Pushover Test Research Articles

Cyclic seismic pushover testing of a multi-story underground station

The experimental approach is crucial for investigating the seismic performance and damage process of underground structures. Considering the shortcomings of the 1-g, centrifuge shaking table and monotonic displacement pushover tests, a large-scale cyclic displacement pushover test method is proposed based on the soil-underground structure dynamic interaction and seismic performance quantification system. Taking a two-story three-span subway station structure as the prototype, the cyclic displacement pushover test device was designed for a 1/7-scale multi-story subway station based on the seismic response characteristics of underground structures. The corresponding numerical simulations and experiments were conducted. Typical numerical results (including the seismic damage process, capacity curves of the structural columns, and strain response) and test results (the macroscopic phenomenon of structural damage development, strain response, and deformation response) are interpreted. The results show that the proposed cyclic displacement pushover test is better than the monotonic displacement pushover test, the damage process of the tested station structure conforms to the description of the inter-story drift ratio (IDR) quantification system of seismic performance. Meanwhile, the column has greater strain amplitudes than other components, and the column strain curves reach their peaks before other components. Furthermore, the tested station structure has a similar damage pattern to the Daikai subway station. The reliability and feasibility of the proposed cyclic displacement pushover test method are verified.

WITHDRAWN: Out-of-plane push-over testing of masonry wall reinforced with prestressed wedges

Masonry partition walls are vulnerable to lateral loads, including seismic, wind, and concentrated push loads. Various strengthening metal fittings have been proposed to improve lateral load resistance, particularly against seismic loads. This study introduces the use of prestressed wedges as a novel reinforcement method for masonry partition walls to enhance lateral load resistance. The resistance of the masonry partition wall against lateral-concentrated push loads was assessed using an out-of-plane push-over test on specimens sized 2300 × 2410 × 190 mm3. The presence or absence of wedges and wedge spacing were set as variables. The push-over test results showed that both the unreinforced specimen and the specimen reinforced with 300 mm spaced wedges toppled, while the specimen reinforced with 100 mm spaced wedges remained upright. Peak loads were measured to be 0.74, 29.77, and 5.88 kN for unreinforced specimens and specimens reinforced with 100 mm and 300 mm spaced wedges, respectively. Notably, a tighter reinforcement spacing yielded a similar strength, as expected, which was attributed to the increased friction force between the masonry wall and steel frame. The W-series specimens exhibited a trend comparable to that of the displacement ductility ratio. Overall, the findings validate that prestressed wedges improve the out-of-plane strength of masonry partition walls.

Seismic risk assessment of highway bridges in western Canada under crustal, subcrustal, and subduction earthquakes

This study conducts seismic risk assessment of highway bridges in western Canada. The performance-based earthquake engineering (PBEE) framework is enhanced to assess the expected annual repair cost ratio (ARCR) and annual restoration time (ART) of a benchmark bridge class under the region’s three types of earthquakes - shallow crustal earthquakes (CEs), deep subcrustal earthquakes (SCEs), and megathrust Cascadia subduction earthquakes (CSEs). First, event-specific seismic hazard models are considered, whereas event-consistent ground motions are selected for non-linear time history analyses. Compared with those from CEs and SCEs, CSE ground motions feature a much longer duration. This long-duration effect is captured by validating the numerical model of the bridge column against (1) a cyclic pushover test under standard versus long-duration loading protocols and (2) a shaking table test excited by six consecutive ground motions. Besides, the Park and Ang damage index is utilized as the column’s engineering demand parameter (EDP) and updated as a demand-capacity ratio model when reaching four different damage states. A comprehensive list of ground motion intensity measures (IMs) is considered where the spectra acceleration at one second, Sa(1.0), is chosen as the most suitable IM based on its performance in proficiency, efficiency, practicality, and EDP-IM correlation across all three earthquake events. Subsequently, component- and system-level fragility models are derived under each earthquake type using the cloud analysis that convolves the seismic demands with capacity models for multiple bridge components. To further quantify and propagate the epistemic uncertainty associated with the development of probabilistic seismic demand models (PSDMs), the bootstrap resampling technique is utilized to generate numerous seismic demand datasets and develop a stochastic set of seismic fragility curves. Finally, the bootstrapped, event-dependent fragility models are combined with the respective hazard models and probabilistic loss functions to assess the expected ARCR and ART for the benchmark bridge class. This study underscores the significantly higher seismic risk of highway bridges when facing CSEs, followed by CEs and SCEs.

