传统铁箍墩接法加固底部糟朽木柱轴压试验

铁箍墩接是古建木柱柱根糟朽的一种传统加固方法。为探讨该方法的有效性,以故宫内某古建木柱为例,进行了试验研究。制作了1根完好木柱、3根铁箍墩接法加固木柱,开展了轴压试验,获得了木柱加固前后的承载力-变形曲线,以及延性、应变、刚度等参数,讨论了铁箍墩接法的加固机理。结果表明:传统铁箍法加固底部糟朽木柱后,由于铁箍对木柱的侧向约束作用,木柱的极限承载力可恢复约91.8%,延性性能可恢复约95.2%,水平极限应变恢复率约97.2%,竖向极限应变恢复率约84.1%,且加固木柱的刚度要大于完好木柱。由于铁箍材料与木材强度差别较大,使得加固后木柱的整体性能要低于完好木柱,因而加固效果有限。结果可为古建筑木结构的保护与维修提供理论参考。     

Site Classification Using Horizontal-to-vertical Response Spectral Ratios and its Impact when Deriving Empirical Ground-motion Prediction Equations

We classify sites based on their predominant period computed using average horizontal-to-vertical (H/V) response spectral ratios and examine the impact of this classification scheme on empirical ground-motion models. One advantage of this classification is that deep geological profiles and high shear-wave velocities are mapped to the resonance frequency of the site. We apply this classification scheme to the database of Fukushima et al. [2003] Fukushima, Y., Berge-Thierry, C., Volant, P., Griot-Pommera, D. A. and Cotton, F. 2003. Attenuation relation for west Eurasia determined with recent near-fault records from California, Japan and Turkey. Journal of Earthquake Engineering, 7(3): 1–26.  [Google Scholar], for which stations were originally classified as simply rock or soil. The calculation of average H/V response spectral ratios permits the majority of sites in the database to be unambiguously classified. Soft soil conditions are clearly apparent using this technique. Ground-motion prediction equations are then computed using this alternative classification scheme. The aleatoric variability of these equations (measured by their standard deviations) is slightly lower than those derived using only soil and rock classes. However, perhaps more importantly, predicted response spectra are radically different to those predicted using the soil/rock classification. In addition, since the H/V response spectral ratios were used to classify stations the predicted spectra for different sites show clear separation. Thus, site classification using the predominant period appears to be partially mapped into the site coefficients of the ground-motion model.     

Microtremor HVSR Study of Site Effects in the Ilirska Bistrica Town Area (S. Slovenia)

The region of Ilirska Bistrica is one of the most seismically active areas of Slovenia, where 15 damaging earthquakes with maximum intensity equal or greater than V EMS-98 have occurred in the last 100 years. These earthquakes have shown that strong site effects are characteristic of the parts of the town that are built on soft Pliocene clay and sand overlain by Quaternary alluvium. Since there is a lack of boreholes and geophysical and earthquake data, the microtremor horizontal-to-vertical spectral ratio (HVSR) method was applied to a 250 m dense grid of free-field measurements over an extended area and to a 200 m dense grid in the town area in order to assess the fundamental frequency of the sediments. Measurements were additionally performed in ten characteristic houses to assess the main building frequencies. The effects of wind and artificial noise on the reliability of the results were analyzed. The map of the fundamental frequencies of sediments shows a distribution in a range of 1–20 Hz. The lower frequency range (below 10 Hz) corresponds to the extent of Pliocene clays and sand overlain by alluvium, which form a small basin, and the higher frequencies to flysch rocks, but variations within short distances are considerable. The measurements inside the buildings of various heights (2–6 stories) showed main longitudinal and transverse frequencies in the range 3.8–8.8 Hz. Since this range overlaps with the fundamental frequency range for Pliocene and Quaternary sediments (2–10 Hz), the danger of soil-structure resonance is considerable, especially in the northern part of the town. Soil-structure resonance is less probable in the central and southern part of the town, where higher free-field frequencies prevail. These observations are in agreement with the distribution of damage caused by the 1995 earthquake (M_L?=?4.7, I_max?=?VI EMS-98), for which a detailed damage survey data is available.     

