Load Time History Research Articles

An improved plasma generation optimization algorithm for optimal design of reinforced concrete frames under time-history loading

Plasma Generation Optimization (PGO) is a newly developed meta-heuristic algorithm that is inspired by the process of plasma generation. This study attempts to enhance the performance of the PGO in order to improve solution accuracy, reliability, and convergence speed. The new method, called improved plasma generation optimization (IPGO), is tested on a benchmark structural optimization problem. Then, the optimal design of reinforced concrete (RC) frames is performed using the PGO and IPGO algorithms. The variables of the problem are taken as the geometry of the cross-sections and reinforcing bars of the members. These variables are considered discrete and are selected from a predetermined section database (DB). The objective function consists of the construction material costs of the structural elements. The plannar frames with different stories are designed according to the standards and requirements of the American Concrete Institute’s Building Code (ACI 318-8). Time history analysis is used to analyze the structures. The main goal of the optimization is to find the most economical design of the frame structure under strong seismic actions. The results show that the proposed approach is feasible to find the optimum design of the structural members of RC frames and the IPGO algorithm is more competent than the PGO algorithm.

Measurement of loads acting on a hydropower unit during stationary and transient operations

Load measurements on hydropower turbines and generators are usually expensive and troublesome due to the need of sensor installation on the rotating parts. Often a longer stop of the unit is required when the sensors are to be installed or removed from the turbine in the waterway, which results in production disturbances.This article presents a methodology and results for estimation of loads acting on turbines and generators during operation without installation of sensors on the turbine or in the generator. The measured quantities are shaft displacement in the bearings and the bending moment on the shaft between the turbine guide bearing and the lower generator guide bearing.To enable calculation of the loads acting on the turbine and the generator, it is necessary to have as many measurement points as the number of unknown loads acting on the shaft train. By using the displacement in the three radial bearings, six of the eight unknown forces can be determined. The remaining two unknown loads can be calculated by measuring the bending moment in the shaft connecting the generator to the turbine. In journal bearings the stiffness is a non-linear function of eccentricity, which means that the obtained stiffness function must be integrated to achieve the force as a function of the shaft eccentricity.By applying the measured time history of displacement and moment on the numerical model, the time history for the unknown loads can be solved. The time history of the external loads is then converted into load spectra.

Finite Element Analysis of Impact-Induced Damage in Pressurized Hybrid Composites Pipes

The high mechanical performance of filament wound hybrid composite pipes can be adversely affected by their low resistance to accidental impact. Loads of dynamic origin are dangerous and cause consequences on the operation of pipes because the damage is often not detected and can affect the structural integrity of composite pipes. In this work, a finite element (FE) model of pressurized hybrid composite pipe is developed and performed to predict the damage initiation and evolution under low-velocity impact through simulations with ABAQUS/Explicit. Hashin’s failure theory is used as failure criterion. At the first stage, load–time histories, and impactor displacement time limits are estimated in an FE model. The numerical results are confronted with the experimental values published in the literature. Once the model was validated, damage initiation and propagation were analyzed, where it is observed that damage mainly occurs by matrix cracking, damage evolution in the matrix in tension, compression and shear are presented and discussed in detail.

An investigation of impact resistance capacity of polypropylene (PP) added plasterboard subjected to soft-body impact

Plasterboard is one of the most-commonly used construction materials because of its low cost and easy installation characteristics. Although the low strength and fragility make plasterboard functioned as non-load-bearing components. Plasterboard walls are always required to satisfy impact resistance against impact from accidental body strike, hard-body impact from wheelchair, etc. during its usage. This study investigates the impact resistance capacity of plasterboards being subjected to soft-body impact load. Laboratory sandbag impact tests are conducted to examine the responses of plasterboard and PP fibre strengthened plasterboard system at different velocities. Detailed numerical models of plasterboards are also generated to assist the analysis. Different damage and failure modes are observed on the plasterboards when subjected to impactor strike at different velocities. It is found that the coupled deformation of plasterboard and sandbag leads to different impact load time histories from sandbag soft impact, which results in the different failure modes. The PP fibre strengthened board exhibits better impact resistance than conventional plasterboard. Parametric study is then conducted to quantify the peak central deflections under different strength and thickness variances on plasterboard. An empirical formula is then derived based on the parametric results for preliminary assessment of the plasterboard impact resistance capacity.

