Stiffness Distribution Research Articles

Experimental recognition of seismic performance of simple bolt-connected precast frame by comparing with equal stiffness cast-in-situ frame

Multi-story industry buildings composed of precast concrete members and simple bolt-type connections are widely constructed in the world, and there is enormous need for future construction market. Based on the equal stiffness and stiffness distribution along the height, a 1/5-scale precast RC frame utilizing simple bolt-type connections and a reference cast-in-situ monolithic RC frame were designed and constructed to conduct shaking table tests. According to the test results, the seismic responses, failure modes and energy dissipation behaviors of the two models were analyzed and compared. The vibration modes of the precast frame performed bending deformation pattern, while the cast-in-situ one presented shear deformation pattern. The damage and inter-story drift of the precast frame was comparatively uniformly distributed along the stories, whereas that of the cast-in-situ frame was concentrated in the first story. The energy dissipation capacity of the precast frame was inferior to that of the reference frame, as the bolt-type connections couldn't have plastic damages and plastic hinges that occurred in the cast-in-situ joint zones. In general, the global seismic performance of the precast frame was close to the reference frame and its collapse resisting capacity under severe earthquakes was reliable. Some serious local failure modes and less of energy dissipation capacity of the precast frame should be concerned in high seismic regions.

Finite Element Analysis of Curved Beam Elements Employing Trigonometric Displacement Distribution Patterns

A finite element analysis (FEA) model was developed for Euler and Timoshenko curved beam elements by incorporating trigonometric displacement distribution patterns. Local polar coordinate stiffness matrices were derived based on force-displacement relations and static equilibrium conditions. By employing the kinetic energy theorem and triangular displacement functions, an expression for the consistent mass matrix of a curved beam element was obtained. A coordinate transformation matrix for the curved beam element was established by relating the local polar coordinate system to the global polar coordinate system. Calculation programs were implemented in the Fortran language to evaluate the static–dynamic performance and natural frequency characteristics of curved beam bridges. The obtained results were then compared with those obtained using ANSYS solid models and “replace curve with straight” beam element models. The comparison demonstrated a strong agreement between the results of the Euler and Timoshenko curved beam element models and those of the ANSYS solid models. However, discrepancies were observed when comparing with the results of the “replace curve with straight” beam element model, particularly in terms of lateral static displacement. This discrepancy suggests that the characteristic matrix derived in this study accurately represents the stiffness and mass distribution of the curved beam, making it suitable for mechanical performance analysis of curved beam bridges. It should be noted that the “replace curve with straight” method overlooks the initial curvature and the bending–torsion coupling effects of a curved beam, resulting in calculation deviations. On the other hand, the use of curved beam elements in numerical analysis provides a simple and practical approach, which facilitates further research in areas such as vehicle–bridge coupling vibrations and seismic analysis of curved beam bridges.

Effect of Additional Mass on Natural Frequencies of Weight-Sensing Structures.

The phenomena of variability and interference in the natural frequencies of weight-sensing structures applied in complex working conditions must solve the problem of reducing or eliminating resonance under low-frequency vibrations to maximize stability, accuracy and reliability. The influence laws of the additional mass with relevant characteristics on the natural frequencies, which include the components of mass, stiffness and center-of-mass distribution, etc. Firstly, the theoretical formulas of the mathematical model are given based on different characteristics of the weight-sensing structure, and various combinations of additional masses on the weight-sensing structures are adjusted in the X-, Y-, and Z-directions. The key factors to be specifically considered in the theoretical formulas are discussed through simulation analysis and experimental validation. Secondly, the locking strength of the fastening screws of some components was changed, and another component was placed on the experimental platform in the experiment. The results show that the mass, center-of-mass, stiffness distribution and other factors of the additional mass have different effects on the natural frequencies, which are important for the demand for high-precision, high-stability weighing measurement. The results of this research can provide an effective scientific evaluation basis for the reliable prediction of natural frequencies.

