) analysis in the case of existing irregular masonry buildings is validated through a comparison with results from nonlinear dynamic analyses, assumed as reference because considered as able to represent the actual seismic behavior. After the selection of a regular prototype case study building, different irregular configurations are defined (in terms of plan irregularity and finite stiffness of horizontal diaphragms). Specific proposals are considered for the selection of load patterns to be used in pushover analysis and the definition of limit states on the capacity curve. A general overview of possible approaches for modelling and analysis of masonry buildings is presented in the introduction.

Our knowledge of earthquake ground motions of engineering significance varies geographically. The prediction of earthquake shaking in parts of the globe with high seismicity and a long history of observations from dense strong-motion networks, such as coastal California, much of Japan and central Italy, should be associated with lower uncertainty than ground-motion models for use in much of the rest of the world, where moderate and large earthquakes occur infrequently and monitoring networks are sparse or only recently installed. This variation in uncertainty, however, is not often captured in the models currently used for seismic hazard assessments, particularly for national or continental-scale studies.

In this theme lecture, firstly I review recent proposals for developing ground-motion logic trees and then I develop and test a new approach for application in Europe. The proposed procedure is based on the backbone approach with scale factors that are derived to account for potential differences between regions. Weights are proposed for each of the logic-tree branches to model large epistemic uncertainty in the absence of local data. When local data are available these weights are updated so that the epistemic uncertainty captured by the logic tree reduces. I argue that this approach is more defensible than a logic tree populated by previously published ground-motion models. It should lead to more stable and robust seismic hazard assessments that capture our doubt over future earthquake shaking.

Near-fault ground motions exhibiting forward directivity effects are critical for seismic design because they impose very large seismic demands on buildings due to their large-amplitude pulselike waveforms. The current challenge in seismic design codes is to recommend simple (easy-to-apply) yet proper rules to explain the near-fault forward directivity (NFFD) phenomenon for seismic demands. This effort is not new and has been the subject of research for over two decades. This paper contributes to these efforts and proposes an alternative set of rules to modify the elastic design spectrum of 475-year and 2475-year return periods for NFFD effects. The directivity rules discussed here are evolved from a relatively large number of probabilistic earthquake scenarios (probabilistic seismic hazard assessment, PSHA) that employ two recent directivity models. The paper first gives the background of the probabilistic earthquake scenarios and then introduces the proposed NFFD rules for seismic design codes. We conclude the paper by presenting some cases with the proposed rules to see how spectral amplitudes modify due to directivity.

Even though the significance of the spatial variability of seismic ground motions for the response of lifelines and its modeling from array data have been addressed for more than half a century, there are still issues associated with its use in engineering applications, which are the focus of the present paper. Common approaches for the simulation of spatially variable seismic ground motions are reviewed, and their corresponding uncertainties are discussed in detail. The importance of the consideration of rotational ground motions in the seismic excitation of structures, and the significance of the kinematic soil-structure interaction in the modification of the foundation input motions are addressed. In addition, difficulties with absorbing boundary conditions and one-dimensional deconvolution methods, when the spatial variability of the ground motions is considered in the seismic analysis of structures, are elaborated upon, and the necessity of developing three-dimensional coherency models is noted. This critical investigation provides insight into and facilitates the appropriate simulation of spatially variable seismic ground motions in engineering applications.

The development of earthquake early warning systems over the last decade has seen a number of studies that have focused either on improving the real-time estimation of seismological parameters, or on the rapid characterization of the possible damage suffered by a structure. However, the rapid increase in real-time seismic networks with stations installed in both the free field and inside buildings now offers the opportunity to combine the experience gained from these activities to develop a comprehensive real-time damage assessment scheme that, depending upon the time frame and spatial scale of interest, can provide useful information for a risk-based early warning system or for rapid loss assessment. Furthermore, newly developed instruments, with their enhanced computing capabilities, also offer the chance to combine early-warning procedures with the monitoring (during seismic crises) of a structure’s behavior. In this paper, an overview of the state of the art in this multidisciplinary field will be given, and an outlook provided as to possible future developments.

This paper presents a number of geotechnical issues encountered in earthquake design of offshore structures and subsea facilities. Parallel with construction of traditional structures such as jackets and gravity-based structures, a considerable effort has recently been put to field developments in deep water. This has brought about other challenges that are largely dependent on geotechnical knowledge. This paper addresses some of the more recent approaches and solutions in geotechnical earthquake design of both shallow water and deep-water structures and facilities such as platforms with large bases, pipelines traversing slopes and seabed installations. It is demonstrated how incorporation of radiation damping and nonlinear soil-structure interaction in offshore installations could optimize the design. Considering the importance of earthquake stability of slopes in deep water development, special attention is given to highlighting several key issues in the earthquake response of submarine slopes including strain softening and three-dimensional shaking.

