Near-fault Pulse-like Motions Research Articles

Performance of double skin FRP-concrete-steel self-centered segmental bridge piers subjected to forward-directivity near-fault ground motion

This paper investigates the damping energy and behavior of a proposed damage-resistant self-centering unbonded post-tensioned (PT) bridge pier system for accelerated bridge construction. The system is investigated with and without the addition of external energy dissipaters using two piers. The cross-section of each pier was a double-skin consisting of two tubes: outside glass fiber reinforced polymer (GFRP) tube and an inside steel tube. Self-consolidating concrete (SCC) was poured in between. The piers system has the benefits of accelerating construction using segments, self-centering capability due to rocking, and high energy dissipation from the steel bars. The piers were tested under ascending intensities of scaled ground motions. A near-fault pulse-like motion was chosen to examine the velocity impact effect on the piers. After being subjected to a series of ground motions of up to 250% of the design earthquake (DE), the piers had self-centering capability with almost no residual drift and no noticeable damage. The peak drift was 8.85%. The rocking motion reveals a complex induced dynamic behavior, in which a small change in the system can cause chaotic and butterfly effects. Hence, the energy of the system was investigated to better comprehend the dynamic behavior of rocking under forced vibrations. Two analytical models were developed to simulate the static and dynamic behaviors of the self-centering unbonded post-tensioned bridge piers.

Influence of Near-fault Pulse-like Motion Characteristics on Seismic Performance of Tall Pier Bridges with Fragility Analysis

ABSTRACT This paper focuses on the effects of near-fault motion characteristics on the seismic performance of tall pier bridges. Using the analytical models calibrated by shake table tests, fragility analysis was conducted investigating the influence of near-fault motion features (i.e., period (Tv) and number (N) of velocity pulses). Results show that seismic fragility of the tall pier under near-fault motions is highly correlated to Tv and the damage states considered, while the influence of N is less significant. Consequently, when considering the seismic behavior of tall piers under near-fault pulse-like motions, choosing records with respect to Tv may lead to more reliable results.

Performance evaluation of existing isolated buildings with supplemental passive pseudo-negative stiffness devices

A new configuration for passive pseudo-negative stiffness device (PPNSD) is proposed to work as a supplemental device for the seismic protection of existing isolated buildings. The proposed PPNSD reproduces the hysteretic behaviors of semi-active pseudo-negative stiffness devices (PNSD). Using the ratchet-pawl mechanism, a prototype was realized and tested under cyclic loading, validating the feasibility of the proposed PPNSD. Seismic reduction effect of the PPNSD on single-degree-of-freedom (SDOF) systems was verified through seismic response analysis. The PPNSD shows superiority in suppressing the peak acceleration response of long period structures, which is unlikely to be achieved even with large supplemental damping. Then, the performance of isolated buildings with PPNSD was evaluated in comparison to those of isolated buildings with traditional passive isolators under far-field (FF), near-fault pulse-like (NFP), and near-fault non-pulse-like (NFNP) ground motions. It is shown that the use of PPNSD not only simultaneously reduced the base drift and the acceleration of superstructures, but also was effective for all the FF, NFNP, and NFP motions.

Interpretations of ground motion records from the 2017 Mw 7.3 Ezgeleh earthquake in Iran

This study examines the time and frequency features of strong motion records obtained during the 12th November 2017 Mw 7.3 Ezgeleh (Iran) Earthquake. Different parameters including peak ground acceleration (PGA), peak ground velocity (PGV), spectral acceleration, significant duration, Arias intensity and Fourier amplitude are illustrated to provide a clear view on main characteristics of this event. As an important finding, a broad-band frequency content characteristic is obvious from the analysis of Fourier amplitude spectra obtained at four closest stations records. According to the results, the record obtained at Sarpol-e Zahab station could be considered as near-fault pulse-like motion. The amount of input energy and its corresponding level of frequency are discussed based on wavelet analysis. Results showed that more than 50% of earthquake energy at Sarpol-e Zahab city has been concentrated around 0.45 s which is close to the natural vibration period of low-to mid rise structures in the region. Finally, seismic intensity in terms of modified Mercalli intensity was extracted based on the Fourier spectrum amplitude and the peak ground motion parameters (PGA, PGV) in Sarpol-e Zahab city which was the worst affected area.