A CPT-based multi-spring model for lateral monopile analysis under SLS conditions in sand

Monopiles are the most common Offshore Wind Turbine (OWT) foundations due to their simplicity in design, fabrication, and installation. However, large new-generation turbines have led to significant changes in monopile dimensions, necessitating extensive finite element analyses and ground investigations to meet design requirements. While Cone Penetration Test (CPT)-based p-y methods can analyse slender pile lateral behaviour, they often miss additional resistance mechanisms relevant to rigid monopiles. This paper introduces CPT-informed resistance mechanisms for monopiles to incorporate additional lateral resistances beyond p-y modelling capabilities. Distributed moment–rotation (m-θ) springs are defined by repurposing CPT-based axial capacity estimation methods for piles; and pile tip shear and moment springs are informed by approximating a residual bearing stress post-installation using local CPT qc values. The performance of the multi-spring model is appraised against data reported from monotonic pile pushover tests conducted at two sand sites. Results show that the multi-spring model is capable of predicting pile head deflections reasonably well within serviceability deflection limits against the reported test data, but ultimate failure loads cannot be predicted using the proposed model. A clear sensitivity in pile response to local variations in CPT qc is demonstrated.

Margin of bearing capacity of one-way straight mortise-tenon joints based on inspection of apparent damage and rotation angle

In essence, safety evaluation of damaged mortise-tenon joints (MJs) involves checking whether the joints have sufficient residual bearing capacity. However, the load, which is necessary in the calculation process, is difficult to observe in practice, resulting in insufficient reliability of the evaluation results. Thus, this study proposes the concept of a margin of bearing capacity (MBC) of an MJ and presents a calculation method for assessing the safety of one-way straight mortise-tenon joints (OSMJs). In the calculating process, the load was replaced with the observed apparent damage and rotation angle. First, the MBC includes the influence of joint damage and rotation angle on the residual moment bearing capacity of the joint. Second, the apparent damage to an OSMJ is simplified to average section loss rates in the height and width directions, which reduces the amount and difficulty of data acquisition. Third, the damage model of an OSMJ included in the proposed calculation method establishes the transforming relationship between the measured average section loss rate and the damage setting on the model, and avoids the influence of uneven distribution of the actual size reduction on the calculation of the residual moment bearing capacity. The results of unidirectional pushover tests of several damaged and intact OSMJ models showed that most absolute errors between the calculated and measured MBCs were less than 0.04, indicating the accuracy of the proposed method for practical safety assessment of damaged OSMJs.

Quasi-static model test and numerical simulation of a single-story double-span underground subway station

The model test method is crucial for studying the seismic performance of underground structures. Presently, model tests for the seismic response of underground structures are primarily performed with 1-g and N-g centrifuge shaking table. The 1-g shaking table model test is limited by the stress similarity in the soil-structure interaction system and cannot reproduce the seismic damage process and extent of the underground structure. The small table size of the N-g centrifuge model test makes it difficult to reflect the spatial effects of the seismic response of large underground structures. Seeking a large-scale model test method that can directly simulate the seismic damage process is essential for investigating the seismic performance of underground structures. Herein, based on the seismic response characteristics of underground structures, a quasi-static pushover test device applicable to underground structures was designed, and the corresponding tests and numerical model calculations were executed using the Daikai subway station as a prototype. Based on the macroscopic phenomena of structural damage development of the subway station in the test, the damage mechanism of the station structure was analyzed and compared with the numerical model. The strain amplitudes, displacements, and digital scatter strain color maps were analyzed to verify the reliability of the cyclic pushover loading test simulating the seismic performance level and damage process of the underground structure. Simultaneously, the rationality of the proposed quantification system for classifying the seismic performance level of single-story underground structures was verified.

