Brace In-plane Bending Research Articles

Performance and design of full-width RHS X and T joints under brace axial tension and brace bending

Within the research framework for updating the international design standard ISO 14346 for hollow section joints, this study examines brace failure and chord sidewall failure in full-width rectangular hollow section (RHS) X and T joints under brace axial tension, brace in-plane bending and brace out-of-plane bending. The codified design rules for full-width RHS joints are reviewed, with their limitations highlighted. Design resistance equations for brace failure, based on the modified effective width method, and also for chord sidewall failure, using the modified bearing–buckling method and the Lan–Kuhn method, are then proposed. Up-to-date experimental and numerical results are collated from the literature, which cover a wide range of geometrical parameters, steel grades ranging from S235 to S960, varying weld details, and loading cases of brace axial tension, brace in-plane bending and brace out-of-plane bending. The compiled results are adopted to evaluate the performance of full-width RHS joints and to assess suitability of the proposed design resistance equations. It is shown that weld details can significantly affect the deformation capacity and static strengths of full-width RHS joints. The proposed design resistance equations yield conservative and reliable strength predictions for full-width RHS joints. Welding guidance and user-friendly design rules, in which an extension to include Class 3 cross-sections is included, are suggested for full-width RHS X and T joints.

Structural testing and numerical modelling of T-joints between cold-formed S690 circular hollow sections under brace in-plane bending

This paper presents a comprehensive investigation into the structural behaviour of T-joints between high strength S690 cold-formed circular hollow sections (CFCHS) under brace in-plane bending, and deformation characteristics of these T-joints under both monotonic and cyclic actions are examined experimentally and simulated numerically. These T-joints between S690 CFCHS were found to deform with a high level of resistances and ductility under both monotonic and cyclic actions.Through an integrated numerical modelling approach developed by the authors, advanced three-dimensional finite element models of CFCHS with solid elements were established. With a proper definition of weld collars at the brace/chord junctions of the T-joints between S690 CFCHS with different diameters and thicknesses, these models were readily adopted to perform a heat transfer analysis, a thermomechanical analysis and a structural analysis sequentially with compatible element types and meshes. Hence, the effects of the welding-induced residual stresses at the welded brace/chord junctions onto the structural behaviour of these T-joints were readily assessed. Consequently, these advanced models are demonstrated to be able to predict rationally deformation characteristics of these T-joints between S690 CFCHS under monotonic and cyclic actions of brace in-plane bending with a high degree of structural accuracy.

Design of chord sidewall failure in RHS joints using steel grades up to S960

It is well known that the current design rules adopted by international design codes such as ISO 14346 and design guides, e.g., the CIDECT design guide No. 3, for chord sidewall failure in mild steel rectangular hollow section (RHS) joints under brace axial compression are considerably conservative, if the RHS joints are adequately supported out-of-plane. This paper presents an investigation into chord sidewall failure in RHS joints using steel grades up to S960. Representative existing design methods for chord sidewall failure in RHS joints are reviewed, and two alternative design methods, i.e., the modified bearing–buckling method and the Lan–Kuhn​ method, are proposed. Up-to-date test and numerical results reported in the literature are compiled. A wide range of geometric parameters, steel grades up to S960 and loading cases of brace axial loading, brace in-plane bending and brace out-of-plane bending are covered. The existing and proposed design methods are assessed against the collated results. The effects of brace-to-chord height ratio, brace angle, steel grade and chord stress ratio are evaluated. It is shown that the proposed design methods can provide more consistent resistance predictions for chord sidewall failure in mild steel and high-strength steel RHS joints under brace axial compression. Corresponding user-friendly design rules are suggested. The design of chord sidewall failure in RHS joints under brace axial tension, brace in-plane bending and brace out-of-plane bending is discussed. Further required research on, in particular, high-strength steel RHS joints is highlighted.

Stress concentration factors of RHS T-connections with galvanizing holes under in-plane bending

The hot spot stress method recommended by the CIDECT Design Guide 8 cannot be directly used in the fatigue design of galvanized tubular steel connections since: (1) the hot-dipping process can sometimes significantly change the residual stress magnitude and distribution at the welded joint; and (2) sufficiently large vent and drain holes must be specified at the welded joint location prior to galvanizing, and quite often at the locations recommended by the design guide for calculation of hot spot stresses. Hence, the existing formulae and charts for determination of Stress Concentration Factors (SCFs) may not be applicable. In this study, Finite Element (FE) modelling was performed to examine the existing SCF formulae for welded rectangular hollow section (RHS) T-connections under brace in-plane bending. The FE models were validated by comparison of the experimental results from six large-scale connection tests. A parametric study including 192 FE models with varied hole locations, brace-to-chord width ratios, chord wall slendernesses, and brace-to-chord thickness ratios, shows that the existing SCF formulae can lead to unsafe predictions. Critical hot spot stress locations are thus identified. The effects of both brace in-plane bending and chord loading were studied. New design formulae that take the vent and drain holes into account are proposed. These formulae are shown to provide more accurate predictions of SCFs in such connections.