A Framework for Vibration Attenuation in Traffic Mast Arm Structures under Wind Loads

Traffic signals and information signs play a vivid role in maintaining safe travel by guiding drivers on highways and urban roads. Traffic mast arm structures are susceptible to vibrations induced by wind loads. Fatigue-induced failure is common in these structures. However, it is crucial to ascertain the functionality of signal support structures. This paper lays the foundation for a fully computational framework to model and mitigate wind-induced vibrations in traffic lighting support structures by conducting computational fluid dynamics (CFD) simulations, dynamic modeling, and damping enhancement. We validated the CFD simulations by aerodynamic testing. The dependence of flow pattern and aerodynamic loads on Reynolds number reveals the importance of full-scale CFD with large eddy simulation for wind load estimation on mast arm structures. Distributed tuned mass dampers were created by employing available weights of lighting boxes. The results show that distributed tuned lighting boxes are effective for vibration suppression. Besides, damping enhancement can significantly reduce vibration-induced stresses and hence extend the fatigue life. The proposed mitigation technique promises to save construction costs and improve the safety of the traveling public. The procedure followed for creating time histories of wind loads integrated with finite element modeling applies to other vibration lessening techniques for potential inclusion in the AASHTO standard.

An Automatic Program of Generation of Equation of Motion and Dynamic Analysis for Multi-body Mechanical System using GNU Octave

Multi-body dynamics is used to calculate the physical quantities required for component design, such as calculating the dynamic response of mechanical components and the time history of dynamic loads. Advances in analysis software, including DADS, ADAMS, RecurDyn, and DAFUL, have made it possible to easily calculate dynamic responses by defining relationships between components and operating environments from 3D modeling on user-created components. However, when the understating of dynamic analysis is lacking, it is difficult to apply multi-body dynamics analysis in the design process, and it is difficult to analyze the acquired response data. In this study, we developed an automatic code to derive equations of motion in the matrix format and calculate dynamic responses of multi-body systems using GNU Octave, a free high level language. In particular, the process of defining matrices and vectors such as inertia matrix, stiffness matrix, and external force vector concerning the degrees of freedom of components by using Euler-Lagrange equations is shown to understand the structure and process of dynamic analysis. The code application by explaining how to use the code in a different mechanical system is also shown to help understand the usage method for who wants to study Multi-body dynamics.

Synthesis of landing dynamics on land-base high performance aircraft considering multi-variate landing conditions

Consideration of standard landing cases from technical guideline document is the traditional design practice followed to ascertain aircraft landing loads. A rationale approach on landing load generation and its impact on flexible airframe dynamic response is explored, considering multi-variate landing conditions for the first time on land-base high performance aircraft. Methodology on nonlinear landing dynamic response simulation is developed by amalgamating inputs such as nonlinear nose and main landing gear dynamic models, aircraft rigid body dynamics, low speed aerodynamic and thrust forces. True representation of landing gear nonlinear oleo-pneumatic strut and tire characteristics, variable geometric strut structural stiffness and nonlinear spin-up friction resulted in satisfactory correlation of vertical and longitudinal dynamic response simulation with experimental drop test results. The adequacy of lateral dynamic model is established from the compatibility condition of landing gear and tire dynamics. A down selection methodology is worked out considering total probability and first order sensitive multi-variate landing parameters such as pitch, roll and vertical descent speed, taking into account the plausible statistical variation. Transient dynamic response simulation is then performed for the critical landing cases, on a flexible airframe finite element model by inputting landing load time history as base excitation motion. The dynamic amplification of acceleration response on flexible fuselage nose location from the simulation, is verified with flight data accelerometer frequency response. The application of flexible dynamic responses in ensuring the structural integrity of airframe is elaborated. The proposed methodology on landing load generation is pragmatic, in comparison with the traditional design practice and provides insight during early design phase thereby achieving landing gear subsystem development in-unison with airframe structural clearance aspects.

