Journal of Earthquake Engineering

Selected case histories recording the behaviour of piled foundations in lateral-spreading fields are reviewed. From these observations and from laboratory experiments in the centrifuge and on the shaking table, it is clear that the resistance of liquefied soil is very small and that the critical points for a pile are at the bottom as well as near the top of the liquefied layer. It is also apparent that liquefaction does not occur until a significant threshold level of shaking is exceeded, and that even then it does not occur instantaneously. Thus damaging motions can be felt by the pile and transmitted to the superstructure before liquefaction and lateral spreading occurs. There is a critical period early in the shaking when the soil has softened enough for ground motion to be amplified but is still stiff enough to induce large bending moments in the pile, near the top of the liquefiable layer. Liquefaction is certainly not a dependable base-isolation mechanism. A controversial issue concerns the continuity or localisation of displacement within the liquefied layer. There is evidence that, in the majority of cases, displacement within the liquefied layer is continuous with depth; although when a sufficiently impermeable layer overlies the liquefiable one, a layer of free water can accumulate and localise displacement. Another old controversy, which now seems to be resolved, concerns the generation of passive earth pressures in non-liquefiable surface layers. There is clear field and laboratory evidence for the development of the passive state in superficial layers. For use in every-day design, two simple methods of analysis have recently emerged. They are the earth pressure method and the seismic deformation method. They have been calibrated against the Kobe data and appear to work well in the few cases where they have been checked on other soils. It remains for them to be verified against a broader range of soils and for further soil-spring models.

Deterministic and probabilistic seismic hazard assessment are frequently represented as irreconcilably different approaches to the problem of calculating earthquake ground motions for design, each method fervently defended by its proponents. This situation often gives the impression that the selection of either a deterministic or a probabilistic approach is the most fundamental choice in performing a seismic hazard assessment. The dichotomy between the two approaches is not as pronounced as often implied and there are many examples of hazard assessments combining elements of both methods. Insistence on the fundamental division between the deterministic and probabilistic approaches is an obstacle to the development of the most appropriate method of assessment in a particular case. It is neither possible nor useful to establish an approach to seismic hazard assessment that will be the ideal tool for all situations. The approach in each study should be chosen according to the nature of the project and also be calibrated to the seismicity of the region under study, including the quantity and quality of the data available to characterise the seismicity. Seismic hazard assessment should continue to evolve, unfettered by almost ideological allegiance to particular approaches, with the understanding of earthquake processes.

The role of joint damage and collapse in the seismic response assessment of existing reinforced concrete frame buildings is herein investigated. Based on recent results from experimental investigations on frame system and subassemblies designed for gravity-load-only, considerations on structural performance based on hybrid local and global failure mechanisms related to joint damage are provided, with particular attention to displacement demand, interstorey drift and damage distribution. Effects of bond deterioration and slip of reinforcing bars passing through an interior joint are discussed in terms of local hierarchy of strength and sequence of events. A simple analytical model for joint response is proposed and adopted for preliminary investigations on frame systems with substandard structural details. The occurrence of a "shear hinge" in the joint might protect to some extent soft-storey mechanisms, reducing the interstorey drift demand, with no significant effects on the global displacement demand. On the other hand, typical inadequacies of structural details (i.e. end-hook anchorage in beam bars combined with use of smooth bars) might cause severe strength degradation leading to particularly brittle failure mechanism.

Reinforced concrete frames designed according to early seismic provisions or, sometimes, without any seismic provision, have usually low strength and, in most cases, show limited ductility. Very often details are poor and, consequently, the critical zones do not behave in a ductile way, showing brittle mechanisms of failure. Because of these problems, the assessment of existing reinforced concrete (RC) structures requires a refined procedure. A summary of models that permit the analysis of the non-linear behaviour of RC structures is discussed. An innovative numerical model is presented wich takes into account the most important mechanical phenomena affecting the non-linear behaviour of the RC frames. In conclusion, the influence of different strength and deformation sources on the global behaviour of existing buildings is studied and the needed capabilities of the numerical models are underlined.

The paper examines the requirements for inelastic static and dynamic analysis applied to earthquake design and assessment. Conventional pushover, with various load distributions, as well as advanced adaptive concepts are examined and compared to incremental dynamic analysis. Regions of applicability of each are discussed and suggestions on which method is better suited under a given set of conditions are qualitatively made. It is concluded that there will always be a class of structure-input motion pairs where inelastic dynamic analysis is necessary. Future developments should aim at reducing the regions where dynamic analysis is needed, hence static analysis may be used with confidence in other cases.

The core of the paper consists of the illustration of a method for seismic reliability analysis of non-linear structures. The method, which is a development over a recent previous proposal, is quite comprehensive in scope since it includes consideration of randomness in the input, in the mechanical properties of the structure and in the limit-state, or capacity, conditions. Essentially, the problem is formulated as the out-crossing of the response process out of a (scalar) safe threshold, and this problem is solved by time-invariant FORM methods. An application to an idealised five-storey building demonstrates the salient theoretical and computational features of the method. The new approach is presented in a broader framework, which involves a discussion on present trends and capabilities in the area of probabilistic seismic design. For the sake of this discussion, two approaches just appeared in the literature are outlined, which are less demanding from a theoretical standpoint, and hence closer to engineering practice, but also less general in scope. It is argued that their introduction on code-assisted design would be feasible immediately with obvious advantages, while approaches of more rigorous nature would be given some more time to mature and to become more accessible to professional use.

Seismic assessment of a viaduct severely damaged in the Duzce (Turkey) earthquake of 12 November 1999 is described. The purpose of the assessment was to determine whether the bridge could be repaired and retrofitted to a state where it could survive a design earthquake larger than the Duzce event, and more than twice as intense as the original design earthquake, while responding within the serviceability limit state. A number of controversial issues were raised as a consequence of this design brief, and are described in this paper.

Large seismic isolation bearings and/or dampers are currently used to seismically retrofit several long-span bridges. These large devices must be thoroughly tested at full-scale with real-time seismic demands, necessitating new, larger seismic testing facilities. Accurate performance characterisation of these devices depends on thorough characterisation of the seismic testing facility itself. One component of this essential systems' characterisation is a mathematical simulation model. Although linear modelling techniques have traditionally been used to characterise seismic facilities, many of the large system's unique behaviours, parameters and non-linearities significantly reduce the accuracy of linear modelling techniques. These significant characteristics are discussed in detail. A more comprehensive non-linear modelling approach is required for these large facilities, and a simulation model using a non-linear time-history dynamic analysis approach is outlined.