The uplifting and rocking of slender, free-standing structures when subjected to ground shaking may limit appreciably the seismic moments and shears that develop at their base. This high-performance seismic behavior is inherent in the design of ancient temples that consist of slender, free-standing columns which support freely heavy epistyles together with the even heavier frieze atop. While the ample seismic performance of rocking isolation has been documented with the through-the-centuries survival of several free-standing ancient temples; and careful post-earthquake observations in Japan during the 1940’s suggested that the increasing size of slender free-standing tombstones enhances their seismic stability; it was Housner (Bull Seismol Soc Am 53(2):404–417, 1963) who more than half century ago elucidated a size-frequency scale effect and explained that there is a safety margin between uplifting and overturning and as the size of the column or the frequency of the excitation increases, this safety margin increases appreciably to the extent that large free-standing columns enjoy ample seismic stability. This article revisits the important implications of this post-uplift dynamic stability and explains that the enhanced seismic stability originates from the difficulty of mobilizing the rotational inertia of the free-standing column. As the size of the column increases the seismic resistance (rotational inertia) increases with the square of the column size; whereas, the seismic demand (overturning moment) increases linearly with size. The same result applies to the articulated rocking frame given that its dynamic rocking response is identical to the rocking response of a solitary free-standing column with the same slenderness; yet larger size. The article concludes that the concept of rocking isolation by intentionally designing a hinging mechanism that its seismic resistance originates primarily from the mobilization of the rotational inertia of its members is a unique seismic protection strategy for large, slender structures not just at the limit-state but also at the operational state.

The seismic performance of a two-story 2D frame and a five-story 3D frame–shear-wall structure founded on spread (isolated) footings is investigated. In addition to footings conventionally designed in accordance with “capacity-design” principles, substantially under-designed footings are also used. Such unconventional (“rocking”) footings may undergo severe cyclic uplifting while inducing large plastic deformations in the supporting soil during seismic shaking. It is shown that thanks to precisely such behaviour they help the structure survive with little damage, while experiencing controllable foundation deformations in the event of a really catastrophic seismic excitation. Potential exceptions are also mentioned along with methods of improvement.

The development of large civil engineering projects in active seismic areas often face the challenge of designing foundations that must sustain large seismic forces while preserving the functionality of the superstructure. The natural solution for such foundations seems to lie in the adoption of piles. However, end bearing piles are not always feasible and piled foundations are also subject to adverse effects which may not make them so attractive. Recent projects have shown that alternative, often innovative solutions, may lie in a combination of solutions coupling at least two of the following elements: shallow foundation, soil improvement, caissons, piles, etc…

The lecture details the pros and cons of the “classical” foundation solutions and illustrate on actual projects how combination of solutions may advantageously get rid of adverse effects while still providing a safe design and preserving constructability of the foundations.

Structural Health Monitoring (SHM) of civil-engineering structures is becoming more and more popular both in Europe and worldwide mainly because of the opportunities that it offers in the fields of construction management and maintenance. More precisely, SHM offers several advantages in terms of reduction of inspection costs, because of a better understanding of the behavior of both structures and infrastructures under dynamic loads, seismic protection, observation in real or near real-time, of the structural response and of evolution of damage. Therefore, it is possible to produce post-earthquake scenarios and support rescue operations. In this context, this paper provides a review of different technical aspects of SHM summarizing some sensor validation methodologies for SHM. Following that, recent progresses on SHM of buildings subjected to seismic actions and relevant ways to detect damage are recalled. Moreover, some aspects of SHM of tunnels and bridges are covered. Some related applications that use sensor networks designed by the University of Trento and a startup are described, pointing out the solutions adopted to build reliable SHM systems. Finally, concluding remarks and promising research efforts are underlined.

Composite construction in steel and concrete offers significant advantages over the conventional one based exclusively on either steel or concrete. This paper provides a comprehensive overview of the state of research in analysis and design of composite steel/concrete building structures involving concrete-filled steel tubular (CFT) columns and steel beams. Experimental and analytical/numerical research on the seismic behavior and simulation of CFT columns and composite framed structures under strong ground motions are all considered with emphasis on recent works of the authors. The paper also discusses seismic analysis/assessment methodologies and performance-based seismic design (PBSD) methods that enable engineers to produce composite structures with deformation and damage control.