Investigation of impact behavior of reinforced concrete beam to column connection strengthened with carbon fiber‐reinforced polymer strips

AbstractReinforced concrete (RC) members may expose to impulsive dynamic loads due to the reasons such as the explosions occurring in the interior or exterior part of them, rockfall, the vehicle crash to the bridges, the collision of masses with the effects of floods and landslide. Many studies have investigated the effects of impulsive dynamic loads on the beam, column, and slab RC structural elements have been investigated in the literature. However, the authors have not encountered any study focused on the impact behavior of beam to column connections of the frames constructing the bearing system of reinforced concrete structures. Therefore, an experimental study has been planned to investigate RC beam impact behavior to column connections strengthened with carbon fiber‐reinforced polymer (CFRP) strips. The concrete compressive strength, shear reinforcement spacing, CFRP strip spacing, and input impact energy applied to test specimens were taken as experimental variables. The time histories of impact load acting on test specimens, accelerations, displacements, and the strains measured from CFRP strips have been recorded in experiments. The experimental variables' effect on dynamic responses of RC beam to column connections strengthened with CFRP strips subjected to impact load has been interpreted in detail. The study's scope and improved numerical analysis procedure have also been introduced to verify experimental results. Good agreement between numerical and experimental results demonstrated that the presented numerical procedures could be safely used for evaluation of impact behavior of RC beam to column connections strengthened with CFRP strips.

Parametric reduced order models for output-only vibration-based crack detection in shell structures

In this work parametric reduced order models (pROMs) for cracked shells are developed and applied to crack detection problems. Mesh morphing is employed to allow for parameterization of the models with respect to the crack location and size, while a clustering approach is adopted to partition the parameter space in sub-domains, for which efficient local reduced order models (ROMs) are constructed. Subsequently, the proposed ROMs are employed as forward simulators within an inverse problem setting, aiming to identify the location of the fault (crack). An output-only scheme is adopted, in the absence of information regarding the loading time history, based on transmissibility functions. The resulting scheme is tested through numerical examples involving realistic geometries, consisting of curved shells or multiple components.

Component-based fragility analysis of transmission towers subjected to hurricane wind load

The fragility model of transmission towers is one of the key elements for the hurricane risk and resilience assessment of regional power utility infrastructure. The current methods for fragility curve assessment of transmission towers rely on either static analysis or dynamic analysis with simplified demand parameters and limit state functions. The details of tower connections, such as joint eccentricities, rotational stiffness and slippage are often overlooked in fragility analyses due to the associated complexity of modeling and large computational effort. The purpose of this study is to model the realistic performance of transmission towers under hurricanes and develop a fragility curve estimation framework balancing between accuracy and efficiency.The correlated multivariate wind load time histories on the tower are simulated, and the load transferred from the cables to the cross-arms of the tower are generated using an effective analytical model. Two types of three dimensional finite element models of transmission towers are developed in the OpenSees platform. One is a complex model with all the joint details, which requires a very large computational effort to conduct nonlinear dynamic analyses, and the other one is a simple model without all the joint details, which is more computationally efficient. A small-scale dynamic probabilistic simulation with the two models is conducted, and a method to replace the complex model with the simple model is developed and validated. Using a high-performance computing cluster, medium-scale nonlinear dynamic time history analyses are carried out to obtain the demands of individual structural members and their correlation. Component-level failures are obtained via large-scale numerical simulations and are linked to global collapse of the tower through element removal and progressive collapse analyses. A case study of two transmission towers under hurricane is carried out. The results indicate that, compared to a traditional tip displacement method, the proposed component-based method with detail driven finite element model and large-scale simulations leads to a more accurate assessment of the transmission tower fragility curve.