Hot spot stress and fatigue behavior of bird-beak SHS X-joints subjected to brace in-plane bending

Abstract This paper deals with the stress concentrations and fatigue behaviour of X-shape bird-beak joints under brace in-plane bending. Static load was first implemented to record the strain distribution along the hot lines, and then stress concentration factors (SCFs) were obtained through quadratic extrapolation method. Numerical analysis was then developed to further determine key parameters affecting SCFs and to propose corresponding SCF design formulas. During the subsequent high-cycle fatigue test, the fatigue cracking process including crack initiation, crack propagation, and fatigue lives, as well as the rigidity degradation were captured. Results show that crack initiation always occurred at the crown area where the SCFs are proved to be the greatest. The through-thickness lives may be utilized to conservatively represent the fatigue design lives of SHS bird-beak X-joints. Bird-beak SHS X-joints demonstrated better fatigue behaviors than those for traditional joints if the same service load is applied.

A toughness based deformation limit for fatigue-cracked X-joints under in-plane bending

This study reports a deformation limit for the initiation of ductile fracture failure in fatigue-cracked circular hollow section (CHS) X-joints subjected to brace in-plane bending. The proposed approach sets the deformation limit as the calculated crack driving force in a fatigue crack at the hot-spot location in the tubular joint reaches the material fracture toughness measured from standard fracture specimens. The calibration of the proposed approach and the numerical procedure utilizes two large-scale X-joint specimens with fatigue generated surface cracks. The subsequent numerical investigation covers X-joints with two different brace-to-chord intersection angles, a wide range of geometric parameters and a practical range of material parameters. The development of the deformation limit includes a non-dimensional material toughness, which covers both the geometric parameters and material properties. The lower-bound deformation limit thus developed exhibits a linear relationship with respect to the crack depth ratio and indicates consistent values among X-joints with different brace-to-chord intersection angles.

A deformation limit based on failure assessment diagram for fatigue-cracked X-joints under in-plane bending

This study develops a deformation limit based on the failure assessment diagram (FAD) for fatigue-cracked circular hollow section (CHS) X-joints subjected to brace in-plane bending. The proposed approach calibrates against two reported experimental tests on CHS X-joints. The subsequent numerical study considers CHS X-joints with two different brace-to-chord intersection angles. The parametric study focuses on both the material toughness level and the geometric parameters. The proposed, dimensionless deformation limit covers a wide range of material fracture-toughness and geometric parameters. The option 1 and option 3 failure assessment curves provide consistent deformation limits compared to the toughness-based deformation limits proposed in a previous study. The lower bound deformation limit exhibits an approximately linear relationship with respect to the crack-depth ratio for X-joints with different brace-to-chord intersection angles. The extrapolated deformation level at a zero crack-depth ratio represents a conservative estimation on the deformation level for intact joints.

Local joint flexibility of completely overlapped tubular joints under in-plane bending

This paper presents parametric equations to predict the local joint flexibility (LJF) of completely overlapped tubular joints under lap brace in-plane bending (IPB). Finite element (FE) package MARC is adopted to simulate the joint with 8-node thick shell elements. The FE method is calibrated and verified based on the existing experimental database of completely overlapped tubular joints and simple T/Y-joints, respectively. A comprehensive parametric study is subsequently conducted to investigate the effects of non-dimensional parameters on the local joint flexibility coefficient (fLJF). Significative recommendations are presented to reduce the LJF of the joint. A set of parametric equations are developed based on 5184 8-node thick shell FE models and have been assessed against the acceptance criteria of the Fatigue Guidance Review Panel. The comparison of fLJF between the proposed parametric equations and the existing T/Y-joint parametric equations shows that the use of the latter to predict the LJF of completely overlapped tubular joints may not necessarily be appropriate.

Fatigue performance of tubular X-joints with PJP+ welds: I — Experimental study

This study examines the fatigue performance of tubular joints fabricated using a new type of enhanced partial joint penetration weld details under constant-amplitude brace in-plane bending actions. The experimental program includes four cyclic tests on two large-scale X-joints, each with a different surface treatment near the weld toe. The experimental results confirm the satisfactory fatigue performance of the tubular X-joints welded using enhanced partial joint penetration welds in comparison with the S-N curves developed for tubular joints with complete joint penetration welds, and demonstrate the significant improvement on the fatigue life rendered by the weld surface grinding and toe grinding. Root fatigue cracking occurs only in specimens with post-weld toe-grinding treatment which enhances the fatigue life for toe cracking.

Fatigue performance of tubular X-joints with PJP+ welds: II — Numerical investigation

This paper summarizes the numerical investigation on the fatigue crack propagation in CHS X-joints under constant amplitude brace in-plane bending, in parallel with the experimental investigation reported in the companion paper. The finite element procedure examines the effects of the crack propagation angle, the crack-front profile and the interaction between adjacent fatigue cracks on the fatigue driving force, measured by the stress-intensity factors. The Paris type estimation provides a close prediction on the propagation life of weld toe cracks observed in the experiment. The initiation of the root crack, based on the Paris prediction, occurs much later than that of the toe crack.

