Unstiffened Joints Research Articles

Hysteretic behavior of eccentric RHS beam-to-column joints under cyclic in-plane bending

This paper conducted experimental and numerical studies on the hysteretic behavior of eccentric rectangular hollow section (RHS) beam-to-column joints under cyclic in-plane bending. The study began with the design of both stiffened and unstiffened joints, followed by hysteresis tests that revealed failure modes like web weld tearing and stiffener fracture. The seismic performance of the joints was evaluated using hysteresis curves, skeleton curves, ductility coefficients, strength degradation coefficients, and energy dissipation coefficients. Compared to unstiffened joints, the stiffened joints exhibited a 30 % increase in peak load and a 18 % improvement in maximum energy dissipation coefficient. Subsequently, finite element (FE) analysis was performed, and the influence of key parameters on the hysteretic behavior was studied using validated FE models. The results indicated that the hysteretic behavior was positively correlated with the flange width ratio of beam to column, the ratio of beam section height to column flange width, and the thickness ratio of beam to column, while it was negatively correlated with the width-to-thickness ratio of the column. Finally, a mathematical model for the hysteretic behavior of both stiffened and unstiffened joints was developed and validated through experimental and FE comparison, offering practical applicability in engineering design.

Simplified methods for the strength of ring-stiffened tubular T/Y-joints

ABSTRACT The method of design the ring-stiffened joints has not been addressed by design codes similar to unstiffened joints. However, API RP 2A has made some suggestions for design ring-stiffened joints such as shear capacity and equivalent thickness approaches. The shear capacity approach uses the shear strength of ring webs in addition to unstiffened capacity, while the equivalent thickness approach uses the unstiffened capacity method with increased thickness due to rings. The results obtained from parametric studies using these two approaches have been presented and compared with the results of the empirical approach proposed by Ragupathi and Nallayarasu and API RP 2A. It is found that the shear capacity approach is under- and over-predicting capacity for axial and moment. The equivalent thickness grossly violates the joint configuration and hence shall not be used.

Experimental and numerical investigation of ultimate strength of ring-stiffened tubular T-joints under axial compression

The design procedure based on empirical equation for computing the chord strength of tubular joints in offshore structures has been established by various researchers and adopted by American Petroleum Institute and other organizations. Local stiffening of chord may be required to carry the loads from braces from various directions. One such method is to introduce internal circular rings at the brace-chord interface. In the present study experimental and numerical investigation on a ring-stiffened tubular joint have been carried out using 1:8 scale model of commonly adopted configuration used in the industry. Experiments have been conducted to measure the failure loads of stiffened and unstiffened tubular joints of (T-geometry) with plain and flanged ring-stiffeners fitted inside the chord. Models were fabricated using steel material and tests were conducted to measure the chord deformation using displacement transducers. First peak in load-displacement relationship has been used to determine the ultimate capacity. Numerical simulations have been carried out using ABAQUS software based on nonlinear stress-strain relationship of steel. The load-displacement characteristics for stiffened and unstiffened joints have been established. Results obtained from numerical simulation have been compared with that obtained from the experimental studies. Strength enhancement factors for stiffened joints have been calculated and compared with that of the unstiffened joints. The enhancement factors for plain and flanged internal ring-stiffener are noted to be 1.66 and 2.03 respectively.

Development of parametric equations for ultimate capacity of internally ring-stiffened tubular T/Y-joints under axial and moment load

ABSTRACT Tubular joints in offshore structures is being designed using semi-empirical equations developed based on experimental and numerical studies and same were recommend for use in API-RP-2A and other codes. The parametric equation does not cover the tubular joints reinforced with internal ring-stiffeners. Hence, an attempt is made in the present study to develop parametric equations as a capacity enhancement factor (Qr ) to ultimate strength equation recommended for unstiffened joints by API-RP-2A. The development of Qr for T/Y-joints under axial and moment load has been carried out using non-linear finite element. Numerical model has been validated using measured results from a scale model test conducted for this purpose and matches reasonably well. Parametric studies have been performed for various geometric non-dimensional parameters. The proposed parametric equations for ring-stiffeners in axial and moment load works well with the unstiffened equations specified in API-RP-2A enabling easy design procedure for ring-stiffened joints.

