High Slenderness Research Articles

A NURBS-based isogeometric formulation for geometrically nonlinear analysis of shells

Due to their high slenderness, shell structures are vulnerable to collapse caused by loss of stability, hence nonlinear analysis is crucial to ensure a safe design. The advantage of isogeometric analysis (IGA) to exactly describe the geometry of the problem independently of the degree of discretization and allow easy refinements has led to its increasing application in shell analysis. However, IGA does not remove the shear and membrane locking presented by fully integrated shell elements based on Reissner-Mindlin's theory. Thus, several alternatives have been proposed in the literature to solve the locking problem, such as mixed formulation and reduced integration. This work aims to study the effectiveness of different reduced integration schemes to eliminate or alleviate locking in the context of stability and geometrically nonlinear analysis of Reissner-Mindlin shells based on the degenerated solid approach. The proposed formulation is applied to the stability analysis of plates and shells, where critical loads and nonlinear equilibrium paths are evaluated and compared to solutions available in the literature or obtained by the Finite Element Method. Excellent results were obtained, demonstrating that a very accurate and efficient approach for nonlinear analysis of plates and shells can be achieved by using high regularity basis functions obtained by k-refinement and an appropriate reduced integration scheme. This approach not only avoids locking but also reduces the computational cost of nonlinear analysis of shells.

Finite prism approach for stability analysis of laminated glass fins

The use of laminated glass (LG) as a structural material for load-bearing components within buildings has been growing over the last decades. LG can be obtained by bonding two or more pieces of glass by means of polymeric interlayers (e.g., poly vinyl butyral – PVB). This building process gives LG elements a huge advantage over the same ones made of fully monolithic glass with respect to safety, since after breakage the fragments usually remain attached to the interlayer, reducing the risk of injuries. As LG members usually present a high slenderness, they are susceptible to flexural buckling under compression, requiring specific calculation methods and verification procedures in order to guarantee appropriate and safe structural performance. Compared to literature, this work presents a novel approach that is focused on the prediction of the elastic critical buckling load of LG columns without any lateral restraints at the tensioned edges. The buckling loads of LG columns have been determined using Finite Element Method, generally using shell elements, or analytical models. Usually, such approaches make use of the concept of effective thickness, in which the thickness of a monolithic element with equivalent bending properties in terms of stress and deflection is employed. However, the effective thickness approach is either difficult to apply or inaccurate. Therefore, this work presents an efficient and accurate approach to evaluate elastic critical buckling loads of LG columns applying Finite Prism Method (FPM). The accuracy of the proposed approach is assessed comparing the results obtained using solid finite element and analytical models for fully monolithic glass columns, as well as LG columns composed by multiple glass layers.

A new uniaxial material model for the simulation of lateral buckling of steel rebars

Lateral buckling of longitudinal rebars is a crucial phenomenon in the response of reinforced concrete structures since it often anticipates crushing of the core concrete and triggers failure of the member. Owing to this, lateral buckling has been simulated in some uniaxial steel models available in the literature. However, most of these models concentrate their novelties on the compression backbone curve of rebars characterised by low-to-medium slenderness ratios and inaccurately simulate the response of bars characterised by high slenderness ratio or the transition from the compression backbone curve to tension. Moreover, all these models neglect the effects of the concrete cover on the lateral buckling of the rebar. This paper presents a new uniaxial material model of reinforcing steel bars with the aim of achieving a more accurate evaluation of the response of rebars including the effects of lateral buckling. The proposed model is calibrated on the results of numerical analyses conducted on refined models of steel rebars characterised by low-to-high slenderness ratios. The proposed model incorporates the possibility to simulate the behaviour of bare and embedded rebars. The response is path-dependent and described by means of smooth curves so as to guarantee numerical stability in nonlinear finite element simulations. The accuracy of the model has been validated by comparison with the response of laboratory tests performed by other researchers on steel rebars. Finally, applications are shown on six reinforced concrete columns in order to highlight the effects of the simulation of buckling of the steel rebars on the global and local responses.

