High Load Ratio Research Articles

Explicit analytical model for seismic shear strength of RC bridge columns

An explicit analytical model was developed to predict the seismic shear strength of reinforced concrete (RC) bridge columns that failed in different modes including flexure, shear and flexure–shear failure. Shear demand was calculated without iteration based on sectional analysis. The shear capacity was divided into three contributions: the concrete compression zone (CCZ), concrete tension zone, and transverse reinforcements. Rankine's concrete failure criteria were used to determine the contribution of the CCZ, while modified compression field theory was applied to predict the other contributions. The three contributions were found to be dependent on the bottom section curvature based on axial-shear–flexure interaction. Seismic shear strength was obtained with a degraded shear capacity equal to the shear demand. Numerous tests were used to verify the proposed model and to assess the methods proposed in existing codes. Based on the proposed model, an application example and parameter analyses were conducted. The results showed that the proposed model produced better agreement with the test data than the methods of current codes. The contribution of the CCZ dominates RC bridge columns with low transverse reinforcements or high axial load ratios. The influence of the axial load ratio on seismic shear strength can be revealed accurately by the proposed model.

Simple Thermal Energy Storage Tank for Improving the Energy Efficiency of an Existing Air-conditioning System

This study aims to improve the energy efficiency of heating, ventilation, and air-conditioning (HVAC) system in existing building by adding a thermal energy storage (TES) tank. By installing the TES tank with volume of 1 m3 in the chilled water loop of HVAC system. it can make charge/discharge surplus heat from heat source equipment. The chilled water can be delivered not only to secondary side but also to TES tank, while the required heat load is less than the rated capacity of the heat source equipment, it does not need to shrink its output and can be operated under the relatively higher load ratio without falling down efficiency. The charge/discharge progress is considered occurring within short-term less than an hour. During the discharge progress the heat source equipment can be turn off or idling operation yields reduce operation hour of heat source equipment and achieve energy-saving. In this paper, a case study is conducted to evaluate the energy efficiency improvement by adding TES tank into HVAC system using system energy simulation, coupling two-dimensional, steady-quasi TES numerical model that is verified by comparison with the measurement data. The error of the TES numerical model compared with the experiment was less than 5%. From the case study results, it is found that the TES tank can improve energy efficiency of HVAC system because of increasing the COP of the heat-source equipment reduces its energy consumption and reducing operation time of the primary chilled water pumps.

Shear behavior of short square tubed steel reinforced concrete columns with high-strength concrete

Six shear-critical square tubed steel reinforced concrete (TSRC) columns using the high-strength concrete (fcu,150 = 86.6 MPa) were tested under constant axial and lateral cyclic loads. The height-to-depth ratio of the short column specimens was specified as 2.6, and the axial load ratio and the number of shear studs on the steel shape were considered as two main parameters. The shear failure mode of short square TSRC columns was observed from the test. The steel tube with diagonal stiffener plates provided effective confinement to the concrete core, while welding shear studs on the steel section appeared not significantly enhancing the seismic behavior of short square TRSC columns. Specimens with higher axial load ratio showed higher lateral stiffness and shear strength but worse ductility. A modified ACI design method is proposed to calculate the nominal shear strength, which agrees well with the test database containing ten short square TSRC columns with shear failure mode from this study and other related literature.

Investigation on fatigue crack growth behavior for commercial pure titanium at different crack tip plastic deformed levels

In this work, fatigue crack growth behavior of commercial pure titanium (CP-Ti) under different load levels is carried out considering the degree of plastic deformation of crack tip. Results show that there are great differences in the stage of crack stable growth in different loading conditions. In the same crack growth rate, strain energy and plastic zone in high load ratio are much higher than those in low load ratio, when crack growth rate is low. However, when crack growth rate is high, the differences of strain energy and plastic zone between these two loading conditions are much smaller. More plastic strain energy will be consumed and a wider range of plastic deformation in crack tip will be required in the high load amplitude. It seems that ΔK and ΔJ cannot describe these phenomena, while ΔJac and ΔJac′ proposed in this paper may be more suitable to describe these phenomena in the fracture surface. In different loading conditions, at the late stage of crack growth, the fracture morphologies are all dimples. The data in all the conditions obey the Paris’ law mode, with the same coefficient and index in the rapid growth stage using the parameter ΔJac′.