NUMERICAL ANALYSIS OF WELDED BEAM-COLUMN JOINTS IN ALUMINUM STRUCTURES

The objective of this study is to compare the performance for welded beam-column joints in aluminum structures using moment-rotation diagrams and to evaluate the influence of welding on joint rigidity. A nonlinear numerical calculation of welded beam–column joints is performed using the finite element method. Material and geometric nonlinearities were included in the model. The connection was analyzed for two cases: variants with and without welded stiffener plates. Two material models were used. The first was derived from the applicable standard for aluminum structures, while the other was based on the results of a previous material study and calibrated using a separate numerical calculation. From the results of the calculations, moment-rotation diagrams for different geometries and material properties of joints were constructed and compared. The obtained moment-rotation diagrams for the stiffened and unstiffened joints demonstrate that stiffening of the joints in aluminum structures improves the properties of the joint in terms of higher rigidity, resistance, and ductility, regardless of the existence of the heat affected zone.

Ultimate capacity of bulge formed T-joints under brace axial compressive loading

An innovative tubular joint reinforced by a bulge plate was proposed for the first time, named the bulge formed joint. Different from the unstiffened joint, an additional bulge plate is employed to connect the braces and the chord. To investigate the ultimate capacity, firstly full scale experiments and finite element (FE) analyses were conducted for a bulge formed T-joint and its counterpart (an unstiffened joint). Verified by test, the FE method was used to investigate the ultimate capacity of more bugle formed T-joints under brace axial compressive loading. Totally 190 FE models of the bulge formed T-joints and 146 FE models of the unstiffened joints (the counterpart) were analyzed. As a result, four failure modes were found for the bulge formed T-joints, and the ultimate capacity of the bulge formed T-joint was proved to be increased up to 200% compared with that of the unstiffened joint. Finally, the effects of geometrical parameters (τS, η1, η2, α, β, γ and τ) on the ultimate capacity were discussed, among these geometrical parameters, the first three are particularly defined for the bulge plate, while the other four are same as those of the unstiffened joint.

Stress concentration factors (SCFs) of bulge formed K-joints under balanced axial loads

A novel stiffened joint called bulge formed joint was put forward. Compared with the unstiffened joint, an additional bulge plate is utilized to connect the chord and braces. Based on the finite element (FE) and experimental method, stress concentration factors (SCFs) were investigated for both a bulge formed K-joints and the corresponding unstiffened joint. By the verification of experimental data, the FE models were used to investigate the SCFs of the bulge formed joints. The maximum SCF of the bulge formed K-joint under balanced axial loads is located at the position φ=105°, which is close to but not exactly the saddle. The SCFs around the intersection weld can decrease remarkably compared with the unstiffened joint by the change of geometrical parameters of the bulge plate. Then 2117 FE analyses were conducted to investigate the geometrical effects on SCFs of the bulge formed joint. These dimensionless geometrical parameters include τs, η1, η2, θ, β, γ, τ, among which, the first three parameters are typical of the bulge formed joints, while the others are same as the definitions in the unstiffened joints. Finally, a set of SCF parametric formulas were obtained by nonlinear regression analyses.

Behaviour of square hollow section brace-H-shaped steel chord T-joints under axial compression

To research mechanical behaviour of square hollow section (SHS) brace-H-shaped steel chord T-joints under axial compression, eight different β joint specimens with different external stiffeners arrangements at the foot of SHS brace were tested under axial compression in brace. The axial compression force was applied to the top end of the SHS brace member, which was weld to the center of the chord member. The test procedures, joint failure modes, jack load–vertical displacement curves and jack load–strain intensity curves were presented in the paper. The effect of β (defined as the brace width to the flange of H-shaped steel chord width ratio) and the external stiffeners at the foot of SHS brace on the mechanical behaviour of the T-joints was also studied. The corresponding finite element analysis was also performed and calibrated against the test results. An extensive parametric study was carried out to evaluate the effect of geometric parameters on mechanical behavior of the T-joints under axial compression. Results of these tests show that local buckling failure of brace member was the main failure mode observed during the tests. 45° oblique external stiffeners could not improve the ultimate capacity of unstiffened joints under axial compression, while 90°normal external stiffeners could effectively improve the ultimate capacity of unstiffened joints with large β, even decrease the ultimate capacity of unstiffened joints with small β. Increasing the thickness of stiffeners can slightly improve the ultimate axial compressive capacity of stiffened joints. It is shown that the design strengths predicted by the current design rules are conservative for the unstiffened test specimens. The joint strengths obtained from the parametric study were compared with the design strengths calculated using current Eurocode 3. It is shown from comparison that the current Eurocode 3 is dangerous for the SHS brace-H-shaped steel chord T-joints under axial compression. The new design equations were proposed for the SHS brace-H-shaped steel chord T-joints under axial compression, which were verified to be safe and accurate.

