Failure Of Bars Research Articles

Fire resistance of concrete-filled steel plate composite (CFSPC) walls

Concrete-filled steel plate composite walls (CFSPC walls) have recently emerged as promising alternatives to conventional reinforcement concrete shear walls due to the benefits of effective space use, less construction time/effort, and higher ductility. While the basic load carrying capacity and seismic performance of CFSPC walls have been receiving great attention, little work has been conducted so far on their fire performance. This paper aims to bridge the knowledge gap by performing 12 fire tests on CFSPC walls, where shear stud spacing, wall height, wall thickness, steel ratio, and fire scenario (uniform or single-sided fire) were considered as the main test parameters. For the uniform fire tests, the specimens exhibited reasonably good fire resistance, where the fire resistance time exceeded 2h. Three dominating failure modes were observed, namely, local buckling of steel plates (LB), severe cracking (‘tearing’) of the weld along the corner line of the wall (WC), and global instability (GI). For the specimens subjected to single-sided fire, minor local buckling of the heated steel plate accompanied by tie bar failure in the buckled areas were observed, but no further load bearing failure was induced by the end of the heating process. The fire resistance time, which exceeded 2.5h for the specimens exposed to single-sided fire, was governed by the ‘thermal insulation criterion’. After 2.5h of heating, some specimens lost their thermal insulation capacity. Based on the test phenomena, preliminary design recommendations are proposed, and future research directions are also outlined.

Strut-and-tie model for interior RC beam-column joints with substandard details retrofitted with CFRP jackets

A softened strut-and-tie model (STM) is developed for interior reinforced concrete (RC) beam-column joints without any steel hoops in the joint or intermediate longitudinal column steel reinforcement, and discontinuous bottom beam flexural steel reinforcement. The STM model is extended to identical joints retrofitted with Carbon Fiber-Reinforced Polymer (CFRP) composites. The STM model is compared to experiments of two full-scale RC beam-column interior joints, one of which was retrofitted with CFRP composites. Failure modes for the original joint included anchorage failure of bottom steel beam bars, crushing of concrete nodal zones, diagonal joint shear failure, and for the retrofitted joint CFRP laminate delamination and crushing of joint core concrete. The STM model is based on the joint reinforcement details and experimental performance, including column axial load effects and the contribution of CFRP horizontal and diagonal laminates as tension ties. STM model assumptions were verified with strain gauge measurements. The STM model was successful in estimating the ultimate shear strength of the original and retrofitted joints. Recommendations are presented for evaluating the strut width for both original and retrofitted joints that include the quality of the bond of beam steel reinforcement to concrete in the beam-column joint.

Finite Element Analysis of Rail-End Bolt Hole and Fillet Stress on Bolted Rail Joints

A rail joint typically is one of the weakest elements of a track superstructure, primarily because of discontinuities in its geometric and mechanical properties and the high-impact loads induced by these discontinuities. The development of continuously welded rail has significantly reduced the number of rail joints, but many bolted joints remain installed in rail transit systems. Because of the unique loading environment of a rail transit system (especially high-frequency, high-repetition loads), defects related to bolted rail joints (e.g., joint bar failures, bolt hole cracks, and cracks in the upper fillet) continue to cause service failures and can pose derailment risks. Recent research in the Rail Transportation and Engineering Center at the University of Illinois at Urbana–Champaign has focused on investigating crack initiation in the bolt hole and fillet areas of bolted rail joints. Stress distribution was investigated at the rail-end bolt hole and upper fillet areas of standard, longer, and thicker joint bars under static loading conditions. Numerical simulations were organized into a comprehensive parametric analysis performed with finite element modeling. Preliminary results indicated that the longer joint bar performed similarly to the standard joint bar but the thicker joint bar reduced rail vertical displacement and rail upper fillet stresses compared with the standard joint bar. However, the thicker joint bar also may generate higher stresses at the rail-end bolt hole. Additionally, joint bar performance was dependent on the rail profile and bolt hole location.