Dynamic performance of retrofitted steel beam-column connections subjected to impact loadings

In this paper, three steel beam-column connections (welded flange-bolted web (WFB) connection, welded flange plate (WFP) connection and reduced beam section (RBS) connection) were experimentally examined under impact loadings. The impact tests using drop-hammer were carried out to simulate the dynamic behaviour of structural progressive collapse. The influence of strengthening or reducing methods on the impact resistance of steel frame substructure was discussed. The failure modes and the dynamic responses of the impact load and deformation time histories as well as the dynamic rotation and energy dissipation of the substructures were investigated. Test results showed that the upper flange of all specimens experienced local buckling near the plastic hinge section, while no welding seam cracked. For retrofitted connections, the out-of-plane and distortional deformation of steel beam flange and web at the plastic hinge section were measured. All the three connections with weld access holes recommended by American Federal Emergency Management Agency (FEMA) had good rotation abilities and impact loading resistances, and their ultimate rotations were not less than the FEMA350 design limits. The ductility and energy dissipation of two retrofitted beam-column connections were improved, which were better than those of un-retrofitted connections. The finite element analysis (FEA) models of steel beam-column connections were also established using ABAQUS software. The internal force development of connections during the impact loading was examined and the results showed that the strengthening or reducing methods had a profound influence on the formation of catenary action of beam-column connections under the dynamic progressive collapse scenario.

The effect of carbon nanotubes modified polyurethane adhesive on the impact behavior of sandwich structures

AbstractSandwich panels with aluminum honeycomb cores have emerged as an essential class of lightweight building materials in a wide variety of structural engineering areas, including marine, railway, aerospace, and automotive structures. Although sandwich structures offer remarkable mechanical properties, for instance, high specific strength and stiffness, these structures are open to the damage created by the low‐velocity impact. This study investigates the dynamic behavior of aluminum honeycomb sandwich panels with different core cell sizes based on neat and multi‐walled carbon nanotube (MWCNT) reinforced polyurethane (PU) adhesives that are aimed. The enhancement in impact performance of honeycomb sandwich structures obtained by the inclusion of MWCNTs in the PU adhesive is investigated. The effect of the MWCNT reinforcements on square sandwich panels at 50 J energy level is examined using an instrumented drop tower test setup. After acquiring load–time histories, the numerical integration method is implemented to obtain load–deflection and energy–time curves. Also, energy balance modeling (EBM) is utilized to characterize the effectualness of MWCNT reinforcement to PU adhesive on the impact behavior of sandwich panels. Theoretical EBM is confirmed that absorbed energy in the shear and bending deflections increased as the percentage of MWCNT‐added PU adhesive increases. Understanding the damage event implicated in absorbing and dissipating the impact energy is determined and compared for panels with different cell sizes, including nanoparticles, using the macroscopic optical images and microscopic scanning electron microscopy images of sandwich panels.

An accurate approach for obtaining spatiotemporal information of vehicle loads on bridges based on 3D bounding box reconstruction with computer vision

Vehicle load is an important basis for bridge design, condition assessment, and maintenance and strengthening, and its statistical laws can help us further understand the behavior of bridges under vehicle loads. Therefore, it is important to obtain accurate vehicle weight and vehicle spatiotemporal information that reflects the load time history. As of now, vehicle weight can be obtained using WIM technology, but there are still some issues in the methods of obtaining vehicle information. The coordinate transformation method is simple to operate, but sacrifices accuracy. The existence of a certain distance from the tail of vehicles to bridge decks makes the results based on dual-target detection differ from the actual. Therefore, an accurate approach for obtaining spatiotemporal information of vehicle loads on bridges based on 3D bounding box reconstruction with computer vision is proposed in this paper. To achieve this, a deep convolutional neural network (DCNN) and the You Only Look Once (YOLO) detector are used to detect vehicles and get the 2D bounding box. By establishing the relationship between 2D and 3D bounding box of the vehicle, an algorithm for 3D bounding box reconstruction of vehicles is proposed to get the sizes and position of vehicles. The spatiotemporal information of the vehicle loads is finally obtained by using multiple objects tracking (MOT). To verify the accuracy and reliability of the proposed approach, a bridge vehicle loads identification system (BVLIS) was developed and tested on a cable-stayed bridge in operation. The results show that the approach is accurate and reliable, and can be used to obtain vehicle information and provide vehicle load boundary conditions for bridge finite element modeling.

