Nonlinear behaviour of unprotected composite slim floor steel beams exposed to different fire conditions

An efficient nonlinear 3D finite element model has been developed to investigate the structural performance of composite slim floor steel beams with deep profiled steel decking under fire conditions. The composite steel beams were unprotected simply supported with different cross-sectional dimensions, structural steel sections, load ratios during fire and were subjected to different fire scenarios. The nonlinear material properties of steel, composite slim concrete floor and reinforcement bars were incorporated in the model at ambient and elevated temperatures. The interface between the structural steel section and composite slim concrete floor was also considered, allowing the bond behaviour to be modelled and the different components to retain its profile during the deformation of the composite beam. Furthermore the thermal properties of the interface were included in the finite element analysis. The finite element model has been validated against published fire tests on unprotected composite slim floor steel beams. The time–temperature relationships, deformed shapes at failure, time–vertical displacement relationships, failure modes and fire resistances of the composite steel beams were evaluated by the finite element model. Comparisons between predicted behaviour and that recorded in fire tests have shown that the finite element model can accurately predict the behaviour of the composite steel beams under fire conditions. Furthermore, the variables that influence the fire resistance and behaviour of the unprotected composite slim floor steel beams, comprising different load ratios during fire, cross-section geometries, beam length and fire scenarios, were investigated in parametric studies. It is shown that the failure of the composite beams under fire conditions occurred for the standard fire curve, but did not occur for the natural fires. The use of high strength structural steel considerably limited the vertical displacements after fire exposure. It is also shown that presence of additional top reinforcement mesh is necessary for composite beams exposed to short hot natural fires. The fire resistances of the composite beams obtained from the finite element analyses were compared with the design values obtained from the Eurocode 4 for composite beams at elevated temperatures. It is shown that the EC4 predictions are generally conservative for the design of composite slim floor steel beams heated using different fire scenarios.     

Horizontally curved composite plate girders with trapezoidally corrugated webs

This paper is concerned with horizontally curved composite plate girders with trapezoidally corrugated webs. Finite element analysis using the computer package LUSAS is employed to investigate the behaviour and ultimate strength capacity of the girders. The presence of web openings of different proportions and their effects on the behaviour and ultimate strength of the girders are also investigated. Parameters that affect the behaviour of these girders are slenderness of the web d/t, web panel aspect ratio b/d, width to the depth of corrugation ratio b_h/h and size of openings. Influence of these parameters on the collapse behaviour is examined. Results are obtained in terms of ultimate strength, failure mechanism and load-deflection curves from the finite element analyses and, some typical results are presented herein. An approximate method to compute the shear capacity of these girders is presented. Comparison of the results with those predicted using the finite element method established the accuracy of the proposed method.     

Incorporation of corrosion effects in reliability-based optimization of composite hybrid plate girders

Available data suggest that corrosion may have significant effects on the reliability of structural components and systems including highway bridges, offshore platforms, and hydraulic structures. Therefore, reliability-based design and/or optimization of a structure or component placed during its intended service life in a corrosive environment must take the effects of this environment into account. A reliability-based optimization approach to the design of composite hybrid plate girders under corrosion effects is developed herein. The proposed approach incorporates all constraints specified in the AASHTO bridge code considering time-dependent corrosion effects. Numerical examples illustrate the application of the proposed approach to time-dependent reliability-based optimum design of both unstiffened and stiffened composite hybrid plate girders for highway bridges.     

Strengthening of steel–concrete composite girders using carbon fiber reinforced polymer plates

Strengthening and rehabilitation of structures is a major concern for researchers in the civil engineering community in recent years due to the aging of these structures and the need for effective methods of strengthening. This paper presents the results of an experimental study of the behavior of strengthened steel–concrete composite girders using Carbon Fiber Reinforced Polymers (CFRP) plates. Strengthening was achieved by attaching the CFRP plates to the bottom flange and in some beams the CFRP plates were also attached to the beam web. Two different types of CFRP plates were used being mainly different in the tensile modulus of elasticity. Shear stress distribution along the bond line between CFRP plates and steel was recorded and reported. The test results showed that using lightweight CFRP plates could enhance the strength and stiffness of steel–concrete composite girders up to 45% of the original 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.     