Physical and numerical modeling of seismic soil-structure interaction of prefabricated subway station structure

Physical and numerical studies were conducted to explore the seismic performance characteristics of multi-story and multi-span prefabricated subway station structures, and to identify its weak parts and damage mode during earthquakes. A two-story three-span prefabricated subway station structure was designed and fabricated with a scale ratio of 1:10 and was subjected to a static pushover test that simulated the soil-structure interaction. The test results were interpreted to determine the horizontal resistance - interstory displacement capacity curve of the station, and the interstory displacement ratio (IDR) limit values under each seismic performance state. The experimental results revealed that the upper layer of the prefabricated subway station structure was more severely damaged than to the lower layer, and that the column ends of the upper layer were the weakest parts of the structure in terms of seismic resistance. The splicing joints were generally reliable, but the side wall-middle span splicing joint showed visible cracks. Furthermore, a finite element model was established based on the physical test configuration and was utilized to analyze the pushover tests. The numerical results indicated that the vertical load significantly accelerates the prefabricated subway station to reach the peak point and limit point of its horizontal bearing capacity. The IDR value of subway station seismic performance level 4 determined by the bearing capacity curve of the column was 30% lower than the IDR value obtained by the structural bearing capacity curve.

Seismic Retrofitting of Indonesian Masonry Using Bamboo Strips: An Experimental Study

Unreinforced masonry (UM) is well known as a vulnerable structure against earthquakes. However, it remains a popular structural system for low-rise residential housing in many high-seismicity areas, particularly in developing regions due to its low cost and easy construction. In the present study, a retrofitting strategy using locally available material, bamboo strips, was proposed. In addition to its fast-growing rate, the tensile strength of bamboo is considered high, nearly comparable to its steel counterpart. A series of experimental tests were performed in this study, including the bamboo tensile test, the mortar flexural test, the diagonal compressive shear test on the masonry assemblages, and the in-plane pushover test on masonry wall specimens without and with bamboo reinforcement. The retrofitted specimens with different volumes of bamboo reinforcement were also considered. The results show that the application of bamboo reinforcement, at a proper volume, significantly increases the ultimate strength and the ductility of the masonry wall. Such results indicate that the brittle failure of UM structures can be avoided by means of bamboo retrofitting.

Failure evaluation of transmission tower with asymmetrical legs by Scaled–Down pushover experiment

Power transmission towers are essential for the maintenance of daily living activities. Natural disasters such as wind storms, earthquakes, and landslides may damage these towers and cause considerable economic losses. To prevent these losses, transmission towers with asymmetrical legs, which have great flexibility on mountainous terrain, have been considered. Consequently, a scaled-down numerical model was first designed on the basis of a full-scale tower to understand the basic failure behavior of a transmission tower with asymmetrical legs. In addition, an experimental tower model was also conducted by pushover test to observe the failure mechanism and ductility. The failure mechanisms of the model were compared with the numerical simulations obtained from the finite element analysis. To accommodate the effects of asymmetrical legs, the component size should be increased appropriately for body extension. The results have demonstrated the feasibility of implementing transmission towers with asymmetrical legs at unique geographical locations.

Push ‘o ver: numerical simulation of the Castel di Lama pushover test through a force-based equivalent frame model

The experimental pushover test performed on the Castel di Lama building is studied by means of a finite element procedure based on the equivalent frame model with the aim to test the ability of this widespread model to reproduce the experimental results and the collapse mechanisms. Pier and spandrel macroelements are modelled as Timoshenko beams with plastic hinges to take into account shear and bending failures. Assuming the nodes as infinitely rigid and resistant, it is possible to model them by introducing properly sized offsets at the ends of pier and spandrel macroelements. A force-based formulation is adopted for the macroelement, taking advantage of its higher performances in terms of accuracy and efficiency with respect to the classical displacement-based method, and the capability of naturally avoiding shear-locking problems.The response of the building walls, either unreinforced or reinforced, is analysed in terms of resisting forces, story displacements and damage patterns, and compared with the experimental results.

