External Stiffeners Research Articles

Multi-Degree-of-Freedom Force Sensor Incorporated into Soft Robotic Gripper for Improved Grasping Stability.

In recent years, soft robotic grippers have emerged as a promising solution for versatile and safe manipulation of objects in various fields. However, precise force control is critical, especially when handling delicate or fragile objects, to avoid excessive grip force application or to prevent object slippage. Herein, we propose a novel three-degree-of-freedom force sensor incorporated within a soft robotic gripper to realize stable grasping with force feedback. The proposed optical sensor employs lightweight and compact optical fibers, thereby allowing for cost-effective fabrication, and a robust sensing system that is immune to electromagnetic fields. By innervating the soft gripper with optical fibers, a durable system is achieved with the fibers functioning as a strengthening layer, thereby eliminating the need for embedding an external stiffening structure for efficient bending actuation. The innovative contact-based light loss sensing mechanism allows for a robust and stable sensing mechanism with low drift (<0.1% over 9000 cycles) that can be applied to soft pneumatic bending grippers. We used the developed sensor-incorporated soft gripper to grasp various objects, including magnetic materials, and achieved slip detection along with grip force feedback without any signal interference. Overall, this study proposes a robust measuring multi-degree-of-freedom force sensor that can be incorporated into grippers for improved grasping stability.

Axial Capacity of CHS T-Joints Reinforced with Welded External Stiffeners Under Load

Axial Capacity of CHS T-Joints Reinforced with Welded External Stiffeners Under Load

Development of lightweight CFS composite built-up beams: Tests and flexural response

Although previous attempts for enhancing the buckling resistance of cold-formed steel (CFS) built-up beams through intermittent stiffening, external stiffening and adoption of concrete as composite sections were successful, they may compromise the lightweight constructional feature of these elements. This paper presents a test program aimed at developing CFS composite built-up beams with superior buckling performance and low self-weight for ensuring lightweight construction. Unlike concrete, lightweight stiff boards of timber and glass fibre reinforced polymer (GFRP) were sandwiched between the CFS channels in the web and flange region to construct the lightweight composite beams. All of the beam specimens were subjected to four-point bending with simply supported boundary conditions. Peak strength, load-deformation curves, initial stiffness, strength-to-weight ratios, failure modes and the normalized yield and plastic moment strengths were compared. This study established the structural feasibility of developing lightweight CFS composite built-up beams from timber and GFRP boards, indicating that the flexural strength and stiffness are significantly improved by 150% and 80%, respectively, as compared to the unstiffened CFS built-up beams. Moreover, the studied composite beams were shown to attain their full yield moment capacities, and in some cases more than 90% of their plastic moment capacity, resulting in built-up members with a higher structural efficiency.

An BESO Approach for Optimal Retrofit Design of Steel Rectangular-Hollow-Section Columns Supporting Crane Loads

In this paper, we propose a cost-effective optimal-topology retrofitting technique for hollow-steel-section columns to sufficiently support industrial running cranes. A so-called bi-directional evolutionary structural optimization (BESO) method was encoded within the MATLAB modeling framework, with a direct interface with an ANSYS commercial finite-element analysis program, to determine the optimal topology of double external steel plates connected to columns in a 3D space. For the initial ground structure, we have adopted standard uniform double U-shaped external stiffener plates located at the top and bottom flange layers of an I-beam to box-column connection (IBBC) area. The influences of inelastic materials and the incorporated nonlinear geometry can effectively describe the premature (local buckling) failures of the columns in an IBBC area. The applications of the proposed optimal-topology BESO-based stiffening method are illustrated through the retrofitting of three hollow-steel-section columns, characterized by non-slender and slender compression sections. Some concluding remarks are provided on the pre- and post-retrofitted responses of the columns, with the results showing both the accuracy and robustness of the proposed external stiffening schemes.

Using the higher-order shear deformation theory to analyze the free vibration of stiffened rotating FGM conical shells in a thermal environment

In this paper, free vibration analysis of rotating stiffened truncated conical shells with functionally graded materials (FGM) in a thermal environment is presented based on the higher-order shear deformation theory (HSDT). Assuming Coriolis acceleration and the centrifugal force, the governing equations of stiffened rotating FGM truncated conical shells are extracted utilizing the HSDT, the Donnell kinematics assumptions, and the smeared stiffeners technique. The partial differential equations are discretized into a set of ordinary differential equations employing Galerkin’s approach. The characteristic equation is computed as a tenth-order polynomial equation in terms of natural circular frequency. Regarding the characteristic equation, the free vibration of the stiffened rotating shell is analyzed to investigate the natural circular frequency. The results are presented and compared with the latest pertained developments found in the literature. Also, the effects of the internal and external stiffener, volume-fraction index, temperature changes, and different vertex angles on the frequency response curve are examined for various rotating speeds.