Welder-optimized CJP-equivalency welds for tubular connections

This paper will present the experimental and numerical investigation on the fatigue crack failure at the root of the enhanced partial joint penetration welds in the circular hollow section X-joint specimens subjected to constant-amplitude brace in-plane bending actions. The enhanced partial joint penetration welds, which include a portion of the brace wall as the inherent “back-plate” for the welds, reduce the workmanship requirement on the welding procedure and improve the quality control of the welded joint. This study compares the fatigue crack developments observed at both the weld toe and the weld root in three large-scale X-joint specimens with different post-weld surface treatments. Root cracking does not take place in as-welded tubular X-joint specimens, of which the fatigue life depends on the weld toe failure. In specimens with post-weld grinding treatments to enhance fatigue resistance of the material near the weld toe, however, the finite-length root face triggers the initiation and propagation of fatigue cracks with an applied number of cycles more than that required to initiate a toe crack in the same tubular X-joint. Ultimate strength testing of specimens with large fatigue cracks, and post-test sectioning, reveals that fatigue failure weld-toe fatigue cracks remain more critical than the root cracks.

Fatigue performance of tubular X-joints: Numerical investigation

The cyclic actions due to waves, currents and winds on offshore platforms frequently cause fatigue cracks at hot-spot locations. The remaining fatigue life of such connections depends significantly on the fatigue driving force often measured by the stress-intensity factors. This study examines the fatigue performance of tubular joints fabricated using a new type of enhanced partial joint penetration weld details under constant-amplitude brace in-plane bending actions. The numerical study ascertains the crack propagation angle, the crack-front profile and the interaction between adjacent cracks on the stress-intensity factors.

Ductile tearing assessment of high-strength steel X-joints under in-plane bending

This study reports an experimental investigation of a fatigue-cracked, pre-notched circular hollow section X-joints fabricated from high strength steels (with the yield strength higher than 800MPa) subjected to brace in-plane bending. The circular hollow section X-joint entails a prefabricated V-notch near the weld toe at the crown position. The experimental procedure applies a fatigue pre-cracking cyclic load followed by a monotonic brace in-plane bending, which leads to brittle through-thickness crack propagation after some amount of ductile tearing. The ductile tearing assessment, integrating the fracture resistance curve obtained from the small-scale fracture specimens and the crack extension in the large-scale tubular joint, predicts closely the load level at which unstable crack extension takes place. The generic level 2A curve outlined in the BS7910 provides an un-conservative estimate on the failure load of the X-joint specimen. The parametric numerical investigation reveals that the strength definition for the cracked joints imposes a significant effect on the shape of the failure assessment curve.

New API RP2A Tubular Joint Strength Design Provisions

The development of the new API RP2A (22nd edition) parametric static strength prediction equations for planar circular hollow section tubular joints is described. Prediction equations are presented for brace axial, brace in-plane bending, and brace out-of-plane bending loads. The prediction equations are based on screened test databases, augmented, and extended by an extensive new series of validated nonlinear finite element simulations for nonoverlapping K joints, double tee (DT/X) joints, and T joints. The increased reliability (reduced scatter) provided by the new static strength formulation was used to justify a reduction of the load factor of safety to 1.6 from the previous value of 1.7.

Stress and strain concentration factors of completely overlapped tubular joints under lap brace IPB load

This paper presents parametric equations to predict the stress concentration factor (SCF) of completely overlapped tubular joints under lap brace in-plane bending (IPB) load. A completely overlapped tubular joint specimen is first tested for the verification and calibration of finite element (FE) models. The experimental results showed that the strain distribution near the weld toe is fairly linear. The maximum strain concentration factor (SNCF) of the joint occurs at the lap brace crown heel under IPB load. For the FE analysis, both 8-node thick shell and 20-node solid elements are suitable for modelling the joint. A set of parametric equations is proposed for predicting the SCF of completely overlapped tubular joints based on 5184 FE models. The parametric equations are verified against the acceptance criteria of Fatigue Guidance Review Panel. The assessment of the proposed equations is based on the database of FE analysis and test results. The comparison of SCF between the proposed parametric equations and the existing T / Y -joint parametric equations showed that the results do not agree reasonably well under IPB load.

Multiaxial fatigue of welded shaft-flange connections of stirrers under random non-proportional torsion and bending: Sonsino, C.M. and Pfohl, R. Int. J. Fatigue Sept. 1990 12, (5), 425–431

This study examines the fatigue performance of tubular joints fabricated using a new type of enhanced partial joint penetration weld details under constant-amplitude brace in-plane bending actions. The experimental program includes four cyclic tests on two large-scale X-joints, each with a different surface treatment near the weld toe. The experimental results confirm the satisfactory fatigue performance of the tubular X-joints welded using enhanced partial joint penetration welds in comparison with the S-N curves developed for tubular joints with complete joint penetration welds, and demonstrate the significant improvement on the fatigue life rendered by the weld surface grinding and toe grinding. Root fatigue cracking occurs only in specimens with post-weld toe-grinding treatment which enhances the fatigue life for toe cracking.