Non-Iterative Equivalent Linearization Based on Secant Period for Estimating Maximum Deformations of Existing Structures

The capacity spectrum method of ATC-40 uses the secant period as the equivalent period of equivalent linear systems. Therefore, it results in a direct graphical comparison. The maximum inelastic displacement and acceleration demands of structures can be simultaneously obtained from the intersection of the demand and capacity diagrams. However, for evaluation of existing structures, the demands need to be determined through iterations since the equivalent period and damping of the equivalent linear systems currently available are both a function of the (displacement) ductility ratio, which is unknown and is the target of evaluation. In addition, the equivalent damping used in the capacity spectrum method is independent of periods of vibration. It may lead to poor estimations of maximum responses especially for short-period systems. This article proposes two equivalent linear systems based on the secant period to estimate the maximum displacement and acceleration responses of existing structures. Both the recommended equivalent period and damping are defined by the strength ratio (elastic lateral strength/yield lateral strength), rather than the ductility ratio. Because the strength ratio of existing structures is a known parameter, the maximum displacement and acceleration responses of these structures can be determined without iterations. Besides, effects of periods of vibration on the equivalent linear systems are also included in this study. The equivalent damping is derived from statistical analyses for bilinear single-degree-of-freedom (SDOF) systems with different periods of vibration, strength ratios and post-yield stiffness based on 72 earthquake ground motions recorded on firm sites. Procedures and examples for applications of the proposed equivalent linear systems on nonlinear static analysis procedures are also provided.     

Liquefaction Resistance of Silty Sands and Dynamic Properties of Cohesive Soils from Düzce,Turkey

This article presents results from a laboratory investigation into the cyclic and dynamic properties of soils from Düzce, Turkey, conducted after the destructive November 12, 1999 earthquake. The investigation was mainly conducted by means of monotonic and cyclic triaxial, as well as resonant-column tests. The triaxial tests allowed the determination of the liquefaction resistance of silty sands, as well as their critical state behavior, whereas the resonant-column tests allowed the determination of shear modulus and damping ratio of cohesive soils. The results are presented and then discussed together with their pertinence to soil behavior when subjected to earthquake loading.     

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.     

Completeness Verification of Complex Response Spectrum Method for Underdamped and Overdamped Multiple-Support Systems Regarding the Decoupled Damping as Mathematical Parameter without Physical Meaning

The completeness of the complex response spectrum method for both formally underdamped and overdamped modes is theoretically proved, and the physical meanings of the decoupled modes as well as involved parameters are recognized and clarified in this paper. For the system with relatively large non-classical damping, the eigenvalue pairs generated by the complex mode decomposition method are real and the so-called modal damping ratios are larger than unity. In this paper, we firstly clarified that the decoupled modes are virtual and the so-called modal frequency and damping ratio are mathematical parameters that have no physical meaning. Then, the completeness of the complex response spectrum method for both formally underdamped and overdamped modes is rigorously proved by allowing the “damping frequency” to be an imaginary number. For the virtually overdamped modes, Duhamel integral involved in the calculation for formally underdamped modes automatically convert to hyperbolic Duhamel integral. A numerical example taken from the published literature is given to verify this method. Structural responses for the system with coupled damping under multi-support seismic excitations are further analyzed and numerical results indicate the accuracy of complex response spectrum method.     

Revisiting the Great Compression: Wage inequality in the United States, 1940–1960

Seminal work by Goldin and Margo (1992 Goldin, C. D. and R. A. Margo.1992. The great compression: The wage structure in the United States at mid-century. Quarterly Journal of Economics 107 (1):1–34. https://doi.org/10.2307/2118322.[Crossref], [Web of Science ®] , [Google Scholar]) used the first available microdata samples of the United States decennial census to document the narrowing of the wage structure between 1940 and 1960, a pattern they refer to as the “Great Compression.” We revisit their findings using newly available, substantially enlarged samples of the decennial censuses covering this period. Our findings largely replicate the patterns initially reported by Goldin and Margo. However, differences emerge when estimating rates of return to education and experience for specific groups and in a decomposition exercise. A second goal is to indicate directions for future research that might benefit from the use of the complete count census data.     

战国秦汉觯、卮、樽辨析

樽、卮(含觯、卮两种器物)很容易被混淆,我们通过综合传世文献、出土文献、考古出土实物三方面材料,认为觯、卮共存且器形有别,觯附三足,卮为平底,且觯的等级高于卮,目前考古出土的玉卮应为玉觯。通过数据分析,我们发现樽与觯的区分要点是容器的径高比差异,樽口大腹浅,便于盛取液体,而觯口小腹深,便于饮用液体。