Seismic Risk Analysis of Regular Girder Bridge Subjected to Mainshock-Aftershock Sequences

When a structure is subjected to an earthquake sequence, the high rate of aftershocks after the mainshock and cumulative damage caused by the earthquake sequence make the structure very dangerous. Considering the uncertainty in seismic occurrences, structural damage is often predicted using a seismic risk analysis. This approach has become a main measure for seismic disaster assessment, and provides a reasonable reference for post-earthquake emergency response decision-making and pre-earthquake seismic design. Therefore, it is of great significance to study a seismic risk analysis considering the effect(s) of aftershocks. In this study, the aftershock hazard is estimated for a post-mainshock environment based on an aftershock probabilistic seismic hazard analysis. Considering the uncertainty regarding the mainshock and aftershock occurrences, in addition to the functional relationship between the mainshock and aftershock parameters, the aftershock seismic hazard is estimated for the pre-mainshock environment. The mainshock fragility and aftershock fragility of regular girder bridges are evaluated based on the Kunnath damage model. Finally, considering the damage accumulation in bridge structures, the seismic hazard and seismic fragility are combined to establish a post-mainshock aftershock seismic risk framework and pre-mainshock mainshock-aftershock seismic risk analysis framework. Based on these, the mainshock risk and mainshock-aftershock risk are compared to verify the importance of considering the aftershock effects in seismic disaster assessments. The aftershock risks for the bridges of different post-mainshock damage states are compared, and the influence of the initial damage after the mainshock on the damage to the structure in the post-mainshock environment is studied.

Empirical Correlations of Spectral Input Energy with Peak Amplitude, Cumulative, and Duration Intensity Measures

ABSTRACT In earthquake engineering, it is acknowledged that a vector of intensity measures (IMs) can better predict seismic structural responses than a single measure. Hence, a vector of IMs is widely applied in a number of applications, such as probabilistic seismic hazard analysis, probabilistic seismic risk analysis, and ground-motion selections. Spectral input energy (EI) has been demonstrated as a promising IM in earthquake engineering, especially in the energy-based seismic design of structures. However, this important measure has not been included in the vector of IMs. Therefore, it is worthwhile to incorporate EI with other important IMs by examining correlations. This study analyzes the empirical correlations of spectral EI with peak amplitude-based IMs, cumulative-based IMs, and duration-related IMs. It is found that spectral absolute EI has strong correlations with peak ground velocity at all investigated periods. However, spectral EI is negatively correlated with duration-based IMs. To demonstrate the applicability of the examined correlations, a simple example is finally presented by employing EI for the ground-motion selections and seismic hazard assessment based on the generalized conditional intensity measure approach.

Urban Seismic Risk Analysis Using Empirical Fragility Curves for Kerend-e-Gharb After Mw 7.3, 2017 Iran Earthquake

This study provides the seismic risk map of the small mountain city of Kerend-e-gharb. This innovative study evaluates the existing methods and formulations with the effects of a devastating earthquake using empirical fragility curves. A seismic vulnerability map is developed at the urban block-scale at a Geographic Information System for a variety of steel, reinforced concrete, confined masonry, unreinforced masonry, and adobe residential buildings. The estimated seismic damages are validated by comparing them with the surveyed building damages in the Sarpol-e-zahab earthquake. Moreover, the damages to the population, including the probability of homelessness, death, severe injuries that need immediate hospital treatment, moderate injuries that require hospital treatment, and light injuries without hospitalization, are assessed. This research also investigated the economic losses and estimation of debris. Finally, the mean damage index map is presented. The mean damage index map is modified due to renovation and retrofitting at higher damage levels, and it is increased at moderate damage levels that have not been structurally improved. The results of this study can be used in seismic crisis management planning in natural hazards to achieve a sustainable city.