Evaluation of Electromagnetic Performance of Emerging Failures in Electrical Machines

In the present paper, electromagnetic simulations of electric motors with early failures are presented. One of the most common failures in induction motors with squirrel cage rotor is considered in this paper. These failures are: broken rotor bars and short circuit in the inter-turn winding. The broken rotor bars failure was simulated with two consecutive broken bars in order to see how the magnetic flux density is affected. The inter-turn short circuit was simulated with the 40% reduction of winding coil in an inter-turn short circuit for one of the phases. In both cases the simulation was performed with the finite element software FEMM and the waveform of the flux density in both the motor air gap and the magnetic core was calculated. Comparative test of the real failure was performed in order to verify the results obtained from the simulation, good agreement between simulation and experimental measurements was obtained.

Discerning broken rotor bar failure from low-frequency load torque oscillation in DTC induction motor drives

This paper proposes a method for separation of broken rotor bar failures from low-frequency load torque oscillation in direct torque control (DTC) induction motor drives by using vq voltage and iq current components’ spectra. The effect of load torque oscillation should be considered in induction motor drives for reliable broken bar fault detection. Induction machine drivers are run in DTC mode to control its torque and speed. In practice, the presence of load torque fluctuation may sometimes cause false positive alarms on stator current spectrum. However, discerning of broken rotor bar failure from low-frequency load variation for DTC drives remains unexplored. Experimental results show that by using the proposed method broken rotor bar failure can be reliably detected in the presence of low-frequency load torque oscillation in DTC induction motor drives.

Experimental Evaluation of Seismically and Non-Seismically Detailed External RC Beam-Column Joints

An experimental investigation was undertaken to study the seismic performance of external reinforced concrete (RC) beam-column joints having representative details for mid-rise RC frame buildings in developing countries such as Iran that were designed and constructed prior to the 1970s. Three half-scale external RC beam-column joints were tested by applying lateral cyclic loading of increasing amplitudes. Tested specimens were comprised of one unit having seismic reinforcement detailing in accordance with the seismic requirements of ACI 318-11, and two units having non-seismic reinforcement detailing in accordance with the 1970s construction practice in many developing countries, such as Iran. Two typical defects were considered for the non-seismic units, being the absence of transverse steel hoops and insufficient bond capacity of beam bottom reinforcing bars in the joint region. Test results indicated that the non-seismically detailed specimens had a high rate of strength and stiffness degradation when compared to the seismically detailed specimen, which was attributed primarily to the joint shear failure or bond failure of the beam bottom bars. The non-seismically detailed specimens also showed a 30% reduction in both average strength and ductility and a 60% loss of energy dissipation capacity in comparison to the seismically detailed specimen.

An experimental study on impact of anchor bars at the steel frames with infilled walls

In this study, a series of experimental study was conducted to investigate the effect of anchor bars on steel frame systems where the connections were provided by anchor bars between frame and infilled walls. Seven one over four scaled specimens having one story and one bay of frames were tested. Experimental study was carried out by damage-controlled and incrementally applied load up to loading cracks. The test results relieved that with the help of using anchor bars the capacity of energy absorption with initial stiffness were increased. It has been found that the frames without using anchor bars failure at the loading edge, the crushing behavior of infilled walls and separations at free edges were occurred. These observed failure behaviors replies with tensile cracking for the frames having anchor bars. There for it should be underlined that anchor bars have a significant effect on improving the behavior of the frames.

Experimental and numerical investigation on the behaviour of prestressed high strength concrete pile-to-pile cap connections

Six Prestressed High Strength Concrete (PHC) pile-to-pile cap connections were tested to evaluate their damage process and failure modes under low cyclic loading. The hysteretic behaviour, rotation angle, ductility and bearing capacity were observed and analyzed. In addition, the effect of connecting forms and cutting-off piles on seismic performance of pile-cap connections was considered and investigated. The experimental results showed that flexural bending failure occurred in all connections. There were two main failure modes. One was tensile rupturing of prestressed bars and headings, resulting in loss of bearing capacity of the connections. The other type was more severe damage due to anchor bar yield and appearance of plastic hinge. Moreover, bond-slip failure of anchor bars was not observed in the cap, and the length of anchor bars met requirements. The analysis indicated that the bearing capacity and energy dissipation capacity increased along with the rotation capacity of the pile connections. Nonlinear finite element models were established to analyze the mechanical properties of these connections using OpenSees. The load-displacement curves obtained from the numerical analysis agreed with the experimental results. The effects of various parameters on the behaviour of the connections were conducted based on the numerical model.