Large-stroke snap-through instability in the axial direction of a bi-stable structure with high slenderness

Mechanical snap-through instability of bi-stable structures may find many practical applications such as state switching and energy transforming. Although there exist diverse bi-stable structures capable of snap-through instability, it is still difficult for a structure with high slenderness to undergo the axial snap-through instability with a large stroke. Here, an elastic structure with high slenderness is simply constructed by a finite number of identical, conventional bi-stable units with relatively low slenderness in series connection. For realizing the axial snap-through instability with a large stroke, common scissors mechanisms are further introduced as rigid constraints to guarantee the synchronous snap-through instability of these bi-stable units. The global feature of the large-stroke snap-through instability realized here is robust and even insusceptible to the local out-of-synchronization of individual units. The present design provides a simple and feasible way to achieve the large-stroke snap-through instability of slender structures, which is expected to be particularly useful for state switching and energy transforming in narrow spaces.

A unified confinement-based direct design approach for novel double-skin composite columns

Concrete-filled stiffened steel tubular short columns, with cold-formed stiffened steel tubes for the outer section, have been widely used in Asia in large commercial facilities and warehouses. This paper focuses on the response of concrete-filled double-skin stiffened steel tubular (CFDSST) short columns and concrete-filled dual stiffened steel tubular (CFDST) short columns, with either square or circular inner tubes. A detailed database of existing information is assembled and presented, as well as a thorough analysis of existing design expressions. Additional examination of the concrete confinement has been undertaken which highlight the differences in behaviour between CFDSST and CFDST cross-sections. The major aim in this work is to provide a unified design approach, which accounts for concrete confinement and accounts for the material and geometric properties of the section, and offers improved accuracy over the existing approaches given in the European, American, British and Chinese design codes. The newly proposed unified confinement-based direct design approach for axially-loaded CFDSST and CFDST short columns is discussed, including the consideration given to the lateral confining pressure provided by the steel tubes. This method is shown to provide more accurate depictions of the load resistances of CFDSST and CFDST short columns relative to the international specifications, with lower scatter and greater range of applicability. It has also been shown that the CFDSST sections with a stiffened outer steel tube provides greater confinement and additional strength compared with equivalent unstiffened circular tubes of high relative tube slenderness. Given the easier fabrication and installation of beam-to-square column joints compared with circular columns, this is likely to extend their range of application in the future.

Experimental Study and Design Method of Cold-Formed Thin-Walled Steel Unequal-Leg Angles under Axial Compression

An experimental study of cold-formed thin-walled steel unequal-leg angles (CFTWS-ULAs) under axially oriented pressure is presented in this paper. Firstly, the initial imperfections and material properties of the angle specimens were measured in detail. The angle specimens were tested under fixed-ended conditions. The results of the experiments showed that the angle specimens with small slenderness ratios were susceptible to local buckling, the angle specimens whose legs had high slenderness ratios and low width–thickness ratios were found to easily suffer from the occurrence of flexural buckling, and the angle specimens whose legs had high width–thickness ratios were found to easily suffer from the occurrence of interactive buckling between local buckling and flexural buckling. The finite element analysis of the ULAs was conducted using ABAQUS6.14 finite element software by creating a model. The buckling modes and ultimate bearing capacities of the test specimens were compared, and the finite element analysis verified that the established model built using the finite element is credible and subsequent parametric analysis was performed. The slenderness ratio had the most significant impact on the ultimate bearing capacities of the unequal-leg angles, as indicated by the analysis results. When the width–thickness ratio and the width ratio of the legs fell within a specific range, the ultimate bearing capacities of the unequal-leg angles increased as the width–thickness ratio and the width ratio of the legs increased. Finally, the comparison results showed that the design strengths predicted by the specifications were very conservative, because the local buckling and torsional buckling were calculated at the same time. Therefore, a recommendation was proposed that the calculation of the load-carrying capacity of an unequal-leg angle should ignore torsional buckling.

