Section Dimensions Research Articles

Experimental study on multi-cellular L-shaped CFST slender columns under biaxial eccentric compression

Multi-cellular L-shaped concrete-filled steel tubular (MCL-CFST) column is an improved L-shaped CFST column, which is composed of multiple rectangular steel tubes welded by U-shaped steel plates and then filled with concrete. The global stability capacity of MCL-CFST slender columns is vital for engineering applications. However, few investigations have been carried out on the stability performance of the MCL-CFST slender columns. This paper mainly focuses on the global stability performance of MCL-CFST slender columns. Firstly, six hinged supported MCL-CFST slender columns were designed and tested under biaxial eccentric loading. The load-displacement curves, failure modes, strain development and second-order effects of MCL-CFST slender columns were reported. Then, a refined finite element (FE) model of the MCL-CFSTs was established and verified by experimental results. Moreover, based on the validated FE model, the effects of eccentricity, column height and sectional dimension on the stability performance of MCL-CFSTs under eccentric loading were investigated, and the bending resistance curves Mx/Mxu-My/Myu in two directions under various parameters were obtained. These results can provide valuable references for engineering practice.

Experimental and numerical studies on the fire resistance of prestressed reactive powder concrete beams

The prestressed reactive powder concrete beam (PRPCB) is an advanced structural member that can take full advantage of the ultra-high compressive strength of reactive powder concrete (RPC) and the high tensile strength of tendons. The fire safety of PRPCBs has been mainly studied at the material level, with relatively few experimental studies being conducted on the structural member level with the principal reason of serious fire-induced spalling. This paper proposed an innovative curing method to prevent RPC spalling under high stress and extreme heating rates. A total of eight full-scale beams were tested in ISO 834 fire, considering the parameters of the cover thickness of tendon, load ratio, partial prepressing ratio (PPR), and bonded and unbonded prestress classifications. Cross-sectional temperatures, mid-span deflections, effective prestressing, crack patterns, failure modes, and fire resistances of the beams were obtained. Subsequently, fine thermal–mechanical coupling nonlinear finite element (FE) models were developed and verified, and an extensive parametric study was performed to simulate the fire responses of PRPCBs. The results indicated that the load ratio, cover thickness, and sectional dimensions notably affected the fire safety of PRPCBs, and PRPCBs fabricated as under-reinforced beams at ambient temperature may fail as lightly reinforced beams under fire exposure. According to these parametric studies, Eurocode 2: BSEN 1992-1-2 might overestimate the fire resistance of PRPCBs with small sectional dimensions by up to 3–15%. Therefore, fire resistance suggestions for PRPCBs are presented in this paper.

Shaping Effects on Long Span Bridge Deck Aerodynamics

An aerodynamic circumstance of wind pressure surrounding the long-span bridge allocates many theoretical and experimental research to this topic. Determination of the materials and optimal cross-sectional shape of bridge decks that affected a dynamic behavior of long span bridge deck is still included in current research issues and works to be continued in this path. These include the Lack of sufficient awareness of wind forces, stemming from complex nature, and the unpredictability of the wind nature. In this study, in addition to recognizing the aerodynamic behavior of the flutter, the acting pressure forces on the bridge deck are investigated. The geometrical shape of decks, wind velocity, and flutter conditions are adopted as design variables that affected the dynamic forces exerted on bridge decks. A common type of geometric sections of the long-span bridge deck and effective aerodynamic phenomena are examined. The hollow box steel suspended deck and double cells box girder linked via upper flanges and cells linked via the top and bottom flanges are adopted for Computational Fluid Dynamic (CFD) approach. Thus, aerodynamic instability and turbulent torsional flutter flows, as well as a trail of shedding vortices around the bridge decks, are investigated. By changing some geometrical parameters of commonly used bridge sections, the optimal cross-section in terms of turbulence created above and below the deck section is examined and an optimal cross-sectional shape variable is proposed. The shape variable and section dimensions adopted for CFD-Simulations are similar to the dimensions and materials used in previous laboratory specimens of wind tunnels to be able to interpret the results and possibly verify them with the result of the current study.

