Tubular T-joints Research Articles

Welded joints under multiaxial non-proportional loading

The present paper examines the fatigue behaviour of a fillet-welded tubular T-joint in the so-called H structural component of an agricultural sprayer. Since the experimental measurements, performed in some control locations of the equipment, have highlighted a stress/strain field of random nature, an equivalent deterministic cyclic loading is here defined in order to simplify the fatigue analysis. Such an equivalent loading is able to accumulate damage values equal to the experimental ones in correspondence of the above control locations. The present study shows that it is possible to neglect the actual nature of loading provided that a suitable equivalent deterministic cyclic loading is taken into account.

Axial hysteretic behavior of doubler-plate reinforced square hollow section tubular T-joints

Doubler-plate reinforcement does not impinge on the appearance of tubular joints, and thus is widely used to improve their strength. To understand the influence of doubler-plates on the axial hysteretic behavior of square hollow section (SHS) tubular joints, two groups of unreinforced and doubler-plate reinforced (DPR) joints were experimental tested and numerically simulated under axial cyclic loading. Parametric studies were conducted to identify key parameters that influence the hysteretic behavior of DPR joints. The design of the specimen and set up of the tests are presented, and failure sequences from the experimental tests are described. The hysteretic curves, skeleton curves, energy-dissipation capacity, ductility, strength, as well as the hysteretic mechanisms of the joints, are discussed. The doubler-plate is found to effectively prevent a crack from developing towards the chord flange and to protect the integrity of the chord, which maintains the compressive capacity of the DPR joints after cracking. The doubler-plate can also increase the strength of the joints, but with a decrease in the energy dissipation capacity and ductility. The brace-chord width ratio and the doubler-plate to chord width ratio are the key parameters that influence the load transfer mechanisms and failure modes of the DPR joints, and the configuration suggestions for DPR joints are proposed.

Experimental studies on behaviour of tubular T-joints reinforced with grouted sleeve

Tubular joints have been widely used in offshore platforms and space structures due to their merits such as easy fabrication, aesthetic appearance and better static strength. For existing tubular joints, a grouted sleeve reinforced method was proposed in this paper. Experimental tests on five tubular T-joints reinforced with the grouted sleeve and two conventional tubular T-joints were conducted to investigate their mechanical behaviour. A constant axial compressive force was applied to the chord end to simulate the compressive state of the chord member during the tests. Then an axial compressive force was applied to the top end of the brace member until the collapse of the joint specimens occurred. The parameters investigated herein were the grout thickness, the sleeve length coefficient and the sleeve construction method. The failure mode, ultimate load, initial stiffness and deformability of these joint specimens were discussed. It was found that: (1) The grouted sleeve could change the failure mode of tubular T-joints. (2) The grouted sleeve was observed to provide strength enhancement up to 154.3%~172.7% for the corresponding un-reinforced joint. (3) The initial stiffness and deformability were also greatly improved by the grouted sleeve. (4) The sleeve length coefficient was a key parameter for the improved effect of the grouted sleeve reinforced method.

Dynamic performance of collar plate reinforced tubular T-joint with precompression chord

This paper examines the dynamic behaviour of collar plate reinforced tubular T-joints with precompression chord by means of experimental and numerical studies. An experimental programme involved one test on unstiffened tubular T-joint and three on stiffened configurations is carried out through a high-performance drop hammer machine. A series of disc springs is installed to apply axial compression to the chord. The dynamic response of four tubular T-joints are described and discussed with emphasis on the effect of the stiffener, especially for dimensional parameters of collar plate. Based on the experimental results, the key behavioural patterns including the development of impact force, deformation and strain, as well as deformation modes are identified. The impact mechanism is also investigated. The finite element (FE) models of the tubular T-joints subjected to impact loadings are then developed by commercial software of ABAQUS, where nonlinear material behaviour and pre-compressive loads are all considered. The numerical findings exhibit close agreement with experimental results in terms of failure pattern as well as the development of impact force and the corresponding displacements. Finally, the dynamic performance of unreinforced and collar plate reinforced tubular T-joints is revealed based on the numerical simulation. Particularly, the joint deformation and impact resistance are examined by the equivalent area method and the formula in NORSOK Standard, respectively. In general, the presence of collar plate stiffeners significantly improves the impact resistance of tubular T-joints.