The European standards for the design and verification of structures are currently under revision. Indeed, after 10 years from their final issue, some criticisms arose as a consequence of both the scientific findings and the design experience gained in Europe. For these reasons, an European program for the revision and harmonization of Eurocodes (mandate M/15 “Evolution of the Structural Eurocodes”) is currently ongoing within the framework of CEN/TC 250, which shall be completed by 2020. The revision process of the rules for the seismic design of steel and steel-concrete structures (currently given by Chaps. 6 and 7 of Eurocode 8 part 1) is supported by a specific working group (WG2) of the commission TC250/SC8 that, in cooperation with the ECCS Technical Committee 13 (TC13), will provide the relevant preliminary documents to the Project Team in charge of drafting the new version of the code. In this perspective, this paper summarizes the critical issues and main aspects that require further insights into the field of steel structures in seismic areas, as well as the recent and currently ongoing relevant research activities that justify the relevant normative updates.

According to Eurocode 6, the lateral capacity of unreinforced masonry walls is calculated using a simple equation, accounting exclusively of a shear failure mode and based on the shear strength of masonry (friction analogy). With the purpose of checking the efficiency of the Code formula, experimental results are collected from the Literature and an effort is made to predict the lateral capacity of the tested walls. This comparison between predicted and experimental shear resistances shows that the Code formula systematically overestimates the lateral capacity of shear walls. The interpretation of this inconsistency is attempted, a qualitative re-evaluation of selected test results is offered and, finally, proposals for further actions within relevant Code Committees are submitted.

A critical overview is provided of current trends in codes for seismic design of bridges, with emphasis on European practice. It is discussed whether the current Eurocode 8-2 provisions are performance-based and what, if anything, is really missing or lagging behind the pertinent state-of-the-art. Two different approaches recently proposed by the author for performance-based design (PBD) of bridges are presented and the feasibility of incorporating them in the next generation of codes, such as the new EC8-2 (currently in the evolution process), is discussed. The first procedure is in line with the exigencies of ‘direct DBD’ wherein stiffness and subsequently strength of the bridge are determined to satisfy a target displacement profile, with due account of the effect of higher modes. The second procedure is ‘deformation-based design’ wherein local deformations of dissipating components are an integral part of the design; two versions of this procedure are presented, one for bridges with ductile piers and one for seismically isolated bridges. Both PBD procedures are applied to a code-designed bridge and comparisons are made in terms of feasibility, cost, and performance.

Earthen and masonry structures are usually heavy and do not possess an integral behavior. A consequence of these characteristics, in combination with the adopted materials featuring low tensile strength and ductility, is that such structures often collapse in a quasi-brittle way, with local failures, usually out-of-plane. This paper first addressed the seismic assessment of these structures, by providing some recent shaking table tests and blind predictions. Obvious limitations were found in providing a good estimate of collapse. Subsequently, techniques for retrofitting and strengthening are addressed, with applications shown in a real case study.

External jacketing of columns with Fiber Reinforced Polymers (FRPs) is a promising retrofitting technique for improving seismic performance of sub-standard reinforced concrete (RC) buildings. The enhancement in deformation capacity and shear strength of jacketed members helps to prevent the brittle collapse mechanism of buildings with inadequate ductility. This paper provides an overview on the retrofitting of columns with FRP jacketing for particularly ductility enhancement and gives a brief summary of seismic strengthening recommendations of various design documents. In addition, a recent full-scale test conducted simultaneously on an as-built and a FRP retrofitted building, which are identical with design geometry, material quality and seismic deficiencies, is briefly presented and the performance of the retrofitting technique is evaluated. Finally, analytical behavior obtained through nonlinear static analyses executed using FRP-confined concrete models recommended in different technical documents are reviewed in comparison with the experimental behavior.

Currently, 8 out of the 10 most populous megacities in the world are vulnerable to severe earthquake damage, while 6 out of 10 are at risk of being severely affected by tsunami. To mitigate ground shaking and tsunami risks for coastal communities, reliable tools for assessing the effects of these hazards on coastal structures are needed. Methods for assessing the seismic performance of buildings and infrastructure are well established, allowing for seismic risk assessments to be performed with some degree of confidence. In the case of tsunami, structural assessment methodologies are much less developed. This stems partly from a general lack of understanding of tsunami inundation processes and flow interaction with the built environment. This chapter brings together novel numerical and experimental work being carried out at UCL EPICentre and highlights advances made in defining tsunami loads for use in structural analysis, and in the assessment of buildings for tsunami loads. The results of this work, however, demonstrate a conflict in the design targets for seismic versus tsunami-resistant structures, which raise questions on how to provide appropriate building resilience in coastal areas subjected to both these hazards. The Chapter therefore concludes by summarizing studies carried out to assess building response under successive earthquakes and tsunami that are starting to address this question.