Effect of Temperature and Dwell Time on Fatigue Crack Growth Behavior of CP-Ti

In this paper, the effects of temperature and dwell time on the Fatigue Crack Growth (FCG) behavior of commercial pure titanium were studied under high and low load ratios. Besides, combined with the fracture surface morphology, the specific characteristics of FCG were analyzed under pure fatigue and dwell fatigue conditions. The experiment results show that the FCG rate of commercial pure titanium (CP-Ti) increases with the temperature under low load ratio, and the dwell time increases the FCG rate. Also, the enhancement of the dwell time increases as the temperature rises. The dwell effect tends to be saturated when the temperature rises to 200 °C. Under high load ratio, the FCG rate of CP-Ti also exhibits a temperature-sensitive enhancement. The enhancement effect of the dwell time on the FCG rate under high load ratio is more significant. However, the effect of the hold time on the FCG rate does not increase at 300 °C. The da/dN–ΔK/E FCG curves for CP-Ti have a tendency to approach each other under different load ratios, which indicates that the E-modulus is an important factor for the difference. The effect of dwell time on the FCG behavior of CP-Ti is dominated by the creep deformation mechanism under different load ratios from room temperature to 300 °C. At the same time, the oxidation effect gradually becomes significant as the load ratio increases to 300 °C. The fracture surface morphology shows that the secondary cracks and the roughness increase with temperature or dwell time under low load ratio condition, while, under high load ratio, the effect of creep deformation on the FCG behavior is more obviously enhanced, and plastic deformation is gradually significant with increase of the dimples.

Comparing fire behaviour of restrained hollow stainless steel with carbon steel columns

Although structural fire performance of hot-rolled carbon steel columns is already well known, limited design specifications are available for stainless steel columns. European fire design guidelines allow for a different material strength and stiffness degradation at high temperatures for stainless steel and, yet, the same buckling curve is recommended for stainless steel and carbon steel columns. This paper reports an experimental investigation on the structural behaviour of hollow steel columns under fire conditions for assessing mainly their fire resistance, critical temperatures and failure modes. The main variables tested included the: (a) cross-section type (circular, square); (b) axial load ratio (0.30 and 0.70); and (c) steel type (carbon and austenitic stainless steel). The structural fire behaviour of stainless steel columns was hence compared with similar carbon steel columns. Furthermore, European fire design methods both for the prediction of the temperature evolution on stainless steel members and for the estimation of the critical temperature of this type of column were also used to compare with experimental data. As well as that, the experimental results were still compared with predictions from other available analytical models, taking into account changes in the buckling curve established in the European design rules. Finally, such design rules were found to be over-conservative for high load ratios and so the development of a different design buckling curve might be useful for an accurate prediction of the fire resistance of stainless steel columns.

Experimental and Numerical Analysis of Large-Scale Circular Concrete-Filled Steel Tubular Columns with Various Constructural Measures under High Axial Load Ratios

To investigate the effect of constructional measures (including horizontal and vertical stiffeners, rebar cages, embedded steel tubes, and cavity welded steel plates) under high axial load ratios on the seismic performance of concrete-filled steel tubular (CFST) columns, quasi-static tests for six large-scale CFST columns with various constructional measures are performed. All specimens are subjected to identical axial forces. The failure mode, hysteresis characteristics, bearing capacity, stiffness degradation, ductility, and energy dissipation of specimens are analyzed. The study shows that the horizontal stiffener delays the occurrence and severity of column base buckling, the vertical stiffener improves the bending resistance capacity and initial stiffness of the member, the rebar cage improves the ductility, and the embedded circular steel tube significantly improves the member’s bearing capacity, ductility, and energy dissipation. When an internal circular steel tube and cavity welded steel plate are applied in tandem, the section steel ratio increases by 4.42% and the bearing capacity improves by 42.72%. A finite element model is created to verify test results, and simulation results match the test results well.

Kinetic analysis of the role of selective NOx recirculation in reducing NOx emissions from a hydrogen engine

The kinetics of NO formation is used to study the effect of NO injected into a hydrogen engine on its emission characteristics. Considering the engine to be a variable-volume reactor, it is shown that the injection of NO reduces its further formation. In contrast to the earlier studies, it is highlighted that a reduction in the amount of NO can be achieved even if the reactions between the fuel and NO do not take place. Additionally, the predicted reduction in NO is not due to the lowering of temperature, which is the focus of most of the exhaust gas recirculation studies. The NO conversion efficiency is predicted to be low for equivalence ratios in the range of 0.4–0.5. However, for higher equivalence ratios between 0.65 and 0.9, the model predicts NO conversion efficiencies above 90% for various amounts of NO injected. Consistent with the data reported in the literature, the model predicts that the NO reduction is a weak function of the engine speed and that the NO conversion efficiency is almost independent of the amount of NO injected for equivalence ratios greater than 0.6. A significant reduction in the NO emissions can be achieved, provided that the temperature generated is high enough for the NO formation reactions to approach equilibrium. It is deduced that conditions such as high equivalence ratio, high load, and high compression ratio will result in a high reduction of NOx. These results can be used to design selective recirculation systems for an enhanced removal of NOx. Even though H2 is used as a fuel in the present work, the insights obtained are equally applicable to other fuels as well.