Detection of rotor fault in three-phase induction motor in case of low-frequency load oscillation

This paper proposes a method for the separation of broken rotor bar failure and low-frequency load fluctuation in line-fed three-phase induction motor. In practice, the presence of load fluctuation at $$2sf_\mathrm{s}$$ has the same effect on a stator current of induction motor as a broken rotor bar fault. In such cases, the detection of broken rotor bar failure becomes difficult. To discern rotor fault and load oscillations, the analytical signal angular fluctuation (ASAF) method, which is a combination of Hilbert transform and the space vector angular fluctuation method, is used. The presented experimental results prove that low-frequency load oscillation and rotor fault can reliably be discriminated using the ASAF signal spectrum.

Bond strength of glass fibre-reinforced polymer ground anchors with helically wound ribs in different bonding agents

Bond strength of glass fibre-reinforced polymer (GFRP) bars with helically wound ribs was experimentally investigated in this study and compared to that of smooth glass fibre-reinforced polymer bars and steel bars. A total of 63 pullout specimens are tested. Variables of the tests involved four types of bars, namely, glass fibre-reinforced polymer bar with helically wound ribs, smooth glass fibre-reinforced polymer bar, ribbed steel bar and smooth steel bar, as well as different bonding agents, including normal concrete, high-performance concrete, grout and epoxy resin. Test results showed that bond failure of glass fibre-reinforced polymer bars with helically wound ribs is because of the shearing off of the ribs with small slips, and the ribs helically wound can improve the bond strength of glass fibre-reinforced polymer bars by 3∼14 times, which is around 13, 9·31 and 15·7 MPa in concrete, grout and epoxy resin, respectively. In addition, polypropylene fibres in high-performance concrete can improve the bond behaviour of anchors.

Post-heating behavior of concrete beams reinforced with fiber reinforced polymer bars

The present paper investigates the post heating behavior of concrete beams reinforced with fiber reinforced polymer (FRP) bars, namely carbon fiber reinforced polymer (CFRP) bars and glass fiber reinforced polymer (GFRP) bars. Thirty rectangular concrete beams were prepared and cured for 28 days. Then, beams were either subjected (in duplicates) to elevated temperatures in the range (100 to <TEX>$500^{\circ}C$</TEX>) or left at room temperature before tested under four point loading for flexural response. Experimental results showed that beams, reinforced with CFRP and GFRP bars and subjected to temperatures below <TEX>$300^{\circ}C$</TEX>, showed better mechanical performance than that of corresponding ones with conventional reinforcing steel bars. The results also revealed that ultimate load capacity and stiffness pertaining to beams with FRP reinforcement decreased, yet their ultimate deflection and toughness increased with higher temperatures. All beams reinforced with FRP materials, except those post-heated to <TEX>$500^{\circ}C$</TEX>, failed by concrete crushing followed by tension failure of FRP bars.

Microstructural influence on the scatter in the fatigue life of steel reinforcement bars

Fatigue damage accumulation and failure of steel reinforcement bars (rebars) is a stochastic process. Scatter can be influenced by the sensitivity of the short crack growth to the microstructural features, especially near the fatigue limit. This work investigates the scatter inherent to the microscopic conditions near the fatigue limit of ferrite–pearlite and martensite microstructures found in the outer layer of rebars. An adapted Navarro–De Los Rios model within a Monte-Carlo framework is used to simulate the short crack growth in material grains. Grain size variation, grain orientation factor and multiple phases i.e., ferrite–pearlite and martensite were considered in the model. The results are compared with the scatter found in fatigue tests on hot-rolled-cold worked (HR-CW) as well as quenched and tempered (QST) rebars. It is shown that microstructural effects explains part of the observed scatter in the fatigue tests.

Computational Modeling of Common Failures in Electrical Induction Machines

One of the most common types of failures in induction motors with squirrel cage rotor is examined in this paper. Generally, such failure is characterized by: broken rotor bars and short circuit in the inter-turn winding. The broken rotor bars failure was simulated with two consecutive broken bars in order to see how the magnetic flux density within be affected. The inter-turn short circuit was simulated with 40% reduction of winding coil in an inter-turn short circuit for one of the phases. In both cases the simulation was performed with the finite element software FEMM© , which helped in obtaining the waveform of the flux density in the motor airgap and the magnetic core. The result obtained allows clear identification of the type of failures that occurs depending of type that will present in the simulation.