Classical monuments are articulated structures consisting of multi-drum columns made of discrete stone blocks that are placed one on top of the other without mortar. Despite the lack of any lateral load resisting mechanism except friction, classical monuments are, in general, earthquake resistant, as proven from the fact that they have survived several strong earthquakes over the centuries. However, in their current condition, they present many different types of damage that affect significantly their stability. This chapter presents the results of theoretical and experimental research on the earthquake resisting features and the assessment of the vulnerability of these structures, which is not straightforward due to the high nonlinearity and the sensitivity of the response. Recent trends towards a performance-based philosophy for the seismic risk assessment of these structures, based on conditional limit-state probabilities and seismic fragility surfaces, are also discussed.

This paper discusses two issues related to the seismic performance of code-conforming structures from the probabilistic standpoint: (i) the risk structures are implicitly exposed to when designed via state-of-the-art codes; (ii) which earthquake scenarios are expected to erode the portion of safety margins determined by elastic seismic actions for these structures. Both issues are addressed using recent research results referring to Italy.

Regarding (i), during the last few years, the Italian earthquake engineering community is putting effort to assess the seismic risk of structures designed according to the code currently enforced in the country, which has extended similarities with Eurocode 8. For the scope of the project, five structural typologies were designed according to standard practice at five sites, spanning a wide range of seismic hazard levels. The seismic risk assessment follows the principles of performance-based earthquake engineering, integrating probabilistic hazard and vulnerability, to get the annual failure rates. Results, although not fully consolidated yet, show risk increasing with hazard and uneven seismic reliability across typologies.

With regard to (ii) it is discussed that, in the case of elastic design actions based on probabilistic hazard analysis (i.e., uniform hazard spectra), exceedance of spectral ordinates can be likely-to-very-likely to happen in the epicentral area of earthquakes, which occur relatively frequently over a country such as Italy. Although this can be intuitive, it means that design spectra, by definition, do not necessarily determine (elastic) design actions that are conservative for earthquakes occurring close to the construction site. In other words, for these scenarios protection is essentially warranted by the rarity with which it is expected they occur close to the structure and further safety margins implicit to earthquake-resistant design (i.e., those discussed in the first part).

On August 24th, 2016, a severe, very long seismic sequence started in Central Italy. It was characterized by nine major shocks M5+, two of which with moment magnitude Mw 6.0 (August 24th, 2016) and 6.5 (October 30th, 2016). A complex seismogenic fault system was activated, with the rupture of several segments. The affected area, which develops in NNW-SSE direction along the Apennines, was very large, due to both the large magnitude values and the distance among the epicenters of the nine major shocks. The maximum observed (cumulated) intensity was XI in both MCS and EMS scales. After 1 year, 78,500 seismic events had been recorded by the National Institute of Geophysics and Volcanology national seismic network. 299 people lost their life, all due to the first main shock. Devastating damage was experienced by buildings, cultural heritage, roads and other lifelines, resulting in huge economical direct losses.

The emergency response was coordinated, according to the Law 225/1992, by the Italian National Department of Civil Protection. The main scientific features of the sequence and the main technical emergency activities are shown, discussed and, when possible, compared to the main recent Italian earthquakes, i.e., 1997 Umbria-Marche, the 2009 Abruzzo and 2012 Emilia earthquakes, pointing out analogies and differences.

The development of the 2nd generation of Eurocodes is under way, under a mandate of the European Commission to CEN. The history of the development of the Eurocodes since 1975 until their release in 2005 as European standards EN (1st generation) is put into perspective. It is pointed out that the evolution of the texts during successive updates of Eurocode 8 leaves open discussions on certain topics for the development of the 2nd generation. After having explained how the work in progress is organised, the most important topics for the current or future work are discussed, with the solutions that are proposed.

Most of the literature on resilience is devoted to its assessment. It seems time to move from analysis to design, to develop the tools needed to enhance resilience. Resilience enhancement, a close relative of the less fashionable risk mitigation, adds to the latter, at least in the general perception, a systemic dimension. Resilience is often paired with community, and the latter is a system. This chapter therefore discusses strategies to enhance resilience, endorses one of prevention rather than cure, and focuses in the remainder on the role played by systemic analysis, i.e. the analysis of the built environment modelled beyond a simple collection of physical assets, with due care to the associated interdependencies. Research needs are identified and include challenges in network modelling, the replacement of generic fragility curves for components, how to deal with evolving state of information.