CFRP修复火灾后混凝土填充钢管的抗压和弯曲性能

对单向CFRP修复火灾后混凝土填充钢管的轴心抗压和弯曲进行试验。试验采用圆形和方形两种试件来研究修复效果,试验结果显示CFRP加固有效提高了损伤柱的承载力。同时,由于CFRP的约束,柱的刚度也有提高。然而,对于梁来讲,试验结果显示修复效果没有柱的修复效果好,从试验结果来看,对于受到火灾严重损伤的混凝土填充钢管梁或承受较大弯矩的构件,建议采用其他合适的修复方法。     

Compressive and flexural behaviour of concrete filled steel tubes after exposure to standard fire

This paper describes a series of new compression and bending tests carried out on concrete filled steel tubes (CFST) after exposure to the ISO-834 standard fire. A theoretical model that has been previously developed is used to predict the post-fire load versus deformation relationships of CFST stub columns and beams. The predicted curves of load versus deformation are in good agreement with the new test results.The previously developed theoretical model had been used to investigate the influence of a number of important parameters on the residual ultimate strength and flexural stiffness of the composite sections and the results of the parametric studies were used to develop formulas for calculating the composite section residual ultimate strength under axial compression or flexural bending and the composite section residual flexural bending stiffness. In these formulas, the ambient temperature compression resistance, bending moment capacity and initial flexural bending stiffness of the composite section should be calculated using an existing design code. In this paper, these formulas are applied to the new test data to assess the suitability of using several different design codes: AIJ-1997, AISC-LRFD-1999, BS5400-1979, DBJ13-51-2003 and EC4-1994.     

Cyclic performance of repaired concrete-filled steel tubular columns after exposure to fire

This paper provides new test data of cyclic behavior of repaired concrete-filled steel tubular (CFST) columns after exposure to fire, the fire-damaged CFST columns being strengthened by wrapping the original columns by concrete and a thin-walled steel tube. The test parameters included the cross-section type (circular, square and rectangular), and the axial load level (0, 0.3, 0.6). It was found that all the test specimens behaved in a ductile manner and testing proceeded in a smooth and controlled way. Based on the experiment measurements, the ultimate lateral strength, flexural stiffness, dissipated energy and ductility of the columns are analyzed and compared. The test results indicate that the ultimate lateral strength and flexural stiffness of concrete-filled hollow structural columns decrease after exposure to fire, however, the ductility of the columns was not adversely affected due to the fire exposure. The test results also indicate that the strength and stiffness of the fire-damaged columns can be restored over the original level of the specimens.     

Cyclic behaviours of concrete-filled steel tubular columns with pre-load after exposure to fire

This paper reports experimental results of cyclic behaviours of eight concrete-filled steel tubular (CFT) columns after exposure to fire under sustained axial load. The specimens were heated by a stackable electrical furnace and blowing liquefied petroleum gas fire into the furnace simultaneously in order to simulate the real fire attack. The furnace heating can be controlled easily and safely in such a hybrid heating method using gas and electricity that the average temperature in the furnace followed as closely as possible the ISO-834 standard fire curve. After the CFT columns had been axially loaded and heated for a specified duration of time, the specimens were cooled down to room temperature according to the ISO-834 fire standard while the axial load was kept constant. Finally, the columns were subjected to lateral cyclic loading under constant axial load. The sustained pre-load led to significant residual deformations of CFT columns during the cooling phase. Two columns were found to have suffered local buckling of steel tube and three columns have suffered steel weld rupture during the fire exposure. Therefore, it is recommended that more than two vent holes or vent holes with larger diameter should be used in a CFT column and engineers should pay much attention to reasonable selection and construction of steel sections. Compared with the fire-damaged columns without pre-load and that at room temperature, the post-fire ductility and energy absorption capacity of CFT with pre-load had no obvious deterioration and the axial load level had no obvious influence on the residual strength of fire-damaged CFT. The experimental results can provide a basis for fire-damage assessment of CFT columns.     

Behaviour of fire-exposed concrete-filled steel tubular beam columns repaired with CFRP wraps

This paper presents the test results of fire-exposed concrete-filled steel tubular (CFST) beam columns repaired by unidirectional carbon fibre reinforced polymer (CFRP) composites. Both circular and square specimens were tested to investigate the repair effects of CFRP composites on them. The test results showed that the load-bearing capacity of the fire-exposed CFST beam columns was enhanced by the CFRP jackets to some extent, while the influence of CFRP repair on stiffness was not apparent. The strength enhancement from CFRP confinement decreased with the increasing of eccentricity or slenderness ratio. Ductility enhancement was also observed except those axially loaded shorter specimens with rupture of CFRP jackets at the mid-height, occurred near the peak loads. However, the strengths of all repaired specimens have not been fully restored due to the long exposure time of them in fire. From the test results, it is recommended that, in repairing severely fire-damaged CFST members, slender members or those members subjected to comparatively large bending moments, other appropriate repair measures should be taken.     