A hybrid NMOS/PMOS capacitor-less low-dropout regulator with fast transient response for SoC applications

In this paper, a new architecture of a fully integrated low-dropout voltage regulator (LDO) is presented. It is composed of hybrid architecture of NMOS/PMOS power transistors to relax stability requirements and enhance the transient response of the system. The LDO is capable of producing a stable output voltage of 1.1 V from 1.3 V single supply with recovery settling time about 680 nsec. It can supply current from 10 µA to 100 mA consuming quiescent current of 20.5 µA and 95 µA, respectively. It supports load capacitance from 0 to 50 pF with phase margin that increases from 43° at low load (10 µA) to 74° at high load (100 mA) and power supply rejection ratio (PSRR) less than −20 dB up to 100 kHz. The proposed LDO is designed in 130 nm CMOS technology and occupies an area of 0.11 mm2. Post layout simulations show better performance compared with other reported techniques.

Fire Resistance of Gypsum Board Protected Steel Columns with High Load Ratios

AbstractA new relationship was developed for estimating fire resistance ratings of gypsum board protected steel columns with high load ratios. The proposed formula can be used to adjust the limitin...

Effect of pre-strain on fatigue crack growth behavior for commercial pure titanium at ambient temperature

In this paper, the effects of pre-strain on fatigue crack growth rate are investigated considering wide ranges of load amplitudes and load ratios for commercial titanium alloy (CP-Ti). Considering the influences of load ratio and load amplitude, the effect of pre-strain on the strain energy and crack tip plastic zone size, and on the fatigue fracture mechanism have been clearly demonstrated. Besides these, how the pre-strain affects fatigue crack propagation, and therefore affects whole fatigue life have also been clearly stated. Results demonstrate that pre-strain may be more sensitive to the enhancement of fatigue crack growth resistance under high load ratio and high peak load conditions. Fatigue lives increase with increasing in the pre-strain level. With the increase of load ratio and load amplitude, pre-strain has more and more significant influence on the plastic strain energy of specimen. A high pre-strain level of 8% and a high load ratio of R = 0.8 are more sensitive to the fatigue life. Static load or static damage has the most significant contribution to the crack tip plastic deformation than fatigue load. The production of twin crystal and dislocation structure after pre-strain process will greatly ensure the enhancement of fatigue crack resistance under all loading conditions. This research will provide support to the insight of pre-strain effect on fatigue crack growth behavior of CP-Ti.

Experimental study of rectangular multi-partition steel-concrete composite shear walls

Multi-partition composite shear walls have the strong points, such as high load-carrying capacity, good ductility, excellent energy dissipation capacity and fast construction speed. Even if they were used in the buildings, there was a little research work on this kind of composite shear walls. To study the axial behavior and cyclic behavior of multi-partition composite shear walls, six axially loaded specimens were tested, and two specimens were tested under horizontal cyclic load. Through testing of specimens, the damage progression, failure mechanism, stiffness and energy dissipation of composite shear walls were investigated. The experimental results showed that this kind shear walls under axial load had high load-carrying capacity and good ductility. The steel section could confine the core concrete, whose capacity was higher than that the summation capacity of steel section and core concrete. The shear walls exhibited excellent energy dissipation ability under high axial load ratio. Because of the confinement of steel section, the specimen showed higher energy dissipation capacity with the increasing of axial load ratio. However, the ductility of the specimen under cyclic load decreased with the increasing of axial load ratio. The experimental results were compared with the calculated results calculated by EC4, AISC and CECS. The results showed that all design codes underestimated the axial compressive capacity of the specimens. For specimens subjected to cyclic load, the results based on EC4 were the nearest to the experimental results.