Reliability Analysis of an Uncertain Single Degree of Freedom System Under Random Excitation

In practical engineering problems, uncertainty exists not only in external excitations but also in structural parameters. This study investigates the influence of structural geometry, elastic modulus, mass density, and section dimension uncertainty on the stochastic earthquake response of portal frames subjected to random ground motions. The North-South component of the El Centro earthquake in 1940 in California is selected as the ground excitation. Using the power spectral density function, the two-dimensional finite element model of the portal frame’s base motion is modified to account for random ground motions. A probabilistic study of the portal frame structure using stochastic finite elements utilizing Monte Carlo simulation is presented using the finite element program ABAQUS. The dynamic reliability and probability of failure of stochastic and deterministic structures based on the first-passage failure were examined and evaluated. The results revealed that the probability of failure increases due to the randomness of stiffness and mass of the structure. The influence of uncertain parameters on reliability analysis depends on the extent of variance in structural parameters.

Residual stresses of Q235 steel wallboard - Q460 high-strength steel column structural system

For the wallboard - column structural system on the side of box-type steel structures, the residual stress distribution of the high-strength steel (HSS) column supported by stressed skin wallboards is affected by the welding between columns and wallboards, making it significantly different from an independent welded H-section member. To investigate and model the residual stress in Q235 steel wallboard - Q460 HSS column structural system, to which little attention has been paid, an experimental study was conducted using both the hole-drilling method and sectioning method. The residual stress magnitude and distribution were obtained. Comparison of the two measurement methods indicates that the maximum tensile residual stresses near the welds measured using the hole-drilling method are much greater than those obtained from the sectioning method. The welding between column and wallboards reduces the maximum tensile residual stress in the middle of column rear flange as well as the compressive stress distribution range in the rear flange. The magnitudes of compressive residual stresses decrease with increasing width-to-thickness ratio and component plate thickness, while no significant correlation with geometries was observed for the tensile stresses. The residual stresses in the combination of the rear flange and wallboards, front flange, and web can be regarded as being in self-equilibrium respectively. In addition, a simplified distribution model is proposed and compared with the experimental results. The new stress distribution model can provide a foundation for investigating the buckling behavior of HSS columns supported by stressed wallboards. • Residual stresses in Q235 steel wallboard - Q460 high-strength steel column structural system were measured. • Results obtained by the hole-drilling method and sectioning method were compared. • Effects of welding between wallboards and column, sectional dimensions, and the interaction among plates were investigated. • A simplified residual stress distribution model for the structural system was proposed.

Vision Assisted Control of Lower Extremity Exoskeleton for Obstacle Avoidance With Dynamic Constraint Based Piecewise Nonlinear MPC

This article proposed a humanoid obstacle passability strategy (OPS) to enhance human-exoskeleton integrated system to cross over multi-obstacle in sagittal plane. A hybrid bounding box integrated with closeness regression in L-shape section and synchronous convergence in convex hull search (HBB-LC) is designed to recognize geometric features of staggered obstacles with partial surface occluded. A human-in-the-loop gait pattern planning mechanism based on nonlinear model predictive control (NMPC) is designed for decoupling mapping. To overcome local oscillation and achieve humanoid behavior transition during NMPC optimization, an intermediate switching criterion based on instantaneous capture point (ICP) transfer principle and guiding interpolation inflection is established to trigger the external intervention of piecewise NMPC models. Exploitation of OPS achieves the contributions below: 1) Sectional attitude and dimension features of spatial multi-obstacle are accurately extracted, with size deviation within 3.4–16.1 mm, recognition accuracy within 89.2%–96.8%. 2) Traversal avoidance region of swing toe/heel orbit (SHO/STO) is enhanced by 28.7%–86.8% (with uplift/sidesway range (UR/SR) by 0.071–0.154 m/0.057–0.092 m). 3) Security margin between STO/SHO and obstacle vertex is improved by 0.05–0.19 m (53.3%–161.1%). 4) Elapsed time of crossing obstacles is reduced by 0.62–1.03 s (with gait phase by 36.2%–43.5%). Experimental verification proves significant augmentation for crossing obstacles by OPS.