Push ‘o ver: a pushover test program on an existing brickwork construction

An experimental pushover was carried on two very similar unreinforced masonry constructions. These constructions were obtained from an existing two-story brickwork building, located at Castel di Lama and to be demolished due to the damage suffered during the seismic sequence that affected central Italy from August 2016 to January 2017. The central stairwell was demolished in a controlled manner and, thanks to the symmetry of the original building, two almost identical portions were delivered: one was simply repaired, by repointing the cracks caused by the earthquakes, the other was seismically strengthened using Fibre Net reinforced plaster or composite reinforced mortar. The plaster was applied on both faces of the load-bearing walls of the first level and only on the external face at the second level. Preliminary dynamic characterization tests were carried out on both constructions. Then, pushover tests were carried out until collapse on both portions, using a steel structure placed on a reinforced concrete mat supported by four piles, which allowed to apply increasing horizontal forces at the two floors. The experimental results were analysed carefully and compared with those obtained from suitable numerical models.

Push ‘o ver: in situ pushover tests on as built and strengthened existing brickwork constructions

In the last decades several experimental tests were performed to analyse the seismic capacity of unreinforced masonry, mostly involving small size structural elements or small-scale building models, not accounting for the actual complexity of existing constructions. This paper illustrates experimental pushover tests on two very similar portions of an existing masonry building. The first portion was tested as built, while the second one was tested after a strengthening intervention with Composite Reinforced Mortar (CRM) system with Glass Fiber Reinforced Polymer (GFRP) components. The retrofit consisted in the application of a reinforced plaster on both sides of the walls of the first storey and on the external side only of the second storey, a solution that preserves the continuity of use. Comparisons between the experimental response of the two portions as well as the observed behaviour of the intervention at the first storey (both wall sides) and second storey (only outer side) provide very interesting insights and preliminary information on the degree of upgrading, essential steps to support the assessment of seismic vulnerability reduction.

On the seismic response of rack structures affected by pinching

The nonlinear static and dynamic behavior of rack structures affected by joint pinching are studied using a simplified single-degree-of-freedom system. Since the joints of steel rack structures play a crucial role in the capacity design methodology, as they are the main source of energy dissipation, their behavior is widely studied in the literature, highlighting their reduced hysteresis loop. However, no systematic study investigates and associates these characteristics with a possible increase in displacement demand. Using a single-degree-of-freedom system focuses attention on the type of cyclic behavior and allows a comparison of linear and nonlinear responses, the latter evaluated using both elastic-plastic and pinching laws. At prescribed ductility, the elastic-plastic and pinching responses can be compared with the prescriptions of Italian and European building codes. This simplified method was also applied to case studies of experimental pushover tests on racks found in the literature. Results showed that neglecting the joint pinching behavior for a rack structure can lead to an underestimation of the displacement demand in the case of the application of the nonlinear static seismic analysis method and hence to the overestimation of the q-factor for the application of linear seismic analysis methods.

Behaviour of masonry lateral loaded walls made with sediment-based bricks from the Usumacinta river (Mexico)

A feasibility study on Usumacinta River sediment-based bricks for resilient housing adapted to both climate change and seismic risks is performed on laterally loaded reduced-scale walls. Mortar formulations with different volume ratios of lime and sand (2/1, 1/2, 1/4, 1/6) are prepared while different types of sediment-based bricks are thoroughly investigated, manufactured and strength characterized. Commercial fired bricks as reference bricks completed the set of bricks tested (FRS: Fired River Sediment-based brick (at 850 °C); LRS: Lime treated River Sediment-based unfired brick (1.5% of lime addition); NRS: Natural fiber Reinforced river Sediment-based unfired brick and FR: commercial Fired Reference brick made without sediment). Walls of different sizes were built with these types of bricks and mortars. Tests under lateral in-plane loads of walls i.e. pushover tests , were conducted. The most significant results are as follows:(1) the addition of lime increases the strength of the mortar; (2) the wall size effect is observed for wall built with reference bricks (FR) and mortar ratio (1/2), where, large walls (LW), intermediate walls (IW) and small walls (SW) exhibiting maximum load of about 1200 N, 440 N and 260 N respectively. Rocking failure mode is the most observed unlike shear and sliding mode which are rarer. (3) maximum load measured during a pushover test performed on walls with sediment-based bricks provides a satisfactory average of 130 N, which exceeds the strength recommended for light buildings; The failure mode is sliding for fired bricks and rocking for unfired bricks. (4) the sediment-based bricks walls give an interesting ductility to pushover test while (2.6–40). These results show promising perspectives on the possibility of using the Usumacinta River sediment-based bricks in walls for light constructions under seismic loading. • The use of Usumacinta River sediment-based bricks is suitable in light buildings (80). • Fired, unfired: lime treated and fibered sediment-based bricks are used for wall tests (86). • The sediment-based brick wall gives satisfactory pushover resistance (68). • The ductility of the walls demonstrates significant seismic resistance (69).

