Seismic Damage to Structures in the 2015 Nepal Earthquake Sequences

The 7.8 Mw Gorkha earthquake struck the east of Lamjung in Nepal, followed by a sequence of powerful aftershocks. Chinese Team Six including the authors inspected the seismic damage to civil structures along 10 paths in densely populated areas with a seismic intensity of VII to IX, 40 days after the main shock. The damage was categorized according to structure types and described in detail. Several conclusions are made: powerful aftershocks can significantly affect the failure patterns; geological conditions, structure types, and height have great influence on the level of damage; and the local risky retrofitting technique needs improvement badly.     

Performance-Based Vulnerability Assessment of Multi-Story Reinforced Concrete Structures Exposed to Pre- and Post-Earthquake Fire

Post-earthquake fire can potentially bring about much more damage than the earthquake itself. Performing a vulnerability assessment for a structure that has already sustained damage in an earthquake and is then exposed to fire is therefore of importance. This paper describes a performance-based investigation in which applied loads to a structure are appropriately quantified. To do so, a sequential structural analysis is performed on the Life Safety performance level of a three-story reinforced concrete frame selected from a building. For the analysis to be more realistic, the slab is also included in the frame analysis through the concept of effective length. The frame is first subjected to an earthquake load with the PGA of 0.30 g followed by a fire analysis, using the ISO834 fire curve and the iBMB fire curve. The time needed for the structure weakened by the earthquake to collapse under fire is then calculated. As a benchmark, fire-only analysis is also performed for the undamaged frame. Moreover, the effect of thermal spalling is considered in the slabs. The selected frame is evaluated under various failure criteria such as load capacity, displacement, and rate of displacement. The results show that no failure is observed when the frame is exposed to fire alone, either when using the ISO curve or the iBMB curve under various failure criteria. It is also shown that while the PEF resistance based on load capacity criteria under the ISO curve is around 120 minutes, it reduces to about 95 min under the iBMB curve. However, considering the rate of deflection failure criteria, the PEF resistance is around 103 min and 75 min under the ISO and the iBMB curves, respectively. It is then concluded that in the PEF analysis, the iBMB curve is more compatible with the concept of performance-based design than the ISO curve is.     

Seismic Fragility Evaluation of Lightly Reinforced Concrete Beam-Column Joints

Fragility functions play an essential role in evaluating the seismic vulnerability of structures. To establish the seismic fragility functions of lightly Reinforced Concrete (RC) beam-column joints, the Park-Ang Damage model has been amended to quantify the damage states and the modified Bouc-Wen-Baber-Noori model has been employed and implemented in ABAQUS to predict the structural hysteresis behavior. Following successful calibration of the numerical results of a RC test frame from literature, the proposed model has been utilized to assess the seismic fragility curves of low to mid-rise RC frames in Singapore for 30 scaled ground motions using incremental dynamic analysis approach.     

Experimental Study on Self-Centering Concrete Wall with Distributed Friction Devices

A self-centering concrete wall with distributed friction devices is proposed to achieve seismic resilient building structures. Unbonded post-tensioned tendons, running vertically through wall panels, provide a restoring force that pulls the structure back toward its undeformed plumb position after earthquake. Two steel jackets are installed at wall toes to prevent concrete spalling and crushing. Friction devices are distributed between the wall and its adjacent gravity columns to achieve controllable energy dissipation, and these devices are readily replaceable. Desirable self-centering and energy dissipation capacities were observed in low-cyclic loading tests, and influences of various parameters on the hysteretic behavior were investigated.     

Fiber-Based Modeling of Circular Reinforced Concrete Bridge Columns

This article presents the application of fiber-based analysis to predict the nonlinear response of reinforced concrete bridge columns. Specifically considered are predictions of overall force-deformation hysteretic response and strain gradients in plastic hinge regions. This article discusses the relative merits of force-based and displacement-based fiber elements, and proposes a technique for prediction of nonlinear strain distribution based on the modified compression field theory. The models are compared with static and dynamic test data and recommendations are made for fiber-based modeling of RC bridge columns.     

Numerical Simulation and Seismic Fragility Analysis of a Self-Centering Steel MRF with Web Friction Devices

This article presents the seismic fragility analysis of a self-centering steel moment-resisting frame (SC-MRF) with web friction devices. A detailed numerical model of the SC frame was developed using the Open System for Earthquake Engineering Simulation (OpenSees) and the elastoplastic responses of the SC-MRF were studied, including the strength degradation under cyclic loading, tendon rupture, beam buckling, bolt bearing and friction loss, etc. The proposed simulation approach is validated by comparing the simulated results with those in existing hybrid-simulation tests, quasi-static pushover test and low cyclic tests, where good agreement is observed. In addition to the well-established performance limit states (i.e., immediate occupancy, collapse prevention and global dynamic instability), two unique performance limit states (i.e., the recentering and repairable limit states) are defined for the SC-MRF. Finally, incremental dynamic analyses are conducted to evaluate the seismic fragilities regarding the five performance limit states.     