Optimal Design of Reinforced Concrete Frame Structures Using Cascade Optimization Method

Optimal design of structures is one of the goals of a structural engineer. Since a large number of design variables are involved, the high computational time for optimization process is one of the main problems of using single heuristic or meta-heuristic algorithms utilized for this purpose, especially in large-scale structures. Cascade optimization replaces the single optimization problem with a multiple stage process. Each step starts with the previous optimum design, with the number of variables being reduced thus increasing the speed of the convergence in each stage of optimization. In this article a Cascade optimization method is developed for optimum design of RC frame structures. By using this algorithm in place of a single optimization method and ensuring its efficiency, the optimum section dimensions are determined by a new method. Then, genetic algorithm is utilized for optimizing of rebar arrangement. According to the results, this method performs optimum design and reduces the elapsed time considerably. Optimization of large-scale concrete structure is carried out by the developed method. It should be noted that the present method works with MATLAB and ETABS interfacing.

Strength of Composite Columns Consists of Welded Double CF Sigma-Sections Filled with Concrete—An Experimental Study

In-filled tubes section is a very successful configuration for axially loaded members such as columns and struts. Steel shell tube filled with concrete has many advantages, such as eliminating the need for shuttering, reinforcement bars or ties besides increasing both flexural and axial capacities and enhancing the ductility. The main disadvantage of in-filled tubes is the need for a shell thick enough to prevent the local buckling and hence the local decomposition. Previous studies tried to solve this problem using intermediate stiffeners or shear connectors. This research presents another approach to solve this problem using double cold-formed sigma-sections (face to face) as steel shell tubes. Sixteen specimens with different lengths, cross section dimensions and shell thicknesses were tested under both concentric and eccentric compression loads. Ultimate capacities, lateral deformations and normal strains were recorded. The theoretical capacities were calculated using AISC-LRDF-94, EN-1994-04 and CSI-COL software considering full composite action, and the deviations from the experimental results were 24%, 24% and 13%, respectively.

The effect of longitudinal hole shape and size on the flexural behavior of RC beams

The building's electro-mechanical piping systems can pass through transverse or longitudinal holes inside the reinforced concrete (RC) secondary beams, thus increasing the available floor height. These holes induced a reduction in the beam stiffness and strength. This work analytically and experimentally investigates the impact of longitudinal hole shape on the flexural behavior of RC beams. The interaction between hole shape and hole location has also been studied. The test program includes nine beams with the same cross section dimensions and steel reinforcement. Holes have been made using two circular PVC pipes (50 and 75 mm in diameter), and wooden medium-density fiberboard (MDF) square tubes (50×50 mm) and rectangular tubes (50×100 mm). Each hole shape was installed at two locations (90 and 160 mm) from the top of the beam. All beams were tested to failure using a four-point bending test. The results show that the hole shape and size up to half the beam width does not affect the failure mode compared to the solid control beam. The reduction in the ultimate strength of hollow beams compared to the control beam depends on the hole shape and size, with a maximum reduction of 20%. The developed analytical models were validated using experimental test results and can be employed to accurately predict the full flexural behavior of hollow RC beams, including cracking, yielding, and ultimate moments/loads.

Prediction of fragility deterioration of earthquake-damaged RC structure based on damaged section analysis

Evaluating quantitatively the performance of earthquake-damaged structures is significant for facilitating post-earthquake reconstruction. However, it is difficult to quantitatively assess the effect of earthquake-induced damage on structural performance. In this paper, a new method to establish the earthquake-damaged structure model (EDSM) based on the damaged section analysis (DSA) procedure is proposed. DSA can consider the effect of the seismic damage on material properties and sectional dimensions, which are the primary difference between DSA and classical moment-curvature analysis. Furthermore, the DSA is verified on two RC columns by the comparison of hysteretic curve between damaged column models and test columns under cyclic loadings. The EDSM is established by replacing the plastic hinge regions of the damaged components with “Hysteretic” material. This method is applied to a RC frame structure and is verified for different ground motions with various intensities. Moreover, by the comparison of vulnerability curves between undamaged structure model (UDSM) and EDSM, the vulnerability of EDSM regarding the moderate limit state is more fragile than the vulnerability of UDSM regarding the slight limit state. A reduction index is proposed to quantitatively evaluate the deterioration of seismic performance of EDSM. It can be identified that the reduction in vulnerability decreases gradually as the limit state increases, which shows the earthquake-induced damage has a more significant effect on the elastic properties of structure.

