Behaviour of precast concrete floor slabs exposed to standardised fires

This paper quantifies the thermal movements of 14 simply supported precast reinforced concrete floor slabs of 4.5 m span and 900 mm width exposed to two standardised heating regimes used in fire resistance furnace tests. The tests were designed to show the effect of varying the slab thickness, type of concrete, imposed load, soffit protection and nature of fire exposure on the mid-span flexural deflection and axial movements of the slab ends. Measured deflections showed that during the 90 min design period of fire resistance thermal bowing was dominant and the effect of the 1.5 kN/m² design imposed load was small. The NPD hydrocarbon fire exposure caused a doubling of the flexural deflections achieved using the standard BS 476: Part 8 (now Part 20) fire exposure in the first 20 min of exposure.     

Behaviour of grouted sleeve connections at elevated temperatures

Tubular steel jointing system that incorporates prestressed grout sleeve connections has superior strength under both static and dynamic loading. This paper reports an investigation into the effect of elevated temperatures on the load carrying capacity of such connections. Eleven specimens were heated in a furnace and the load was applied through an Instron machine. Three different grout lengths were chosen. The load deflection behaviour at different temperatures was compared. It was found that the ultimate load reduces almost linearly as temperature difference (TD) between outer and inner tubes increases. It is encouraging to observe the ductile behaviour of grouted connections at elevated temperature. Thermal analysis was also conducted to predict the temperature field in the connection.     

Performance of cellular composite floor beams at elevated temperatures

This paper describes an experimental and numerical study at both ambient and elevated temperatures on the behaviour of full-scale composite floor cellular steel beams. A total of four specimens, comprising two different steel geometries and loading conditions were tested under monotonic loading and at elevated temperatures. All beams were designed for a full shear connection between the steel beam and the concrete flange using headed shear studs. The beams were designed to fail by web-post buckling, which was observed in all the tests. Failure temperature observed in the fire tests indicated that failure by web-post buckling of cellular beams in fire cannot simply be estimated by applying temperature dependent reduction factors on stiffness, as given in codes. A finite element model is then established with both material and geometrical non-linearity using shell elements to compare against the experimental results. The comparison between the finite element prediction and actual test results are quite good in terms of failure modes, load deflection behaviour and ultimate loads.     

高温下钢筋桁架楼承板中栓钉抗剪性能研究

对15个试件进行常温和高温下推出试验,研究钢筋桁架楼承板中栓钉的抗剪性能,得到混凝土楼板和栓钉不同位置处的温度分布以及栓钉受剪承载力随温度的退化规律。试验结果表明,钢筋桁架楼承板中栓钉在常温和高温下的破坏均为栓钉剪断破坏,栓钉根部混凝土局部压碎,但是与平板混凝土板中栓钉的破坏位置不同,所研究的栓钉剪断破坏的位置在钢梁上翼缘处,而不是在栓钉根部焊缝处,这也在很大程度上导致了钢筋桁架楼承板中栓钉受剪承载力比平板混凝土板中栓钉低。钢筋桁架楼承板中栓钉的受剪承载力和刚度均随温度的升高而降低。通过对试验数据的分析,提出了钢筋桁架楼承板中栓钉高温下受剪承载力和荷载-滑移曲线的计算方法。     

Design of screwed steel sheeting connection at ambient and elevated temperatures

Cold-formed profiled steel roof sheeting can be directly connected to the top chord of a steel roof truss through self-tapping screws. At ambient temperatures, neither EN 1993-1-8 nor EN 1993-1-3 can be used directly for this type of connection. Besides, no design rules are available in EN 1993-1-2 for designing screwed connections in fire. A 3D Finite Element (FE) model for a single-lap shear screwed connection is developed using ABAQUS software. After the validation by tests, the model is used to predict the ultimate resistance of connections at both ambient and elevated temperatures. Further, the effects of edge and end distances on the connection resistance are investigated. Based on the analyses results, revised design equations for predicting the connection resistance are proposed. The design resistance is calibrated by testing and FE analyses results according to the procedure given in EN 1990 and the partial safety factor is derived.     

Design of screwed steel sheeting connection at ambient and elevated temperatures

Cold-formed profiled steel roof sheeting can be directly connected to the top chord of a steel roof truss through self-tapping screws. At ambient temperatures, neither EN 1993-1-8 nor EN 1993-1-3 can be used directly for this type of connection. Besides, no design rules are available in EN 1993-1-2 for designing screwed connections in fire. A 3D Finite Element (FE) model for a single-lap shear screwed connection is developed using ABAQUS software. After the validation by tests, the model is used to predict the ultimate resistance of connections at both ambient and elevated temperatures. Further, the effects of edge and end distances on the connection resistance are investigated. Based on the analyses results, revised design equations for predicting the connection resistance are proposed. The design resistance is calibrated by testing and FE analyses results according to the procedure given in EN 1990 and the partial safety factor is derived.     

