Seismic fragility of bridges: An approach coupling multiple-stripe analysis and Gaussian mixture for multicomponent structures

An approach is developed to build multivariate probabilistic seismic demand models (PSDMs) of multicomponent structures based on the coupling of multiple-stripe analysis and Gaussian mixture models. The proposed methodology is eminently flexible in terms of adopted assumptions, and a classic highway bridge in Eastern Canada is used to present an application of the new approach and to investigate its impact on seismic fragility analysis. Traditional PSDM methods employ lognormal distribution and linear correlation between pairs of components to fit the seismic response data, which may lead to poor statistical modeling. Using ground motion records rigorously selected for the investigated site, data are generated via response history analysis, and appropriate statistical tests are then performed to show that these hypotheses are not always valid on the response data of the case-study bridge. The clustering feature of the proposed methodology allows the construction of a multivariate PSDM with refined fitting to the correlated response data, introducing low bias into the fragility functions and mean annual frequency of violating damage states, which are crucial features for decision making in the context of performance-based seismic engineering.

Effects of Brace Configuration and Structure Height on Seismic Performance of BRBFs Based on the Collapse Fragility Analysis

The brace configuration and structure height are two factors that have a significant effect on the seismic behavior of braced frame buildings. In the present study, the buckling-restrained braced (BRB) frames were considered to estimate the effect of these two parameters using probabilistic seismic assessment methods. The uncertainty in the different parameters involved in the seismic design of the structural system was also considered. Four, six, and ten-story buildings with the Chevron and inverted Chevron bracing configurations were designed, and their responses due to various ground motions were estimated using incremental nonlinear dynamic analyses. Fragility curves, mean annual frequency of exceeding immediate occupancy (IO), and collapse prevention (CP) states were generated using probabilistic seismic analysis, fragility curves concept, and drift hazard curves. The results demonstrate that the inverted Chevron type BRBFs has better structural performance than Chevron bracing types. Furthermore, an increase of the height of structures, despite lower drift’s hazards, increases the fragility probability.

Long-Term Ground-Based Measurements of Aerosol Optical Depth over Kuwait City

We analyze ten years (2008–2017) of ground-based observations of the Aerosol Optical Depth (AOD) in the atmosphere of Kuwait City, in Middle East. The measurements were conducted with a CIMEL sun-sky photometer, at various wavelengths. The daily average AOD at 500 nm (AOD500) is 0.45, while the mean Ångström coefficient (AE), calculated from the pair of wavelengths 440 and 870 nm, is 0.61. The observed high AOD500 values (0.75–2.91), are due to regional sand and dust storm events, which are affecting Kuwait with a mean annual frequency of almost 20 days/year. The long-term record analysis of AOD500 and AE, shows a downward and upward tendency respectively, something which could be attributed to the continuous expansion and industrialization of the main city of Kuwait, in combination with the simultaneous increase of soil moisture over the area. By utilizing back trajectories of air masses for up to 4 days, we assessed the influence of various regions to the aerosol load over Kuwait. The high aerosol loads during spring, are attributed to the dominance of coarse particles from Saudi Arabia (AOD500 0.56–0.74), a source area that contributes the 56% to the mean annual AOD500. Other dust sources affecting significantly Kuwait originated from the regions of Iraq and Iran with contribution of 21%.

A Spectrum-to-Spectrum Method for Calculating Uniform Hazard Floor Response Spectra

In codes’ provisions and design procedures for acceleration-sensitive nonstructural components, seismic demand is commonly defined by means of floor response spectra expressed in terms of pseudo-acceleration. Depending on the considered analysis method, floor response spectra may be derived from floors’ acceleration histories, based on structural response-history analysis, or calculated using a predictive equation from a given input ground motion spectrum. Methods for estimating floor response spectra that are based on the second alternative are commonly called spectrum-to-spectrum methods. The objective of this paper is to briefly review these methods, and to discuss the main assumptions they are based on. Both predictive equations from selected seismic codes and proposals from the literature are included in the review. A new probability-based method, recently developed by the Authors for generating uniform hazard floor response spectra, namely, floor response spectra whose ordinates are characterized by a given target value of the mean annual frequency of being exceeded, is also described. By using this method floor spectra are determined through closed-form equations, given the mean annual frequency of interest, the damping ratio of the spectra, the modal properties of the structure, and three uniform hazard ground spectra. The method is built on a proposal for a probabilistic seismic demand model that relates the ground spectral acceleration with the floor spectral acceleration, and is able to explicitly account for the ground motion variability of the nonstructural response. Results for a case study consisting of a service frame of a visbreaking unit in an oil refinery are presented to show the good predictive accuracy of the method with respect to exact uniform hazard floor response spectra obtained through a standard probabilistic analysis.

Ice core melt features in relation to Antarctic coastal climate

Measurement of light intensity transmission was carried out on an ice core S100 from coastal Dronning Maud Land (DML). Ice lenses were observed in digital pictures of the core and recorded as peaks in the light transmittance record. The frequency of ice layer occurrence was compared with climate proxy data (e.g. oxygen isotopes), annual accumulation rate derived from the same ice core, and available meteorological data from coastal stations in DML. The mean annual frequency of melting events remains constant for the last ∼150 years. However, fewer melting features are visible at depths corresponding to approximately 1890–1930 AD and the number of ice lenses increases again after 1930 AD. Most years during this period have negative summer temperature anomalies and positive annual accumulation anomalies. The increase in melting frequency around ∼1930 AD corresponds to the beginning of a decreasing trend in accumulation and an increasing trend in oxygen isotope record. On annual time scales, a relatively good match exists between ice layer frequencies and mean summer temperatures recorded at nearby meteorological stations (Novolazarevskaya, Sanae, Syowa and Halley) only for some years. There is a poor agreement between melt feature frequencies and oxygen isotope records on longer time scales. Melt layer frequency proved difficult to explain with standard climate data and ice core derived proxies. These results suggest a local character for the melt events and a strong influence of surface topography.