A simplified approach for predicting temperatures in insulated RC members exposed to standard fire

Fire resistance of concrete structural members can be enhanced through the application of external fire insulation on the surfaces of concrete member. For evaluating fire resistance of such insulated RC members, temperatures in concrete and steel reinforcement are to be known. This paper develops a simplified approach for predicting cross-sectional temperatures in an insulated RC structural member exposed to standard fire. The approach is derived by replacing the insulation layer into an equivalent concrete thickness layer and then undertaking statistical regression analysis on temperature data of modified concrete section. The effect of critical parameters, including geometry of concrete member and insulation, thermal properties of concrete and fire insulation, and duration of fire exposure is accounted for in temperature equations. The validity of the approach is established by comparing predictions from the proposed equation with data generated from fire tests and finite element analysis. These comparisons show the proposed equation gives reasonable prediction of temperatures, within a range of ±10%, in insulated concrete members. The applicability of the proposed approach in design situations is illustrated though a numerical example. The simplicity of the proposed method makes it attractive for use in design situations and for incorporation in design manuals.     

Reliability analysis of reinforced concrete columns exposed to fire

A reliability analysis is conducted on reinforced concrete columns subjected to fire load. From an evaluation of load frequency of occurrence, load random variables are taken to be dead load, sustained live load, and fire temperature. Resistance is developed for axial capacity, with random variables taken as steel yield strength, concrete compressive strength, placement of reinforcement, and section width and height. A rational interaction model based on the Rankine approach is used to estimate column capacity as a function of fire exposure time. Various factors were considered in the analysis such as fire type, load ratio, reinforcement ratio, cover, concrete strength, load eccentricity, and other parameters. Reliability was computed from 0 to 4 h of fire exposure using Monte Carlo simulation. It was found that reliability decreased nonlinearly as a function of time, while the most significant parameters were fire type, load ratio, eccentricity, and reinforcement ratio.     

Nonlinear analysis of reinforced concrete cross-sections exposed to fire

A nonlinear structural analysis of cross-sections of three-dimensional reinforced concrete frames exposed to fire is presented. The analysis includes two steps: the first step is the calculation of the transient temperature field in cross-sections exposed to fire and the second step is the determination of the mechanical response due to the effect of thermal and mechanical load. A nonlinear finite-element procedure is proposed to predict the temperature field history. In this thermal analysis, the effect of moisture has been taken into account by introducing a water vapor fraction function to define the variation of enthalpy. A mechanical nonlinear analysis of the cross-sections is performed for each temperature distribution and for the applied exterior load using an algorithm of arc-length control. The mechanical and thermal properties of concrete and steel are taken according to the European Standard ENV 1991-1-2 [ENV. Eurocode 2, design of concrete structures, part 1–2: general rules—structural fire design. ENV 1992-1-2, 1995]. In order to validate the proposed thermal and mechanical models, comparisons between numerical and experimental results have been performed. The agreement found is in both cases, fairly good. In addition, a numerical example of the structural analysis of several cross-sections of a reinforced concrete waffle slab under external load and fire is shown.     

A simplified model for predicting the shear response of reinforced concrete membranes

Presented in this paper is a study to provide a practical method for predicting the distinctive points observed in the response of RC membranes without tracing the load-deformation history. Features of the proposed model, based on the solution of the governing equations derived in the secant form, include the consideration of the tension stiffening effect and multidirectional reinforcement. For practical purposes, due to simplicity of the modeling and ability to obtain the response of the RC membrane with a minimum amount of computation, the proposed method offers attractive alternatives to the use of available complicated models. The reliability of the proposed model is confirmed by comparison between predicted and test results.     

A simplified approach for predicting the buckling strength of centrally compressed members

In the last few years a number of codes of practice have adopted multiple-column curves which are dependent on material and geometrical imperfection level as well as steel strength.This paper presents three proposals aimed at simplifying the design process. These proposals are based on the concept of adopting one strength curve which is dependent only on out-of-straigthness. Other effects are incorporated by modifying the effective slenderness of the members.Design tables are presented for members of different cross-section and a design example is given.     