预应力碳纤维板加固钢筋混凝土梁的受弯性能

为研究预应力碳纤维板加固钢筋混凝土梁的受弯性能,进行3根碳纤维板加固钢筋混凝土梁和1根对比梁的试验研究。研究预应力以及梁底锚固方式对混凝土梁的受弯承载力、刚度、裂缝发展情况、碳纤维板利用率的影响。试验结果表明:预应力碳纤维板加固可明显提高试件的受弯承载力,提高碳纤维板的利用率,减小裂缝宽度,其延性有所下降,在加固构件设计时需特别关注;两种锚固方式均满足正常使用要求,综合考虑无需开槽的MJ-2锚具适用性更强。     

Numerical simulation of concrete encased steel composite columns

This paper investigates the behaviour of pin-ended axially loaded concrete encased steel composite columns. A nonlinear 3-D finite element model was developed to analyse the inelastic behaviour of steel, concrete, longitudinal and transverse reinforcement bars as well as the effect of concrete confinement of the concrete encased steel composite columns. The interface between the steel section and concrete, the longitudinal and transverse reinforcement bars, and the reinforcement bars and concrete were also considered that allowed the bond behaviour to be modeled and the different components to retain their profile during the deformation of the column. Furthermore, the initial overall (out-of-straightness) geometric imperfection was carefully incorporated in the model. The finite element model has been validated against published experimental results. The main objective of the study was to understand the structural response and modes of failure of the columns and to assess the composite column strengths against current design codes. The study covered slender, non-slender, stub and long concrete encased steel composite columns. The concrete strengths varied from normal to high strength (20-110 MPa). The steel section yield stresses also varied from normal to high strength (275-690 MPa). Furthermore, the variables that influence the composite column behaviour and strength comprising different slenderness ratios, concrete strength and steel yield stress were investigated in a parametric study. It is shown that the increase in structural steel strength has a small effect on the composite column strength for the columns having higher relative slenderness ratios due to the flexural buckling failure mode. The composite column strengths obtained from the finite element analysis were compared with the design strengths calculated using the American Institute for Steel Construction AISC and Eurocode 4 for composite columns. Generally, it is shown that the EC 4 accurately predicted the design strength for the concrete encased steel composite columns having a concrete cylinder strength of 30 MPa and structural steel yield stresses of 275 and 460 MPa, which are in the limits of the code, which otherwise, was generally conservative. The AISC predictions were quite conservative for all the concrete encased steel composite columns.     

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.     

Eccentrically loaded concrete encased steel composite columns

This paper presents a nonlinear 3-D finite element model for eccentrically loaded concrete encased steel composite columns. The columns were pin-ended subjected to an eccentric load acting along the major axis, with eccentricity varied from 0.125 to 0.375 of the overall depth (D) of the column sections. The model accounted for the inelastic behaviour of steel, concrete, longitudinal and transverse reinforcement bars as well as the effect of concrete confinement of the concrete encased steel composite columns. 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 incorporated in the model. The finite element model has been validated against existing test results. The concrete strengths varied from normal to high strength (30–110 MPa). The steel section yield stresses also varied from normal to high strength (275–690 MPa). Furthermore, the variables that influence the eccentrically loaded composite column behaviour and strength comprising different eccentricities, different column dimensions, different structural steel sizes, different concrete strengths, and different structural steel yield stresses were investigated in a parametric study. Generally, it is shown that the effect on the composite column strength owing to the increase in structural steel yield stress is significant for eccentrically loaded columns with small eccentricity of 0.125D. On the other hand, for columns with higher eccentricity 0.375D, the effect on the composite column strength due to the increase in structural steel yield stress is significant for columns with concrete strengths lower than 70 MPa. The strength of composite columns obtained from the finite element analysis were compared with the design strengths calculated using the Eurocode 4 for composite columns. Generally, it is shown that the EC4 accurately predicted the eccentrically loaded composite columns, while overestimated the moment.     

Experimental study on fire protection methods of reinforced concrete beams strengthened with carbon fiber reinforced polymer

In this paper, two reinforced concrete (RC) beams strengthened with carbon fiber reinforced polymer (CFRP) and attached with thick-painted fire resistant coating were tested for fire resistance following the standard fire testing procedures. The experimental results show that the specimen pasted with the insulated layer of 50 mm in thickness could resist fire for 2.5 h. It is also demonstrated that the steel wire mesh embedded in the insulated layer can effectively prevent it from cracking and eroding under firing. __________ Translated from Journal of Tongji University (Natural Sciences), 2006, 34(11): 1452–1456 [译自: 同济大学学报 (自然科学版)]     

Numerical simulation on seismic retrofitting performance of reinforced concrete columns strengthened with fibre reinforced polymer sheets