Fatigue behaviour of blind bolts under tensile cyclic loads

This study presents an experimental investigation on the fatigue performance of a blind bolt under axial-tension cyclic loads. Static, constant-amplitude, and variable-amplitude fatigue tests on standard bolts and blind bolts are carried out. The bolt failure modes, static tensile force–displacement relationships, and fatigue life are presented. Based on the experimental results, S–N curves of the bolts under constant-amplitude fatigue tests are obtained by means of regression analysis, and the influence of the load amplitude as well as the load ratio on the fatigue performance of blind bolts are examined. It is found that S–N curves follow a power function with negative exponent, and exhibit similar S–N curves to standard bolts under higher load ratio. Meanwhile, the fatigue life of blind bolts under variable-amplitude loadings is estimated by using Miner's linear cumulative-damage theory. The fatigue strength of blind bolts under variable-amplitude fatigue loading is lower than the corresponding value for constant-amplitude fatigue tests. Finally, the fatigue strength of blind bolts is assessed by comparing the test results with existing specifications. It is concluded that the fatigue performance of blind bolts can be conservatively estimated by the corresponding specification of design codes.

Computational study on prestressed concrete members exposed to natural fires

The present paper is aimed at investigating the structural behaviour in bending of prestressed concrete members exposed to natural fires, i.e. fires with a heating and a cooling phase. The fire scenarios considered are characterized by a heating phase that coincides with the ISO834 standard fire and a linear cooling branch. To accurately track the structural behaviour, the usual constitutive models for concrete, ordinary and prestressing steel at high temperature are adapted to account for the different behaviour of the materials upon unloading and cooling. Parametric analyses are carried out on typical prestressed sections (an I-girder and a double-tee), in order to highlight the detrimental effect of longer fire durations (up to 120 min) and lower cooling rates (3 °C/min) as well as the variability of the structural behaviour with the variation of the load level.The results show that in members characterized by massive sections (I-girder in the present study) and exposed to natural fires, limiting the attention to the heating phase is not sufficient, as the maximum temperature in the prestressing steel may be reached long (even hours) after the onset of cooling (in accordance with tests reported in the literature), leading to delayed failure. Moreover, within the range of variation of the cooling rate (3–10 °C/min, ranging from slow to fast cooling) and load level (M/Mu = 0.15–0.30, ranging from low to high load ratio), the structural behaviour exhibits significant variations in the cooling phase of the fire, from an almost complete recovery of the initial configuration to runaway failure.

Effect of Severe Temperatures and Restraint on Instability and Buckling of Elliptical Steel Columns

This paper presents the findings of an experimental research to investigate the performance of axially restrained elliptical hollow (EHS) steel columns subjected to severe hydrocarbon fire. The test programme involved 12 steel columns presenting 2 oval sections 200 × 100 × 8 mm and 300 × 150 × 8 mm and yielding 2 slenderness λ = 51 and 33. The 1800 mm columns were tested under loading ratios ranging between 0.2 and 0.6 of the ultimate strength determined using EC3 and under axial restraint degree ranging from 0 to 0.16. The obtained results of axial displacements, lateral displacements, measured restraint forces, and high temperatures are presented in the paper. It was found that introducing restraint to the columns with elliptical section produces high restraint forces which reduce the time to lose lateral stability. This is more evident in cases of lower load ratios than the higher load ratios. The numerical study presented in this paper involved building a finite element model to simulate the columns behaviour in fire. The model was validated using the test results obtained from unrestrained and restrained columns fire tests. The model demonstrated good agreement in the prediction of failure times and failure mechanisms of local and overall buckling. The FEM model was then used to conduct a parametric analysis involving factors of slenderness, restraint and loading. The conclusions drawn for this research are presented at the end of the paper.

Numerical analysis of lateral displacement of beam-column joints in concrete frame structures subjected to fire

In this article, numerical analysis has been conducted for multi-story reinforced concrete frames under different fire scenarios to investigate the time-dependent lateral displacement of beam-column joint, which is almost the same as the lateral displacement at column end. Based on the numerical results, an empirical model has been proposed to approximately determine the time-dependent lateral displacement of the beam-column joint, and the calculated results using this model are compared with the numerical and test results provided by other researchers. Finally, the fire performance of axially-and-rotationally restrained reinforced concrete columns with time-dependent sidesway is numerically analyzed and compared with that without sidesway. It is found that (1) when the target floor is on fire, the influence of its non-adjacent floors’ exposure to fire on the lateral displacement of the joints at the target floor can be neglected, and the biggest error induced by this neglect is less than 10%; but the exposure of its adjacent floor(s) to fire leads to larger lateral displacement of these joints; and (2) fire endurance of the restrained reinforced concrete column with high nominal axial load ratio (e.g. 0.6) decreases significantly with the increasing of the column’s sidesway.