(Invited) From Material Discovery Towards a Prototype Hydrogen Generator

This contribution illustrates the approach that has been taken to develop a pre-commercial prototype of a water electrolyzer. The aim has been to develop and integrate novel nanostructured materials for both electrodes and membranes, demonstrating their upscaling potential by integrating them together in a water electrolysis cell with an active surface are of 50 cm² in order to proof their potential for further advancing the state of the art of green hydrogen generation.Based on industrial experience with medium flow hydrogen generators, the boundary conditions for the new electrode and membrane components were set, safeguarding their integration potential in a scalable stack. A nano-porous intrinsically conductive membrane material has been developed, that allows to drastically reduce the thickness of the separator, reducing ohmic losses without compromising gas separation quality during operation at elevated pressures (up to 20 barg). This was combined with nanostructured electrodes of 4-10 μm thickness on both sides of the membrane. Given the vast range of dimensional scales, from 10-8 to 10-1 m, special attention was needed for the interconnections of the materials to establish the required electronic and ionic coupling together with adequate mechanical robustness. Two phase fluid flow modelling was done do have an optimal reactant distribution and gas evacuation from the electrode surfaces. Aspects like contact resistance, elasticity, stiffness, and overall electric field distribution inside the cell were considered. To characterize the cell in operation, a dedicated balance of plant has been built (Fig.1). The installation is automated and takes care of the electrolyte solution conditions and of the process window settings in terms of applied current, electrolyte flow and operational temperature at a pre-set pressure. The resulting cell potential, the gas purity of both oxygen and hydrogen streams and the differential pressure between anolyte and catholyte chambers are continuously monitored. The upscaled cell demonstrates a high hydrogen generation efficiency of 85% (HHV) that outperforms currently installed alkaline systems in terms of productivity and gas purity when operated at an elevated pressure 20 bar. The cell robustness, documented over 800 hours, reveals further areas of improvement for both the internal construction of the stack and the control logic of the balance of plant. Figure 1

Influence of local features on twist springback of high-strength steel long channels

With the wide use of high-strength steel (HSS) in structural body parts, the twist springback of high-strength steel parts has received extensive attention. Unlike automotive panels, structural body parts are arranged with many local features to meet functional requirements. The aim of this study is to investigate the influence of local features on twist springback to optimize the design of local features to reduce twist springback. In this paper, a typical long channel part (A-pillar upper inner plate) is taken as the object of study. A suitable location division method for this type of part is proposed to investigate the changes in twist springback and cross-sectional stresses in the part before and after the addition of local features at different locations through finite element simulations. The results show that the local features affect the stiffness and stress distribution of the parts and significantly affect the twist springback of the parts. Finally, the research results guided the optimization of the local feature design of an A-pillar upper inner plate, resulting in a remarkable 37% and 61% decrease in twist springback at the left and right ends of the optimized part, respectively. In addition, stamping molds were developed for optimized parts, and experiments were carried out to verify the effectiveness of the law and the reliability of the finite element model.

Performance Analysis and Optimization of a Modified Stewart Platform for the Qitai Radio Telescope

This paper presents a modified Stewart platform that enables precise adjustment of the sub-reflector for the Qitai Radio Telescope (QTT). QTT demands a parallel platform capable of carrying a heavy load (≥4000 kg), and moving in elevation from 5° to 88° together with the primary reflector while precisely adjusting the sub-reflector’s position in five degrees of freedom. To meet these requirements, a modified Stewart platform with two separated actuators is proposed, and the comprehensive performance is analyzed and optimized for practical application. The performance of the modified platform is compared to that of the traditional Stewart platform, and the results demonstrate that the modified platform has superior load-bearing capacity and stiffness over the entire elevation angle with more uniform actuator load and stiffness distributions. These results indicate that the modified Stewart platform is well-suited for practical application in QTT.