Fatigue life estimation of structures under statistically and spectrally similar variable amplitude loading

A new fatigue life prediction framework provides an improved life prediction under statistically and spectrally similar irregular variable-amplitude loading for a notched beam model. It enables the cumulative damage rule to account for the load sequence effects by modifying the probability density function of the stress-amplitude history through (1) identification of overloads based on the rainflow-counting algorithm; (2) analytical characterization of the overload retardation effects; and (3) correction to the damage rule using overload amplitude rate characterization. The fatigue lives estimated from experimentally acquired and synthetically generated load-time histories are compared to the ones generated from simulations that qualitatively reproduce the fatigue lives in physical experiments. The notable improvement in prediction accuracy outperforms the Palmgren–Miner’s rule and power-spectrum-based life estimation. The demonstrated application to the field acceleration data substantiates its use for in-service structural health monitoring and damage prognosis. This framework does not require a priory knowledge of the applied load, and it can be applied to other engineered structures with known structural and defect properties.

A time-domain stepwise fatigue assessment to bridge small-scale fracture mechanics with large-scale system dynamics for high-speed maglev lightweight bogies

Maglev trains with an operation speed higher than 600 km/h show a great advantage during the next generation ground transportation. To tackle the structural integrity of maglev bogies made of forged 6082-T6 aluminum alloys, a time-domain stepwise fatigue assessment (TSFA) approach was proposed to consider complex dynamic fatigue loads. Firstly, a vehicle-bridge coupled maglev vehicle dynamic model was established to obtain the time history of service-simulation fatigue loads. Secondly, these variable-amplitude fatigue spectra were introduced to examine the damage accumulation lifetime of the maglev bogie using classical nominal stress method, the swing bar of high-speed maglev bogies was then identified as the safety critical component. To further enhance the service safety and reliability, the remaining life of the swing bar was investigated using damage tolerance method in terms of a real fatigue failure accident from Shanghai maglev trains. Calculation results indicate that the remaining life is sharply reduced from 34.1 years to 19.7 years when an initial crack length of 3.0 mm is generated at the swing bar. More conservative but practical fatigue life can be acquired when using the new TSFA framework instead of individual damage tolerance or safe-life method.

Fatigue Simulation of Railway Car Bogie Frame Based on Multi-body Dynamics and Finite Element Analysis

This paper proposes to combine multi-body dynamics and finite element method to study the fatigue strength and fatigue life of the bogie frame from the perspective of taking the vehicle as a whole. The analysis model established by the multi-body dynamics simulation software SIMPACK, the dangerous positions are determined from FEA software ANSYS, Based on the load-time history and Miner cumulative damage theory, the fatigue strength prediction is carried out using n-soft software, the study shows that the time-load history of each key position obtained by simulating working conditions can predict the life of components more accurately, this method can be applied to the initial stage of product design.

Methods to test and generate the wear load spectrum of journal bearing in concrete pump

Load spectrum is the basis of wear analysis and life prediction in journal bearing. This paper proposed a method to generate the wear load spectrum from the measured loads that avoiding the errors introduced by the assumptions of traditional constant load. An axle pin load sensor that can recognize the load value and its orientation is developed to test the original wear loads of journal bearing in a concrete pump under working conditions. The interactive influence of perpendicular components in the measured loads is involved by introducing two effect coefficients. Subsequently, the amplitude distribution of the wear load and its lasting time are extracted from the measured complex load-time history using the rain-flow counting method and Weibull distribution. The wear load spectrum is generated by dividing the continuous load into eight load steps. The validity of the approach is supported by finite element analysis on the wear simulation of journal bearing using the generated load spectrum and the measured load. In addition, the effects of load types and load sequence on the wear are discussed based on the proposed method and the FE model. This study provides guidelines to efficiently generate the wear load spectrum for journal bearing, and enhances its reliability of wear prediction in the experimental tests and theoretical analysis.