Experimental and numerical characterization of pultruded GFRP structural elements with stiffened Web-Flange junctions

The research presented herein pertains to the experimental characterization and numerical simulation of the behavior of pultruded Glass Fiber-Reinforced Polymer (GFRP) structural elements, with specific attention devoted to enhancing ultimate capacity of the Web-Flange Junctions (WFJs). An experimental campaign, performed on as-manufactured I-beam specimens is conducted to serve as a baseline for comparing behavior of pultruded GFRP members with the proposed stiffening strategy. The specimens were produced through the addition of external elements, using bonded L-shaped profiles with variable lengths that were installed in the proximity of the WFJ zone of the I-beams. The experimental tests aimed at highlighting the inherent premature failure that potentially occurs at the WFJ as a result of lack of reinforcement continuity between the flanges and the web that are required to withstand localized radial stress concentration at these locations. To this end, the top flanges of the thin-walled GFRP I-beams were clamped while a localized concentrated load was applied on the bottom flange by means of a steel jaw. Two sets of tests were performed, namely Mid-Point (MP) and End-Point (EP), according to the location of the applied load that caused delamination, and eventually separation of the bottom flange from the web. Failure modes observed in the tests, together with experimental load–displacement curves, are used to investigate the influence of external stiffeners length on the ultimate strength and stiffness of the I-beams. In executing the tests, Digital Image Correlation (DIC) techniques were used to track the spatial distribution of the deformation in the elements. Based on the results of the experimental campaign, a numerical investigation was conducted, to characterize the influence of different stiffeners’ arrangements on the mechanical response of the GFRP elements. The computational model was validated by comparing the observed response, both in terms of measured force–displacement curves, and bi-dimensional displacement maps obtained by means of the DIC technique. Numerical results highlighted the importance and influence of an explicit representation of the mechanical response of the bonding agent (i.e., the epoxy resin used to attach the stiffeners to the I-beams) to correctly characterize the deformation behavior of the strengthened WFJs.

Numerical Investigation of the Flexural Performance of Slender CFST Beams with Varied Shapes of External Stiffeners

The concrete-filled steel tube (CFST) beams are recommended for adopting in the modern composite structure since it achieves a higher load capacity and ductility behaviour than the conventional structural members. Using CFST beams with slender steel cross-section led to reduce the overall cost of the structures compared to those with compact cross-sections; however, it is more likable to buckle under the compression stress. Therefore, providing external stiffeners along the four sides of CFST beams expected to reduce the overall self-weight also can provide more stiffening to the steel’s section that led to improve the loading capacity. In this study, a finite element (FE) method was adopted for numerically investigated the flexural performance of the square CFST beams stiffened with different shapes of external grooves that provided along the beam’s sides. The FE software named ABAQUS was adopted for this purpose where the original model of CFST beam analysed and verified with the corresponding tested specimen. After that, additional CFST model have been built and analysed to investigate further parameters including the effects of varied tube’s thickness, steel yielding strength, concrete compressive strength and different external grooves shape (V-shaped, U-shaped, C-shaped, double V-shaped). The results established from the numerical analyses showed that the ultimate bending capacity (Mu) of the stiffened CFST models was generally enhanced by increasing their concrete strength, steel yield strength and tube’s thickness but in varied percentages. For example, increasing the tube thickness from 1.5 mm to 3.0 mm achieved an improvement in Mu values of about 72.3\%. while fewer improvement percentages have been recorded of about 11.9\% when only the core concrete strength increased from 25 MPa to 55 MPa. Furthermore, the shape and number of grooves have positive impact on the bending capacity of stiffened CFST models, where using U-shape, C-shape and V-shape external grooves were led to improve the Mu value of about 2.8\%, 10.7\%, and 11.1\%, respectively. Thus, the energy absorption of these models was improved accordingly.