Seismic Performance of Prestressed High Strength Concrete Pile to Pile Cap Connections

Six pile-cap connections were tested to evaluate performance of prestressed high-strength concrete piles (PHC pile)-cap connection under cyclic loading. The failure modes, hysteretic performance, ductility and bearing capacity were observed and analyzed. In addition, the seismic performance of pile-cap connection with different connections and cutting-off pile was studied. The test results showed that the connections appear to be bending damage. The two flexural modes were tensile rupturing of prestressed bar and yielding of anchor bars. The connections were severely damaged because of the yielding of anchor bars and appearing of the plastic hinge. The bond slippage failure of anchor bars was not observed in the cap, and the length of anchor bars meet requirements. It shows that the bearing capacity and energy dissipation capacity increased with the rotation capacity of pile connection. Nonlinear finite element models were established to analyze the mechanical properties of these connections using ABAQUS. The moment-displacement curves obtained from the numerical analysis compared well with the experimental results. The effects of parameters on the behavior of the connections were conducted based on the numerical model.

Fatigue Behavior of Reinforced Concrete T-Beam

AbstractEqui-amplitude fatigue loading experiments on five reinforced concrete T-beams were conducted to observe the fatigue damage development. Three stages of fatigue damage were seen in the experiments. The damage rapidly developed early and late periods of the experiment but was relatively stable in between. The beam failed because of the brittle failure of the steel bars. The number of cracks, and the width and height also follow three stages: rapid development, stable development, and failure. The growth of almost all of the cracks stopped at the time of beam failure, except one or two main cracks that continued to grow. The deflection of the test beam and strain of the steel bars and concrete increased rapidly early in the fatigue cycles, with increasing fatigue cycles, whereas they were relatively stable in the middle stage but increased rapidly near the failure point. Finally, the S–N curve of the reinforced concrete T-beam was obtained.

Mechanical behaviour of corroded RC beams strengthened by NSM CFRP rods

Corrosion of steel in reinforced concrete leads to several major defects. Firstly, a reduction in the cross-sectional area of the reinforcement and in its ductility results in premature bar failure. Secondly, the expansion of the corrosion products causes concrete cracking and steel–concrete bond deterioration and also affects the bending stiffness of the reinforced concrete members, causing a reduction in the overall load-bearing capacity of the reinforced concrete beams. This paper investigates the validity of a repair technique using Near Surface Mounted (NSM) carbon-fibre-reinforced polymer (CFRP) rods to restore the mechanical performance of corrosion-damaged RC beams. In the NSM technique, the CFRP rods are placed inside pre-cut grooves and are bonded to the concrete with epoxy adhesive.Experimental results were obtained on two beams: a corroded beam that had been exposed to natural corrosion for 25years and a control beam, (both are 3m long) repaired in bending only. Each beam was repaired with one 6-mm-diameter NSM CFRP rod. The beams were tested in a three-point bending test up to failure. Overall stiffness and crack maps were studied before and after the repair. Ultimate capacity, ductility and failure mode were also reviewed. Finally some comparisons were made between repaired and non-repaired beams in order to assess the effectiveness of the NSM technique. The experimental results showed that the NSM technique improved the overall characteristics (ultimate load capacity and stiffness) of the control and corroded beams and allowed sufficient ductility to be restored to the repaired corroded elements, thus restoring the safety margin, despite the non-classical mode of failure that occurred in the corroded beam, with the separation of the concrete cover due to corrosion products.

Diagnosis of broken rotor bars in induction motors based on harmonic analysis of fault components using modified adaptive notch filter and discrete wavelet transform

This paper proposes a method for fast and accurate detection of broken rotor bars (BRBs) in a three-phase squirrel cage induction motor. The fundamental component of the stator current signal is extracted using a linear time-invariant filter. The resultant residual signal which contains the harmonic components of the current is then used to detect the BRBs, by means of discrete wavelet transform (DWT). Since in experiment it is not possible to break the rotor bars while the motor is under load, finite element method and MATLAB/Simulink are employed to accurately demonstrate the behavior of the running machine as the BRB happens. To get more accuracy, differential evolution (DE) optimization algorithm is used to obtain the corresponding fault impedance for the rotor external circuit of the MATLAB model. Detail coefficients (DCs) of the wavelet decomposition are employed as the new fault indicators. Simulation results show that using DCs of the harmonic component signal rather than the actual current signal, leads to more distinctive fault signatures in the wavelet decomposition. The obtained results suggest that the proposed fault detection scheme can be employed as a highly reliable technique for diagnosing rotor bar failures in running machines.

