Hui Jiao

Imperfection, residual stress and yield slenderness limit of very high strength (VHS) circular steel tubes

This paper investigates the initial geometrical imperfection, residual stresses and yield slenderness limit of very high strength (VHS) circular steel tubes with the yield stress of 1350 MPa. The out-of-straightness is found to be about L/6800 on average, where L is the length of the stub column. The measured residual stress is found to be approximately 4% on average of the yield stress, with the outside surface in compression and the inside surface in tension. Eight stub columns were tested with the element slenderness ratio ranging from 135 to 258. New element slenderness limit (ley) of 258 and non-dimensional local buckling parameter limit of 3.1 are proposed for VHS tubes. The very large value of ley is obtained partly because of the very large yield stress used in the definition of element slenderness and partly because of the very low residual stress in the section. Two stub columns of non-heat-treated tubes were also tested in the same manner. The results were compared with those of the VHS tubes. Both VHS tubes and non-heat-treated tubes have a similar level of geometrical imperfections. The residual stress pattern obtained for non-heat-treated tubes is very different from that of VHS tubes.  2002 Elsevier Science Ltd. All rights reserved.

Material ductility of very high strength (VHS) circular steel tubes in tension

Abstract This paper investigates the material ductility of very high strength (VHS) circular steel tubes under tension in terms of the ultimate strength to the yield stress ratio, the percentage elongation, the fracture to ultimate load ratio and the ultimate to yield strain ratio. 15 tensile coupon tests and 12 full section tests on VHS tubes were carried out. The tested VHS tubes have a diameter ranging from 31.8 mm to 75 mm with wall thickness ranging from 1.6 mm to 2.0 mm. The non-heat-treated tubes, which were used to make VHS tubes, were also tested for comparison purposes. Different failure modes were observed for VHS tubes and non-heat-treated tubes. The ultimate strength to yield stress ratio of VHS tubes was compared with that of various cold-formed hollow sections, sheet steels and quenched and tempered steels. The test results were compared with ductility requirements in various codes. It has shown that the VHS tubes satisfied the material ductility requirement specified in the Australian/New Zealand Standard for Cold-Formed Steel Structures AS/NZS4600.

Tension Capacity of Very High Strength (VHS) Circular Steel Tubes after Welding

The details and results of an investigation into the tension capacity of very high strength (VHS) circular steel tubes after welding are presented. Two types of welded connections are used in the study: (1) complete penetration butt-welded VHS tubes, and (2) VHS tubes fillet welded to a steel plate along the whole perimeter of the tube. Both butt welded connections and transverse fillet welded connections failed in the heat-affected-zone (HAZ). The failure modes are compared with those of welded cold-formed square hollow sections. Significant strength reduction was observed in the HAZ compared with the strength of the VHS tubes. A strength reduction factor is proposed in this paper to modify the existing design rules. The FOSM (First Order Second Moment) reliability analysis method is used to calibrate the reduction factor. It seems that the tension member rules in the existing standards can be used along with the proposed reduction factors. The issue of matching between the parent metal and weld metal is also discussed.

Experiments on Dented Steel Tubes under Bending

Steel tubular sections are extensively found in many kinds of civil and mechanical engineering structures such as columns and bracing elements, offshore industry, bridge elements, etc. Due to the extensive usage, a considerable amount of research has been carried out on such structural members. Although many studies focused on the bending stability of steel members, very little research can be found on the effect of surface defects, such as dent imperfection, on the bending behavior of such tubular members. This research aimed to evaluate the effect of dent-shaped defects on the flexural capacity of CHS members. The data obtained in this paper can be applied to evaluate the capacity of large scale CHS members with similar D/t ratio when a dent is formed on the tubes during the service life of such members. An interrelation between the capacity and the details of the dent, i.e. size and position of the dent, was proposed.

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.

Concrete-filled circular steel tubes with a timber infill under axial compression

Over the past two decades, there has been significant interest in research relating to concrete-filled tubes, and a corresponding penetration of this technology into practice. This paper aims to expound upon the effect of timber cores on the structural response of concrete-filled circular tubes under compression. A timber infill with different shapes and geometries surrounded by concrete and encased in a steel tube was employed. The effects of the combination of infill elements on the failure, axial capacity, ductility, and structural efficiency (weight versus capacity) are exhaustively set forth. For the specimens with the highest timber to concrete ratio, the capacity was enhanced by about two times the capacity of the hollow steel specimens. For these specimens a significant reduction in the total weight of the composite element was obtained relative to the fully concrete-filled specimens. These specimens showed the highest ductility among the other specimens. In addition, greater ratios of energy absorption to the mass were obtained for the specimens with different timber cores in comparison to the equivalent values for fully concrete-filled tubes, which is quite desirable in many practical scenarios. It is found that the use of timber as an inner core element in this new composite yields promising results in decreasing the weight and yet enhancing the capacity, ductility, and energy absorption, and can be a good alternative to double-skin concrete-filled steel tubes.

Flexural behaviour of locally post-tensioned reinforced concrete beams

Abstract This study investigates the behaviour of locally post-tensioned reinforced concrete beams. Local post-tensioning is an innovative alternative to existing conventional post-tensioning methods that achieves prestressing of concrete beams using conventional reinforcing steel. Four concrete beams were locally post-tensioned and tested. The theoretical predictions were made based on AS3600-2009. A new theoretical approach is proposed to determine the cracking moment of tested beams, since the current design guide was not applicable to this post-tensioning method.

An investigation into the effect of roughness conditions and materials on bond strength of CFRP/steel double strap joints

Abstract Different combinations of carbon fibre-reinforced polymer (CFRP) and adhesives were employed to connect two steel plates in double strap joints with various roughness levels. Three different bond lengths were also examined to decide which one was preferred for further testing. All specimens were subjected to static tensile test until failure. It was found that variations of surface roughness level between 0.73 and 7.75 μm had no significant effect on the ultimate loading capacity of the specimens. However, the CFRP type played a very important role in the bond strength and the failure modes. The best performing CFRP type was the CFRP plate. It was also found that the effective bond length was approximately 100 mm when using CFRP plates.