Numerical analysis of the effect of internal and external stiffeners on the behavior of concrete-steel composite connections

Numerical analysis of the effect of internal and external stiffeners on the behavior of concrete-steel composite connections

Experimental and numerical study on compressive behavior of welded hollow spherical joints with external stiffeners

Welded hollow spherical joints are one of the most popular joints used in reticulated shell structures. Welded hollow spherical joints in service may need to be strengthened due to age-related damage or unexpected overloading. Adding external stiffeners to the welded hollow spherical joint is an effective way to strengthen it. In this paper, experimental and numerical studies are performed to investigate the enhanced behavior of welded hollow spherical joint with external stiffeners. Axial compression tests of five specimens are performed with a particular focus on the effects of the number, height, and thickness of the external stiffeners on the load-bearing capacity of the joint. A 3D finite element model is developed and calibrated against the experimental results. Parametric studies on a number of design variables are conducted using the calibrated finite element model. The results show that the external stiffeners significantly enhanced the compressive capacity of the joint, and the compressive capacity of the joint is increasing with the number, height, and thickness of the external stiffeners increasing. It is also found that the diameter of the hollow sphere has little impact on the enhanced behavior of the welded hollow spherical joint with external stiffeners. Practical formulas for axial load-bearing capacity and initial axial stiffness of the welded hollow sphere joint with external stiffeners are developed, based on the results of parametric studies.

Bearing Capacity Test and Calculation Theory on CHS X-Joints Stiffened with Joint-Plates

In this study, three circular hollow sections (CHS) tubular X-joints strengthened with different external stiffeners or not were investigated by experimental and nonlinear finite element anlaysis (FEA) to study the bearing capacities, failure modes, and the strengthening effect of joint-plates. The test results showed that the bearing capacity of the X-joints could be improved by using the stiffened joint-plate, and the joint failure process was delayed, and the cambered surface of chord was straightened by tensile braces. Compared with the tested results, the nonlinear FEA could simulate the failure modes and its bearing capacity behavior of steel tubular X-joints well. To estimate the contribution of joint-plates on bearing capacity, four kinds of parameters were selected, and the corresponding FEA of stiffened X-joint were performed. Then, the influencing law on bearing capacity of different parameters was investigated. According to the superposition method of bearing capacities, the annular calculation model and the multi-parameter calculation theory of stiffened circular steel tubular X-joints were proposed, in which the coefficients were determined through regression analysis. Lastly, the comparisons on tested, simulated, and theoretical bearing capacities of stiffened X-joints were carried out, and there had good agreement between them.

A unified strong spectral Tchebychev solution for predicting the free vibration characteristics of cylindrical shells with stepped-thickness and internal–external stiffeners

In this paper, a strong spectral Tchebychev (ST) solution is developed to investigate the free vibration behaviors of the cylindrical shell with stepped-thickness (CS-S) and cylindrical shell with internal–external stiffeners (CS-IES) subject to arbitrary boundary constraints. The spring parameter technique is used to impose combined connections and arbitrary boundaries. The potential and kinetic energy expressions of CS-S and CS-IES are derived based on the framework of the first-order shear deformation theory (FSDT). Finally, the unified governing differential equations of CS-S and CS-IES are acquired by the Hamilton’s variational principle. According to the Gauss–Lobatto point discretization process, the Tchebyshev polynomials are used to perform spectral expansion on the axial variables of admissible displacement functions. Circumferential variables are represented by sine and cosine series. In this way, the unified free vibration mathematical model of CS-S and CS-IES is obtained. On the basis of confirming that the model results have reached the state of convergence, the numerical results are compared with those calculated by the reference and the finite element method, which fully validates that the strong spectral Tchebychev model obtained in this paper is feasible for analyzing the free vibration characteristics of CS-S and CS-IES subject to arbitrary boundary constraints. Last but not least, the paper carries out the parametric analysis of how the change of the geometric parameters of the stiffeners will affect the natural frequencies of CS-IES.

