X.L. Zhao

Concrete-filled circular steel tubes subjected to pure bending

Current design codes and standards provide little information on the flextural behaviour of circular concrete filled tubes (CFT) as there have been few experimental studies. There are significant differences in d/t-limits recommended in various codes for CFT under bending. This paper presents an experimental investigation of the flexural behaviour of circular CFT subjected to large deformation pure bending where d/t = 12 to 110. The paper compares the behaviour of empty and void-filled, cold-formed circular hollow sections under pure plastic bending. It was found that for the range of d/t40, void filling prevented local buckling for very large rotations, whereas multiple plastic ripples formed in the inelastic range for specimens with 74d/t110. In general, void filling of the steel tube enhances strength, ductility and energy absorption especially for thinner sections. Based on the measured material properties, the plastic d/t-limit was found to be 112. A simplified formula is provided to determine the ultimate flexural capacity of CFT. The existing design rules for the ultimate moment capacity of CFT may be extended conservatively to a new slenderness range of 100ls 188.  2001 Elsevier Science Ltd. All rights reserved.

Plastic mechanism analysis of circular tubes under pure bending

Abstract There are a number of solutions available to predict the response of a circular steel tube under pure bending. However, most of these solutions are based on an elasto-plastic treatment, which is complex and difficult to use in any routine design. This paper describes a theoretical treatment to predict the moment-rotation response of circular hollow steel tubes of varying D / t ratios under pure bending. The Mamalis et al. (J. Mech. Sci. 1989;203:411–7) kinematics model for a circular tube under a controlled moment gradient was modified to include the effect of ovalisation along the length of the tube. Inextensional deformation and rigid plastic material behaviour were assumed in the derivation of the deformation energy. The plasticity observed in the tests was assumed to spread linearly along the length of the tube. Two local plastic mechanisms (Star and Diamond shapes) were studied to model the behaviour observed in the tests especially during the unloading stage. The theoretical predictions are compared with the experimental results recently obtained by Elchalakani et al. (Quartral. J. Struct. Eng. 2000;3(3):1–16). Good agreement was found between the theoretical predictions and experimental moment-rotation responses, particularly for the Star shape mechanism. A closed-form solution is presented suitable for spreadsheet programming commonly used in routine design.

Bending tests to determine slenderness limits for cold-formed circular hollow sections

There are significant differences in slenderness limits recommended in various codes for circular hollow sections (CHS) under bending as there have been little experimental studies. In this paper an attempt is made to establish more accurate slenderness limits for cold-formed circular hollow sections. This paper describes a series of bending tests to examine the influence of section slenderness on the inelastic bending properties of cold-formed CHS. Twelve bending tests were performed up to failure on different sizes of CHS with diameter-to-thickness ratio d/t ranging from 37 to 122. This range of d/t was obtained by machining as-received coldformed circular hollow sections grade C350L0. The test results are compared with other experimental data and the design rules given in various steel specifications. The slenderness limits were established to define Class 1 (compact), 2, 3 (non-compact) and 4 (slender). These limits were based on modifications of criteria for rotation capacity commonly used for steel structures. A design curve was developed and recommended for the design of cold-formed CHS under pure bending.  2002 Elsevier Science Ltd. All rights reserved.

Fatigue Behaviour of Steel Elements Strengthened with Stand CFRP Sheets

CFRP sheets are extensively used to strengthen defected steel members through a wet layup process. Different techniques were used to control the quality of bonding, such as vacuum bags and rollers. Recently strand CFRP sheets were applied in the strengthening of steel elements with manufacturer suggested epoxy resin and a primary resin. This study investigated the fatigue behaviour of steel beams and double strap steel joints bonded with strand CFRP sheets. Eight beams were tested under four point bending and eight double strap steel joints were tested under axial tension. Comparisons were made on the failure modes and the fatigue life for specimens bonded with and without the primer epoxy resin. Results showed that the fatigue life of defected steel beams and double strap steel joints bonded with strand CFRP sheets are comparable to that of specimens strengthened with CFRP plates and other high modulus CFRP sheets, whereas a linear reduction in fatigue life was observed when the primer resin was applied on the steel surface of the samples.

Evaluation of Stress Intensity Factor for CFRP Bonded Steel Plates

Recent studies on the application of carbon fibre reinforced polymer (CFRP) materials to defected steel structures have demonstrated the potential for significant reduction of stress intensity factor (SIF) values at crack tips leading to extended fatigue lives. However, most of the previous research relied on experimental and numerical methods, which were either expensive or time-consuming. In this paper, the SIF values at crack tips of steel plates strengthened with bonded composite materials were evaluated using linear elastic fracture mechanics. The analysis was based on the classical solution of SIF values of plain steel plates, considering load share effect and geometry correction factor change resulted from the overlay patch. The effect of different parameters were demonstrated and compared with experimental results, including initial damage degrees of specimens, geometric and mechanical properties of retrofitting materials and bond locations. Good agreement with the experimental data indicated that this approach could conservatively predict the SIF values with reasonable accuracy. A parametric study on variables including the CFRP modulus, the bond width and bond length was conducted based on this method to further investigate their effect on the SIF values.

Finite Element Analysis to Determine Stress Concentration Factors of Dragline Tubular Joints

This paper presents a finite element analysis (FEA), by using ANSYS 10.0 (SAS IP Inc 2005), to determine the stress concentration factor (SCF) of the dragline tubular joints. Different load cases, element types, weld sizes and shapes are considered in the analysis. The predicted SCF are compared with the experimental values based on full-size laboratory testing. Finite element (FE) model for the dragline tubular joint consists of the main chord and bracing members with all the attachments like, end plates, load cells, nuts and rods, with concave weld shape and equal weld leg length has been selected to determine SCF. Two layers of 10-node tetrahedral element (SOLID187) and 2 layers of 20-node hexahedral element (SOLID186) have been used in the FE model. The FEA results for all the laboratory test specimens are particularly good when loaded under load case 1 (LC1) and reasonable when loaded under load case 2 (LC2).