Finite Element Modelling of the Behaviour of a Certain Class of Composite Steel-Concrete Beam-To-Column Joints / Skonczenie-Elementowe Modelowanie Zachowania Sie Pewnej Klasy Wezłów Zespolonych Stalowo-Betonowych W Połaczeniach Rygli Ze Słupami

Abstract Beam-to-column end-plate joints can be classified as rigid (fully restrained), semi-rigid (partially restrained) or pinned, depending on their type, configuration and the connector arrangement. Fully restrained joints are needed for rigid frames in which there is assumed that the frame joints have sufficient rigidity to maintain - under the service state - the angles between the intersecting members, ensuring the full moment transfer. In contrast in semi-continuous frames, partially restrained joints are characterized by relative rotations occurring between the intersecting members so that the bending moment can only be transferred partially. In recent years, the idea of using partially restrained, unstiffened joints in building structures has gained momentum since this idea appears to be more practical and economical. Semi-continuous frames can resist actions by the bending moment transfer in partially restrained joints, allowing in the same time for a certain degree of rotation that enhances the overall ductile performance of these structures. One of the effective ways that affects ductility of end-plate beam-to-column joints is to use thinner end-plates than those used nowadays in practical applications. In the current study, a certain class of steel-concrete composite joints is examined in which the thickness of end-plates is to be equivalent to approximately 40-60% of the bolt diameter used in all the composite joints investigated in the considered joint class. This paper is an extension of the authors’ earlier investigation on numerical modelling of the behaviour of steel frame joints. The aim of current investigations is to develop as simple as possible and yet reliable three-dimensional (3D) FE model of the composite joint behaviour that is capable of capturing the important factors controlling the performance of steel-concrete end-plate joints in which the end-plate thickness is chosen to be lesser than that used nowadays in conventional joint detailing. A 3D FE model constructed for composite joints of the considered joint class is reported in this paper and numerical simulations using the ABAQUS computer code are validated against experimental investigations conducted at the Warsaw University of Technology. Comparison between the nonlinear FE analysis and full scale experimental results of the considered class of composite joints is presented which conclusively allows for the accuracy assessment of the modelling technique developed. Comparison between the FE results and test data shows a reasonable agreement between the numerical FE model developed and physical model of experimentally examined joint specimens. Finally, practical conclusions for engineering applications are drawn.

Offshore Tubular T-Joints Reinforced with Internal Plain Annular Ring Stiffeners

Although the use of internal ring stiffeners to enhance the strength of tubular joints is common practice in older platforms, no definitive guidance on the design of such joints is found in any of the major offshore design codes. This is the result of an acute lack of research in this area. In this paper, the findings of an analytical study on the strength of tubular T-joints with centrally located plain stiffeners under brace compression loading are presented. Data on the strength of 32 stiffened joints and their associated unstiffened joints were generated using a well-validated numerical model. It was found that the stiffening could accentuate chord bending failure and that strength prediction based on pure shear yield was grossly conservative. The influence of the various geometric parameters on stiffener strength was examined and the observed trends form the basis of an accurate predictive equation for the strength of the stiffeners, which was derived using regression analysis. A methodology for predicting the strength of stiffened joints based on the summation of the stiffener strength and the strength of the unstiffened joint is proposed.

Comparative behaviour of stiffened and unstiffened welded tubular joints of offshore platforms

The paper presents the results of an experimental investigation conducted on welded tubular joints, that are employed in offshore platforms, to study the behaviour and strength of these joints under axial brace compression loading. The geometrical configuration of the joints tested were T and Y. The nominal diameter of the chord and brace members of the joint were 324 and 219 mm respectively. The chord thickness was 12 mm and the brace 8 mm. The tested joints are approximately quarter size when compared to the largest joints in the platforms built in a shallow water depth of 80 m in the Bombay High field. Some of the joints were actually fabricated by a leading offshore agency which firm is directly involved in the fabrication of prototype structures. Strength of the internally ring-stiffened joints was found to be almost twice that of the unstiffened joints of the same configuration and dimensions. Bending of the chord as a whole was observed to be the predominant mode of deformation of the internally ring-stiffened joints in contrast to ovaling and punching shear of the unstiffened joints. It was observed in this investigation that unstiffened joint was stiffer in ovaling mode than in bending and that midspan deflection of unstiffened joint was insignificant when compared to that of the internally ring stiffened joint. The measured midspan deflection of the unstiffened joint in this investigation and its relation with the applied axial load compares very well with that predicted for the brace axial displacement by energy method published in the literature. A comparison of the measured deflection and ovaling of the unstiffened joint was made with that published by the author elsewhere in which numerical prediction of both quantities have been made using ANSYS software package. The agreement was found to be quite good.