Investigation of concrete encased steel composite columns at elevated temperatures

This paper presents a nonlinear 3-D finite element model investigating the behaviour of concrete encased steel composite columns at elevated temperatures. The composite columns were pin-ended axially loaded columns having different cross-sectional dimensions, different structural steel sections, different coarse aggregates and different load ratios during fire. The nonlinear material properties of steel, concrete, longitudinal and transverse reinforcement bars as well as the effect of concrete confinement at ambient and elevated temperatures were considered in the finite element models. The interface between the steel section and concrete, the longitudinal and transverse reinforcement bars, and the reinforcement bars and concrete were also considered allowing the bond behaviour to be modelled and the different components to retain its profile during the deformation of the column. The initial overall (out-of-straightness) geometric imperfection was carefully included in the model. The finite element model has been validated against published tests conducted at elevated temperatures. The time–temperature relationships, deformed shapes at failure, time–axial displacement relationships, failure modes and fire resistances of the columns were evaluated by the finite element model. It has been shown that the finite element model can accurately predict the behaviour of the columns at elevated temperatures. Furthermore, the variables that influence the fire resistance and behaviour of the composite columns comprising different load ratios during fire, different coarse aggregates and different slenderness ratios were investigated in parametric studies. It is shown that the fire resistance of the columns generally increases with the decrease in the column slenderness ratio as well as the increase in the structural steel ratio. It is also shown that the time–axial displacement relationship is considerably affected by the coarse aggregate. The fire resistances of the composite columns obtained from the finite element analyses were compared with the design values obtained from the Eurocode 4 for composite columns at elevated temperatures. It is shown that the EC4 is conservative for all the concrete encased steel composite columns, except for the columns having a load ratio of 0.5 as well as the columns having a slenderness ratio of 0.69 and a load ratio of 0.4.     

Experimental behaviour of the reverse channel joint component at elevated and ambient temperatures

The reverse channel connection appears to have the best combination of desirable features under fire loading: moderate construction cost, ability to develop catenary action and extremely high ductility through deformation of the web channel (Ding and Wang, 2007). This paper presents the results of an experimental investigation of a reverse channel component conducted at the University of Coimbra as part of the European RFCS COMPFIRE Project, the main focus of which is to characterise the behaviour of steel joints that connect steel beams to concrete-filled tubular columns under natural fire loading. A series of tensile and compressive tests at ambient and elevated temperatures was conducted. The purposes of the experimental tests were to characterise the strength, stiffness and ductility of this joint component and to establish a relationship between force, displacement and temperature.     

Behaviour of column web component of steel beam-to-column joints at elevated temperatures

This paper presents an experimental investigation on stocky column web panels of semi-rigid beam-to-column joints exposed to fire conditions in order to verify an analytical prediction model. Recent experimental studies show that the degradation of material properties and high axial forces, due to restrained thermal expansion of beams at elevated temperatures, significantly affect the moment–rotation response of the joints. The component method originally established for the evaluation of the joint behaviour at ambient conditions can be adopted for elevated-temperature cases. Recently developed mechanical models for joints may not be accurate as the column web component is simultaneously subjected to bending moments and axial forces. This paper focuses on the component column web in shear in order to identify the key parameters which affect joint behaviour in shear at elevated temperatures. This experimental work was conducted on three extended end-plate connections subjected to both ambient and elevated temperature conditions. After validations by test results, detailed finite element simulations were performed for a series of parametric studies at other elevated temperatures. Both experimental and numerical results are finally compared with analytical predictions.     

Behaviour of column web component of steel beam-to-column joints at elevated temperatures

This paper presents an experimental investigation on stocky column web panels of semi-rigid beam-to-column joints exposed to fire conditions in order to verify an analytical prediction model. Recent experimental studies show that the degradation of material properties and high axial forces, due to restrained thermal expansion of beams at elevated temperatures, significantly affect the moment–rotation response of the joints. The component method originally established for the evaluation of the joint behaviour at ambient conditions can be adopted for elevated-temperature cases. Recently developed mechanical models for joints may not be accurate as the column web component is simultaneously subjected to bending moments and axial forces. This paper focuses on the component column web in shear in order to identify the key parameters which affect joint behaviour in shear at elevated temperatures. This experimental work was conducted on three extended end-plate connections subjected to both ambient and elevated temperature conditions. After validations by test results, detailed finite element simulations were performed for a series of parametric studies at other elevated temperatures. Both experimental and numerical results are finally compared with analytical predictions.     

