Experimental study on fire performance of axially-restrained NSC and HSC columns

Bo Wu,Yi-Hai Li 국제구조공학회 2009 Structural Engineering and Mechanics, An Int'l Jou Vol.32 No.5

This paper describes fire performance of eight axially restrained reinforced concrete (RC) columns under a combination of two different load ratios and two different axial restraint ratios. The eight RC columns were all concentrically loaded and subjected to ISO834 standard fire on all sides. Axial restraints were imposed at the top of the columns to simulate the restraining effect of the rest of the whole frame. The axial restraint was effective when the column was expanding as well as contracting. As the results of the experiments have shown, the stiffness of the axial restraint and load level play an important role in the fire behaviors of both HSC and NSC columns. It is found that (a) the maximum deformations during expanding phase were influenced mostly by load ratio and hardly by axial restraint ratio, (b) For a given load ratio, axial restraint ratio had a great impact on the development of axial deformation during contraction phase beyond the initial equilibrium state, (c) increasing the axial restraint increased the value of restraint force generated in both the NSC and HSC columns, and (d) the development of column axial force during the contracting and cooling phase followed nearly parallel trend for columns under the same load ratio.

Experimental study on fire performance of axially-restrained NSC and HSC columns

Wu, Bo,Li, Yi-Hai Techno-Press 2009 Structural Engineering and Mechanics, An Int'l Jou Vol.32 No.5

This paper describes fire performance of eight axially restrained reinforced concrete (RC) columns under a combination of two different load ratios and two different axial restraint ratios. The eight RC columns were all concentrically loaded and subjected to ISO834 standard fire on all sides. Axial restraints were imposed at the top of the columns to simulate the restraining effect of the rest of the whole frame. The axial restraint was effective when the column was expanding as well as contracting. As the results of the experiments have shown, the stiffness of the axial restraint and load level play an important role in the fire behaviors of both HSC and NSC columns. It is found that (a) the maximum deformations during expanding phase were influenced mostly by load ratio and hardly by axial restraint ratio, (b) For a given load ratio, axial restraint ratio had a great impact on the development of axial deformation during contraction phase beyond the initial equilibrium state, (c) increasing the axial restraint increased the value of restraint force generated in both the NSC and HSC columns, and (d) the development of column axial force during the contracting and cooling phase followed nearly parallel trend for columns under the same load ratio.

Effect of Axial Restraints on Top-Seat Angle Connections at Elevated Temperatures

Leong Siong Hean,N.H. Ramli Sulong,Mohammed Jameel 대한토목학회 2016 KSCE Journal of Civil Engineering Vol.20 No.6

In this study, a finite element model of a top-seat angle connection at elevated temperature is developed to study the effect of axial restraints on the connection behavior. Models are first validated with existing fire test results of top-seat angle connection. Non-linear behavior of the materials was modeled with the definition of elastic-plastic multi-linear properties and frictional contact between surfaces is included to simulate actual conditions. Validation of the model behavior shows that the model is in good agreement with existing experimental results and therefore the model is used for further study on the effect of axial restraints towards connection behavior. Both isothermal and anisothermal conditions were analyzed considering the effect of axial restraints. Results from the model shows that an increase in axial restraints has increased connection capacity while the stiffness remains similar for low axial levels. On the other hand, the shrinkage of the beam, may lead to tensional axial loads, causing axial pulling on the connection, resulting in higher deformation and different deformation patterns of the component angle sections. Effect on the design guidelines for connection design with axial restraints provided by Eurocode 3:2005 (Part 1-8) is also discussed at the end.

Effective torsional strength of axially restricted RC beams

Cátia S.B. Taborda,Luís F.A. Bernardo,Jorge M.R. Gama 국제구조공학회 2018 Structural Engineering and Mechanics, An Int'l Jou Vol.67 No.5

In a previous study, design charts where proposed to help the torsional design of axially restricted reinforced concrete (RC) beams with squared cross section. In this article, new design charts are proposed to cover RC beams with rectangular cross section. The influence of the height to width ratio of the cross section on the behavior of RC beams under torsion is firstly shown by using theoretical and experimental results. Next, the effective torsional strength of a reference RC beam is computed for several values and combinations of the study variables, namely: height to width ratio of the cross section, concrete compressive strength, torsional reinforcement ratio and level of the axial restraint. To compute the torsional strength, the modified Variable Angle Truss Model for axially restricted RC beams is used. Then, an extensive parametric analysis based on multivariable and nonlinear correlation analysis is performed to obtain nonlinear regression equations which allow to build the new design charts. These charts allow to correct the torsional strength in order to consider the favourable influence of the compressive axial stress that arises from the axial restraint.

Parametric analysis and torsion design charts for axially restrained RC beams

Luís F.A. Bernardo,Cátia S.B. Taborda,Jorge M.R. Gama 국제구조공학회 2015 Structural Engineering and Mechanics, An Int'l Jou Vol.55 No.1

This article presents a theoretical parametric analysis on the ultimate torsional behaviour of axially restrained reinforced concrete (RC) beams. This analysis is performed by using a computing procedure based on a modification of the Variable Angle Truss Model. This computing procedure was previously developed to account for the influence of the longitudinal compressive stress state due to the axial restraint conditions provided by the connections of the beams to other structural members. The presented parametric study aims to check the influence of some important variable studies, namely: torsional reinforcement ratio, compressive concrete strength and axial restraint level. From the results of this parametric study, nonlinear regression analyses are performed and some design charts are proposed. Such charts allow to correct the resistance torque of RC beams (rectangular sections with small height to width ratios) to account for the favorable influence of the axial restraint.