A novel design of printed ridge gap waveguide‐based 0‐dB backward‐wave coupler

This article presents the design and implementation of a 0-dB coupler based on Gap Waveguide (GW) technology. The proposed coupler is considered as the first backward-wave 0-dB realized using this technology. It operates at a central frequency of 30.5 GHz and maintains good performance regarding isolation and directivity all over the band. A parametric sweep is carried out to study the effect of the coupling section dimensions on the coupler performance to achieve nearly 0-dB coupler. The design is based on finite difference time domain (FDTD) analysis using CST microwave studio and Finite Element Method (FEM) using Ansoft HFSS for validation. The novelty of this article is that the achievable coupling section structure is considered as a genuine design for most 0-dB couplers for different operating frequency bands with its corresponding unit cell design based on GW technology. To validate this concept, the design steps are applied to four different frequency bands without changing the shape of the coupling section. The corresponding GW feeding networks are implemented on different materials for different couplers: Rogers RT5880, Rogers RO3003, and Rogers RO4350B. Furthermore, one of the couplers is fabricated and tested. The measured and simulated results show an acceptable agreement.

Numerical Investigation on the Thermal Behaviour of Steel‐reinforced CFST Columns in Fire

AbstractIn previous research, the authors highlighted the limited fire resistance of slender concrete‐filled steel tubular (CFST) columns. A possible solution for enhancing the fire resistance of such columns consists of embedding an open steel profile within the concrete infill of the CFST section, generating the so‐called steel‐reinforced concrete‐filled steel tubular (SR‐CFST) section, where the inner steel profile results thermally protected by the surrounding concrete, thus delaying its degradation at elevated temperatures. This strategy may result beneficial in the fire situation as compared to other sectional configurations where the steel parts are directly exposed to the heat source.In this paper, a two‐dimensional finite element model will be developed for studying the thermal behaviour of SR‐CFST columns and validated by comparing the temperature distribution results with experimental tests available in the literature. The numerical model will be subsequently used to carry out parametric studies, in order to analyse the influence of the different parameters, such as the cross‐section shape, outer tube thickness or the inner steel profile sectional dimensions over the thermal response of these composite sections when subjected to fire.Finally, a simplified temperature distribution proposal will be developed, which may help practitioners for a fast evaluation of the cross‐sectional temperature field of SR‐CFST columns at a given fire resistance period by assigning an equivalent temperature to each component of the composite section (hollow steel tube, concrete encasement and inner steel profile). This simplified temperature proposal may result helpful when evaluating the sectional capacity of SR‐CFST columns in fire, by using a single strength and stiffness value for each component of the composite cross‐section corresponding to its temperature.

Structural optimization of an aeroelastic wind tunnel model for unsteady transonic testing

The reduction of loads, ultimately leading to a weight reduction and thus an increase in aircraft performance, plays an important role in the design of modern aircraft. To this end, two aeroelastic tailoring methodologies, independently developed at ONERA and DLR and aiming at load reduction by means of a sophisticated application of composite materials, were applied to a common model geometry. A choice was made in favor of the publicly available NASA Common Research Model (CRM) wing, featuring a comprehensive database with respect to geometry, as well as analytical and experimental research results. The span of the wing half to be investigated was set to 0.55 m, limited by the test section dimensions. While wind tunnel testing was part of ONERA’s workshare, the model building was performed by DLR. This paper at hand focuses on the structural, aeroelastic optimization of the DLR wing. It is based on an optimization framework developed and constantly being enhanced and extended at the DLR - Institute of Aeroelasticity (DLR-AE). The paper describes the consideration of different structural objective functions, structural and aeroelastic constraint combinations, design field considerations, as well as the application of an aero load correction applied in the course of the optimization. The final results consist of the selection of an appropriate fiber type, optimized fiber layers represented as stacking sequence tables for the upper and lower wing skins, and the corresponding optimized jig twist distribution, required for manufacturing the lamination molds; in summary, all data required to start the construction of the wind tunnel model.