浅谈三种现浇混凝土空心楼盖板

介绍了三种常见的现浇混凝土空心楼盖板的构造特点和力学特点，着重对三种楼板的计算分析方法做了论述和对比，并对技术经济指标作了比较，以利于取长补短，推广新技术及新的结构体系。     

速成墙板-钢筋混凝土组合楼板的受力性能研究

利用非线性有限元方法,对速成墙板-钢筋混凝土组合楼板受力全过程进行分析,结合实测数据,对计算所得的荷载-挠度曲线、截面应变及承载力进行了对比,并对结果进行分析,建立了力学分析模型,为今后对这种结构进行计算机模拟试验提供了参考依据。     

Stress–strain constitutive equations of concrete material at elevated temperatures

The paper presents a critical review of the currently available models for the mechanical behaviour of concrete at elevated temperatures. Based on these models and experimental data a stress–strain–temperature model is proposed which incorporates the effect of transient strain implicitly. This model can be easily incorporated into existing commercial finite element analysis software. A numerical example on a wall element heated on two opposite faces indicates that at very early stages of heating transient strain does not play an important part, but that as the exposure time increases the effect of ignoring transient strain progressively increases and produces unconservative estimates of load carrying capacity.     

Properties of concrete made with recycled crushed glass at elevated temperatures

The effect of replacement of fine and coarse aggregates with recycled glass on the fresh and hardened properties of Portland cement concrete at ambient and elevated temperatures is studied. Percentages of replacement of 0–100% of aggregates with fine waste glass (FWG), coarse waste glass (CWG), and fine and coarse waste glass (FCWG) were considered. Soda-lime glass used for bottles was washed and crushed to fine and coarse aggregate sizes for use in the concrete mixes. Samples were cured under 95% RH at room temperatures (20–22 °C), heated in the oven to the desired temperatures, allowed to cool to ambient temperatures, and then tested for their residual compressive strength. The compressive strength of the concrete samples made with waste glass was measured at temperatures up to 700 °C. Moreover, the effect of the percentages of replacement with recycled glass on the slump values and initial and final setting time of concrete has also been measured.     

Behaviour of headed stud shear connectors for composite steel-concrete beams at elevated temperatures

The behaviour of composite steel-concrete beams at elevated temperatures is an important problem. A three-dimensional push test model is developed herein with a two-dimensional temperature distribution field based on the finite element method (FEM) and which may be applied to steel-concrete composite beams. The motivation for this paper is to increase the awareness of the structural engineering community to the concepts behind composite steel-concrete structural design for fire exposure. The behaviour of reinforced concrete slabs under fire conditions strongly depends on the interaction of the slabs with the surrounding elements which include the structural steel beam, steel reinforcing and shear connectors. This study was carried out to consider the effects of elevated temperatures on the behaviour of composite steel-concrete beams for both solid and profiled steel sheeting slabs. This investigation considers the load-slip relationship and ultimate load behaviour for push tests with a three-dimensional non-linear finite element program ABAQUS. As a result of elevated temperatures, the material properties change with temperature. The studies were compared with experimental tests under both ambient and elevated temperatures. Furthermore, for the elevated temperature study, the models were loaded progressively up to the ultimate load to illustrate the capability of the structure to withstand load during a fire. It is concluded that finite element analysis showed that the shear connector strength under fire exposure was very sensitive. It is also shown that profiled steel sheeting slabs exhibit greater fire resistance when compared with that of a solid slab as a function of their ambient temperature strength.     