Design Optimization of a Mechanically Improved 499.8-MHz Single-Cell Superconducting Cavity for HEPS

Superconducting 499.8-MHz radio frequency (rf) cavities have been proposed for the High Energy Photon Source (HEPS), a 6-GeV diffraction-limited synchrotron light source currently under construction in Beijing. Being an active third-harmonic system, two cavities shall provide 3.5-MV rf voltage and 400 kW of beam power to enable a complex rf gymnastics required by a novel injection scheme. Adopting the veteran KEKB-type 500-MHz single-cell geometry, the cavity design has been focused on optimizing its mechanical properties. Various cavity wall thickness and external stiffening mechanisms were investigated to reinforce the cavity to ensure compliance to the pressure vessel codes under different working conditions from a bare cavity to a dressed cavity in the cryomodule. Small margins on operational pressure previously reported on this type of cavity have been largely improved with slightly reduced yet acceptable frequency tunability. Extensive design optimizations have been conducted aiming for lower stress, larger elastic buckling pressure, and higher frequencies of mechanical modes to cause microphonics, while the frequency tunability, pressure sensitivity, and Lorentz force detuning were being carefully monitored. In view of the HEPS’s lifetime, cavity fatigue was examined. This constitutes a comprehensive design optimization of the 499.8-MHz single-cell superconducting cavity with improved mechanical properties.

Study on shear capacity of connections with external stiffening rings between square steel tubular columns and steel beams

This paper studied the shear behavior of the connections with external stiffening rings between square steel tubular columns and steel beams by experimental, numerical and analytical methods. Two connections with external stiffening rings were tested under low cyclic loading to investigate the effect of axial compression ratio on the shear behavior and capacity of the connection. The test result showed that the change of the axial compression ratio had little effect on the shear capacity of the connection while the ductility of the connection was decreasing with the increase of the axial compression ratio. Seven nonlinear finite element models were designed to investigate the seismic behavior of the connection under cyclic test. Parametric studies are carried out to study the influence of the following parameters on the shearing capacity and deformation in panel zone: the width and the height of the steel tube in panel zone and the thickness of the external stiffening rings. Finally, based on the model considering the post-buckling strength of the web of the steel tube in panel zone, a calculation formula was fitted by the results of the finite element simulation.

EXPERIMENTAL STUDY ON THE SEISMIC BEHAVIOR OF CFST RECTANGULAR COLUMN TO H-SECTION STEEL BEAM CONNECTIONS WITH EXTERNAL STIFFENERS

In view of the decreasing trend of column widths in steel residential buildings, a new type of concrete filled rectangular steel tubular column-H-section steel beam connections with external stiffeners is proposed. The diaphragm is removed in this connection. Pseudo-static tests of seven new joint specimens were carried out. The test parameters were the section of the steel beam and the thickness and the height of the long side of the external stiffeners. The failure modes, hysteretic curves, skeleton curves, strength, stiffness degradation, strength degradation, ductility and energy dissipation capacity of the joints were studied. The test results show that there were three failure modes of this new joint, namely, tensile failure of beam flange, weld failure between beam flange and external stiffener, and column wall failure near the external stiffener. The ductility coefficient was 2.17 to 3.67, the equivalent viscous damping coefficient was 0.2 to 0.3, and the average drift ratio was 1/49. The seismic performance of the joint met the requirements of strong joints and weak members. Increasing the thickness or the height of the long side of the external stiffeners can increase the strengths of the joints. The premature cracking of the beam flange, the welding seam between beam flange and external stiffener, or the column wall will reduce the ductility and energy dissipation capacity of the joints. It is proposed that the connection coefficient should be 1.4 when checking the ultimate strength of the column wall and joint weld. The strengthening or weakening measures on the flange at the beam ends can be taken to ensure that the new joints have a sufficient plastic deformation capacity.

Study on shear failure of joint with external stiffener under seismic loads

This paper aimed to investigate the shear failure of joint with external stiffener between square hollow section (SHS) column and H-shaped beam under seismic loads. The quasi-static tests were performed on four specimens. The typical shear failure on panel zones was observed. The hysteretic curves and skeleton curves exhibit the excellent capacity of energy dissipation and ductility of this type joint. The ductility coefficient and energy dissipation coefficient were adopted to quantitatively analyze the effects on ductility and energy dissipation of different parameters. The nonlinear finite element models were developed to carry out further parametric studies. The feasibility of FE analysis was validated by the experimental results. The parametric studies systematically show the effect rules of each parameter. A superposed method was proposed to calculate the shear strength of the joint with external stiffener. The theoretical formula provides an accurate result which accords well the test and FE analysis.