Post-fire bond between the steel tube and concrete in concrete-filled steel tubular columns

Push-out tests were carried out on 64 concrete-filled steel tubular columns, which had been exposed to ISO 834 standard fire for 90 min or 180 min, respectively. At the same time, 12 unheated specimens were also prepared and tested for comparison. The variables investigated in the bond tests were selected as (a) fire exposure time; (b) cross-section type; (c) cross-sectional dimension; (d) interface length to diameter (or width) ratio; (e) concrete type; (f) fly ash type; and (g) concrete curing condition. The effects of the above different parameters on the bond behaviour are discussed. The test results indicate that fire exposure had a significant effect on the bond between a steel tube and its concrete core. A decrease in bond strength was generally observed for specimens after a fire exposure of 90 min; however, bond strength recovery was found when the fire exposure time was extended to 180 min. Other factors also had influence to some extent.     

Heat transfer in concrete-filled carbon and stainless steel tubes exposed to fire

Nowadays, performance-based design methods are increasingly used for fire resistance assessment of structures. To implement these methods, it is paramount to determine the temperature development within a structural member exposed to fire as accurately and efficiently as possible. Numerical models are developed in this paper to simulate the temperature development in concrete-filled carbon and stainless steel tubes. It was found that the influence of the moisture content in concrete and the thermal contact conductance at the steel–concrete interface is significant. New models for thermal conductivity of concrete and thermal contact conductance at the interface are proposed in this paper. Comparisons of temperature development are made between numerical simulations and extensive experimental results. Improved agreement with test results is achieved when the proposed models are used in the heat transfer analysis.     

采用CFRP覆层修复的钢管混凝土梁柱的抗火性能

介绍了通过单向碳纤维聚合物加固的钢管混凝土梁柱在高温下的试验结果。为研究CFRP材料的加固作用,对所有圆形和方形的构件进行了测试。试验结果表明,通过CFRP加固后CFST梁柱的承载能力有所提高,但CFRP对其强度的影响却不明显。当偏心率或长细比增加时,CFRP所带来的强度的增幅会减少。当结构处于最大载荷时,除了那些在中间部位没有CFRP覆层的轴向受力短柱外,构件延性也有所增强。但是由于长期暴露在火中,所有修复过的构件都不能完全恢复到以前的性能。试验结果表明,当修复火灾中严重受损的CFST构件为长细构件或者遭受相对较大弯矩的构件时,建议适当考虑其他修复方法。     

Residual strength of concrete-filled RHS columns after exposure to the ISO-834 standard fire

The residual strength of a composite column may be used to assess the potential damage caused by fire and help to establish an approach to calculate the structural fire protection for minimum post-fire repair. The behavior of six rectangular hollow structural steel (RHS) columns filled with concrete, with or without fire protection, after exposure to the ISO-834 standard fire (ISO 834, 1975), subjected to axial or eccentric loads have been experimentally investigated and the results presented in this paper. Comparisons are made with predicted column strengths using the existing codes such as LRFD-AISC-1994, AIJ-1997, EC4-1996 and GJB4142-2000. It was found, in general, that the loss of the strength of the specimens without protections was significantly greater than that of columns with fire protection. A mechanics model is developed in this paper for concrete-filled RHS columns after exposure to the ISO-834 Standard Fire (ISO 834, 1975), and is a development of the analysis used for ambient condition (Han et al., 2001). The predicted load versus mid-span deflection relationship for the composite columns is in good agreement with test results. Based on the theoretical model, influence of the changing strength of the materials, fire duration time, sectional dimensions, steel ratio, load eccentricity ratio, depth-to-width ratio and slenderness ratio on the residual strength index (RSI) is discussed. It was found that, in general, the slenderness ratio, sectional dimensions and the fire duration time have a significant influence on the residual strength index (RSI). However, the steel ratio, the depth-to-width ratio, the load eccentricity ratio and the strength of the materials have a moderate influence on RSI. Finally, formulas suitable for incorporation into a building code, for the calculation of the residual strength of the concrete-filled RHS columns after exposure to ISO-834 standard fire is developed based on the parametric analysis results.     