Failures of cranes due to wind induced vibrations

Self-excited vibrations with large amplitudes in natural wind may occur at slender structural elements with low damping. Because of the different designs (e. g. using solid sections today instead of cables for tension elements in the past) the susceptibility to wind induced oscillations has increased. Those wind induced vibrations of profiles with specific cross section geometry which are motion induced and therefore self-exciting are called “galloping vibrations”. Especially systems with elements that are highly tensile loaded and undamped, like hangers of bridges or tension bars of cranes, are sensitive to wind induced vibrations. Therefore more and more fatigue problems caused by galloping oscillations have occurred in the 1990s. This paper describes exemplary the collapses of two modern cranes of different design and manufacturers. During standstill periods, both cranes suffered from wind induced vibrations of the tension bars, which bear up the counterweights. The failure analysis process to identify and explain the fatigue fractures as well as the comparative experiments and simulation to verify that they were caused by wind induced galloping-vibrations is described. It is shown, which parameters led to galloping-vibrations of the tension bars and how their onset wind speed and the amplitudes can be estimated with more accuracy by a non-linear and non-stationary approach. Furthermore it is shown that such dynamic stresses caused fatigue failure of the tension bars for the counter weights and subsequently collapsing of the cranes.For loss prevention knowledge and results gained by these investigations should be put at disposal to engineers working on this field of design. In the meantime, a contribution to development of appropriate technical standards on structural steelwork was given by the research works on galloping. Although new standards were introduced, which consider wind induced vibrations, such failures still occur. (Reference to the paper “Fatigue crack in railway bridge hanger due to wind induced vibrations – failure analysis, measures and remaining service life estimation” in this same Special Issue “A tribute to A. Martens” 2014).

Redundant robust topology optimization of truss

A common problem in the optimization of structures is the handling of uncertainties in the parameters. If the parameters appear in the constraints, the uncertainties can lead to an infinite number of constraints. Usually the constraints have to be approximated by finite expressions to generate a computable problem. Here, using the example of topology optimization of a truss, a method is proposed to deal with such uncertainties by utilizing robust optimization techniques. This leads to an approach without the necessity of any approximation. Another common problem in the optimization of structures is the consideration of possible damages. The typical approach is to prevent these damages by a convenient structure—this concept is known as safe-life. The method developed here applies the principle of redundancy to resist damages, which is the design philosophy of fail-safe. In general this leads to a high dimensional partitioning problem. By using a linear ansatz we get a computable problem. Finally, robust and redundant methods are combined, and simple numerical examples of typical problems illustrate the application of the methods. Our new redundant robust topology optimization of truss, based on known concepts of different research fields, gives a structure which is not only “safe” for parameter perturbations but for failure of bars, too. This introduces the fail-safe concept to structural optimization.

Effect of reinforcement curtailment on deflections, strain and crack development in RC slabs under catenary action

The present paper is concerned with the development and influence of catenary action in one-way slabs. The testing of two longitudinally restrained large-scale slab specimens subjected to an accidental event (removal of the central support) and subsequent monotonic vertical loading until collapse is described. The observed increase in the load-carrying capacity as well as the change in the structural behaviour in slabs that are restrained against inward movement is investigated. Whereas the first specimen is designed with continuous flexural reinforcement over the entire length, the second slab test is subjected to the same loading scheme and has the same geometrical dimensions, but differs in the reinforcement arrangement, considering a more conventional reinforcement curtailment. Comparisons are made with respect to the development of displacements, strains and cracks in the critical sections. The failure of the top reinforcement bars over the supports occurs at approximately the same load level. In addition, the failure mode (failure of the bottom reinforcement bars over the supports) is the same for both slabs. However, the final collapse load in case of the slab with curtailed reinforcement is strongly reduced due to the reduction of reinforcement in the bottom layer over the supports. The results of this contribution are of particular importance with respect to the validation of numerical and deterministic models related to the development of tensile membrane action in concrete slabs in the framework of robustness assessment.