Seismic risk analysis for large dams in West Coast basin, southern Ghana

Dams are parts of the critical infrastructure of any nation, the failure of which would have a high-risk potential on the people and properties within the dam vicinity. Ghana is one of the most seismically active regions in West Africa and has at least 5 large dams across the region, constructed in strategic locations. The area is characterised by low-to-moderate seismicity, yet historical events suggest that major earthquakes that are potentially damaging have occurred in the study basin. This paper summarises the method used to analyse seismic risk and discusses the seismic hazards of three large dams across the study basin based on the seismicity at the dam sites and their risk due to structural properties and the location of each dam. The peak ground acceleration (PGA) values for the dam sites estimated are within the range of (0.31 g and 0.52 g) for 10,000 years return period. The study shows that one large dam has a high-risk class in the basin. This dam should be inspected and analysed for its seismic safety and people’s protection in the downstream paths.

Full-Wavefield, Full-Domain Deterministic Modeling of Shallow Low-Magnitude Events for Improving Regional Ground-Motion Predictions

ABSTRACT At the fundamental level, seismic risk analysis relies on good modeling tools for predicting the ground motion resulting from hypothetical earthquake events, which is traditionally approximated using many variations of ground-motion prediction equations (GMPEs). The main benefit of these equations lies in their low computational cost, allowing one to run Monte Carlo simulations in which event probabilities are dictated by regional catalogs comprising historical observations. These equations, however, rely on approximations that are only accurate in a statistical sense. In this study, we consider cases in which regional high-resolution 3D earth models are available from exploration reflection seismology. These high-fidelity velocity models allow us to perform deterministic elastic ground-motion simulations at local distances, given a prescribed synthetic earthquake event, and compare the results with those predicted by GMPEs. This full-wavefield full-domain modeling approach is significantly more costly and particularly challenging due to the slow shear-wave velocity at the near surface, which requires fine spatial and temporal discretizations. With the aid of powerful computational resources, we use an adaptive mesh generator and an efficient wave solver to model the 3D elastic and anelastic wave propagation from the hypocenter all the way to the ground surface. This approach can simultaneously account for 3D subsurface structures, near-surface site effects, topographic relief, and the radiation pattern of the source. In areas where observations are sparse, the modeling results can fill the gap between stations and provide a test bed that can be used for improving the development and accuracy of GMPEs. This approach is well suited for areas where shallow low-magnitude-induced seismic events can occur. Lastly, to demonstrate our approach, we consider an observed seismic event at the Groningen gas field and compare the recorded ground motions with both—those predicted by our approach and those predicted by GMPEs.

Performance-Based Seismic Fragility and Risk Assessment of Five-Span Continuous Rigid Frame Bridges

Earthquakes can cause serious damage to traffic infrastructures, among which the impact on bridge structure is the most important. Therefore, in order to assess bridges serviceability, it is important to master their damage mechanism and to analyze its probability of occurrence under a given seismic action. Various uncertainties, like the location of epicentre of future earthquakes and their magnitudes, make this task quite challenging. We are also required to consider different earthquake scenarios and the damaged states of bridge components associated with those earthquakes. To suppress these difficulties, this study proposed a new method of performance-based seismic fragility and risk assessment for bridges. The proposed method included three steps: (1) performance-based seismic fragility estimation of a five-span continuous rigid frame bridge, (2) seismic hazard analysis for locations of the bridge, and (3) seismic risk analysis of the bridge. The proposed method that considered the performance of the bridge and the uncertainty in the location of the earthquake epicentre and magnitudes can provide valuable references for seismic-resistant design of multispan continuous rigid frame bridges in the future.

A Study on the Dynamic Behavior of a Vertical Tunnel Shaft Embedded in Liquefiable Ground during Earthquakes

Since liquefaction was first observed in South Korea during the Pohang earthquake, public concerns regarding the seismic stability of major infrastructure have increased substantially. However, the seismic behavior of tunnel shafts, which are an important element of tunnel structures, has not been properly established, especially under liquefiable soil conditions. In this study, 3D numerical modeling with Fast Lagrangian Analysis of Continua in 3 Dimensions (FLAC3D) was performed to predict the dynamic behavior of a vertical tunnel shaft during liquefaction. This study demonstrates key aspects of the dynamic behavior of tunnel shafts by varying important parameters such as the thickness of the liquefiable soil layer and applied seismicity level. Moreover, important dynamic responses such as excess pore pressure generation, the seismic bending moment of the shaft, and lateral displacements are highlighted. Finally, meaningful discussion of the seismic risk analysis based on damage indices is conducted based on the analysis results.