Effects of heating and loading histories on post-fire cooling behaviour of concrete-filled steel tubular columns

A finite element method (FEM) program is developed and used in this paper to analyse the behaviour of concrete-filled steel tubular (CFST) columns during the entire stage of fire exposure, including: loading at ambient temperature, heating, cooling to the ambient temperature and post-fire loading to failure. The emphasis of this paper is on CFST column behaviours during the cooling and post-fire stages because these behaviours are affected by the loading and heating histories, but they have not previously been studied. This paper will present the mechanical property models for these different loading and heating stages. To validate the FEM program, some experimental data, including fire resistance, axial deformation and ultimate strength of CFST columns are compared and it is found that the FEM program can predict the test results with good accuracy. Using the FEM program, a parametric study is then conducted to investigate the influences of ambient temperature loading and heating history on the cooling and post-fire behaviours of CFST columns. It is concluded that various parameters (such as load ratio and elevating temperature time ratio etc.) affect the residual strength of CFST columns severely. Finally, this paper proposes a set of formulas which can be used to predict the residual strength of CFST columns after going through the whole fire exposure process.     

Seismic performance improvement of circular steel columns using precompressed concrete-filled steel tube

This paper presents a new method aimed at improving seismic resisting characteristics of circular-shaped steel columns representing highway bridge piers and an experimental investigation carried out to validate it. In this method, a special compression member is placed in the middle of the pier in order to take the axial load from the superstructure. As a result, the influence of axial load on the inelastic buckling deformation of plates can be greatly controlled. The special compression member consists of a precompressed concrete-filled steel tube (PC-CFT). Six specimens were tested to check the seismic performances of the proposed column system. It was clear from the test results that the specimens with PC-CFT could deform even up to ten times of their yield displacements without significant load deterioration. They also showed improved ultimate strength, ductility and energy absorption capacities than the corresponding benchmark specimens.     

Nonlinear analysis of circular concrete-filled steel tubular short columns under axial loading

The confinement effect provided by the steel tube in a circular concrete-filled steel tubular (CFST) short column remarkably increases the strength and ductility of the concrete core. The reliable prediction using nonlinear analysis methods for circular CFST columns relies on the use of accurate models for confined concrete. In this paper, accurate constitutive models for normal and high strength concrete confined by either normal or high strength circular steel tubes are proposed. A generic fiber element model that incorporates the proposed constitutive models of confined concrete is created for simulating the nonlinear inelastic behavior of circular CFST short columns under axial loading. The generic fiber element model developed is verified by comparisons of computational results with existing experimental data. Extensive parametric studies are conducted to examine the accuracy of various confining pressure models and the effects of the tube diameter-to-thickness ratio, concrete compressive strengths and steel yield strengths on the fundamental behavior of circular CFST columns. A new design formula accounting for concrete confinement effects is also proposed for circular CFST columns. It is demonstrated that the generic fiber element model and design formula adequately predict the ultimate strength and behavior of axially loaded circular CFST columns and can be used in the design of normal and high strength circular CFST columns.     

Column tests of dodecagonal section double skin concrete-filled steel tubes

A series of tests on dodecagonal section double skin concrete-filled steel columns (DCS) were carried out in this study. Column specimens having different lengths ranged from 1000 mm to 3500 mm were tested. The behavior and strengths of dodecagonal section double skin concrete-filled steel columns were investigated. In addition, local bucking of inner and outer steel tubes were also investigated. Material properties of the concrete and steel used in the test specimens were measured. The test strengths are compared with the design strengths calculated using the proposed methods based on current AISC Specification and Eurocode for the design of composite structural members. The suitability of design method proposed by other researcher for circular section double skin concrete-filled steel columns for dodecagonal section specimens was also evaluated.     

Flexural strength analysis of non-post-tensioned and post-tensioned concrete-filled circular steel tubes

Concrete-filled circular steel tubes (CFT) have recently gained significant attention for their enhanced strength and ductility over the conventional steel and reinforced concrete construction. The concrete compressive strength is significantly increased by the lateral confinement provided by the steel tube and local buckling of the steel tube is restrained by the concrete infill. This paper investigates the further enhancement of these composite actions due to post-tensioning the concrete cores inside circular steel tubes. The flexural behavior of a post-tensioned CFT as well as a non-post-tensioned CFT was studied experimentally and analytically. A numerical algorithm for predicting the moment capacities of circular CFT, with or without the post-tensioning effects, has been validated against the test results by the authors as well as those published in the literature.     

Ultimate capacity of rectangular concrete-filled steel tubular columns under unequal load eccentricities

The paper describes 36 experimental tests conducted on rectangular and square tubular columns filled with normal and high strength concrete and subjected to a non-constant bending moment distribution with respect to the weak axis. The test parameters were the nominal strength of concrete (30 and 90 MPa), the cross-section aspect ratio (square or rectangular), the thickness (4 or 5 mm) and the ratio of the top and bottom first order eccentricities e_top/e_bottom (1, 0.5, 0 and − 0.5). The ultimate load of each test was compared with the design loads from Eurocode 4, presenting unsafe results inside a 10% safety margin. The tests show that the use of high strength concrete is more useful for the cases of non-constant bending moment, whereas if the aim is to obtain a more ductile behavior the use of concrete-filled columns is more appealing in the cases of normal strength concrete with non-constant bending moments because, although they resist less axial force than the members with HSC, they obtain a softened post-peak behavior.     