Numerical Modeling of the Walls Perforation Influence on the Accuracy of Wind Tunnel Experiments Using Two-Dimensional Computational Fluid Dynamics Model

The presented work is dedicated to the development of the mathematical model of a closed-type wind tunnel (WT) in a two-dimensional (2D) formulation based on the computational fluid dynamics (CFD) Unsteady Reynolds Averaged Navier Strokes Equations (URANS) method. The influence of the WT walls perforation percentage on the aerodynamic characteristics of the SC1095 (SC is an abbreviation for “supercritical”) airfoil in a wide range of pitch angles from 0 to 28 degrees has been studied. The obtained data are compared with the aerodynamic characteristics of the airfoil in free flow conditions. It is shown that the use of perforation makes it possible to significantly reduce the influence of the wind tunnel walls on the obtained aerodynamic characteristics of the airfoil bringing them closer to the airfoil characteristics under free flow conditions. The developed low resource-intensive 2D CFD WT model can be used to determine the optimal ratio between the walls perforation percentage, the WT test section dimensions, and the experimental model dimensions for various test modes. Support of WT experiments with the help of preliminary parametric studies based on the 2D CFD model could significantly reduce material costs and increase the accuracy of the obtained results.

Design of cold-rolled aluminium alloy channel beams subject to global buckling

The paper presents the development of new design guidelines for cold-rolled aluminium alloy 5052-H36 channel beams subject to global buckling. The validation data are obtained from the experimental and numerical Finite Element (FE) results as performed in previous studies by the authors. The failure modes in the experiments were observed as flexural–torsional buckling modes, and good agreement was achieved between the experimental and FE results. Based on the reliable and calibrated FE models, a parametric study is carried out in this paper to extend the data range by varying section dimensions and member lengths. Subsequently, all experimental and FE ultimate strengths of the parametric study are compared with nominal member capacities of aluminium beams as predicted by the current Australian/New Zealand, American and European aluminium design standards. Additionally, the Direct Strength Method (DSM) design rules from the Australian and New Zealand standard - AS/NZS 4600 for cold-formed steel structures are applied to predict member bending capacities for comparisons. Finally, reliability analyses are performed to evaluate the efficiency of the abovementioned design methods. It is found that the design guidelines for nominal member bending capacities to the AS/NZS 4600 standard provide accurate predictions and are therefore proposed to be used for the design of cold-rolled aluminium alloy channel beams as investigated in this paper.

The Direct Strength Method for first generation trapezoidal steel sheeting under Interior One Flange and Interior Two Flange web crippling

Trapezoidal steel sheeting is a popular product in the built environment, especially for roofing and cladding. However, at a support the webs of the sheeting may fail due to concentrated force, referred to as web crippling. In this paper it is investigated whether the Direct Strength Method (DSM) can be used to predict this failure. First, an experimental programme, carried out in the past, is selected for its sheet section dimensions, materials, span lengths, and load bearing plate widths which were varied systematically. Two distinct failure mechanisms occurred: the yield arc and the rolling mechanisms. Based on the experiments, in this paper a finite element model is developed to simulate Interior One Flange (IOF) and Interior Two Flange (ITF) web crippling, and a very good correlation is obtained with the experiments. Subsequently, the simulations are used to predict the buckling load (via the first positive Eigenvalue), the yield load (via first plastic strains or stress extrapolation), and the ultimate load for each experiment and additional cases. In addition, for each failure mechanism, an existing theoretical model is used to predict the first order elasto-plastic mechanism initiation load. Six different DSM equations are calibrated for both IOF and ITF web crippling, using the above different buckling and yield loads. For IOF cases, the use of the IOF first positive Eigenvalue for the buckling load, and the first order elasto-plastic mechanism initiation load, results in the best correlation and coefficient of variation. For ITF cases, the ITF Eigenvalue and same yield load shows the best results. Excluding the rolling mechanism, the DSM correlates equally (IOF) or better (ITF) with the numerical results than the AISI S100-16 design rule. Thus, the DSM, as a generalized slenderness approach, is suited for IOF and ITF web crippling of first generation trapezoidal steel sheeting. Furthermore, it could be considered to allow for the use of the well performing FE models explicitly by the codes—e.g. enabling manufacturers to create tables of strength.