Seismic Vulnerability Assessment of Steel Moment-Resisting Frames Based on Local Damage

This paper investigates the seismic vulnerability of steel moment-resisting frames (SMRFs) based on plasticity development in structural components. Pushover analyses with three different lateral load patterns are performed to identify the location of plastic hinges at the collapse state and IDA analysis is employed to find the seismic intensities causing the formation of the specified hinges. An ensemble of 30 near-fault pulse-like motions classified into three groups of short, medium and long period motions and ten ordinary ground motions are used for seismic loading on five sample frames with 3 to 15 stories. Employing this method, the interstory drift ratio coincident to the seismic collapse is evaluated. Then, the critical pulse period producing the most damage to the structures is estimated. Finally, fragility curves are developed for near- and far-fault excitations. The results indicate that maximum interstory drift ratio at the collapse state of the frames ranges from 1.5% to 10%. It is also found that the ratio of critical pulse period to natural period of SMRF structures is between 2.3 and 2.9. Moreover, the fragility analysis reveals that collapse intensity equivalent to 10% of exceedence for near-fault motions is 70% to 85% less than far-fault records.

Effects of wave passage on torsional response of symmetric buildings subjected to near-fault pulse-like ground motions

This article investigates the effects of wave passage on the torsional response of elastic buildings in the near-fault region. The model of the soil-foundation-structure system is a symmetric cylinder placed on a rigid circular foundation supported on an elastic halfspace. The idealized model is subjected to obliquely incident plane SH waves simulating the action of near-fault pulse-like motions. The response of the structure is assessed in terms of the relative twist between the top and the base of the superstructure. A parametric analysis of the maximum relative twist as a function of the input parameters of the seismic excitation and soil-foundation-structure system is performed to identify the parameters that control the torsional response of buildings due to wave passage in the near-fault region. It is shown that the torsional response is most sensitive to a key parameter of the near-fault ground motion referred to as “pulse period”. Specifically, large rotations are observed when the pulse period is close to the torsional period of the structure. It is also demonstrated that the wave passage effects are controlled by the wave apparent velocity, rather than the local site conditions. Furthermore, broadband near-fault ground motions from three hypothetical earthquakes of different magnitude are generated, and the torsional responses due to the simplified pulse-like and broadband ground motions are compared against each other. The results show that the simplified pulse model that describes the coherent seismic radiation is able to represent the main features of the near-fault ground motions that cause large torsional response. The maximum relative twist at resonance is found to be ~10−3rad, a value that is consistent with the upper bound of rotations in structures reported in the literature.

Seismic Response of a Damage-Resistant Recentering Posttensioned-HYFRC Bridge Column

This paper presents the shake-table tests of a damage-resistant posttensioned bridge column designed to rock at the interface with its foundation. It compares the response to that of a conventional bridge column detailed to conform with current California seismic design criteria. The lower portion of the rocking column was built using hybrid fiber-reinforced concrete (HYFRC) and was armored with headed rebars; these features enhanced the compression damage resistance at the column base. Unbonded rebars that crossed the rocking plane provided hysteretic energy dissipation. Both columns were subjected to a sequence of scaled historical triaxial earthquake ground motions including near-fault pulse-like motions. The conventional column formed a flexural plastic hinge with extensive spalling. It accumulated a 6.8% residual drift ratio after a sequence of seven ground motions, which caused a peak drift ratio of 10.8%. The posttensioned-HYFRC column sustained only light damage and accumulated only a 0.4% residual drift ratio after the same seven ground motions, which caused a peak drift ratio of 8.0%.