Gear mesh stiffness and dynamics: Influence of tooth pair structural stiffness asymmetry

There are certainly a lot of different gear mesh stiffness models in the literature, nonetheless, it is not possible to find comprehensive and in-depth gear parametric studies that sweep the gear geometrical domain. To do so, it is required to have a fast and accurate gear mesh stiffness model, which is precisely one of the main developments of this work. By taking advantage of this new model, a parametric study on the influence of tooth pair structural stiffness asymmetry is conducted. This investigation evaluates how the referred stiffness asymmetry affects the gear mesh stiffness, dynamic displacements and load distribution by performing an extensive number of simulations that cover the entire selected gear geometry spectrum — two randomly generated populations of gears, one for spur and another for helical, are tested with and without the tooth pair structural stiffness asymmetry. It is concluded that tooth pair structural stiffness asymmetry cannot be neglected since it can lead to significant modifications on the frequency content of the gear mesh stiffness signal as well as the dynamic behavior of the geared system.

Analysis of Concrete Pavement Slab resting on Non-uniform Elastic Foundation using the Finite Element Method

During the rigid pavement design, the concrete slab working on the base/subbase and subgrade is usually modeled as the slab on the elastic foundation. However, the non-uniform distribution of materials in the base or subgrade layers naturally exists in real conditions and should be considered. In this paper, a finite element method for calculating concrete pavement slabs on an elastic foundation with non-uniform stiffness distribution is developed. This study applies 4-node finite elements and Mindlin plate theory to formulate the finite element equations. The results predicted by the proposed approach are verified analytically. Calculation examples are conducted with the practical settings to investigate the influence of slab and foundation stiffness parameters on the slab displacement.

An orthotropic continuum model with substructure evolution for describing bone remodeling: an interpretation of the primary mechanism behind Wolff's law.

We propose a variational approach that employs a generalized principle of virtual work to estimate both the mechanical response and the changes in living bone tissue during the remodeling process. This approach provides an explanation for the adaptive regulation of the bone substructure in the context of orthotropic material symmetry. We specifically focus upon the crucial gradual adjustment of bone tissue as a structural material that adapts its mechanical features, such as materials stiffnesses and microstructure, in response to the evolving loading conditions. We postulate that the evolution process relies on a feedback mechanism involving multiple stimulus signals. The mechanical and remodeling behavior of bone tissue is clearly a complex process that is difficult to describe within the framework of classical continuum theories. For this reason, a generalized continuum elastic theory is employed as a proper mathematical context for an adequate description of the examined phenomenon. To simplify the investigation, we considered a two-dimensional problem. Numerical simulations have been performed to illustrate bone evolution in a few significant cases: the bending of a rectangular cantilever plate and a three-point flexure test. The results are encouraging because they can replicate the optimization process observed in bone remodeling. The proposed model provides a likely distribution of stiffnesses and accurately represents the arrangement of trabeculae macroscopically described by the orthotropic symmetry directions, as supported by experimental evidence from the trajectorial theory.

Mechanical behavior of glued-in GFRP rod in glubam: Experimental and analytical study

Fiber reinforced polymer (FRP) rod is a reasonable alternative to steel rod used in glued-in rod engineered bamboo structures, mainly attributed to its significantly higher strength-weight ratio and excellent corrosion resistance, while this field of study is almost empty. To fill this research gap, a total of 95 glued laminated bamboo (glubam) joints with single glued-in glass FRP (GFRP) rod were tested in a pull-pull tension loading program, aiming to shed light on the influence of anchorage length, rod diameter, bonding length-diameter ratio, glue-line thickness and rod-to-grain angle on the mechanical behavior. Particular emphasis was placed on the initial stiffness analysis and the explanation of the bond-slip mechanism based on the strain distribution of GFRP rod. Approximately 30% and 65% of the glued-in glubam joints can be categorized as the pure bonding failure at the glubam-adhesive interface and the shear failure in the glubam, respectively. It is found that the shear strength and initial stiffness of glubam joint decrease with increasing of the bonding length, the bonding length-diameter ratio, and the glue-line thickness, but the two indexes increase with increasing of GFRP rod diameter. The shear strength and initial stiffness exhibit an initial upward and then downward trend when the rod-to-main fiber direction angle varied from 0° to 90°. The analysis results reveal that the distribution of the local slip stiffness is non-uniform along the bonding length, also meaning that the local bond-slip relationships are varied at different locations. Based on the single-parameter correlation analysis, a strength model and an initial stiffness model are proposed for predicting the mechanical performance of glubam joints with glued-in GFRP rod for a potential application in evaluating actual glubam joints.

