Changhai Zhai

Quantitative Identification of Near‐Fault Pulse‐Like Ground Motions Based on Energy

Abstract Near‐fault pulse‐like ground motions have long been known to be capable of inducing significant seismic damages to the building structures. Reasonable classification of such ground motions has been a challenge to earthquake engineers. This study serves to propose an energy‐based approach that can be used to identify those ground motions with dominant pulses observed in the velocity time series; and time integral of the squared ground velocity is employed to represent the motion energy. For removing the influence of high‐frequency contents, the potential velocity pulse is first extracted with a pulse model. The starting and ending time points as well as period of the velocity pulse are subsequently determined by the peak‐point method. Records with peak ground velocities above 30  cm/s from a database containing more than 3600 recorded ground motions are selected and utilized to calibrate the final criterion. It is concluded that those ground motions whose dominant velocity pulses hold relative energy values of greater than 0.3 can be satisfactorily classified as pulse‐like. The proposed approach is further used to identify pulse‐like features in arbitrary orientations and pulse‐like ground motions possibly caused by forward‐directivity effects. Online Material: Tables identifying ground motions with near‐fault pulse‐like behavior.

A unified method for modeling of axially and/or transversally functionally graded beams with variable cross-section profile

A novel finite element method is presented to solve static and free vibration problems of axially and/or transversally functionally graded (FG) beams with variable cross-section profile. The presented method overcomes the inaccuracy of the conventional finite beam elements for analysis of the FG beams by introduction of exact displacement interpolation functions. The introduced exact displacement interpolation functions have a unified expression for individual case of axially FG beams, transversally FG beams, varied cross-section beams, as well as their coupled cases. Hence, such cases can be modeled with a unified method and by only one beam element, leading to the presented method easy for modeling the FG beams. The element stiffness matrix is derived according to the internal force equilibrium relationship of the beam element. The element mass matrix and element equivalent nodal loads are derived based on the exact displacement interpolation functions. The method has high accuracy and fast convergence, which is tested with verification examples including a stepped beam, a beam with axially varying material properties and a beam with transversally varying material properties.

Experimental and Finite Element Analytical Investigation of Seismic Behavior of Full-Scale Masonry Infilled RC Frames

This article investigates the seismic behavior of masonry infilled RC frames with/without openings. Four full-scale, single-story, and single-bay specimens were tested under constant vertical loads and quasi-static cyclic lateral loads. The experimental results showed that the infill wall was more influential in stiffness than in load-resisting capacity. The opening increased the ductility ratio of the structure due to the uniform distribution and slow propagation of cracks. Finally, simplified micro finite element models are established to simulate the tested specimens, which effectively predict the load-displacement response of the structures and the crack damage of masonry infill wall with acceptable accuracy.

Dimensional analysis of earthquake-induced pounding between adjacent inelastic MDOF buildings

In this study the seismic pounding response of adjacent multi-degree-of-freedom (MDOF) buildings with bilinear inter-story resistance characteristics is investigated through dimensional analysis. The application of dimensional analysis leads to a condensed presentation of the response, and the remarkable self-similarity property for bilinear MDOF buildings with inelastic collision is uncovered. It is shown that when the response is expressed in the appropriate dimensionless form, response spectra for any intensity of the excitation collapse to a single master curve. The reduced Π set explicitly describes the interaction between the colliding structures. The effect of pounding on the MDOF building’s response is illustrated using three well-divided spectral regions (amplified, de-amplified and unaffected regions). Parametric studies are conducted to investigate the effects of the story stiffness of structures, the story stiffness ratio and mass ratio of adjacent buildings, the structural inelastic characteristics and the gap size values. Results show that (i) the influence of system stiffness ratio to the lighter and more flexible building is more significant in the first spectral region, where the maximum response of the building is amplified because of pounding; and (ii) the velocity and pounding force of the heavier and stiffer building is unexpectedly sensitive to the mass ratio of adjacent buildings.

Constant Ductility Energy Factors for the Near-Fault Pulse-Like Ground Motions

This study is focused on the constant ductility energy factors for bilinear system under the near-fault pulse-like ground motions. The variation of energy factors is studied in consideration of the earthquake magnitude, rupture distance, damping ratios, and post-yield stiffness ratios. The results indicate that the near-fault pulse-like ground motions would increase the energy dissipation of structures. The energy factors are significantly influenced by the earthquake magnitude. The damping ratios have more obvious influences on the energy factors than the post-yield stiffness ratios. A predictive model is proposed for the application of constant ductility energy factors for near-fault pulse-like ground motions.