Out-of-plane performance evaluation of energy-dissipating hysteretic infill wall system under cyclic loading

An innovative energy-dissipating hysteretic (EDH) infill system is conceptualized and designed with the help of slotted concrete blocks interlinked with energy-dissipating link elements. Full-scale reinforced concrete frames infilled with the EDH infill system and the conventional brick infill system are tested initially under in-plane cyclic test followed by out-of-plane cyclic test. A specially designed double-leaf airbag test setup maneuvered with hydraulic actuators is fabricated for out-of-plane testing of the infilled frames. The hysteresis performance of both the infill wall systems is compared experimentally. A cyclic pushover test in the out-of-plane direction is also carried out to assess the amount of damage and the residual capacity of the EDH infill system. The test results reveal that the out-of-plane performance with prior in-plane damage of the EDH infill is significantly improved over the conventional brick infill system in terms of lateral strength, stiffness degradation, deformation capacity, energy dissipation, and effective damping. A numerical model of the EDH infill system is developed based on the elastomeric properties of the energy-dissipating links in the OpenSees software platform.

Effect of base-plate types on system behaviour in down-aisle pallet rack systems

In rack systems, due to operational necessities, bracing of the system is limited in down-aisle direction. In addition, generally thin-walled components with single symmetry axis are used. Therefore, eccentric conditions may occur in the structure, which might affect system stability. There are studies in the literature about the stability aspect of these systems. In terms of stability, use of bracings and support conditions are prominent in rack systems. In this paper, effect of different base-plate types on system behaviour and energy dissipation capacity for low-rise rack systems with different bracing conditions was investigated. In this context, tests and numerical analyses were performed. Numerical models are validated with experimental test. In the test, static pushover has been performed for three two-storey full scaled models with different bracing conditions. For numerical analysis, the static pushover test is performed with the addition of base-plate type changes. In numerical analysis, three types of base-plates are employed with three bracing cases. In this context, it was detected that bracings in the system provide stiffness to the system, however in single-modular rack systems such as the subject of our study, the external bracings cause torsional effects. Therefore, beneficial effect of the bracings can not be utilised efficiently. However, it was also detected that as the rotational stiffness of the base supports increase, torsional effects are restricted. Therefore, it can be suggested that in low-rise, single modular rack systems, use of base-plates with higher stiffness value may provide better system stability as well as more economic solutions.

Nonlinear seismic performance of a large-scale vertical-axis wind turbine under wind and earthquake action

Vertical-axis wind turbines (VAWTs) are being regarded as a complementary technology to the more commercially used horizontal-axis wind turbines (HAWTs). This paper investigates the dynamic response and the collapse pattern of a 1 MW VAWT under the wind and earthquake action. A refined numerical model of the VAWT with three straight blades is created using the finite element (FE) method, and the combined kinematic-isotropic material hardening model is adopted to describe the material nonlinear behavior of the turbine tower. This FE modeling method is validated against a nonlinear pushover test. Two earthquake sets that contain 20 near- and 20 far-field earthquakes are selected as the inputs, and the aerodynamic loads are also calculated for the turbine under the rated wind speed. Time-history dynamic analyses are conducted to compare the nonlinear seismic performance of the operational turbine under the selected earthquakes. The results show that the average fore-aft and side-side displacements at the tower top excited by the near-field earthquakes are 35.3% and 20.1% larger than those excited by the far-field earthquakes. Additionally, the near-field earthquakes are prone to cause the yielding, buckling and collapse of the operational wind turbine. The collapse analyses indicate that the exact failure locations of the tower cannot be predicted by the 1st-order mode pushover analysis, and the failure height triggered by the near-field earthquakes is usually higher than that by the far-field earthquakes.