Multimaterial Topology Optimization of Adhesive Backing Layers via J-Integral and Strain Energy Minimizations

Abstract Strong adhesives often rely on reduced stress concentrations obtained via specific functional grading of material properties. This can be seen in many examples in nature and engineering. Basic design principles have been formulated based on parametric optimization, but a general design tool is still missing. We propose here the use of topology optimization to achieve optimal stiffness distribution in a multimaterial adhesive backing layer, reducing stress concentration at selected (crack tip) locations. The method involves the minimization of a linear combination of (i) the J-integral around the crack tip and (ii) the strain energy of the structure. This combination is due to the compromise between numerical stability and accuracy of the method, where (i) alone is numerically unstable and (ii) alone cannot eliminate the crack tip stress singularity. We analyze three cases in plane strain conditions, namely, (1) double-edged crack and (2) center crack, in tension, as well as (3) edge crack under shear. Each case evidences a different optimal topology with (1) and (2) providing similar results. The optimal topology allocates stiffness in regions that are far away from the crack tip, and the allocation of softer materials over stiffer ones produces a sophisticated structural hierarchy. To test our solutions, we plot the contact stress distribution across the interface. In all observed cases, we eliminate the stress singularity at the crack tip, albeit generating (mild) stress concentrations in other locations. The optimal topologies are tested to be independent of the crack size. Our method ultimately provides the robust design of flaw tolerant adhesives where the crack location is known.

Towards the optimization of passive undulatory locomotion on land: mathematical and physical models.

The current study investigates the body-environment interaction and exploits the passive viscoelastic properties of the body to perform undulatory locomotion. The investigations are carried out using a mathematical model based on a dry frictional environment, and the results are compared with the performance obtained using a physical model. The physical robot is a wheel-based modular system with flexible joints moving on different substrates. The influence of the spatial distribution of body stiffness on speed performance is also investigated. Our results suggest that the environment affects the performance of undulatory locomotion based on the distribution of body stiffness. While stiffness may vary with the environment, we have established a qualitative constitutive law that holds across environments. Specifically, we expect the stiffness distribution to exhibit either an ascending-descending or an ascending-plateau pattern along the length of the object, from head to tail. Furthermore, undulatory locomotion showed sensitivity to contact mechanics: solid-solid or solid-viscoelastic contact produced different locomotion kinematics. Our results elucidate how terrestrial limbless animals achieve undulatory locomotion performance by exploiting the passive properties of the environment and the body. Application of the results obtained may lead to better performing long-segmented robots that exploit the suitability of passive body dynamics and the properties of the environment in which they need to move.

Influence of Positioning of Shear Wall on the Torsional Response of Building

Abstract: It would be ideal if all buildings have their lateral-load resisting elements symmetrically arranged and earthquake ground motions would strike in known directions. Due to scarcity of land in big cities, architects often propose irregular buildings in order to utilize maximum available land area and to provide adequate ventilation and light in various building components. However, it is quite often that structural irregularity is the result of a combination of both types. Most buildings have some degree of irregularity in the geometric configuration or the distribution of mass, stiffness, and/or strength. Due to one or more of these asymmetries, thestructure’s lateral resistance to the ground motion is usually torsionally unbalanced creating large displacement amplifications and high force concentrations within the resisting elements which can cause severe damages and at times collapse of the structure. Eccentric arrangement of non-structural components, asymmetric yielding, presence of rotational component in ground motions and the variations in the input energy imparted by the ground motions also contribute significantly to the torsional response of buildings. So, this research work demonstrates the importance oflocation of shear wall is should be checked before the sequential failure defined by the Response spectrum analysis method. In Numerical Tool like SAP-2000 which are uses worldwide for pushover analysis method.

