Statistical Characterization of Structural Demand under Earthquake Loading. Part 2: Robust Estimation of the Dispersion of the Data

Performance-based earthquake engineering methodologies often require a probabilistic model of structural demand. Since observations masking the probability distribution of the majority of the data are frequently found, robust estimation methods are proposed to estimate the probabilistic model parameters (i.e., central value and dispersion). The performance of thirty-three robust dispersion estimators is evaluated, for different sample sizes, using the chord rotation, curvature, shear force, and inter-story drift demands obtained after analyzing five reinforced concrete structures under real earthquake records scaled to several intensities. Based on the results, combinations involving dispersion and central value (defined in a companion article) estimators are proposed.     

Critical Assessment of Intensity Measures for Seismic Response of Italian RC Bridge Portfolios

The seismic assessment of a road network depends largely on the characterization of the fragility of its bridge components. The accuracy of bridge seismic demand estimates and the use of proper intensity measures (IM) will significantly influence such task. The available literature has mainly focused on buildings or a limited number of bridge configurations and IMs, which may not be representative for bridge portfolio assessment studies. In this paper, the correlation quality between a larger pool of traditional and innovative IMs and the nonlinear dynamic response of typical Italian RC bridges is investigated to identify the best-performing IMs.     

故宫灵沼轩的动力特性及抗震性能研究

故宫灵沼轩是我国最早建造的钢铁-砌体组合结构之一,具有重要的历史、艺术和科学价值。为了评估灵沼轩在地震作用下的结构安全状况,建立了灵沼轩结构的三维有限元模型,并对其进行了动力特性和地震时程分析,得出了其固有频率、模态振型、地震位移响应和地震应力响应。结果表明:灵沼轩整体结构布置对称性较高,扭转刚度较大,对抗震较为有利;在8度多遇地震、设防地震和罕遇地震作用下,灵沼轩的金属结构部分及砌体结构部分的顶点位移和层间位移角均符合现行规范要求,砌体结构部分的第三主应力响应均小于材料的抗压强度,不存在压溃风险。在8度多遇地震和设防地震作用下,砌体结构部分的第一主应力响应均小于材料的抗拉强度,结构不会发生拉裂。但在8度罕遇地震作用下,砌体结构的部分位置拉应力超过材料的抗拉强度,这些位置存在开裂危险。最后,综合动力特性和抗震性能分析的结果,提出了灵沼轩的抗震加固建议。     

Inelastic Displacement Ratios for Seismic Assessment of Structures Subjected to Forward-Directivity Near-Fault Ground Motions

This article presents results of a statistical study focused on evaluating inelastic displacement ratios (i.e., ratio of maximum inelastic displacement with respect to maximum elastic displacement demand) of degrading and non degrading single-degree-of-freedom (SDOF) systems subjected to forward-directivity near-fault ground motions. C_R spectra are computed for normalized periods of vibration with respect to the predominant period of the ground motion to provide a better ground motion characterization. This period normalization allows reducing the record-to-record variability in the estimation of C_R. An equation to obtain estimates of C_R for the seismic assessment of structures exposed to forward-directivity near-fault ground motions is proposed.     

The SAFE Experimental Research on the Frequency Dependence of Shear Wall Seismic Design Margins

The SAFE experimental programme consists of a series of 10 specimens of shear walls, with different reinforcement ratios, tested until their ultimate capacity under seismic input motion by the pseudo dynamic method. A unique input signal is used, calibrated for controlling the seismic demand. Its input central frequency is selected so that for some specimens it is lower than their eignenfrequency, while for other ones it is the opposite. In conclusion there is clear experimental evidence that design margins are much larger in the second case (input central frequency larger than structure eignenfrequency) than in the first one.     

Design Methodology for Seismic Upgrading of Substandard Reinforced Concrete Structures

This article presents a design methodology for seismic upgrading of existing reinforced concrete (RC) buildings. The methodology is based on the modification of the deflected shape of the structure so as to achieve a near-uniform distribution of interstorey drift along the building height, thereby eliminating damage localization. Yield Point Spectra are utilized for the definition of demand and a direct displacement-based design approach is implemented. The fundamental steps of the method are described in detail, including a systematic evaluation of assumptions and limitations. A full-scale tested structure is used as a case study for assessment and verification of the proposed methodology. Alternative retrofit scenarios are set according to target response and performance levels. The role of the target deflected response shape and its influence on the outcome of the retrofit strategy is investigated. The viability of the alternative retrofit scenarios is studied for different ground motions including near-fault earthquake records.     

Critical Issues on Probabilistic Earthquake Loss Assessment

Probabilistic loss modeling can be used to develop risk reduction measures, such as the identification of regions more prone to human and economic losses, or to develop financial mechanisms to transfer the earthquake risk from local governments to the private sector. This study addresses several critical issues in probabilistic loss modeling, and provides recommendations depending on the intended final use of the risk results. Modeling issues related to convergence in probabilistic event-based analysis; consideration of epistemic uncertainties within a logic tree; generation of different types of loss exceedance curves; and derivation of risk maps are thoroughly investigated. The Metropolitan Area of Lisbon is used to explore these issues, and it is demonstrated that different assumptions in the loss modeling process can lead to considerably different risk results. Furthermore, the findings and recommendations of this study are also relevant for institutions that promote the assessment of earthquake hazard and risk, such as the Global Earthquake Model Foundation.     