Effective torsional strength of axially restricted RC beams

Taborda, Catia S.B.,Bernardo, Luis F.A.,Gama, Jorge M.R. Techno-Press 2018 Structural Engineering and Mechanics, An Int'l Jou Vol.67 No.5

In a previous study, design charts where proposed to help the torsional design of axially restricted reinforced concrete (RC) beams with squared cross section. In this article, new design charts are proposed to cover RC beams with rectangular cross section. The influence of the height to width ratio of the cross section on the behavior of RC beams under torsion is firstly shown by using theoretical and experimental results. Next, the effective torsional strength of a reference RC beam is computed for several values and combinations of the study variables, namely: height to width ratio of the cross section, concrete compressive strength, torsional reinforcement ratio and level of the axial restraint. To compute the torsional strength, the modified Variable Angle Truss Model for axially restricted RC beams is used. Then, an extensive parametric analysis based on multivariable and nonlinear correlation analysis is performed to obtain nonlinear regression equations which allow to build the new design charts. These charts allow to correct the torsional strength in order to consider the favourable influence of the compressive axial stress that arises from the axial restraint.

Modeling fire performance of externally prestressed steel–concrete composite beams

Huanting Zhou,Shaoyuan Li,Chao Zhang,M.Z. Naser 국제구조공학회 2021 Steel and Composite Structures, An International J Vol.41 No.5

This paper examines the fire performance of uninsulated and uncoated restrained steel-concrete composite beams supplemented with externally prestressed strands through advanced numerical simulation. In this work, a sequentially coupled thermo-mechanical analysis is carried out using ABAQUS. This analysis utilizes a highly nonlinear three-dimensional finite element (FE) model that is specifically developed and validated using full-sized specimens tested in a companion fire testing program. The developed FE model accounts for nonlinearities arising from geometric features and material properties, as well as complexities resulting from prestressing systems, fire conditions, and mechanical loadings. Four factors are of interest to this work including effect of restraints (axial vs. rotational), degree of stiffness of restraints, the configuration of external prestressed tendons, and magnitude of applied loading. The outcome of this analysis demonstrates how the prestressing force in the external tendons is primarily governed by the magnitude of applied loading and experienced temperature level. Interestingly, these results also show that the stiffness of axial restraints has a minor influence on the failure of restrained and prestressed steel-concrete composite beams. When the axial restraint ratio does not exceed 0.5, the critical deflection of the composite beam is lower than that of the composite beam with a restraint ratio of 1.0.

Post-buckling behaviours of axially restrained steel columns in fire

Guo-Qiang Li,Peijun Wang,Hetao Hou 국제구조공학회 2009 Steel and Composite Structures, An International J Vol.9 No.2

This paper presents a simplified model to study post-buckling behaviours of the axially restrained steel column at elevated temperatures in fire. The contribution of axial deformation to the curvature of column section is included in theoretical equations. The possible unloading at the convex side of the column when buckling occurs is considered in the stress-strain relationship of steel at elevated temperatures. Parameters that affect structural behaviours of the axial restrained column in fire are studied. The axial restraint cause an increase in the axial force before the column buckles; the buckling emperature of restrained columns will be lower than non-restrained steel columns. However, the axial force of a restrained column decreases after the column buckles with the elevation of temperatures, so make use of the post-buckling behaviour can increase the critical temperature of restrained columns. Columns with temperature gradient across the section will produce lower axial force at elevated temperatures.

Plastification procedure of laterally-loaded steel bars under a rising temperature

Huang, Zhan-Fei,Tan, Kang-Hai,England, George L. Techno-Press 2010 Structural Engineering and Mechanics, An Int'l Jou Vol.35 No.6

This paper investigates the structural responses of axially restrained steel beams under fire conditions by a nonlinear finite element method. The axial restraint is represented by a linear elastic spring. Different parameters which include beam slenderness ratio, external load level and axial restraint ratio are investigated. The process of forming a mid-span plastic hinge at the mid-span under a rising temperature is studied. In line with forming a fully plastic hinge at mid-span, the response of a restrained beam under rising temperature can be divided into three stages, viz. no plastic hinge, hinge forming and rotating, and catenary action stage. During catenary action stage, the axial restraint pulls the heated beam and prevents it from failing. This study introduces definitions of beam limiting temperature $T_{lim}$, catenary temperature $T_{ctn}$ and warning time $t_{wn}$. Influences of slenderness ratio, load level and axial restraint ratio on $T_{lim}$, $T_{ctn}$ and $t_{wn}$ are examined.

Plastification procedure of laterally-loaded steel bars under a rising temperature

Zhan-fei Huang,Kang-hai Tan,George L. England 국제구조공학회 2010 Structural Engineering and Mechanics, An Int'l Jou Vol.35 No.6

This paper investigates the structural responses of axially restrained steel beams under fire conditions by a nonlinear finite element method. The axial restraint is represented by a linear elastic spring. Different parameters which include beam slenderness ratio, external load level and axial restraint ratio are investigated. The process of forming a mid-span plastic hinge at the mid-span under a rising temperature is studied. In line with forming a fully plastic hinge at mid-span, the response of a restrained beam under rising temperature can be divided into three stages, viz. no plastic hinge, hinge forming and rotating, and catenary action stage. During catenary action stage, the axial restraint pulls the heated beam and prevents it from failing. This study introduces definitions of beam limiting temperature Tlim, catenary temperature Tctn and warning time twn. Influences of slenderness ratio, load level and axial restraint ratio on Tlim, Tctn and twn are examined.