Shear Failure Of Columns Research Articles

Shear strength and cracking mechanism of precast bridge columns with grouted sleeve connections

Precast columns are widely adopted in Accelerated Bridge Construction (ABC) due to its fast construction speed, environment amity and little interference to existing traffic. The bending behavior of precast columns with grouted sleeve connections under seismic effect has been widely studied. However, studies on the shear behavior of this kind of columns (for example, short columns under seismic effect) are limited, which restricts the application of ABC in seismic regions. This study proposes an approach for evaluating the shear cracking mechanism of sleeve region and the shear strength of precast columns with grouted sleeve connection. In this approach, the sleeve units and concrete units in the sleeve region are established according to the different stiffness between sleeve and concrete, individually. The cracking angle of membrane element of units is solved based on the Modified Compression Field Theory (MCFT). Thus, the crack developing diagram is drawn in light of the cracking range of units. An improved model for computing the shear strength of precast columns with grouted sleeve connection is presented by the predicted cracking method in this paper. A skeleton curve prediction method based on the principal compressive strain in compression zone has also been proposed and verified. In general, the improved MCFT method given in the paper is effective for predicting shear failure of precast bridge columns with sleeve connection.

Eccentric compression tests and design of welded steel tube strengthened RC stub columns with high-strength grout

The post-welded steel tube and high-strength grouting material reinforcing method is a novel rehabilitation approach developed for RC structures with insufficient strength. It could significantly improve the strength without enlarging the original cross-section area. To figure out the mechanical performance of post-welded outer-wrapped steel tube and high-strength grouting material (HGM) strengthened RC (PSGS-RC) stub columns, this paper first reports on an experimental investigation. The influence of critical parameters such as the loading eccentricity and original core concrete area ratio on the behavior of this type of composite was analyzed in detail. The typical failure modes, vertical force versus axial displacement relationship, strain response, and ductility index of several crucial points are presented to better understand the mechanical performance of eccentrically loaded PSGS-PC short columns. Moreover, a detailed discussion on the strength enhancement index and concrete contribution ratio was conducted to figure out the typical feature of the novel rehabilitation method. Finally, for evaluating the resistance of eccentrically loaded PSGS-PC short columns, a design method based on the ultimate equilibrium theory was proposed. The result demonstrated that the typical failure modes of PSGS-PC short columns generally comprised the local buckling of outer-wrapped steel tube, tearing of weld, crushing of HGM and core concrete, shear failure of columns, and buckling of longitudinal rebar. The strength of eccentrically loaded PSGS-PC stub columns was enhanced with a decrease of the load eccentricity and the intrinsically core concrete column area ratio. The ductility property was significantly improved with an increase of the load eccentricity and the core concrete area ratio. This research is expected to provide a reliable design method for repairing structures that possess inadequate strength in engineering practice.

Experimental and numerical study on impact resistance of RC bridge piers under lateral impact loading

Vehicle collisions with piers are main threat to bridge structures, thus the impact resistance of pier is significant for bridge structure safety. This paper presents the experimental and numerical work undertaken for investigating the impact process, damage and failure mode, dynamic behavior, and impact resistance evaluation method of reinforced concrete (RC) piers under lateral impact loading. By using a horizontal impact system, a series of simplified truck model collision test on the square sectional RC piers are performed, in which two main designing parameters, i.e., impact velocity and longitudinal reinforcement ratio, are especially concerned. Moreover, the detailed finite element models are established by the commercial program LS-DYNA, which are verified against the test results. It indicates that, the shape of impact force time-history does not exhibit the platform stage compared with that of the conventional drop hammer impact tests attributed to the shear failure mode occurred for the present columns; the damage level, impact force, displacement at impact position and energy dissipation rise with increasing the impact velocity; increasing the longitudinal reinforcement ratio improves the impact resistance of RC piers effectively; a plastic hinge forms with the hoop reinforcement yielding before the shear failure of column; the impact force causes a considerable change of the axial force of RC piers during the impact process. Finally, based on the energy conservation law, the residual displacement at the impact position is adopted and validated as the reasonable criterion for evaluating the impact resistance of RC piers. The present work can provide helpful references for the anti-collision design and impact resistance evaluation of the urban and highway bridge piers.