This paper evaluates the seismic performance of reinforced concrete columns retrofitted with fibre reinforced polymer (FRP) sheets through numerical simulations of the load–deformation response using two-dimensional finite element analysis (2D-FEA). The relatively rational mesh configuration is verified through comparison of analysis results obtained from the different mesh configurations. The seismic performance of three reinforced concrete (RC) columns strengthened with FRP sheets is assessed through a series of parametric studies, and the applicability of existing crack models and constitutive relationships on crack discontinuity and concrete compressive behaviour are validated. Comparisons of analysis results with tests shows that an equivalent uniaxial strain model and a failure criterion can be used to accurately simulate nonlinear behaviour and the failure of concrete under a biaxial stress state, respectively. Moreover, it is shown that a modified confinement model can be simply adopted to evaluate confined effects from hoop steel and FRP on concrete, which generally operate in three-dimensional confinement. Lastly, the seismic retrofitting performance of RC columns wrapped with FRP sheets is verified by analysing load–deformation responses and the progression of stress–strain at inflection points and bottoms of the columns.     

Evaluation of debonding failure of reinforced concrete girders strengthened in flexure with FRP laminates using finite element modeling

The use of Fiber-Reinforced Polymer (FRP) materials dates back to the early 1940s when they were used in aerospace and naval applications. During the 1970s and early 1980s, FRP started being used in civil engineering applications for new construction, but more importantly for repair and strengthening of existing structures. However, experimental research showed that the typical failure mode of reinforced concrete (RC) structures strengthened with FRP composite materials is due to the debonding that occurs at the interface between concrete and FRP. The bond between FRP and concrete is therefore the key factor controlling the behavior of these structures since it limits the full use of the FRP strength. The paper evaluates the effect of the debonding failure on the response of FRP-strengthened RC beams. A nonlinear RC beam element with bond-slip between the concrete and the FRP laminates is developed and used to analyze several test specimens and to investigate their corresponding failure mode. The model was also used to study the reduction factor of FRP tensile strength of simply supported strengthened RC girders due to debonding failure. This reduction factor proved to be affected by several parameters: (a) the bond strength between FRP and concrete interface; (b) the concrete strength; (c) the thickness of FRP; (d) the modulus of FRP; (e) the width of FRP laminate; and (f) the development length of the FRP sheet. A large number of beam specimens were analyzed in order to conduct a thorough evaluation of debonding failure of RC beams strengthened with FRP laminates. Based on these studies, new equations that account for the aforementioned parameters were proposed to address the reduction in FRP strength due to debonding failure.     

Reliability-based optimum design of reinforced concrete girders

A practical optimization approach to the design of reinforced concrete girders for highway bridges is presented. The approach is entirely based on the American Association of State Highway and Transportation Officials (AASHTO) standard specifications for highway bridges. All behavior and side constraints specified in the AASHTO bridge code are considered in the optimum design process. Two optimization formulations are presented. The first uses load and resistance design factors (LRFD) and the second one is entirely based on a reliability approach. In these formulations, the independent and dependent variables are identified, and the constraints are formulated in both the AASHTO-LRFD format and the AASHTO-reliability-based format. The solutions for both formulations are obtained by using a nonlinear optimization software. Numerical examples illustrate the application of the proposed approach to the design of reinforced concrete T-girders. Sensitivity analyses are also performed for finding the effects of various parameters including steel to concrete cost ratio and allowable reliability level on the optimum solution.     

Geometrically non-linear analysis including shear deformation of composite laminates

Geometrically non-linear deformations of composite laminated plates are computed using the perturbation finite element method (PFEM). The PFEM is more economic in terms of computational time than conventional finite element iterative procedure, and results in semi-analytic solutions because deformations are polynomial functions of external loads, and vice-versa. To account for the transverse shear effect on deformation of a laminated plate, a discrete-layer shear deformation theory is introduced. This approach predicts more accurately the distribution of displacements and stresses through the thickness than single-layer theories. Detailed derivation of the theory is presented in the paper. A three-node triangular element model and computer program have been developed and implemented as part of this study. Computed numerical results of several examples show that the perturbation finite element solutions are in good agreement with exact solution, experimental data and calculated numerical result from other investigators.     

Structural upgrading of RC beams using composite overlays

This paper describes the development and validation of a new design approach for strengthening of structural concrete beams using physically connected concrete overlays. The test results of the present experimental programme showed that each of the strengthened beams acted as a single unit and failed in a flexural ductile manner. The measured extra strength reached up to 154% of the capacity of the existing beams. The proposed approach not only enhances the flexural capacity and ductility but most important prevents the occurrence of brittle shear failures reported with other strengthening techniques.     