Flexural behavior and resistance of uni-planar KK and X tubular joints

The importance of the research on moment-resistant properties of unstiffened tubular joints and the research background are introduced. The performed experimental research on the bending rigidity and capacity of the joints is reported. The emphasis is put on the discussion of the flexural behavior of the joints including sets of geometrical parameters of the joints and several loading combinations. Procedures and results of loading tests on four full size joints in planar KK and X configuration are described in details at first. Mechanical models are proposed to analyze the joint specimens. Three-dimensional nonlinear FE models are established and verified with the experimental results. By comparing the experimental data with the results of the analysis, it is reported reasonable to carry out the structural analysis under the assumption that the joint is fully rigidly connected, and their bending capacities can assure the strength of the members connected under certain limitation. Furthermore, a parametric formula for inplane bengding rigidity of T and Y type tubular joints is proposed on the basis of FE calculation and regression analysis. Compared with test results, it is shown that the parametric formula developed in this paper has good applicability.

Investigations on Internally Ring-Stiffened Joints of Offshore Platforms

The paper presents in detail the experimental and numerical investigations on tubular joints that are stiffened internally with three annular rings in order to study their behaviors and evaluate their strengths under axial brace compression loading. The nominal chord and brace diameters of the tested T and Y-joints were 324 and 219 mm and their thicknesses were 12 and 8 mm, respectively. The tested joints are approximately quarter-size when compared to the largest joints in the platforms built in a shallow water depth of 80 m in the Bombay High field. Some of the joints were actually fabricated by a leading offshore agency that is directly involved in the fabrication of prototype structures. Bending of the chord as a whole was observed to be the predominant mode of deformation of the internally ring-stiffened joints in contrast to ovaling and punching shear of the unstiffened joints. Strengths of the internally ring-stiffened joints were found to be almost twice that of unstiffened joints of the same dimensions. A tri-linear stress-strain model that takes into account the strain-hardening characteristics of the material was developed to assess the strength of the internally ring-stiffened joints. Finite element analysis (FEA) was performed to predict the bending deflection of the internally ring-stiffened joints under axial brace compression loading. This FEA used NISA II software package and was based on the tri-linear model proposed in this study. [S0892-7219(00)00104-7]

Behavior of Internally Ring-Stiffened Joints of Offshore Platforms

As part of a larger experimental program on assessment and rehabilitation of damaged, internally ring-stiffened, steel tubular joints of offshore platforms, experimental investigation was carried out on undamaged, internally ring-stiffened tubular joints to study their behavior, determine their strength, and establish a benchmark. The paper presents, in detail, the experimental investigation carried out on tubular joints that are stiffened internally with three annular rings under axial brace compression loadings. The chord and brace diameters of the tested T and Y joints were 324 and 219 mm and their thicknesses were 12 and 8 mm, respectively. The tested joints are approximately a quarter of the size of the largest joints in the platforms built in a shallow water depth of 80 m in the Bombay High field in the Arabian Sea. Bending of the chord as a whole was observed to be the predominant mode of deformation of the internally ring-stiffened joints, in contrast to the ovaling and punching shear of the unstiffened joints. Strength of the internally ring-stiffened joints was found to be almost twice that of the unstiffened joints of the same dimensions.

Viability of unstiffened offshore deck joints in high strength steel hot-rolled sections

Abstract This paper addresses the numerical analysis of offshore deck joints between hot-rolled H-sections using high strength steels. The viability of using economic unstiffened joints in the offshore industry, particularly in high strength steels, is demonstrated. Comparisons of ultimate load and costs are carried out with various parametric variations (geometry, steel grades Fe E 355 and Fe E 460, and configuration). Also, comparisons with Eurocode 3 and the American Petroleum Institute API-RP2A Load and Resistance Factor Design rules are carried out and deficiencies in the codes highlighted. The insight gained on the application of deck joints with hot-rolled H-sections in high strength steels is discussed.