Performance-Based Assessment Methodology for Retrofit of Buildings

Postearthquake reconnaissance and recent research on seismic risk analysis have shown that nonductile concrete frame structures are much more susceptible to collapse than modern code-conforming frames. The performance-based assessment paradigm has been a persistent research theme over the last decade within the earthquake engineering community in order to estimate seismic fragilities and earthquake loss for these nonductile concrete frames. This paper proposes a nonlinear performance-based methodology to evaluate different retrofit methods considering hazard level, target performance levels, and life-cycle cost estimates. The structural performance is the main parameter considered for the optimization, although a life-cycle cost analysis is also presented. As a case study, the longitudinal frame of an existing building was modeled considering the effect of flexural-shear-axial load interaction in order to capture column shear and axial failures. The presented performance-based procedure identifies the most economic retrofit solution that satisfies structural response requirements for a given performance level.

Seismic mobile shaker testing of full-scale RC building frames with high-strength NSM-FRP hybrid retrofit system

Past seismic events have led to the premature failure of many existing seismically-vulnerable reinforced concrete building frames due to their inadequate column details. To ensure the ductile behavior of the non-ductile building frames, a hybrid retrofit system combining high-strength near surface mounted bars (flexural retrofit) with carbon fiber reinforced polymer sheets (shear retrofit) can be installed to the seismically-vulnerable columns. This study presents the experimental dynamic assessment of the non-ductile building frames retrofitted with the hybrid system. To prevent the column shear failure due to the flexural strengthening, a two-stage design procedure for the hybrid retrofit system was proposed. The full-scale dynamic testing was performed on the retrofitted test frame to realistically measure dynamic responses and quantify the retrofit effects. The hybrid retrofit system reduced the peak inter-story drift of the first-story columns by about 25% and their hinge rotation by approximately 76%. Additionally, the retrofit system can uniformly distribute the damage or drift over the entire structure, i.e. mitigation of soft-story mechanism.

Investigation on improving the shear behavior of columns in masonry infilled RC frames under lateral loads

Presence of infills in reinforced concrete (RC) frames enhances the lateral load behavior of the bare frame system, but makes the columns vulnerable to shear failure under strong ground motions. This local adverse effect of infill is more pronounced in exterior ground storey columns where strong and stiff infills are present on one-side of the columns. Some of the current seismic standards address the local adverse effect of infill using variety of recommendations but their application is limited. Therefore, it is of primary importance to develop methods to counteract the adverse effect of infill. To prevent or delay the shear failure of columns, three methods (increasing the dimensions of the columns taking care of the effect of infill, using weak and soft masonry to reduce the effect of infill on columns, and decreasing the frame-interaction using collector beams in infills) were adopted in the present experimental study. From the results, it was observed that the methods not only enhanced the lateral load behavior, but also delayed the shear failure of columns without compromising the other functional requirements. The three methods can be efficiently used in both existing and new constructions to reduce the seismic vulnerability of masonry infilled RC frame buildings.

Seismic fragility curves for California concrete bridges with flared two-column bents

The increased aesthetic appeal of columns with flares resulted in bridge configurations with flared columns during the 1970–1990 design periods in California. Earthquake reconnaissance reports indicated that the flare is detrimental on the structural functionality of the bridge with some geometric configurations. This research examines the relative vulnerability of bridge configurations with flared and non-flared columns for three-span two-column bent bridges with oblong cross-sections. The numerical model of the columns with and without flare is calibrated with existing experimental results. The calibrated models are used to develop the fragility curves for California bridges. Fragility curves for each bridge class are generated by accounting for the material, geometric, and structural uncertainties, and the effects of column flare on the seismic demand and fragility of bridges are investigated in detail in this paper. Results reveal that the presence of column flare for the case study bridges increases the stiffness and strength of columns because of a low potential of column shear failure, resulting in the reduction of bridge vulnerability and have beneficial effects on the structural functionality. Thus, the flare does not necessarily reduce the seismic performance for all bridge cases and depends on the failure mode associated with the bridge configuration. The fragility curves suggested in this research can be used by stakeholders in deciding the retrofitting and maintenance strategies of bridges with architectural flares.

A novel macromodel for prediction of shear failure in columns of masonry infilled RC frames under earthquake loading

Columns of masonry infilled reinforced concrete frames are prone to shear failure due to the detrimental effect of infill under lateral earthquake loads. This effect is catastrophic in ground storey columns, where the shear force demand is very high. The primary objective of the study is to understand the applicability of the codal recommendations to estimate the shear demand on the columns realistically, and propose an alternative solution for prediction of shear failure of columns by improving the existing macro-modelling techniques. Half-scaled specimens of masonry infilled RC frames tested in an experimental study were considered to evaluate the proposed improvements. Applicability of the improved macromodel was also verified with the results obtained in other past experimental studies. The proposed simplified analytical modelling technique can be used for practical engineering purposes to assess the shear failure of columns and can be utilized for designing or strengthening the shear deficient columns in such frames.