Static pushover test of spring-underground structure system for seismic performance analysis of underground structure

The static test is appropriate for studying the seismic performance of underground structures due to the large size of the model structure and the controllable stress level. By setting elastic elements around the model structure, the static pushover test of the spring-underground structure system could simulate the initial stress state of the underground structure and soil-structure interactions during the pushover process. Taking the Daikai subway station damaged in the 1995 Hanshin earthquake in Japan as the model structure, a series of static pushover tests of underground structures with a geometric scale of 1/5 were performed. The impact of vertical ground motion on the seismic performance of underground structures was studied by conducting two static pushover tests of spring-underground structure system under different vertical loads; the impact of surrounding soil on the seismic performance and failure process of underground structures was studied by conducting static pushover tests with and without springs under the same vertical load. The relationship between the horizontal pushover force and the structural interstory drift, the concrete strain change process of structural weak vertical member, the structural failure phenomenon, and the seismic performance of the test object can be obtained from the tests, which can provide valuable information for experimental research on the seismic performance of underground structures. The test results demonstrate that the vertical ground motion decreases the ductility of underground structure, and the surrounding soil improves the lateral collapse resistance of underground structure.

Experimental and numerical analysis of the lateral response of full-scale bridge piers

This paper presents the results of in-situ testing of two integrated pile-columns of a partially demolished bridge located in Auckland, New Zealand. A series of tests involving lateral monotonic pushover and subsequent dynamic free vibration snapback tests were used to quantify the variation in the stiffness and damping behaviour of the pile-column specimens over a range of lateral load levels. Each testing sequence consisted of incrementally increasing peak monotonic loads followed by the dynamic snapback, with a series of varying peak loads at the end of the testing sequence to evaluate the influence of loading history on the monotonic and dynamic response. The secant stiffness between the monotonic pushover tests performed to the same loading levels before and after the maximum load was applied, reduced by up to 40% in both the pile-columns, primarily due to soil gapping effects, highlighting the significant potential softening of the system prior to pile or column yielding. Progressive reduction in the damping of the system during each snapback test was evident, due to the varying contributions of different energy dissipation mechanisms, and the level of damping varied depending on the peak load applied. These results highlighted the significant influence of soil gapping and nonlinearity on the dynamic response of the system. Numerical models were developed in the open source structural analysis software OpenSeesPy using a Nonlinear Beam on Winkler Foundation approach to further investigate the response of the pile-columns. Models of both the pile-columns using existing p-y curves for clay soils showed good agreement with the experimental data in load-displacement, period and snapback acceleration time histories. Sensitivity analysis showed that the surface soft clay layer had a significant effect on the lateral response and dynamic characteristics of the model, reinforcing the need for good characterisation of the near surface soil profile to capture the behaviour of the system.

An Updated Discrete Element Model for the In-Plane Behaviour of NFRCM Strengthened Masonry Walls

Abstract. Masonry strengthened with natural fabric-reinforced cementitious matrix (NFRCM-strengthened masonry) is investigated by updating an existing discrete element model. Masonry walls are modelled by rigid blocks and elastoplastic interfaces that are able to account for mortar joints and block cracking. The reinforcement is modelled in a simplified manner considering perfect adhesion between wall and reinforcement and by adopting further spring elements connecting block centres. The model is validated by comparing it with an existing FEM based on a multi-step homogenization, where reinforced masonry is considered as a whole. Both approaches are used for performing nonlinear pushover tests with an increasing shear action applied to unreinforced and reinforced panels. The updated discrete model turns out to be able to represent the strength increment given by the reinforcement, but it is less able to represent the corresponding ductility increment.