Design Approach for Eccentrically Loaded Reinforced Concrete Sections

The behavior of eccentrically loaded reinforced concrete sections depends on both the eccentricity and load value. The type of failure can be determined according to the eccentricity-thickness ratio and the applied load value. Interaction diagrams can be used for designing eccentrically loaded section, but the section dimensions are required to use these diagrams. According to the section dimensions and the load value, the type of failure and the required reinforcement can be determined from the interaction diagrams. Most design codes controls the design process to ensure ductile failure. But according to the design procedures of interaction diagrams, the failure type can’t be controlled simply. This paper aims at finding a design procedure for eccentrically loaded reinforced concrete sections that enable the designer to simply control the design process and the failure mode. The proposed design approach makes the design process more direct and simplified.

Examining the relationship between gas channel dimensions of a polymer electrolyte membrane fuel cell with two-phase flow dynamics in a flooding situation using the volume of fluid method

Liquid structures such as droplets and slugs exist inside gas channels of polymer electrolyte fuel cells in low-temperature applications. The efficiency of these electrochemical devices depends on the effective removal of the produced water. The gas channels’ specifications like section geometry, corner angles, and surface wettability properties substantially control the liquid removal process. Here, five channels with various section geometries are modeled and the liquid-slug discharge process is investigated using a transient volume of fluid method. The numerical model consists of a segment of the cathode-side gas channel with the working conditions of an operational fuel cell. The dynamic two-phase flow simulations show that channels with smaller width and height eventuate in proper flow distribution at the gas feed. A channel with the sectional dimensions of 0.5 mm × 0.5 mm results in. 35.18% faster GDL (Gas-Diffusion Layer) clearance, 29.32% faster liquid expulsion compared to other channels having 2–3 times higher dimensions. Therefore, this channel is recommended as the best design for improved fuel cell performance.

Studying the Effects of Negative Skin Friction on Single Piles in Basrah Governorate

The finite element method capable of simulating the behavior of deep foundations subjected to negative skin friction in Basrah soil is investigated. Single piles under drag forces are analyzed using the PLAXIS program with an axisymmetric model. Linear elastic, Soft Soil and Mohr-Coulomb constitutive relations are adopted, where higher order triangular element is chosen for pile and soil clusters. Both pile and soil are modeled using (15)-node triangular elements. Three sites in Basrah province (Umm Qasr Port, Khor Al-Zubair, and Shatt AlArab Hotel) were selected to perform this study. The soil profile and layer characteristics are obtained from the soil investigation reports. Where the negative skin friction is evaluated due to filling loads. It is Conclusion thatSmall relative displacements are necessary to activate the negative skin friction. The elastic shorting for pile effect negative skin friction, due to increase relative displacement. The elastic shorting of the driven pile is more than that of the bored pile due to the less cross-sectional area of the driven pile. The results revealed proportional relation between the developed drag forces and pile section dimensions, interface friction factor, and fill height, with a maximum effect on the section dimension and minimum effect on the interface factor. The locations of neutral points are not sensitive to the above-mentioned factors.

Load-slip behaviour of single-leg bolted angle steel connections at elevated temperatures

Steel lattice transmission towers and communication towers are vital infrastructure components of power grid and telecommunication systems, respectively. They are usually located in rural or urban wildland areas and can be prone to bushfires or wildland fires in the current climate of heatwaves and droughts. However, an understanding of the resilience of steel lattice towers in fire has not received due attention. Such lattice structures are commonly made from steel angle members with bolted connections in which slippage might occur inevitably due to relatively larger size of bolt holes for erection purpose. Therefore, this paper investigates the axial load-slip behaviour of the single-leg bolted angle steel connections at elevated temperatures for the steel lattice structures. A three-dimensional finite element model is developed for the investigation by using ABAQUS software. Temperature-dependent material nonlinearities and the interaction of structural components at elevated temperatures are taken into account in the model. An effective approach of generating bolt pretension and the quasi-static analysis method using the dynamic explicit procedure are adopted for the analysis. The reliability of the developed finite element model is validated by comparing its predictions with available experimental results reported in the open literature. By using the validated finite element model, extensive parametric studies are conducted to elucidate the effects of the change in the angle section dimension, the direction of applied loading, the grade, size and number of the bolts on the load-slip responses of the connections at elevated temperatures. A simplified theoretical model in component formulation form is proposed and verified for predicting the axial load-slip curves of the single-leg bolted angle steel connections at elevated temperatures and would be applicable to advanced analysis of steel lattice structures considering joint slip.