Fragility analysis of hillside buildings designed for modern seismic design codes

SummaryThe structural configurations of hillside buildings are significantly different than those observed on flat terrain. To study the effect of often observed structural configurations of hillside buildings, collapse fragility of regular “flat land (FL)” and irregular “split‐foundation (SF)” and “step‐back (SB)” buildings is studied using incremental dynamic analysis. The effects of building height, seismic zone, and near‐ and far‐field sites on collapse fragility are investigated. It is observed that SF and SB hillside buildings exhibit significant torsional effects at the storey just above the uppermost foundation level. In case of FL buildings, collapse occurs due to the flexural failure of beams and columns. On the other hand, in SF and SB buildings, the collapse occurs due to the combined effects of shear failure of short columns and flexural failure of beams and columns in the storey just above the uppermost foundation level. In general, SF buildings are observed to be the most vulnerable, whereas FL buildings are the least vulnerable. It is observed that high‐rise SF and SB buildings show unacceptably high probability of collapse at maximum considered earthquake, in Seismic Zone V and for the near‐field site in Seismic Zone IV.

Seismic Damage to RC Low-rise Building Structures Having Irregularities at the Ground Story During the 15 November 2017 Pohang, Korea, Earthquake

This study examines the seismic failure of RC low-rise building structures having irregularities at the ground story during the 15 November 2017 Pohang, Korea, earthquake, Mw = 5.4, which is the second strongest since the government began monitoring them in 1978 in South Korea. Some 2,000 private houses were damaged or destroyed in this earthquake. Particularly, serious damage to the piloti story of RC low-rise residential building structures of fewer than five stories was observed within 3 km of the epicenter with brittle shear failure of columns and walls due to severe torsional behavior. Buildings below six stories constructed before 2005 did not have to comply with seismic design requirements, so confinement detailing of columns and walls also led to inadequate performance. However, some buildings constructed after 2005 were damaged at the flexible side of the piloti story due to the high torsional irregularity. Based on these results, this study focuses on the problems of the seismic torsion design approach in current building codes.

Experimental Investigation of the Creep Behaviour of Coal by Monitoring Changes of Acoustic Properties

Time-dependent deformation of coal is a major concern for deep coal mining. When experiencing high stress conditions, the enhanced time-dependent deformation of coal will become a very possible factor to trigger the violent failure of underground openings. For this study, the inner structural variation of an extremely inhomogeneous coal subjected to incremental creep stress has been experimentally investigated by monitoring both the macroscopic deformation and acoustic property. Results show the following: (1) The pronounced inhomogeneity of coal induces considerable irregularities in creep strain data collected during the increase of stress level. These flawed data highlight the shortcomings of conventional strain-time relationship experiments for deciphering the mechanisms that control creep; (2) The mechanical response of samples under creep compression is the result of competition between compaction and cracking. Cracking deteriorates the structure and increases the ultrasonic attenuation, whilst the compaction causes the densification along the axial axis and reduces attenuation; (3) Long-term creep involves multiple structural deformations that alter the internal structure of the coal. This results in fluctuations of the ultrasonic velocities and their dynamic moduli; (4) During uploading phase of short term creep, vertical cracks develop and transform the sample into a structure consisting of separated columns. Further deterioration of the coal increases the possibility of tensile and shear failure of individual columns, which can trigger the violent collapse of an already fragile structure.

Mainshock-aftersh​ock response analyses of FRP-jacketed columns in existing RC building frames

Column jacketing techniques have been widely used to improve the seismic performance of existing reinforced concrete buildings and to mitigate additional damage induced by subsequent earthquakes to earthquake-damaged structures. To numerically assess damage mitigation of structures due to column jacketing under successive earthquakes, this study proposes a numerical modeling technique for shear-critical reinforced concrete columns retrofitted and repaired using fiber-reinforced polymer wraps and validates the simulated results with existing experimental data. A non-ductile reinforced concrete frame with the potential of column shear failure is then selected and modeled using the proposed modeling technique to evaluate the effect of FRP retrofit and repair upon mainshock and aftershock responses. In addition, two rehabilitation parameters such as story-related rehabilitation and number of FRP wraps are selected, and used to create rehabilitated frame models to maximize the effectiveness of FRP installations for the as-built frame under successive earthquakes. This effectiveness is examined in terms of the damage level of maximum and residual deformation.