Tortional Effect in Vertically Mass Irregular RC Frame Structure under Response Spectrum Analysis

Abstract: The structures having discontinuity like distribution of mass, stiffness and geometry of the structure are termed as Irregular structures. Irregular structures contribute a large portion of urban infrastructure. The past earthquakes have shown catastrophic effect on the buildings with irregularities. It was seen that the buildings with irregularities are more prone to earthquake then the regular building. In the present study a special case of vertical mass irregularity are discussed. Six different models including three basic cases of buildings models i.e. vertically regular structure, vertically irregular structure and vertically irregular structure with shear wall at different position are taken for analysis by Response spectrum. It has observed that, the torsion, base reaction, displacement and modal load participation ratio in structure. This study exhibits effect of mass variation aspect of RCC structure

Experimental study of moment redistribution before yielding in precast prestressed concrete beams made continuous

AbstractThe bending stiffness distribution of beams composed of simple‐span precast prestressed beams, which are made continuous by deck reinforcement at intermediate supports, is not constant. As the structure undergoes loading, the deck slab at the intermediate support cracks earlier than the prestressed span soffit and due to that the bending stiffness distribution changes along the beam length. The effect of nonconstant bending stiffness distribution on the moment redistribution of the structure before the yielding is studied experimentally and analytically. Two continuous beams with spans of 10 m + 10 m were manufactured, loaded to failure, and studied. It was concluded that the studied structure undergoes considerable moment redistribution before yielding although it would be designed for zero redistribution at the ultimate limit state. If this elastic redistribution is neglected in the structural analysis, the service limit state sagging moment at midspan may end up on the unconservative side. Elastic redistribution can be predicted quite accurately with the help of nonlinear analysis or roughly with simplified idealization as proposed in this article.

A Three-Dimensional Digital Model for Early Diagnosis of Hepatic Fibrosis Based on Magnetic Resonance Elastography.

Hepatic fibrosis is an early stage of liver cirrhosis, and there are no better non-invasive and convenient methods for the detection and evaluation of the disease. Despite the good progress made with the liver stiffness map (LSM) based on magnetic resonance elastography (MRE), there are still some limitations that need to be overcome, including manual focus determination, manual selection of regions of interest (ROIs), and discontinuous LSM data without structural information, which makes it impossible to evaluate the liver as a whole. In this study, we propose a novel three-dimensional (3D) digital model for the early diagnosis of hepatic fibrosis based on MRE. MRE is a non-invasive imaging technique that employs magnetic resonance imaging (MRI) to measure the liver stiffness at the scanning site through human-computer interaction. Studies have indicated a significant positive correlation between the LSM obtained through MRE and the degree of hepatic fibrosis. However, for clinical purposes, a comprehensive and precise quantification of the degree of hepatic fibrosis is necessary. To address this, the concept of Liver Stiffness Distribution (LSD) was proposed in this study, which refers to the 3D stiffness volume of each liver voxel obtained by the alignment of 3D liver tissue images and MRE indicators. This provides a more effective clinical tool for the diagnosis and treatment of hepatic fibrosis.

Seismic performance of bolted T-stub Beam-to-Box column connection

Using internal continuity plates in box columns due to accessibility problems and complicated detailing of connection was perceived as fraught with shortcomings and limitations. To overcome this deficiency, the effectiveness of external continuity plates in conjugation with T-stub connectors in I-beam to box-column connections under cyclic action is investigated in this study. Three damage control strategies based on the fuse action of T-stubs (T-F), strengthening the connections (T-S), and using reduced beam section (RBS) connections (T-R) are pursued and are compared to a corresponding end-plate connection (E). Nonlinear finite element analyses are conducted using the ABAQUS/STANDARD software on the connections under the qualification cyclic loading conditions to elucidate all the main seismic behavioral aspects, including the strength, initial stiffness, energy dissipation, ductility, von-Mises stress, and equivalent plastic strain distributions of models. First, the accuracy of numerical models is validated by a series of available experimental results. Then, a parametric study is performed on the effects of the moment ratio of the beam to column, the number of continuity plates, stiffeners embedded on the beam, and using high strength materials on the seismic performance of the connections. From the results, it is found that the T-S model was the most efficient among the different connections studied, supplying the stiffness, strength, energy absorption, and ductility increases by 2%, 21%, −17%, and 22%, respectively with respect to the E model, satisfying the requirements of AISC Seismic Provisions for SMF. Increasing the beam-to-column moment ratio and the number of continuity plates, had limited effects by less than 10% on the seismic performance of the connections. In the absence of continuity plates, the connections exhibited worse inelastic seismic performance with decreases in stiffness, strength, and energy dissipation up to 67%, 44%, and 74%, respectively. The T-F connection gained a 13% strength increase and satisfied the AISC requirements when high-strength T-stubs are used. Based on the numerical observations, design strength and rotational stiffness of the proposed connections on the component-based joint design strategies, are finally outlined.