Artificial Neural Network and Kriging Surrogate Model for Embodied Energy Optimization of Prestressed Slab Bridges

The main objective of this study is to assess and contrast the efficacy of distinct spatial prediction methods in a simulation aimed at optimizing the embodied energy during the construction of prestressed slab bridge decks. A literature review and cross-sectional analysis have identified crucial design parameters that directly affect the design and construction of bridge decks. This analysis determines the critical design variables to improve the deck’s energy efficiency, providing practical guidance for engineers and professionals in the field. The methods analyzed in this study are ordinary Kriging and a multilayer perceptron neural network. The methodology involves analyzing the predictive performance of both models through error analysis and assessing their ability to identify local optima on the response surface. The results show that both models generally overestimate the observed values. The Kriging model with second-order polynomials yields a 4% relative error at the local optimum, while the neural network achieves lower root mean square errors (RMSEs). Neither the Kriging model nor the neural network provides precise predictions but point to promising solution regions. Optimizing the response surface to find a local minimum is crucial. High slenderness ratios (around 1/28) and 40 MPa concrete grade are recommended to improve energy efficiency.

Aerodynamic Characterization of a Hypersonic Projectile Using Acceleration-Based Measurements and Numerical Methods

Armor-piercing fin-stabilized projectiles stand out by their exceptionally high slenderness ratios and ground-level flight at super- and hypersonic speeds. As space constraints limit the integration of measurement equipment into such slender test models, nonintrusive measurement techniques become favorable. The present analysis demonstrates a renewed approach to the free-flight technique, which sets models into unconstrained flight through the test section. It enabled the evaluation of the quasi-steady drag, lift, and pitching moment characteristics, and, to some extent, also the dynamic pitch damping characteristics. An alternative to the free-flight technique is the free-oscillation technique, which limits the motion to a rotation around the center of gravity. The free-oscillation technique allowed a higher-fidelity analysis of the static and dynamic pitching moments. Both methods were based on the analysis of the accelerations derived from trajectory tracking. This acceleration-based approach enabled the evaluation of the highly nonlinear characteristics. The high slenderness of the investigated projectile leads to a significant contribution of the viscous forces to the overall drag. These viscous forces are sensitive to the laminar-turbulent boundary-layer state, as well as the thermal boundary conditions. The application of a high-resolution schlieren system enabled the assessment of the local laminar-turbulent boundary-layer state, while the use of low- and high-enthalpy testing facilities enabled the assessment of the aerothermal influence. Steady-state Reynolds-averaged Navier–Stokes simulations, which included laminar-turbulent transition modeling, were employed to replicate the results of the experimental efforts.

On-bottom stability of a radial fin integrated surface-laid pipe-section against vertical penetration

This study focuses on the on-bottom stability of surface-laid submarine pipelines during vertical penetration under thermal-induced buckling. Submarine pipelines are susceptible to buckling due to their high slenderness ratio, and the resistance to such displacement relies on pipeline stiffness and soil resistance. To investigate this, laboratory model tests were conducted using a scaled-down pipe section placed on a clay bed. The clay bed was prepared with remoulded Kaolin clay, ensuring uniform moisture content, and the model pipe was scaled down from a typical industry prototype. These tests were conducted in 2D plane strain conditions within a tank, equipped with observation windows to monitor pipe movement and soil behaviour. A significant improvement in the pipe section's penetration resistance and on-bottom stability was observed after incorporating fin in the pipe. The addition of fins altered the earth pressure distribution beneath the vertically loaded pipe, leading to a larger failure mechanism in the adjacent soil. Furthermore, the soil-surface-heaving progressed towards the tank wall with increasing angular distance between the radial fins. Thus, the research demonstrated the effectiveness of radial fins in enhancing the stability and resistance of submarine pipelines against vertical penetration, shedding light on potential improvements in submarine pipeline design and deployment.

Effectiveness in protecting rigid-block-like objects through horizontal and vertical seismic isolation

In this paper, a double, horizontal and vertical base isolation, is considered for the protection of rigid-block-like objects against seismic excitation. The equations of motion, the uplift and impact conditions are obtained under the hypothesis that the block can undergo full-contact and rocking motions. It is assumed that the horizontal isolation device has a hysteretic behavior, described with the Bouc-Wen model, whereas the vertical isolation has a visco-elastic behavior, modeled through a Kelvin-Voight device. Three rigid-block-like objects, representing a caryatid, a server and hospital cabinets are considered. Moreover, as base excitation, three earthquake horizontal and vertical records are selected accounting for their spectral content and PGA. The results of the seismic analysis are arranged in rocking maps and a comparison among the systems with the horizontal and vertical isolation, with only the horizontal isolation, and with no isolation are performed to check the effectiveness of the proposed seismic protection. Results show that the coexistence of horizontal and vertical isolation in protection against seismic excitation is particularly effective under earthquakes with high vertical PGA and in objects with high slenderness and heaviness. Finally, an analysis that consists of exciting the system with horizontal and vertical one-sine, impulsive base accelerations is performed to build the overturning spectra. Also, in this case, the double, horizontal and vertical isolation has manifested better performances than the other analyzed systems.