Seismic Behaviors of Steel Bar Reinforced Joints of Concrete Filled Steel Tubular Laminated Columns

This paper presents the numerical studies on seismic behaviors of steel bar reinforced joints of Concrete Filled Steel Tubular Laminated Columns (CFSTLC). First, the material constitutive relations and modeling details are introduced. Then, the failure mode, bearing capacity and energy dissipation of CFSTLC joints under cyclic loading are studied, in which the effects of concrete grade, steel grade and reinforcement arrangement are discussed. Finally, the restoring force model is proposed to describe the seismic behaviors of bar reinforced joints. Results show that the bar reinforced joints perform good seismic performance. There are three main failure modes, including flexural failure of beam, shear failure of column and mixed failure. Increasing concrete grade and reinforcement ratio of column can increase the ultimate bearing capacity and energy dissipation capacity. Using bundled longitudinal reinforcement bars in beams can slightly reduce the ultimate bearing capacity, but can improve the energy dissipation capacity of specimens with flexural failure modes. Increasing the reinforcement ratio and steel grade can decrease the degradation rate of structural stiffness. The tri-linear restoring force model can make a good prediction of hysteretic behaviors of CFSTLC joints.

Influence of column shear failure on pushover based assessment of masonry infilled reinforced concrete framed structures: A case study

Structural frames, constructed either of steel or reinforced concrete (RC), are often infilled with masonry panels. However, during the analysis of the structural frames, it has become common practice to disregard the existence of infills because of the complexity in modeling. This omission should not be allowed because the two contributions (of infills and of frames) complement each other in providing a so different structural system. The use of different modeling assumptions significantly affects the capacity as well as the inelastic demand and safety assessment. In specific, the adoption of equivalent diagonal pin-jointed struts leaves open the problem of the evaluation of the additional shear on columns and consequently of the choice of a proper eccentricity for the diagonal struts. In this context, this paper presents the results of a real case study. The seismic performance of the RC structure of a school is evaluated by using concentric equivalent struts for modeling infills and the level of the additional shear on the columns is fixed as a rate of the axial force on them in agreement to a strong correlation obtained after a numerical experimentation. Hence, the applicability of the correlation mentioned before is shown and the form in which the results can be provided is presented. The characteristics of the new approach, first time applied to a real case, are highlighted by a comparison between the performance obtainable with different modeling detail levels of the infills. Through the paper, it is proved that the simplified evaluation of the additional shear demand produced by infills just for the base columns is sufficient to warn that a simplified model disregarding infills or based on the use of concentric struts for the infills may considerably overestimate the structural capacity. Further, by the study of a real case, the paper provides an overview of the models developed by the authors to obtain the capacity of reinforced concrete framed structure for the practical applications.

Low‐seismic damage strategies for infilled RC frames: shake‐table tests

SummaryUnreinforced masonry (URM) infill panels are widely used as partitions in RC frames and typically considered as non‐structural elements in the design process. However, observations from recent major earthquakes have shown that under seismic excitation, the structural interaction between columns and infill walls can significantly alter the structural behaviour, thus causing catastrophic consequences. The purpose of this research was to propose and test an innovative low seismic damage detailing method, which isolates the infill panel from bounding columns with finite width vertical gaps during the infill panel construction phase and deploys steel wire connections in mortar layers anchored to columns. Taking into account the similitude requirements, a total of six one‐third scale, single‐storey single‐bay RC frames with different infill configurations and flexible connection details were carefully designed and tested on a shake‐table. Three real earthquake records were selected and scaled to ascending intensity levels and used as input signals. A series of thorough investigations including dynamic characteristics, hysteretic behaviour, failure mechanisms, out‐of‐plane vulnerabilities and the effect of different gap filling materials and load transfer mechanisms were rigorously studied. The experimental results indicate that the undesirable interaction between infill panels and bounding frame is significantly reduced using the proposed low seismic damage detailing concept. Direct shear failure of columns at an early stage is prevented, and structural redundancy at high levels of excitation can be provided. In general, the structural stability and integrity, and displacement ductility of infilled RC frames can remarkably be improved. Copyright © 2017 John Wiley & Sons, Ltd.