Bridge distributed stiffness identification of continuous beam bridge based on microwave interferometric radar technology and rotation influence line

Continuous beam bridges are a widely used and prevalent type of bridge. As material properties degrade over time, the bridge stiffness decreases, leading to a reduction in bearing capacity. Accurate identification of bridge stiffness is crucial for safe operation, but there are few studies on identifying the stiffness of continuous beam bridges. This paper proposes a novel method to identify regional stiffness distribution using microwave interference radar technology and rotation influence lines. Firstly, the regionally distributed multi-point displacement influence lines (DILs) are procured via an innovatively designed lightweight radar device and the multi-point rotation influence lines (RILs) can be subsequently derived through a difference method. Utilizing the derived multi-point RILs, the continuous beam is decoupled into multiple-span simple beams featuring additional moment couples. Then the moment envelope area (MEA) can be computed by integrating the superposition principle and the curvature envelope area (CEA) can be determined using multi-point DILs. Finally, the bridge distributed stiffness can be obtained by combining the MEA and CEA of the monitored area. To verify the validity of the proposed method, a laboratory experiment involving a continuous beam is conducted to identify its’ bridge distributed stiffness (BDS).

VERTICAL IRREGULAR RC BUILDING SEISMIC ANALYSIS WITH STIFFNESS & SETBACK

The distribution of stiffness, strength, and mass in both vertical and horizontal orientations affects how well a high rise structure performs during powerful earthquake events. A building is considered irregular if there is a difference in stiffness, strength, and mass between the adjoining storeys. The performance and behavior of regular and vertical irregular G+10 reinforced concrete (RC) buildings under seismic loading are the main topics of the current study. In this study, stiffness and setback, two types of vertical abnormalities, are taken into consideration. Response spectrum analysis (RSA) is used to conduct seismic analysis on a total of eight normal and irregular structures. It is possible to get several earthquake reactions, including storey displacement, storey drift, overturning moment, storey shear force, and storey stiffness. These replies have been used to compare the differences between regular and irregular structures. The outcome suggests that a stiff building construction with uneven setbacks causes instability under earthquake stress. In RC construction, a proportional quantity of stiffness is advantageous to regulate the instability. Keyword: Setback, Vertical irregularity, Storey displacement, Storey drifts, Response spectrum analysis.

Genetic Algorithm Optimization of Beams in Terms of Maximizing Gaps between Adjacent Frequencies.

The aim of this paper is to optimize the thickness variation function of simply supported and cantilever beams, in terms of maximizing gaps between chosen neighboring frequencies, and to analyze the obtained results. The optimization results are examined in terms of achieving the objective function (related to eigenvalue problems), but also in terms of their dynamic stiffness (forced vibrations excited by a point harmonic load). In the optimization process, a genetic algorithm was used. Problems related to structural dynamics were solved by FEM implementation into the algorithm. Sample results were presented, and the developed algorithm was analyzed in terms of the results convergence by examining several variable parameters. The authors demonstrated the validity of applying the described optimization tool to the presented problems. Conclusions were drawn regarding the correlation between stiffness and mass distribution in the optimized beams and the natural frequency modes in terms of which they were optimized.