Behaviour of short and slender concrete-filled stainless steel tubular columns

In this paper, a series of tests were carried out on short and slender concrete-filled stainless steel tubular columns to explore their performance under axial compression or combined actions of axial force and bending moment. Empty short steel hollow sections were also tested for comparison. The test results showed that the performance of the composite columns was quite good and have the potential to be used extensively as structural members. Comparisons of the test results were also made with several existing design methods for conventional concrete-filled carbon steel tubular columns as presented in Australian standard AS 5100 (2004), American code AISC (2005), Chinese code DBJ/T 13-51-2010 (2010), and Eurocode 4 (2004), which indicates that all the codes are somewhat conservative in predicting the load-carrying capacities of both short and slender columns.     

Analytical behaviour of concrete-filled double skin steel tubular (CFDST) stub columns

This paper reports a finite element analysis of the compressive behaviour of CFDST stub columns with SHS (square hollow section) or CHS (circular hollow section) outer tube and CHS inner tube. A set of test data reported by different researchers were used to verify the FE modelling. Typical curves of average stress versus longitudinal strain, stress distributions of concrete, interaction of concrete and steel tubes, as well as effects of hollow ratio on the behaviour of CFDST stub columns, were presented. The influences of important parameters that determine sectional capacities of the composite columns were investigated.     

Further study on the flexural behaviour of concrete-filled steel tubes

This paper is a further study on the flexural behaviour of concrete-filled steel tubes based on the former work presented by Han [Han LH. Flexural behaviour of concrete-filled steel tubes. Journal of Constructional Steel Research 2004;60(2):313-37]. A total of 36 composite beam specimens filled with self-consolidating concrete (SCC) were tested. The main parameters varied in the tests are: (1) sectional types (circular and square); (2) steel yielding strength (from 235 to 282 MPa); (3) the ratio of tube diameter (or width) to wall thickness, D/t (from 47 to 105), and (4) the ratio of shear span to depth (from 1.25 to 6). Comparisons are made with predicted beam capacities using the existing methods, such as AIJ-1997 [Architectural Institute of Japan (AIJ). Recommendations for design and construction of concrete filled steel tubular structures. 1997], AISC-LRFD-1999 [AISC. Load and resistance factor design specification for structural steel buildings. Chicago: American Institute of Steel Construction, Inc.; 1999], BS5400-1979 [British Standard Institute: BS5400, Part 5, Concrete and composite bridges. 1979], EC4-1994 [Eurocode 4. Design of composite steel and concrete structures, Part 1.1: General rules and rules for buildings (together with United Kingdom National Application Document). DD ENV 1994-1-1:1994. London W1A2BS: British Standards Institution; 1994] and the method proposed by Han [Han LH. Flexural behaviour of concrete-filled steel tubes. Journal of Constructional Steel Research 2004;60(2):313-37].Applied calculation formulae of moment versus curvature curves and the flexural stiffness of concrete-filled steel tubular (CFST) beams are presented, based on the mechanics model of Han [Han LH. Flexural behaviour of concrete-filled steel tubes. Journal of Constructional Steel Research 2004;60(2):313-37]. Comparisons are made with predicted beam flexural stiffness using different methods, such as AIJ-1997, AISC-LRFD-1999, BS5400-1979, EC4-1994 and the method proposed in this paper. Comparisons are also made between the simplified model and the mechanics model, and generally good agreement is achieved.     

新型钢管混凝土结构的特点及运用

主要介绍了各类新型钢管混凝土结构的特点,受力性能,制作与加工的方法,阐述了各类钢管混凝土结构的应用情况和发展前景,以提高人们对新型钢管混凝土结构的认识,推广钢管混凝土结构的应用。     

Performance of concrete-filled thin-walled steel tubes under pure torsion

In practice, concrete-filled steel tubes (CFST) are often subjected to torsion. To date, such a problem however has not been addressed satisfactorily by design codes. The present study is thus an attempt to study the torsional behaviours of concrete-filled thin-walled steel tubes. ABAQUS software is used in this paper for the finite element analysis (FEA) of CFST subjected to pure torsion. A comparison of results calculated using this modelling shows good agreement with test results. The FEA modelling was used to investigate the influence of important parameters that determine the ultimate torsional strength of the composite sections. The parametric studies provide information for the development of formulae to calculate the ultimate torsional strength, as well as the torsional moment versus torsional strain curves of the composite sections.