The Inelastic Input Energy Spectra for Main Shock–Aftershock Sequences

This study investigates the input energy spectra for inelastic single-degree-of-freedom (SDOF) systems under main shock–aftershock sequences. The input energy spectra quantitatively reveal the effects of aftershocks on input energy, which verifies the necessity of incorporating aftershocks in energy-based seismic design. The investigation selects the sequences including one aftershock or two aftershocks respectively, according to the proposed criteria for selecting earthquake records. Then, the input energy for sequences is normalized by mass, m, and expressed in terms of the equivalent velocity, VE,seq. Next, the variation of VE,seq is studied in consideration of the hysteretic models, ductility values, periods of vibration, site conditions, relative intensities of aftershocks and number of aftershocks. The results indicate that the effects of aftershocks on input energy are significant in almost the whole period region. Finally, a simplified expression of input energy is proposed for incorporating aftershocks in energy-based seismic design.

Selection of the Most Unfavorable Real Ground Motions for Low- and Mid-rise RC Frame Structures

The goal of this article is to select those real (or recorded) ground motions capable of exposing the low- and mid-rise reinforced concrete frame structures to an extreme limit state. By performing correlation analyses, two optimal intensity measures are first selected to represent the ground motion damage potential. Then based on each records damage potential, four subsets of strong ground motions, referred to as the most unfavorable ground motions, are identified and preliminarily confirmed to be applicable to the low- and mid-rise RC frame structures.

Effects of hanging wall and footwall on demand of structural input energy during the 2008 Wenchuan earthquake

Systematic differences in the duration and frequency content of ground motions from the hanging wall and footwall during the 2008 Wenchuan earthquake are investigated, focusing on the influence of these differences on structural input energy based on the elastic and inelastic energy responses of structures. A comparison of the input energy spectra between the hanging wall and the footwall reveal that the structural input energy on the hanging wall is not amplified due to the short duration and low peak ground velocity to acceleration ratio (V/A). However, the larger demand of structural input energy on the footwall in the range of medium and long periods is observed and the demand increases up to 50% relative to the average level of structural input energy for rupture distances larger than 30 km. The importance of considering the footwall effect on structural input energy when comparing ground motions in the range of medium and long periods is recognized.

Evaluation of Displacement-Based, Force-Based and Plastic Hinge Elements for Structural Non-Linear Static Analysis:

This paper evaluates accuracies and applicabilities of frame elements constructed by displacement-based (DB), force-based (FB) and plastic hinge (PH) methods for structural non-linear static analysis. The theoretical bases of the three frame elements are compared and the global response (nodal level) and local response (sectional level) of the three frame elements are evaluated. The results lead to the conclusions that more DB elements than FB elements need to be employed to obtain accurate post-yielding global responses in the cases of sections undergoing hardening or zero post stiffness. Both DB and FB elements fail to provide consistent and rational evaluation of the post-yielding global responses in the case of sections undergoing softening. The post-yielding global responses presented by the PH element are strongly influenced by the preset hinge length. The three frame elements present consistent local response results but different maximum responses in most cases. The preferred numbers of integration points to be used in DB and FB elements are also advised from the points of view of both consistency and stability.

Experimental Research on Biaxial Compressive Strength of Grouted Concrete Block Masonry

Grouted concrete block masonry (GCBM) exhibits distinct directional properties. In total, 66 tests on 5 kinds of macro-elements were conducted for different principal stress ratios under biaxial compression. The surface of peak strength has been obtained in terms of heterogeneity index and stress ratio. The surface can express the anisotropic nature of GCMB. Comparisons of 5 kinds of macro-element failure curves were made and it has been found that the biaxial compressive strength when the applied stress direction and the material axis coincide is much greater than that of applied stress inclined to material axis. On term of heterogeneity index and stress ratio, the law of strength varying with orientation has been obtained. It shows that the degree of anisotropy is affected by the stress state. Strong behaviour occurs only when one principal stress predominates, that is, the stress ratio value is low, and gradually becomes weaken as the difference between the two principal compression stresses decreases, namely the stress ratio is increasing. Under equal biaxial compression, the different macro-element strengths are nearly equivalent and the masonry seems to be isotropic. The work conducted in this paper provides a basis for further theoretical study on the behaviour of GCBM under biaxial compressive stress.