Experimental study of stiffening rings reinforced tubular T-joint with precompression chord

Abstract This paper examines the dynamic behaviour of stiffening rings reinforced tubular T-joints with precompression chord by means of experimental studies. An experimental programme involved one test on unstiffened tubular T-joint and three on stiffened configurations is carried out through a high-performance drop hammer machine. A series of disc springs is installed to apply axial compression to the chord. The dynamic response of four tubular T-joints are described and discussed with emphasis on the dimensional parameters of stiffening rings. Based on the experimental results, the key behavioural patterns including the development of impact force, deformation and strain, as well as deformation modes are identified. In general, the presence of stiffening rings significantly improves the impact resistance of tubular T-joints.

Static test on failure process of tubular T-joints with initial fatigue crack

Fatigue crack initiated in welded tubular joints due to cyclic loading may produce harmful effect on the integrity of the tubular structures. To study such effect, both fatigue and static tests on nine circular tubular T-joint specimens made of carbon steel materials were carried out. The specimens were subjected to tensile loading in both fatigue and static tests. The load-displacement relation, the crack propagation and the failure mode of the specimens are all analyzed. The deterioration of the static strength of the cracked T-joints is also investigated and evaluated through an area reduction factor. Experimental results indicate that the static strength of a tubular T-joint with a surface crack seems to decrease slightly while a through crack has relatively remarkable effect on the reduction of the static strength. Additionally, experimental results also show that the toughness of the materials and the geometry of the specimens play an important role on the failure process of cracked tubular T-joints.

Fatigue strength of welded joints under multiaxial non-proportional loading

Fatigue behaviour of a fillet-welded tubular T-joint in the so-called H structural component of an agricultural sprayer is examined by using experimental strain measurements found in the literature and linear elastic finite element analyses. Two stress-based critical plane criteria are applied at a verification point located near the intersection between the end of brace and chord of the above-mentioned T-joint, where crack initiation and growth is experimentally observed.

Behaviour of carbon fiber reinforced polymer strengthened tubular joints

This paper highlights the experimental and numerical investigations performed on a tubular T-joint fabricated from circular hollow sections under axial compressive loads applied at the brace. Tests were performed on a reference joint and the joint wrapped with Carbon Fiber Reinforced Polymer (CFRP). The Nitowrap EP carbon fiber with Nitowrap 410 resin serve as a composite material is used for wrapping the T-joint. Schematic diagram of the fabricated tubular joint for the experimental test setup, along with the experimental and numerical results are presented. After performing these experiments, it has been demonstrated that the joint wrapped with CFRP has a better strength and lesser deflection than a reference joint. Finite element analysis carried out in Ansys reveals that the results were in good correlation with the experimental values.

Hysteretic behavior of tubular T-joints after exposure to elevated temperature

Experimental tests on circular hollow section (CHS) tubular T-joints under axial loading after exposure to elevated temperature are conducted to investigate their hysteretic behavior. Two full-scale specimens with different brace-to-chord diameter ratios are heated firstly in an electrical furnace to 500°C and then they are cooled down to ambient temperature. After exposure to elevated temperature, the two specimens are tested together with two corresponding identical T-joints without exposure to elevated temperature in order to compare the different hysteretic performance. In the tests, all the specimens experience cyclic axial loading at the brace end while the ends of the chord are pinned. From experimental observation, both the heated and the unheated specimens are found to experience local yielding in compression and cracking in tension around the brace/chord intersection before final failure. To investigate the influence of elevated temperature on the hysteretic behavior, hysteretic curves, skeleton curves, ductility, accumulative energy dissipation, strength and rigidity degradation are all analyzed based on experimentally measured results. The detailed analyses indicate heating and cooling process in fire condition has minor effect on the hysteretic behavior of CHS T-joints subjected to service loading.