Effect of external strengthening on the flexural capacity of cold-formed steel beams

Cold-formed steel (CFS) members face stability problems mainly due to their thin-walled features. Such issues have been addressed by different techniques in the past. Local strengthening of the vulnerable regions by various external stiffening techniques has immense potential to delay the local instabilities experiences at these vulnerable locations. It may not only be effective by economical as well. This paper discusses an experimental investigation on the flexural behaviour of uniquely strengthened open CFS beams, comprising of channels, and tested under pure flexural loading with simply supported end support conditions. The test results were mainly drawn in terms of strength and stiffness characteristics of these CFS open beams, which helped in quantifying the efficiency of such uniquely strengthened beams. Lastly, the theoretical capacities were computed using the current codes, which were later compared against the test strengths. It was concluded that these uniquely strengthening techniques has the potential of improve the flexural strengths of the CFS beams conveniently.

Experimental and Numerical Investigation of Axial Tensile Strength of CHS X-Joints Reinforced with External Stiffening Rings

To study the reinforcement effect of the external stiffening ring reinforced and unreinforced X-joints having a circular hollow section subjected to brace tensile load, an experimental and computer-simulated research has been done. Six full-scale X-joints are examined in this study: three reinforced specimens and three corresponding unreinforced ones. The test specimens cover different brace to chord diameters ratios (β) of 0.25, 0.51, and 0.73. The experimental facilities and detailed parameters are presented. The experimental results including failure modes, load–displacement curves, and ultimate capacity are analyzed and compared. Two failure modes, weld failure and chord plastification, are observed in the tests. The test results reveal an ultimate load enhancement in the reinforced joints by approximately 50% in comparison with the unreinforced joints and an initial stiffness increase of 248.1% for the β of 0.25 and approximately 49–76% for the β of 0.51 and 0.73. A finite element model using shell elements is developed, which accurately predicts the static performance of the X-joints subjected to brace tensile load with and without external stiffening rings.

Buckling Analysis of Stiffened Cone-Cylinder Intersection Subjected to External Pressure

This paper aims to examine the influence of ring stiffener reinforcement on the buckling behavior of cone-cylinder intersection subjected to external pressure. Three different stiffener locations were analyzed; they are (i) cone-cylinder intersection, (ii) cone mid-section, and (iii) cylinder mid-section. Internal and external stiffeners are used in the study. Buckling strength of the stiffened cone-cylinder intersections were obtained with the aid of FE analyses for all the cases above. Cone-cylinder intersections are modelled as elastic-perfectly plastic of Hiduminium alloy (HE-15). Validations of the present results with the published experimental data were presented. Result indicates that the introduction of stiffener reinforcement in externally pressurized cone-cylinder intersections seemed to be desirable in a preliminary design process, thus promotes a stronger shell to resist buckling.

Studies on Axial Tensile Loading Capacity of CHS T-joints Reinforced with External Stiffening Rings

To analyze the effects of stiffening external rings on the brace axial tension capacity of circular hollow section T-joints (CHS-T), loading experiments and finite element model analysis were utilized. Three groups of full-scale specimens (reinforced and unreinforced T-joints) of different brace-to-chord diameter ratios were tested under tension to validate the strengthening effect (Three unreinforced specimens were previously tested and the other three reinforced samples were newly completed). The experimental equipment and parameters are described in detail. Load–ovalization and load–displacement curves in addition to the failure modes were analyzed. It is concluded that the ultimate strength and initial stiffness are significantly enhanced for the reinforced CHS joints compared to the unreinforced specimens. SHELL181 elements were utilized in the finite element analysis to accurately simulate the loading capacity with and without external stiffening rings of the joints, within 10% error.

Stiffened concrete-filled steel tubes: A systematic review

Abstract The behavior of concrete filled steel tubular (CFST) elements with different types of stiffening measures has been wildly investigated during the past two decades through experimental, analytical, and numerical approaches. The previous studies highlighted the importance of the presence of internal and external stiffeners to enhance the structural performance of CFST elements under different loading conditions. In this paper, a systematic methodology is proposed in order to rigorously review the current knowledge in literature about stiffened CFSTs. The reviewed studies include tests of static axial compression, pure bending and combined of static loading actions as well as the cyclic behavior and the fire resistance. A coherent taxonomy regarding the research of stiffened CFSTs is proposed based on the loading conditions besides their cross-sectional shape and type of stiffeners. The study also presents a number of practical applications of stiffened CFSTs in different engineering projects. Finally, concluding remarks, current research gap and future research directions on stiffened CFST are provided and discussed.