Modeling of interior beam-column joints for nonlinear analysis of reinforced concrete frames

Beam-column connections undergo significant shear deformations and greatly contribute to story drifts during earthquake loading, yet their response is typically neglected in traditional frame analyses through the use of rigid end offsets. Although local joint models are available in the literature for the investigation of single, isolated joints, there is a lack of holistic frame analysis procedures simulating the joint behavior in addition to important global failure modes such as beam shear, column shear, column compression, and soft story failures. The objective of this study is to capture the impact of local joint deformations on the global frame response in a holistic analysis by implementing a joint model into a previously-developed global frame analysis procedure. The implemented joint element simulates the joint shear deformations and bar-slip effects. Concrete confinement effects are also considered so that both older and modern joints can be modeled. The developed procedure successfully captures the local load-deflection response of joints within a global frame analysis procedure. The ratio of predicted and observed peak load had a mean of 1.25 before the modification, and a mean of 1.05 after the modification.

RC Short Column Effects on Seismic Performance of Small-Sized Buildings in Korea

This paper investigates seismic performance of a small-sized single story building in Korea. Nonlinear pushover anlaysis is performed to verify shear failure of RC short columns eventually led to performance degradation. Also, nonlinear time history analysis is performed using the same earthquakes from Gyeongju. Similar failure mode was obtained as in the report where a sudden rupture of the RC columns happened.

Seismic performance of older R/C frame structures accounting for infills-induced shear failure of columns

Older infilled frame R/C structures have several deficiencies as compared to seismic code-conforming structures, inherent to the behavior of the structural elements and interaction between structural and nonstructural components, which should be accounted for an accurate performance assessment. In order to investigate the importance of incorporating the contribution of the infills (typically considered as non-structural members) and their interaction with non-ductile columns in a building structural model, a comprehensive performance based earthquake assessment over a range of hazard levels is carried out through Nonlinear static and Incremental Dynamic Analyses of a 2D frame model incorporating: (1) fiber section axial force–moment interaction, (2) cracking and crushing of the infills and (3) infills-induced shear failure of columns. The analyses are performed on three different common structural configurations: (A) bare frame, (B) uniformly infilled frame, and (C) partially infilled frame (pilotis frame). Results show the importance of correctly accounting for shear failure of columns when modeling the infill–frame interaction, and the influence of the inelastic shear law, opening placement, type of infills, and choice of infill constitutive model.

5 May 2014 MW 6.1 Mae Lao (Northern Thailand) Earthquake: Interpretations of Recorded Ground Motion and Structural Damage

A moderate left-lateral strike-slip earthquake of MW 6.1 occurred on 5 May 2014 in northern Thailand, causing damage to the town of Mae Lao and nearby municipalities. Based on an instrumental earthquake catalog, the Mae Lao earthquake is the second largest earthquake in modern Thai history after the MW 6.3 Nan earthquake in 1935. In this study, the strong-motion records from the epicentral area are analyzed in order to investigate the characteristics of the ground motions. In addition, post-earthquake field observations and statistics for typical structural damage in the affected area, such as the soft story effect and the shear failure of columns, are summarized. Besides major structural damage, the majority of structures in the epicentral area withstood the strong motion, which could be due to the low stress drop of the main shock (40 bars) leading to below average ground motions for an MW 6.1 earthquake.

Pseudo-Dynamic and Quasi-Static Testing of an Irregular Steel Concrete Composite Frame with Wing Walls

This paper presents a series of pseudo-dynamic tests (PDTs) and quasi-static tests (QSTs) on a dual wing-walled frame system, represented here by a 1/7-scaled composite moment frame with steel reinforced concrete (SRC) columns and reinforced concrete (RC) wing walls. Special characteristics of this scaled system are irregular story layout, strong-beam weak-column mechanism and large axial load. A series of scaled El-Centro (NS) waves were used as the input ground motion for the PDTs, the results of which showed that the seismic behavior was significantly improved by the RC wing walls. With the strong-beam weak-column connections, severe damages sustained by the longitudinal wing walls (LWW) prevented the potential collapse of column, and the transverse wing wall (TWW) efficiently avoided the fragile shear failure of short columns and panel zone of beam-column joints. The failure mechanisms were identified indicating that wing walls improved the ductility for the bare frame. This study provides a solid experimental support on the evaluation of seismic behavior of irregular SRC frames with RC wing walls, which could be applied in the main factory buildings of thermal power plants (TPP).