Fatigue life estimation of fillet-welded tubular T-joints subjected to multiaxial loading

This study investigates the applicability of a stress-based approach to multiaxial fatigue strength assessment of fillet-welded tubular T-joints, used in the so-called “H” component of an agricultural sprayer. Experimental strain measurements on such a structural component under sprayer service conditions have been performed. A finite element analysis of the component is herein developed under linear elastic hypothesis. Finally, a stress-based critical plane criterion is applied to several material points located in the proximity of the intersection between the end of a brace and a chord of one above-mentioned T-joint, where crack nucleation is experimentally observed.

Discussion on two methods for determining static strength of tubular T-joints at elevated temperature

To predict the static strength of a welded tubular joint at elevated temperature using finite element simulation, two methods in the literature were reported. The first method aims to analyze the static strength of a tubular joint at a specified elevated temperature, and a routine mechanical analysis is carried out by defining the material properties at the specified elevated temperature according to some specifications. This method does not consider the heat transfer process of the tubular joint in a fire condition. The second method is used to determine the static strength of a tubular joint using a combination of transient state heat transfer analysis and mechanical analysis. The tubular joint subjected to a specified load is heated in accordance with ISO 834-1 standard fire curve to fail at a critical temperature, and the specified load is considered as the static strength of the joint at the critical temperature. In this study, a detailed parametric study on the failure process of circular hollow section tubular T-joints at elevated temperature is carried out using finite element method. The static strengths of the circular hollow section T-joint models obtained from the two methods are compared. The comparison shows that the first method produces a higher estimation on the static strength compared to the second method. Finally, the effect of some geometrical parameters, chord stress ratio, and elevated temperature on the difference of the two methods is also investigated.

Hysteretic behaviour of circular tubular T-joints with local chord reinforcement

When a welded circular hollow section (CHS) tubular joint is subjected to brace axial loading, failure position is located usually at the weld toe on the chord surface due to the weak flexural stiffness of the thin-walled chord. The failure mode is local yielding or buckling in most cases for a tubular joint subjected to axial load at the brace end. Especially when a cyclic axial load is applied, fracture failure at the weld toe may occur because both high stress concentration and welding residual stress along the brace/chord intersection cause the material in this region to become brittle. To improve the ductility as well as to increase the static strength, a tubular joint can be reinforced by increasing the chord thickness locally near the brace/chord intersection. Both experimental investigation and finite element analysis have been carried out to study the hysteretic behaviour of the reinforced tubular joint. In the experimental study, the hysteretic performance of two full-scale circular tubular T-joints subjected to cyclic load in the axial direction of the brace was investigated. The two specimens include a reinforced specimen by increasing the wall thickness of the chord locally at the brace/chord intersection and a corresponding un-reinforced specimen. The hysteretic loops are obtained from the measured load-displacement curves. Based on the hysteretic curves, it is found that the reinforced specimen is more ductile than the un-reinforced one because no fracture failure is observed after experiencing similar loading cycles. The area enclosed by the hysteretic curves of the reinforced specimen is much bigger, which shows that more energy can be dissipated by the reinforced specimen to indicate the advantage of the reinforcing method in resisting seismic action. Additionally, finite element analysis is carried out to study the effect of the thickness and the length of the reinforced chord segment on the hysteretic behaviour of CHS tubular T-joints. The optimized reinforcing method is recommended for design purposes.