Mechanical stimulation induces rapid fibroblast proliferation and accelerates the early maturation of human skin substitutes

The clinical treatment of large, full-thickness skin injuries with tissue-engineered autologous dermo–epidermal skin substitutes is an emerging alternative to split-thickness skin grafting. However, their production requires about one month of in vitro cell and tissue culture, which is a significant drawback for the treatment of patients with severe skin defects. With the aim to reduce the production time, we developed a new dynamic bioreactor setup that applies cyclic biaxial tension to collagen hydrogels for skin tissue engineering. By reliably controlling the time history of mechanical loading, the dynamic culturing results in a three-fold increase in collagen hydrogel stiffness and stimulates the embedded fibroblasts to enter the cell cycle. As a result, the number of fibroblasts is increased by 75% compared to under corresponding static culturing. Enhanced fibroblast proliferation promotes expression of dermal extracellular matrix proteins, keratinocyte proliferation, and the early establishment of the epidermis. The time required for early tissue maturation can therefore be reduced by one week. Analysis of the separate effects of cyclic loading, matrix stiffening, and interstitial fluid flow indicates that cyclic deformation is the dominant biophysical factor determining fibroblast proliferation, while tissue stiffening plays a lesser role. Local differences in the direction of deformation (in-plane equibiaxial vs. uniaxial strain) influence fibroblast orientation but not proliferation, nor the resulting tissue properties. Importantly, dynamic culturing does not activate fibroblast differentiation into myofibroblasts. The present work demonstrates that control of mechanobiological cues can be very effective in driving cell response toward a shorter production time for human skin substitutes.

Performance of offshore concrete platforms subjected to internal explosion after gas leak

The paper describes the assessment of existing concrete structures that support large oil platforms in the North Sea when subjected to internal explosion loads. Internal explosion loads are the result of a gas leak from the pipes inside of a hollow concrete column (shafts) supporting large concrete gravity-based oil platforms. Explosion at base, mid-height and close to the deck support of the column were studied following a gas leak at different zones of pipe networks in the shafts. Explosion models generated pressure–time curves for predicting the structural response. The structural response of the shaft during an explosion is dependent on the interaction of fluid and structure. Explosion models generated dynamic pressure–time history loading to be used in structural safety assessments. Transient dynamic analysis was carried out to assess the structural response and strength assessment was carried out using design codes. As the original designs in the past for most of the existing offshore installations did not include the ‘gas leak’ scenario, the study suggested that a suitable risk assessment for existing installations carrying utilities in the column is carried out to ensure that the structural stability is not compromised.

Assessment of service loads and material influence on the lifetime of thick-walled nodular cast iron components

To reduce weight and increase power as well as utilisation of nodular cast iron components, e.g. for wind turbines and heavy industry parts, locally higher strength need to be withstood by the material. This becomes crucial when additional overloads, coming from a storm, a grid loss during operation or a misuse, influence the structure of thick-walled components, causing high local elastic-plastic deformations. In this case, the cyclic, elastic-plastic material behaviour and its development under cyclic loading are important and have to be considered during component design.To assess the material’s local elastic-plastic material behaviour under the influence of overloads, strain-controlled fatigue tests, under constant and variable amplitude loading, were performed under alternating loading, R = −1, with unnotched specimens removed from cast blocks, a main frame and two planet carriers of wind turbines made from EN-GJS-400-18U-LT, EN-GJS-700-2, ADI-800 and ADI-900.By using two different load-time histories with a bulky and a more linear form, derived from local hot spots at a wind turbine’s hub and a main frame, in a first step, the lightweighting potential could be determined by comparing the S-N curves for constant and variable amplitude loading. In order to analyse the influence of an additional overload during usage, in additional test series, a maximum compressive overload of 0.5 and 1.0% total strain under tensile loading was included in the load sequence and the results compared with those without an overload.