Material Modeling in the Dynamic Nonlinear Analysis for Recycled Aggregate Concrete Structures

Dynamic mechanical tests of recycled aggregate concrete (RAC) test units confined by transverse hoop reinforcement are carried out. The effects of the strain rate, hoop reinforcement confinement, and replacement ratio of recycled coarse aggregate (RCA) on the mechanical properties of confined recycled aggregate concrete (CRAC) are thoroughly analyzed and assessed. The strain-rate-dependent constitutive model of CRAC is proposed for the high strain rate representative of seismic conditions. A three-dimensional discrete numerical model, based on the proposed rate-dependent material model of CRAC, is established to investigate and evaluate the dynamic nonlinear behaviors of RAC structures.     

Seismic Retrofitting of Columns with Lap Spliced Smooth Bars Through FRP or Concrete Jackets

The effectiveness of RC jacketing or FRP wrapping for seismic retrofitting of rectangular columns having smooth (plain) bars with 180° hooks lap-spliced at floor level is experimentally investigated. The relatively low deformation capacity and energy dissipation of five unretrofitted columns is found not to depend on lap length, if lapping is not less than 15 bar-diameters. Six columns cyclically tested up to ultimate deformation after RC concrete jacketing demonstrate force and deformation capacity and energy dissipation sufficient for earthquake resistance, regardless of the presence or length of lap splicing in the original column. Another ten columns cyclically tested to ultimate deformation after wrapping of the plastic hinge region with CFRP show that FRP wrapping of the splice region is more effective than concrete jackets for enhancement of the deformation and energy dissipation capacity of old-type columns with smooth bars lap-spliced at floor level, provided that wrapping extends over the member length sufficiently to preclude plastic hinging and early member failure outside the FRP-wrapped length of the column.     

EXPERIMENTAL BEHAVIOUR OF R/C FRAMES RETROFITTED WITH DISSIPATING AND RE-CENTRING BRACES

An extensive program of shaking table tests on 1/4-scale three-dimensional R/C frames was jointly carried out by the Department of Structure, Soil Mechanics and Engineering Geology (DiSGG) of the University of Basilicata, Italy, and the National Laboratory of Civil Engineering (LNEC), Portugal. It was aimed at evaluating the effectiveness of passive control bracing systems for the seismic retrofit of R/C frames designed for gravity loads only. Two different types of braces were considered, one based on the hysteretic behaviour of steel elements, the other on the superelastic properties of Shape Memory Alloys (SMA). Different protection strategies were pursued, in order to fully exploit the high energy dissipation capacity of steel-based devices, on one hand, and the supple-mental re-centring capacity of SMA-based devices, on the other hand. The experimental results confirmed the great potentials of both strategies and of the associated devices in limiting structural damage. The retrofitted model was subjected to table accelerations as high as three times the acceleration leading the unprotected model to collapse, with no significant damage to structural elements. Moreover, the re-centring capability of the SMA-based bracing system was able to recover the undeformed shape of the frame, when it was in a near-collapse condition. In this paper the experimental behaviour of the non protected and of the protected structural models are described and compared.     

Displacement-Based Assessment of Non Ductile Exterior Wide Band Beam-Column Connections

Results from testing two half-scale exterior wide band beam-column sub-assemblages under cycles of lateral displacement are presented in this article. The first subassemblage represents the current level of detailing adopted in low to moderate seismic regions, such as Australia, for connections where seismic provisions are not normally a consideration in design. Minor (inexpensive) detailing changes in the reinforcement distribution and anchorage were introduced to the second test specimen. These changes significantly improved the connection performance in terms of increased displacement capacity and a reduction in strength deterioration. Using a displacement-based assessment approach to assess primary moment-resisting band beam frames of up to eight stories, it was found that the current level of detailing is adequate for the drift demands resulting from the expected Australian seismicity for a 500-year return period. However, for the displacement demands corresponding to a 2500-year return period, the frames sited on very soft soils and frames over four stories sited on intermediate soils would require improved detailing such as that used in the second sub-assemblage. A strength hierarchy of strong column-weak beam was assumed in this assessment.