Static strength of collar-plate reinforced tubular T-joints under axial loading

To study the effect of collar-plate reinforcement on the static strength of tubular T-joints under axial loading, fundamental research work is carried out from both experimental test and finite element (FE) simulation. Through experimental tests on 7 collar-plate reinforced and 7 corresponding un-reinforced tubular T-joints under axial loading, the reinforcing efficiency is investigated. Thereafter, the static strengths of the above 14 models are analyzed by using FE method, and it is found that the numerical results agree reasonably well with the experimental data to prove the accuracy of the presented FE model. Additionally, a parametric study is conducted to analyze the effect of some geometrical parameters, i.e., the brace-to-chord diameter ratio β, the chord diameter-to-chord wall thickness ratio 2γ, collar-plate thickness to chord wall thickness ratio τc, and collar-plate length to brace diameter ratio lc/d1, on the static strength of a tubular T-joint. The parametric study shows that the static strength can be greatly improved by increasing the collar-plate thickness to chord wall thickness ratio τc and the collar-plate length to brace diameter ratio lc/d1. Based on the numerical results, parametric equations are obtained from curving fitting technique to estimate the static strength of a tubular T-joint with collar-plate reinforcement under axial loading, and the accuracy of these equations is also evaluated from error analysis.

Structural analyses of reinforced tubular T-joints

The use of rolled hollow sections has been substantially boosted mainly due to the advantages associated with structural behavior and aesthetics, leading to an intense use in Europe, Southeast Asia, North America, Australia, and now, in Brazil due to the wider supply of these profiles. Therefore, it is important to investigate the structural behavior in order to provide an adequate structural design for the civil engineering community. Thus, this paper presents a parametric analysis of a reinforced T-joint focused on NBR 16239 provisions. Two types of reinforcement plates: Collar and Double were investigated. A wide set of numerical models has been defined varying the thickness of the reinforcing plate, chord and brace members and axial loads applied in the brace. The numerical models have been developed using ANSYS 12.0 software considering both geometrical and material non-linearity. Concerning the results, there was a slight gain of resistance when a double plate reinforcement was used, mainly for small displacement, due to large stiffness provided and a linear response up to the serviceability limit. In addition, Von Mises stress distribution confirmed the type A failure with chord yielding beginning at the upper chord surface. Comparing the numerical results with NBR 16239 provisions, an excessive conservatism was noted for this code. In fact, the Brazilian code only takes into account the reinforcement thickness in joint resistance. However, when the results provided by the new proposal where both thicknesses (chord and reinforcement) are considered, a more realistic assessment of the joint capacity is obtained.

Concrete-Filled Steel and Steel Tubular T-Joints under Cyclic in-Plane Bending

A series of tests on the concrete-filled and steel tubular T-joints under in-plane bending cyclic loading are presented in this paper. Two steel tubes were welded to form T-joint specimens, and then the chord was filled with self-compact concrete. For each test specimen the chord was loaded in a constant compression while the brace was under quasi-static cyclic in-plane bending loading. In total three concrete filled steel T-joints and two pure steel T-joints were tested. The failure modes, hysteretic curves and ultimate strengths of test specimens were obtained. The failure modes of all specimens were punching shear failure of the chord. It was shown that the filled concrete greatly enhanced the stiffness, the yielding strength and the ultimate strengths of T-joints. The energy dissipation ratio of the specimens under cyclic loading was calculated. The energy dissipation ability of concrete-filled specimens was better than that of steel specimens. A method provided by ECCS was used to calculate the yield strength of the test specimens. The ultimate strengths predicted by current AISC Standard and API RP2A specifications were compared with the test results.

Performance of tubular T-joints at elevated temperature by considering effect of chord compressive stress

This paper aims to study the performance of welded circular tubular T-joints subjected to axial compressive loads at the ends of both the brace and the chord at elevated temperature. Different from previous studies on circular hollow section (CHS) T-joints under brace axial load at elevated temperature, the effect of the chord stress is also taken into consideration. Overall six specimens are designed and tested in an environment in accordance with ISO 834 heating process. Different levels of compressive loads are applied in the axial direction of the chord to investigate their effect on the fire resistance of the specimens. Temperature development at some critical positions around the brace/chord intersection is measured during experimental test. The developments of defined global and local deformations during heating process are also obtained. The failure modes of all specimens are found to be characterized by chord plastification around the brace/chord intersection. By introducing an elastic modulus reduction factor and a yield strength reduction factor, the feasibility of some equations provided by Eurocode EN 1993-1-2, CIDECT and API RP 2A WSD for calculating the static strength of the specimens at elevated temperature are evaluated based on the experimental results.