Finite element investigation of shear failure of lean duplex stainless steel plate girders

The use of duplex stainless steel material has gained popularity in the last two decades thanks to its nature that combines well the advantages of both austenitics and carbon steel materials. The duplex grades offer a combination of higher strength than austenitics in addition to a great majority of carbon steels with similar or superior corrosion resistance. However, high nickel prices have more recently led to a demand for lean duplexes with low nickel content, such as grade EN 1.4162. Wide-ranging work is needed to include the lean duplex grade EN 1.4162, into design standards such as EN 1993-1-4. Accordingly, a finite element modelling for full-size lean duplex stainless steel plate girders of non-rigid end stiffeners of Grade EN 1.4162 is presented in this paper. The paper is principally concerned with shear failure mechanism characteristics of this type of plate girders, which is not yet investigated. The ABAQUS 6.6 programme, as a finite element package, is used in the current work. The lean duplex stainless steel material is simulated here based on an accepted stainless steel material model available in the literature. A number of transversely stiffened I-section plate girders having equal depth of 1000 mm in span of 4 m is considered and parametric studies regarding flange width-to-web depth ratio, flange-to-web thickness ratio and web plate slenderness are carried out. However, new conclusions on shear strength of lean duplex stainless steel plate girders are presented.     

碳纤维布加固钢筋混凝土梁非线性有限元分析

在参考了其他学者在钢筋混凝土非线性有限元分析中所做工作的基础上,编制了完整的碳纤维布加固钢筋混凝土梁的非线性有限元分析程序.程序中采用了分离式有限元模型并且考虑了钢筋与混凝土之间粘结滑移关系.通过与试验梁对比,程序计算所得加固与未加固梁的极限承载力、梁的破坏形式、荷载-挠度曲线、加固层应变与试验值都很接近,验证了程序的可行性与适用性.     

Fatigue life prediction of existing crane runway girders

In this study, fatigue life of crane runway girders of a steel mill structure was investigated. Load spectra were generated based on former crane operation records. Detailed finite element models of the crane runway girders were prepared using shell and beam elements. Quasi-static load tests were conducted with the help of overhead cranes that travelled with crawling speed. Strain data was collected by using transducers mounted on preselected locations of the crane runway girders. These data were then used to refine the finite element models. Numerical analyses by means of the calibrated finite element models were performed to evaluate the remaining fatigue life. It was found that due to lack of continuity of vertical stiffeners to upper flange, fatigue life of the crane girders is exceeded. To overcome this problem, fillet welding of the stiffeners to flanges is suggested and this modification is verified by carrying out necessary calculations for the updated detail.     

Numerical investigation into the composite behaviour of over-deformed segmental tunnel linings strengthened by bonding steel plates

Bonding steel plate has been used as a strengthening approach to repair disrupted segmental lining of operational tunnels. This paper introduces numerical investigation into the composite behaviour of the initially deformed segmental tunnel linings strengthened by bonding steel plates using finite element modelling. Cohesive zone modelling was used to simulate the interface bonding behaviour between the segmental linings and steel plates. The full history of the tunnel behaviour before and after strengthening were simulated, where the segmental tunnel lining is initially loaded to create some deformation, then after bonding steel plate, the strengthened tunnel is reloaded until failure occurs. By comparing the results with experimental data from the literature, the proposed model was proved to be capable of simulating the strengthened lining behaviour and able to capture the strengthening failure process in terms of the interface debonding. Subsequently, the segmental lining response and interface shear stress distribution and propagation were analysed to interpret the interaction and failure mechanism of the steel plate strengthened segmental linings. The influence of the initial deformation and the steel plate thickness were investigated and discussed in terms of the strengthened stiffness and capacity. It has been found that the interface shear stress concentration occurred at the positions of the segment joints, where bond damage first initiated. The ultimate failure of the steel plate strengthening happened suddenly once a local debonding zone close to the segmental joint was formed. In addition, the predicted results indicate that a delay in strengthening would result in an increase in strengthened capacity but a decrease in strengthened stiffness. By using thicker steel plates, the strengthened stiffness was improved, while the strengthened capacity could be improved only if the thickness was relatively thin.     

Finite element modelling of plate girders with web openings

This paper is concerned with a finite element model to predict the behaviour and ultimate load of plate girders with web openings. The finite element package is used to model the plate girders with web openings. Accuracy of the model is assessed by applying it to plate girders tested earlier by other researchers. Comparison of analytical results with the available experimental results for yielding patterns, ultimate load values and load–deflection relationships show good agreement between the finite element and experimental results thus validating the accuracy of the proposed model. The proposed finite element method was extended to carry out a parametric study. The study covered parameters such as web slenderness and flange stiffness.