Seismic behavior of preloaded rectangular RC columns strengthened with precambered steel plates under high axial load ratios

In the past 20years, numerous tests have been conducted to demonstrate the effectiveness of additional fiber reinforced polymers (FRPs) or steel jacketing in the seismic retrofit of circular reinforced concrete (RC) columns. However, very few studies have explored the seismic retrofit of rectangular RC columns, in particular under high axial load ratios (ALRs). To address this issue, a simple and innovative approach is used in this study, in which rectangular RC columns subjected to high ALRs are retrofitted with post-compressed steel plates under severe lateral reversed cyclic loads. An experimental study is conducted to validate the effectiveness of this approach for improving the seismic performance of RC columns under high ALRs. Specimens with identical geometric and reinforcement layouts are fabricated and tested. One of the columns, which is not strengthened, is used as the control whereas the others are strengthened by using bolted steel plates. The effects of plate thickness, initial precamber and ALR on the failure mode, as well as ductility, strength degradation and energy dissipation, are investigated. The results demonstrate that this approach is effective in increasing the lateral displacement ductility capacity while maintaining the high axial load-carrying capacity of the columns.

High Load Ratio Fatigue Strength and Mean Stress Evolution of Quenched and Tempered 42CrMo4 Steel

The fatigue strength at a high number of cycles with initial elastic–plastic behavior was experimentally investigated on quenched and tempered 42CrMo4 steel. Fatigue tests on unnotched specimens were performed both under load and strain controls, by imposing various levels of amplitude and with several high load ratios. Different ratcheting and relaxation trends, with significant effects on fatigue, are observed and discussed, and then reported in the Haigh diagram, highlighting a clear correlation with the Smith–Watson–Topper model. High load ratio tests were also conducted on notched specimens with C (blunt) and V (sharp) geometries. A Chaboche model with three parameter couples was proposed by fitting plain specimen cyclic and relaxation tests, and then finite element analyses were performed to simulate the notched specimen test results. A significant stress relaxation at the notch root became clearly evident by reporting the numerical results in the Haigh diagram, thus explaining the low mean stress sensitivity of the notched specimens.

Experimental and numerical investigations on seismic behavior of steel truss reinforced concrete core walls

This paper aims to investigate the seismic behavior of steel truss reinforced concrete (STRC) core walls under cyclic lateral loading. The quasi-static tests on two 1/5-scaled specimens including one steel reinforced concrete (SRC) core wall and one STRC core wall with embedded steel trusses designed according to engineering practice were conducted. The structural behavior of two specimens were compared, including the damage mode, force-displacement relationship, deformation capacity, strength and stiffness degradation, energy dissipation, and strain responses. The test results show that the embedded steel trusses affects significantly the seismic performance of the STRC core walls in terms of lateral load carrying capacity, energy dissipation and damage mode. In addition, finite element (FE) models of SRC and SPRC core walls were developed and validated. The FE models are able to predict the nonlinear responses of core walls with a reasonable level of accuracy. In consideration of the fact that in engineering practice the axial compressive load ratio of STRC core walls would be much higher than that of the specimens in the tests, the verified numerical model was further used in the study on the seismic behavior of STRC core walls with high axial compressive load ratio. The results obtained in this study could provide useful information for the engineering application of STRC core walls.

Data partitioning and association mining for identifying VRF energy consumption patterns under various part loads and refrigerant charge conditions

Variable refrigerant flow systems account for a considerable portion of energy consumption in buildings. In order to improve the energy efficiency and estimate the energy saving potentials of variable refrigerant flow systems this study proposes a data mining based method to identify and interpret the power consumption patterns and associations. Two descriptive data mining algorithms, clustering analysis and association rules mining, are used for data partitioning and association mining. The proposed method consists of four phases: data pre-processing, data partitioning, data association mining and knowledge interpretation. Experimental data collected from a tested variable refrigerant flow system in the standard psychrometer testing room are pre-processed and prepared to examine the proposed method. Three time independent influential factors: part load ratio, refrigerant charge level and cooling condition are analyzed. Results show that the method is able to help identify energy consumption patterns and extract energy consumption rules in variable refrigerant flow systems. Three distinct energy consumption patterns are identified: undercharge fault, low and high part load ratio conditions. For compressor operation frequency switch control and refrigerant undercharge patterns, the energy saving potentials could be estimated by making comparisons between energy patterns and rules in a top-down way.