Cyclic Behavior of Concrete-Filled Tube Columns with Bidirectional Moment Connections Considering the Local Slenderness Effect

In this research, the cyclic behavior of concrete-filled thin tube (CFTT) columns with bidirectional moment connections was numerically studied within the context of thin-walled structures. Novel considerations in the design of CFTT columns with slenderness sections are proposed through a parametric study. A total of 70 high-fidelity finite element (FE) models are developed using ANSYS software v2022 calibrated from experimental research using similar 3D joint configurations. Furthermore, a comparison of different width-to-thickness ratios in columns was considered. The results showed that the models with a high slenderness ratio reached a stable cyclic behavior until 0.03 rad of drift, and a flexural strength of 0.8 Mp was reached for 4% of the drift ratio according to the Seismic Provisions. However, this effect slightly decreased the strength and the dissipated energy of the moment connection in comparison to columns with a high ductility ratio. Moreover, an evaluation of concrete damages shows concrete cracked for cyclic loads higher than 3% of drift. Finally, the joint configurations studied can achieve a good performance, avoiding brittle failure mechanisms and ensuring the plastic hinges in the beams.

Eccentric compression behavior of partially CFRP wrapped seawater sea-sand concrete columns reinforced with epoxy-coated rebars

The combined use of seawater sea-sand concrete (SSC), epoxy-coated rebar (ECR) and fiber-reinforced polymer (FRP) can achieve significant economic and environmental benefits in coastal infrastructure constructions. Thus, this paper first presents an experimental and analytical study on the eccentric compression behavior of partially carbon FRP (CFRP) wrapped SSC columns reinforced with ECR. The effects of load eccentricities, thickness of CFRP strips, clear spacing ratios, and slenderness ratios were comprehensively analyzed. Test results show that as the eccentricity increased from 60 mm to 120 mm, the increment of load capacity of partially CFRP wrapped columns reduced by 27.6 % to 17.1 %. Moreover, a larger lateral deflection was detected for the specimens with a relatively high slenderness ratio, which results in an obvious second-order effect and further aggravates the nonlinearity of axial load-bending curves, especially for the specimens under a small eccentricity. In addition, an eccentricity-dependent stress-strain model for concrete was developed under FRP strip-steel hoop composite confinement, and the lateral deflection formula of eccentrically loaded columns was also derived based on force equilibrium analysis. Finally, a new theoretical formula incorporating the effect of the slenderness ratio was also presented for the determination of load and moment capacity of the partially CFRP wrapped columns based on section analysis. By comparing with the test results, the developed model exhibits satisfactory accuracy.

Investigation on Influence of Die Electric Medium in Electrical Discharge Machining of Monel 400 Alloy

Electrical Discharge Machining (EDM) is a non-conventional thermal energy based erosive process, which primarily applied for machining hard materials. Material Removal Rate (MRR) and surface roughness are the response parameters used to characterize the dielectric nature of the machined surface in EDM process. Addition of ingredients in the dielectric fluid improves the properties of fluid for better machining of the samples. The dielectric fluid medium plays a key role in controlling the electrical discharge and heat absorption, thereby removes the debris and cools the work piece during the machining process. In the current study, comprehensive work is done by investigating the effect of different dielectric fluid medium on machining parameters of EDM process with the addition of different powders in the dielectric fluid, which results in high precise and better topography in the machined part surface. Addition of powders such as Titanium (Ti), Silicon (Si), Graphite (Gr), Copper (Cu) and Aluminium Oxide (Al2O3) in dielectric fluid increases the convection property in the work piece tool interaction with increase in the micro-hardness of material. This work analyses the performance study of Electrical Discharge Machining (EDM) of Monel 400 alloys, which can be improved by adding metallic powder into the dielectric medium. Material Removal Rate (MRR) is measured in the samples machined out of EDM process. In addition, Taguchi L27 Orthogonal Array is formulated for conducting machining in a sequential order to understand the implications of machined process parameters on the material removal rate over different dielectric mediums. It is found that the Aluminium oxide, graphite powder mix with EDM oil gives better material removal rate and less machining time. Furthermore, the introduction of Cu powders in the dielectric fluid provides better machinability response parameters. But it is preferable to parts with high slenderness ratio especially holes.