Generation and application of a standardized load-time history to tubular T-joints in offshore platforms

Marine structures are mostly made of metals and always experience complex random loading during their service periods. The fatigue crack growth behaviors of metal materials have been proved from laboratory tests to be sensitive to the loading sequence encountered. In order to take account of the loading sequence effect, fatigue life prediction should be based on fatigue crack propagation (FCP) theory rather than the currently used cumulative fatigue damage (CFD) theory. A unified fatigue life prediction (UFLP) method for marine structures has been proposed by the authors’ group. In order to apply the UFLP method for newly designed structures, authorities such as the classification societies should provide a standardized load-time history (SLH) such as the TWIST and FALSTAFF sequences for transport and fighter aircraft. This paper mainly aims at proposing a procedure to generate the SLHs for marine structures based on a short-term loading sample and to provide an illustration on how to use the presented SLH to a typical tubular T-joint in an offshore platform based on the UFLP method.

Determining the Limit Plastic Load in T-Joints of Pipelines Subjected to Compound Combined Loading

An applied technique of nonlinear analysis and the results of a determination of the limit plastic loads for tubular T-joints subjected to compound combined loading are presented. A T-joint is considered as an intersection of cylindrical shells. A numerical nonlinear analysis of the connectors is performed with the use of the method of finite elements and shell theory, taking into account geometric nonlinearity in a quadratic approximation, the theory of plastic fl ow, and the iteration procedure of the Newton–Raphson method. A new method based on a maximum criterion of the rate of growth of the relative plastic work is used to determine the limit plastic loads. Graphical results in the form of generalized curves of the limit plastic loads for three different sequences of application of a compound load are presented for a T-joint subjected to the joint action of internal pressure and a bending moment applied to a branch.

Fire resistance behaviour of tubular T-joints reinforced with collar plates

The fire resistance behaviour of tubular T-joints is experimentally and numerically investigated. Two specimens are tested to failure under a constant brace axial load at elevated temperatures. One specimen is reinforced with a collar plate at the brace/chord intersection and the other is not reinforced. Both specimens have identical geometric and material properties for comparison. The failure modes of the specimens without and with reinforcement are the local indent of the chord and the local indent of the chord with axial yielding of the brace, respectively. In the numerical study, a finite element (FE) package ABAQUS with a sequentially coupled thermal stress analysis is adopted to simulate the joint specimens. The FE and experimental results for the critical temperature, fire endurance time and failure mode are in good agreement. The FE method is subsequently used to conduct a parametric study. The failure modes and load ratio have a significant influence on the fire resistance of the joint. Finally, the critical temperatures obtained from the experiment and the numerical simulation are compared with the proposed equation derived from CIDECT and Eurocode 3. It is found that the proposed equation provides a reasonable prediction of the critical temperature of tubular joints reinforced with collar plates at load ratios ranging from 0.2 to 0.6.

Prediction of stress distribution along the intersection of tubular T-joints by a novel structural stress approach

The Zero Point Structural Stress (ZPSS) approach is improved to calculate the structural stress for the fatigue life assessment of tubular joints. In the ZPSS approach, the stress distribution perpendicular to the stress classification line (SCL) can be obtained directly by post-processing the finite element (FE) results. The applicability and accuracy of the approach is verified by comparing the FE results with fatigue experimental test data. High fit degree parametric equations for stress concentration factor calculation are derived based on the ZPSS approach for stress analysis of tubular T-joints in engineering.