Seismic Vulnerability Assessment of Hybrid Steel-Timber Structure: Steel Moment-Resisting Frames with CLT Infill

In this article, seismic vulnerability assessment is carried-out on a novel hybrid structure (steel moment resisting frame (SMRF) and cross laminated timber (CLT) infill panels). For the seismicity of Vancouver, Canada, a three-bay, 3-, 6-, and 9-story height SMRFs are designed for two ductility levels (ductile and limited ductility). To study the seismic vulnerability CLT infilled building, parametric analysis was performed by varying infill configuration (bare frame, one-bay infilled, two-bay infilled, and fully infilled). The structure is modeled in OpenSees and nonlinear dynamic analysis is performed. Peak inter-story drift demand and corresponding FEMA performance limits (capacity) values are used to compute the corresponding fragility curves. From the analyses, it can be seen that as more bays are infilled, the fundamental period and seismic vulnerability is reduced significantly. The results highlight that, within the performance-based earthquake engineering, different objectives can be met with varying the CLT configuration.     

MULTIPLIER ESTIMATES: TO BIAS OR NOT TO BIAS?*

ABSTRACT It is well known that multiplier estimates within an interindustry context may be biased when the input coefficients are stochastic. Several conditions have been derived under which the estimates were shown to be biased, all with the same sign. In contrast to these analytical results, however, simulations using a stochastic transactions table unexpectedly reported the unbiasedness of multiplier estimates. This note argues that the sample sizes were too small. It is shown that for increased sample sizes the multiplier estimates are all positively and significantly biased, in line with the analytical results, but the biases are very small.     

Obsidian hydration dating on the South Coast of Peru

We compare over 230 obsidian hydration readings from 30 individual site components from the Southern Nasca Region (SNR) with independent age estimates based on radiocarbon dates and temporally diagnostic artifacts. Although there are problems with small sample sizes, and readings must be adjusted for elevation, a very strong relationship accounting for nearly 90% of the total variation in the data set is found. This suggests that obsidian hydration dating (OHD) works in the SNR and is a viable means of independently estimating age. Residual values from our regression suggest that hydration age estimates are usually within 15% of the radiocarbon estimates. Finally, we present an equation other scholars can use to estimate age for Quispisisa obsidian in the SNR.     

A Simplified Drift-Based Assessment Procedure for Regular Confined Masonry Buildings in Seismic Regions

This article presents a simplified procedure for assessing the seismic performance of existing low-to-medium rise confined masonry (CM) buildings, which are a typical construction type in Latin-America. The procedure consists of the estimation of the peak roof and first-story inelastic drift demand of CM buildings. The expected peak inelastic displacement demand is related to drift-based fragility curves, which express the probability of being or exceeding two key damage states in the masonry panels, developed from a relatively large experimental database. The proposed procedure could be very useful for obtaining rapid estimates of expected performance during future earthquake events and for assessing the seismic vulnerability of regular confined masonry structures.     

Seismic Demand on Shear Panel Dampers Installed in Steel-Framed Bridge Pier Structures

This study is aimed at investigating the demand on shear panel dampers (SPDs) installed in steel structures under strong earthquake motions to serve as guidance for the recommended capacity of SPDs in seismic design. For this purpose, an extensive dynamic analysis is carried out on steel bridge pier structures with SPD devices. To describe the restoring force characteristics of SPDs, the analysis uses a newly developed combined hardening model based on experimental data. The seismic demands made on SPD devices are examined and then summarized to give recommended values for determining the necessary deformation capacity of SPDs.     

Establishment of a Time-Varying Modified Kanai-Tajimi Non Stationary Stochastic Model of Seismic Records Based on the S-transform

This article investigates the time-varying characteristics of seismic records. Time-varying amplitude and energy spectrums are defined to reflect the time-frequency dependence, and a general formulation of the S-transform is introduced. The S-transform is tested with various window functions to analyse the Kobe seismic records. The results indicate that using a complex window function with properly adjusted parameters gives favorable outcomes. Analyses of three soil sites show that sites with hard soil feature seismic records with shorter stationary durations, higher frequency centers, and broader frequency bands than other soil sites. The average time-varying spectrums of the seismic records are simulated using a uniform non stationary stochastic model and a time-varying modified non stationary Kanai-Tajimi stochastic model. Empirical formulas are established for the time-varying spectrums of the earthquake records from these sites by nonlinearly fitting stochastic models to the record data. The values of the time-varying spectrum factors for different earthquake intensities and sites agreeing with the Chinese Seismic Code are obtained. Based on these analyses and observations, we propose using the solutions to the stochastic models to simulate non stationary ground motions.     