Bending fatigue performance of high-strength seven-wire monostrand

Large-amplitude cable vibrations are remarkably common in cable-stayed bridges subjected to wind and mechanical loading due to low inherent damping of these structures. Although the high slenderness ratio of cables means that bending moments are not normally significant over their free length, local effects near the cable anchorage induce non-negligible bending stress variation under dynamic loading, which could lead to fatigue failure of stay cables. In modern parallel strand stay cables, the fatigue performance depends on the fatigue properties of the individual seven-wire monostrands which together make up the cable. We introduce a new bending fatigue testing device for seven-wire monostrand which can control the fatigue loading in the axial and transverse directions of the cable independently. The fatigue endurance of monostrand with guide deviators is significantly longer than without guide deviators present, especially for smaller bending fatigue amplitudes. We found that bending fatigue life of monostrand is also reduced by increased constant or dynamic axial stress, but over-tensioning of the monostrand (e.g. during construction) had no consistently adverse effect on fatigue life. For monostrand loaded in severe bending, fatigue occurs due to the bending stress variation caused by the combination of bending moments and axial forces at the extreme edges of the monostrand cross-section, rather than due to relative movement and fretting between adjacent wires within the monostrand.

A fully coupled regularized mortar‐type finite element approach for embedding one‐dimensional fibers into three‐dimensional fluid flow

SummaryThe present article proposes a partitioned Dirichlet‐Neumann algorithm, that allows to address unique challenges arising from a novel mixed‐dimensional coupling of very slender fibers embedded in fluid flow using a regularized mortar‐type finite element discretization. The fibers are modeled via one‐dimensional (1D) partial differential equations based on geometrically exact nonlinear beam theory, while the flow is described by the three‐dimensional (3D) incompressible Navier‐Stokes equations. The arising truly mixed‐dimensional 1D‐3D coupling scheme constitutes a novel approximate model and numerical strategy, that naturally necessitates specifically tailored solution schemes to ensure an accurate and efficient computational treatment. In particular, we present a strongly coupled partitioned solution algorithm based on a Quasi‐Newton method for applications involving fibers with high slenderness ratios that usually present a challenge with regard to the well‐known added mass effect. The influence of all employed algorithmic and numerical parameters, namely the applied acceleration technique, the employed constraint regularization parameter as well as shape functions, on efficiency and results of the solution procedure is studied through appropriate examples. Finally, the convergence of the two‐way coupled mixed‐dimensional problem solution under uniform mesh refinement is demonstrated, a comparison to a 3D reference solution is performed, and the method's capabilities in capturing flow phenomena at large geometric scale separation is illustrated by the example of a submersed vegetation canopy.

Computational study of a homogenized nonlinear generalization of Timoshenko beam proposed by Turco et al.

<p>Mechanical metamaterials are most often assemblies of stocky beam elements connected through rigid connections, hinges, or flexural joints. The description of these materials through classical beam theories is challenging because of the wide variety of complex phenomena observed in the severe deformation regime mechanical metamaterials must undergo and because most classical beam theories can only be applied to elements with sufficiently high slenderness. In the spirit of Hencky, Turco et al. (2020) has recently formulated an intrinsically discrete nonlinear elastic model suitable for the design of mechanical metamaterials. The objective of this contribution was to present a numerical study of the nonlinear generalization of the Timoshenko beam that results from the asymptotic homogenization of the discrete model introduced by Turco et al. The present numerical study took into account several loading cases and elucidated the sensitivity of the homogenized continuum with respect to axial, bending, and shear stiffness parameters, as well as to load imperfections, in terms of mechanical behavior, including buckling onset and post-critical behavior. It was found that the predictions obtained with the homogenized model in the large deformation regime matched excellently with those of the discrete model proposed by Turco et al.</p>

