T. C. Fung

Ultimate capacity of completely overlapped tubular joints: I. An experimental investigation

Abstract This paper presents the experimental finding of a full-scale completely overlapped joint tested to failure under lap brace axial compression. The failure mode of the joint was due to the through brace face plastification. An ovalising effect of the through brace cross-section and bending on the gap region were noted. The onset of non-linear behaviour occurred at 72% of the ultimate load. The capacity of the joint was found to be 7.3% higher than that of the simple gap K-joint, which was determined from the ultimate strength formula. The deformation limit of the joint was 3.82 times the lap brace yield deformation. In the numerical study, the 8-node thick shell element was used to model the joint. No weld element was modelled, but the material with strain hardening property was necessary in the model. The comparison of the load–displacement curve with the experimental results showed a reasonably good agreement, with 5% less for both the ultimate load and the corresponding lap brace axial displacement. As the finite element method was verified to be reliable and accurate, a detailed parametric study was performed to intensively investigate the ultimate load behaviour of the completely overlapped joint as described in the companion paper on behavioural study.

Hysteretic behavior of completely overlap tubular joints

Abstract The completely overlap joints used in an eccentric offshore jacket were investigated by both the experimental method and finite element analysis. A full scale specimen was tested under cyclic quasi-static loadings to understand its performance under seismic loading. In the test, local buckling was first observed at the through brace in the intersection area. Then, a crack was found in the weld toe between the lap brace and the through brace. The propagation of the crack led to final failure of the joint. Some indicators to assess the performance of the joint were evaluated within the context of seismic resistance. The energy analysis indicated that the local buckling at the intersection area was the main energy dissipating mechanism. Comparison with an N-joint shows that the completely overlap joint performed better under seismic loadings. After calibration by the experimental results, an extensive finite element analysis was carried out to determine the main factors that have significant influence on the hysteresis behavior of the joint.

Ultimate capacity of completely overlapped tubular joints. II: Behavioural study

Abstract This paper presents numerical results on the ultimate strength behaviour in terms of load–displacement characteristics and failure mechanisms of completely overlapped tubular joints. The effect of the gap length on the behaviour of the joints is also investigated. In the study, the end of the lap brace was axially loaded. Criteria were set up to determine the minimum failure load. It was found that a joint subjected to compression load gave the lowest ultimate capacity. The geometric parameters of the chord were shown to have an insignificant impact on the joint behaviour. The joint strength increased as the gap reduced. A sharp increase in the joint strength at small gap was noted for a relatively high βTL(=dL/dT=0.768). The Y-joint capacity was reached at a sufficiently large gap. τTL(=tL/tT) had a crucial influence on the joint failure behaviour. A relatively mild βTL(=dL/dT=0.616) and τTL(=tL/tT=0.560) resulted in lap brace failure prior to joint collapse. The failure mechanisms changed from “chord” (through brace) bending, followed by localised plastification of the through brace face to yielding of the lap brace when the gap was reduced from a large gap to a small gap of 51 mm.

Numerical study of multiplanar tubular DX-joints subject to axial loads

Abstract In this paper, 20 multiplanar DX-joints (double X) with in-plane braces and out-of-plane braces subjected to various combination of axial loads were analysed. The multiplanar effect of loaded and unloaded out-of-plane braces and the load-interaction of multiplanar DX-joints were investigated. The influence of geometric parameters on the stress concentration factors of the joints was also studied. The results obtained from the analyses of multiplanar DX-joints are compared with those of the corresponding uniplanar X-joints. It is found that, when the out-of-plane braces are not loaded, the multiplanar effect is not significant when the brace diameter is much smaller than the chord diameter. When the out-of-plane braces are loaded, the hot spot stresses are found to be lower or higher than the values predicted based on uniplanar joints. The deviation can be as much as 87% or 94%, respectively, depending on the combination of the load conditions and geometrical parameters.

Rankine approach for steel columns in fire: numerical studies

Abstract A finite element model has been extensively applied to verify the accuracy of the proposed Rankine formula for determining the fire resistance of bare steel columns. The parameters involved include relative slenderness ratio, load eccentricity, steel grade, and initial imperfections of initial crookedness and residual stresses. Through these parametric studies, it is possible to establish the safety limits on the application of the proposed Rankine formula. It is realised that incorporating a deflection control criterion in the evaluation of column elastic-critical loads helps to improve the overall performance of the formula. The Rankine formula is then compared to a series of 68 eccentrically-loaded column tests from the literature. The comparison study shows good agreement with both test results and analytical predictions.

Time step integration algorithms with predetermined coefficients for second order equations

Abstract In this paper, the effect of using the predetermined coefficients in constructing time step integration algorithms suitable for linear second order differential equations based on the weighted residual method is investigated. The second order equations are manipulated directly. The displacement approximation is assumed to be in a form of polynomial in the time domain and some of the coefficients can be predetermined from the known initial conditions. The algorithms are constructed so that the approximate solutions are equivalent to the solutions given by the transformed first order equations. If there are m predetermined coefficients (in addition to the two initial conditions) and r unknown coefficients in the displacement approximation, it is shown that the formulation is consistent with a minimum order of accuracy m + r . The maximum order of accuracy achievable is m +2 r . This can be related to the Pade approximations for the second order equations. Unconditionally stable algorithms equivalent to the generalized Pade approximations for the second order equations are presented. The order of accuracy is 2 r −1 or 2 r and it is required that m +1⩽ r . The corresponding weighting parameters, weighting functions and additional weighting parameters for the Pade and generalized Pade approximations are given explicitly.