Investigation of the Seismic Behavior of a Historical Masonry Minaret Considering the Interaction with Surrounding Structures

This study examines the seismic behavior of the minaret of the well-known historical structure “Sultan Ahmed Mosque” under strong earthquake motion. Despite their slenderness and height, minarets are towers with well-established earthquake resistance. In general, these structures were constructed adjacent to the main structure and/or its components. Hence, it is expected that the dynamic behavior of the minarets is influenced by the dynamic characteristics of the adjacent structures as well as the contact conditions. In the presented study, the dynamic behavior of the M6 minaret of the Sultan Ahmed Mosque, which is in contact with the portico that surrounds the courtyard of the mosque, is considered. Ambient vibration tests were conducted on site in order to identify the individual and coupled vibration modes of the minaret and the portico. A finite/discrete model was developed and seismic analysis was carried out. The comparative study reveals considerable differences in responses of different models under strong and very strong earthquake motion.     

Seismic Reconnaissance and Observed Damage after the Mw 6.8, 24 August 2016 Chauk (Central Myanmar) Earthquake

On 24 August 2016, Mw 6.8 earthquake occurred near Chauk, Central Myanmar. This earthquake caused a significant amount of damage over a very large number of historical monuments. After providing a general summary of the regional tectonic settings and seismicity, the observed ground motion has been discussed, and performance of structures in the epicentral area is addressed, focusing on the damage observed in both historical and recent constructions. The observed damage patterns and their extent are analyzed and interpreted in light of observed damage that was found. Lastly, seismic fragility curves of local buildings have been derived.     

Seismic Resistance Evaluation of Unreinforced Masonry Buildings

The article presents seismic resistance evaluation study of unreinforced brick masonry buildings. The study was carried out as part of the Ph.D. research work of the first author. As part of the study, in addition to the standard laboratory tests, a dynamic field test was carried out on single-story, single-room unreinforced masonry structure. The model structure was tested in actual ground conditions against simulated earthquake vibrations produced through controlled explosions, especially designed for this purpose. Based on masonry properties accrued from lab and field tests, finite element models of the brickwork system were also studied. Finally, the software named, “Shear Damage Index (SDI),” developed as part of this study, was used to plot contours of shear demand (shear stress) to shear capacity (shear strength) ratio on the numerical model and hence to identify potential weak zones in the model for possible strengthening of those locations.     

2015尼泊尔地震后加德满都谷地世界文化遗产破坏特征

2015尼泊尔地震对加德满都谷地的世界文化遗产造成不可估量的损失。为了探究谷地文化遗产震害整体分布特征,对谷地世界文化遗产建筑的震害状况开展实地调查,划分了三类文化遗产震害等级标准,形成了六处世界文化遗产的震害图并对其进行统计分析,加德满都、帕坦和巴德岗三处杜巴广场震后保存基本完好的文化遗产建筑占各自总数的39.5%、53.3%、50%,斯瓦扬布纳特寺、昌谷纳拉扬寺和博达哈大佛塔主体保存基本完好,附属建筑不同程度地破坏。加德满都、帕坦和巴德岗三处杜巴广场地层的松软沉积层放大地震效应使建筑严重破坏,斯瓦扬布纳特寺、昌谷纳拉扬寺震害主要受地形特征影响。考察成果为我国同类遗产建筑的抗震防护提供参考。     

Site Effects at Long Periods from Digital Strong Motion Records of the KiK-net,Japan

The high-quality digital records of the Japanese KiK-net were examined, with the aim of studying the influence of local site conditions on the displacement spectral ordinates at long periods. The attention was limited to those records for which the velocity profiles up to 100–200 m depth were known, and corresponding surface and borehole accelerograms were available. Based on the available records and with the support of 1D numerical simulations, different aspects that may have an influence on the amplification of long period spectral ordinates were studied, including the bedrock velocity profile, the site classification using V_s,30, and the earthquake magnitude.

Small amplification factors at long periods were found, ranging from 1 to 1.3, with median value from 1.05 to 1.1, for Eurocode 8 site classes B and C, respectively. Only for two records on soft soils (at the boundary between Eurocode 8 classes C and D), from small magnitude earthquakes, large amplification factors were obtained, up to about 4. A good correlation was found of the amplification levels with the response spectral ratio D(T₀)/D(10), where D(T₀) and D(10) are displacement spectral ordinates of the input signal at bedrock, at the fundamental period T₀ of the soil profile and at T = 10 s, respectively.     

Critical Issues in Earthquake Scenario Loss Modeling

The assessment of human or economic losses due to single events (scenario) may effectively support decision makers in the development of important risk mitigation actions. The study presented herein sheds light on several problems and limitations in the current practice of scenario loss modeling, such as: the number of simulations required to achieve convergence; epistemic and aleatory uncertainty in the ground motion prediction and vulnerability models; and consideration of the earthquake rupture geometry. These issues are investigated using the Metropolitan Area of Lisbon (Portugal), and it has been observed that distinct assumptions in the loss modeling can lead to considerably different results. The findings of this study are also pertinent for probabilistic seismic risk analyses in which a large number of stochastically generated events are employed to assess probabilistic losses.