A Design Framework for Semi-Active Structural Controlled Adjustable Constant Force Mechanisms

Abstract Semi-active adjustable constant force mechanisms (ACFMs) are an emerging alternative in applications where energy-efficient control of constant force environments is required. However, there is a lack of design strategies in the literature for semi-active ACFMs. This study addresses this gap by presenting a design strategy for ACFMs that semi-actively tunes the constant force by structural control. A design framework is presented, which consists of an optimization of a high slenderness large stroke constant force mechanism (CFM) followed by a parametric study on adjusting constant force through slenderness reduction by repositioning the boundary condition location. The design framework was able to change constant force ranging from two to four times with a stroke of 11–26% of the mechanism footprint. A selected design with a larger force magnitude was fabricated and experimentally tested, demonstrating a change in constant force of 2.01 times, which is comparable to that of active control designs and improved compactness, i.e., stroke of 11% of the footprint of the mechanism. In conclusion, the proposed ACFM design framework maximizes the initial CFM stroke and achieves constant force tuning by changing beam slenderness, resulting in compact and efficient ACFM designs.

The Effects of Freezing Rain on Forest Stands Administered by Zalău Forestry Department During 2014-2022 Period

Freezing rain is a rare and extremely dangerous meteorological phenomenon, consisting of the fall of liquid precipitation, while the air temperature at ground level falls below 0oC, favoring the freezing of raindrops and forming an ice sheet on the surfaces on which they are deposited. In Romania, the areas affected by the freezing rain phenomenon undergo significant changes regarding the structure and composition of the forest vegetation. The main purpose of this work is to figure out how to manage the forest stands affected by the freezing rain identifying optimal solutions for ecological reconstruction and prevention of significant damages. For the research of the affected areas, we placed 7 experimental areas, having the size of 500 m2 each, all the trees that were in this radius were inventoried. In this research work we evaluate the presence of this phenomenon which was reported in the Stejarul Zalău Forest District, in 2014, then in 2017, but with a lower intensity. Out of a total of 1627.6 ha, 500 ha were affected by the freezing rain, most of them being surfaces with beech forest stands with a consistency of 0.9, having a high slenderness index. Following the results obtained, it was observed that the most affected trees were the youngest. These having a reduced diameter, thin branches and a high slenderness coefficient.

Beam-column behavior and design equations for high-strength composite filled tube (CFT) members: Investigating the interaction between section and member slenderness ratio

This paper investigates the influence of the section and member slenderness ratio on the axial-moment interaction behavior for high-strength concrete-filled tube (CFT) members. An extensive parametric study is conducted with the aim of extending the material strength limitations in the AISC 360-22 for composite members and investigating the reduction of strength as the member slenderness increases. A 2D fiber-based model using OpenSees software was developed and calibrated based on experimental studies to conduct an extensive parametric study, which includes; (1) member (global) slenderness; (2) section slenderness; (3) section's aspect ratio; (4) material strength; (5) level of axial loading. The study demonstrates that the unconservative error in the current design equations can be as high as 25 % for high member slenderness, while the conservatism in the high section slenderness can be more than 200 %. The proposed interaction curve and the design equations demonstrate high accuracy in predicting the maximum strength of the CFT members under combined loading conditions for a wide range of parameters, including the conventional strength CFT members. A fundamental insight into behavior is presented along with the results. This paper discusses the potential simplification of these findings into design equations that can be used in everyday practice.

Embodied Energy Optimization of Prestressed Concrete Road Flyovers by a Two-Phase Kriging Surrogate Model.

This study aims to establish a methodology for optimizing embodied energy while constructing lightened road flyovers. A cross-sectional analysis is conducted to determine design parameters through an exhaustive literature review. Based on this analysis, key design variables that can enhance the energy efficiency of the slab are identified. The methodology is divided into two phases: a statistical technique known as Latin Hypercube Sampling is initially employed to sample deck variables and create a response surface; subsequently, the response surface is fine-tuned through a Kriging-based optimization model. Consequently, a methodology has been developed that reduces the energy cost of constructing lightened slab bridge decks. Recommendations to improve energy efficiency include employing high slenderness ratios (approximately 1/28), minimizing concrete and active reinforcement usage, and increasing the amount of passive reinforcement.