Performance of axially restrained concrete encased steel composite columns at elevated temperatures

The structural performance of axially restrained concrete encased steel composite columns at elevated temperatures is investigated in this study. An efficient nonlinear 3-D finite element model was presented for the analysis of the pin-ended axially loaded columns. The restraint ratios varied from 20% to 100% of the axial stiffness of the composite columns at ambient temperature. The finite element model was verified against published test results on axially restrained concrete encased steel composite columns at elevated temperatures. The columns investigated had different cross-sectional dimensions, different coarse aggregates and different load ratios during fire. The nonlinear material properties of steel, concrete, longitudinal and transverse reinforcement bars as well as the effect of concrete confinement at ambient and elevated temperatures were considered in the finite element model. The interface between the steel section and concrete, the longitudinal and transverse reinforcement bars, and the reinforcement bars and concrete were also considered allowing the bond behaviour to be modelled and the different components to retain its profile during the deformation of the column. The initial overall geometric imperfection was carefully included in the model. The time-temperature relationships, deformed shapes at failure, time-axial displacement relationships, failure modes and fire resistances of the columns were evaluated by the finite element model and compared well against test results. Furthermore, the variables that influence the fire resistance and behaviour of the axially restrained composite columns comprising different axial restraint ratios, different load ratios during fire, different coarse aggregates and different slenderness ratios were investigated in a parametric study. It is shown that axially restrained composite columns behave differently in fire compared to the unrestrained columns since the typical “runaway” failure was not predicted from the finite element analysis. The fire resistances of the composite columns obtained from the finite element analysis were compared with the design values obtained from the Eurocode 4 for composite columns at elevated temperatures. It is shown that the EC4 is generally conservative for all the axially restrained concrete encased steel composite columns, except for some columns with higher load and slenderness ratios.     

Research at Imperial College on the effect of elevated temperatures on concrete

Long-standing research at Imperial College into the effect of elevated temperatures on concrete for fire and nuclear reactor applications has recently been intensified. A wide range from the thermally unstable gravel aggregate concrete to the thermally more stable lightweight included mechanical properties, microstructural analysis and conditions within the test specimen.The transient thermal strain of concrete during first-time heating under load to 600 °C can be described by two main components, the free thermal strain (FTS) and the load-induced thermal strain (LITS), which possess distinct and different properties but which were not significantly influenced by the age at testing. A ‘master’ LITS curve for concrete was found to exist for temperatures up to about 450 °C.Basic creep studies at constant temperatures indicated a marked increase in creep above 550–600 °C for cement paste and lightweight concrete which suggests that the structural, though not necessarily the refractory, usefulness of Portland cement-based concretes in general is limited to temperatures below 550–600 °C.Initial studies of the effect of heating on the strength of concrete show certain trends which may differ from those expected from current knowledge. This suggests that it may be possible to design concretes in the future which retain a larger proportion of their initial strength after heating than is achieved at present.     

Calculation of the fire resistance of composite concrete floor and roof slabs

A calculation method is described for the determination of the fire resistance of composite floor and roof slabs. Using this method, simple approximate formulas are derived for the calculation of the thermal fire resistance and minimum cover thickness to the steel of reinforced and prestressed concrete slabs, consisting of two layers of concrete, one lightweight and one normal weight. Comparison with test results shows good agreement between calculated and experimental fire resistances.     

Behaviour of the reinforced concrete face slabs of concrete faced rockfill dams during impounding

The primary goal of this study was to examine the relative effects of various factors that affect the cracking of the face slab of a concrete faced rockfill dam (CFRD). Rigorous simulation of the construction and impoundment stages were conducted in sequential phases during which the rockfill and the rockfill-face slab interface were modelled with elasto-plastic models that were calibrated to existing test data. The face slab was modelled using embedded reinforcements with a total strain crack model. The results of the analyses showed that the stiffness of the rockfill significantly changed the crackwidth on the face slab, but not the extent of cracking. Increasing the reinforcement ratio was the most effective remedy for decreasing the average crack width on the slab. The properties of the rockfill-slab interface and the location of the reinforcement within the slab did not significantly affect the cracking on the slab.     

Effects of elevated temperatures on properties of concrete

Concrete material in structures is likely exposed to high temperatures during fire. The relative properties of concrete after such an exposure are of great importance in terms of the serviceability of buildings. This paper presents the effects of elevated temperatures on the physical and mechanical properties of various concrete mixtures prepared by ordinary Portland cement, crushed limestone, and river gravel. Test samples were subjected to elevated temperatures ranging from 200 to 1200 °C. After exposure, weight losses were determined and then compressive strength test was conducted. Test results indicated that weight of the specimen significantly reduced with an increase in temperature. This reduction was very sharp beyond 800 °C. The effects of water/cement (w/c) ratio and type of aggregate on losses in weight were not found to be significant. The results also revealed that the relative strength of concrete decreased as the exposure temperature increased. The effect of high temperatures on the strength of concrete was more pronounced for concrete mixtures produced by river gravel aggregate. The results of the physical and mechanical tests were also combined with those obtained from differential thermal analysis, and colour image analysis.