火灾下钢筋混凝土剪力墙温度场分析

对火灾下钢筋混凝土剪力墙的温度场进行了有限元计算分析,得到了剪力墙截面温度场分布规律,计算结果与试验结果符合较好。在此基础上,分析了混凝土热工参数、迎火面对流系数及综合辐射系数对混凝土剪力墙温度场的影响。结果表明:已有文献给出的混凝土热工参数对剪力墙温度场影响不大,但是温度场对迎火面对流系数及综合辐射系数的变化较为敏感。     

Modeling of insulated CFRP-strengthened reinforced concrete T-beam exposed to fire

The use of Fiber Reinforced Polymer (FRP) has been established as one of the possible techniques to strengthen concrete beams in flexure and shear. Carbon Fiber Reinforced Polymer (CFRP) has been identified as the material of choice in civil infrastructure applications. The fire performance of such CFRP-strengthened members and their resistance to heat transfer and to various environmental exposure factors need to be investigated. In this paper, a detailed finite element model of a CFRP-strengthened reinforced concrete T-beam is developed. The model accounts for the variation in thermal and mechanical parameters of the beams’ constituent materials with temperature, including CFRP and insulation materials. Nonlinear time domain transient thermal-stress finite element analysis is performed using the commercial software ANSYS to study the heat transfer mechanism and deformation within the beam for fire conditions initiating at the bottom of the beam. To relate the simulation to an actual case, a reinforced concrete T-beam strengthened with CFRP and fire-tested by other investigators is modeled. The progression of temperature in the beam, CFRP, reinforcing steel, and along the CFRP–concrete interface is compared to the observed fire test data. Overall, the predicted temperature results are in good agreement with the measured ones. In addition, the mid-span deflection increases nonlinearly during the fire exposure time due to the increase in the total strain on the tension side of the beams and due to concrete cracking. Successful FE modeling of this structure provides an economical, alternative solution to expensive experimental investigations.     

Design equation for predicting fire resistance of reinforced concrete beams

An approach for evaluating the fire resistance of reinforced concrete (RC) beams is presented in this paper. A macroscopic finite element model is applied to study the influence of various parameters on the fire resistance of RC beams. Data from parametric studies is utilized to develop a simplified expression for evaluating the fire resistance of an RC beam as a function of influencing parameters. The validity of the proposed approach is established by comparing the fire resistance predictions with those obtained from finite element studies as well as from fire resistance tests. Predictions from the proposed equation are also compared with fire resistance estimates from current codes of practice. The applicability of the approach to design situations is illustrated through a numerical example. The proposed rational approach expresses fire resistance in terms of conventional structural and material design parameters, and thus facilitates easy evaluation of fire resistance. The proposed approach provides better estimates than those from current codes of practice and thus can be used to evaluate the fire resistance of RC beams with an accuracy that is adequate for design purposes.     

高强混凝土的抗火灾高温性能研究概述

综述目前高强混凝土的抗火灾高温性能的研究进展,并对聚丙烯纤维、橡胶粉、钢纤维等对改善高强混凝土的高温性能情况进行比较,指出研究中所存在的问题和今后的研究发展方向,对高强混凝土的高温性能研究和应用起到指导作用.     

钢纤维混凝土高温应力损伤性能

研究高温对不同钢纤维掺鼍的钢纤维混凝土抗拉、劈拉强度的影响结果,对影响机理进行了简单的分析.试验结果表明:钢纤维混凝土高温后的抗压、劈拉强度随所受最高温的升高而缓慢下降,400℃以后下降稍快.与素混凝土相比,其残余强度率分别可提高30%和20%左右.采用有限元软件ANSYS对混凝土加热过程中的温度场与应力场进行分析,提出了混凝土高温下开裂损伤判据.     

A numerical approach for modeling the fire induced restraint effects in reinforced concrete beams

In this paper, a model to predict the influence of fire induced restraints on the fire resistance of reinforced concrete (RC) beams is presented. The three stages, associated with the fire growth, thermal and structural analysis, for the calculation of fire resistance of the RC beams are explained. A simplified approach to account for spalling under fire conditions is incorporated into the model. The validity of the numerical model is established by comparing the predictions from the computer program with results from full-scale fire resistance tests. The program is used to conduct two case studies to investigate the influence of both the rotational and the axial restraint on the fire response of the RC beams. Through these case studies, it is shown that the restraint, both rotational and axial, has significant influence on the fire resistance of the RC beams.     

火灾下混凝土柱的温度场影响分析

利用SAFIR有限元分析软件对火灾下钢筋混凝土柱的温度场进行分析和计算,探讨高温下保护层厚度、截面周长、受火时间和截面配筋率对柱截面温度场的分布规律,为进一步认识钢筋混凝土柱的高温力学性能创造条件,并为开展火灾后钢筋混凝土柱的修复加固提供理论依据。     

A softened membrane model for torsion in reinforced concrete members

The Softened Membrane Model (SMM), developed for predicting the behavior of reinforced concrete (RC) membrane elements under shear, is extended to RC members subjected to torsion. This new analytical method, referred to as the Softened Membrane Model for Torsion (SMMT), takes into account the strain gradient of concrete struts in the shear flow zone by making two modifications to the constitutive relationships of concrete. First, in the tensile stress–strain relationship of concrete, the pre-cracking stiffness and the strain at peak stress should each be increased by 45%. Second, the Hsu/Zhu ratio for torsion is taken as 80% of the Hsu/Zhu ratio for shear. Similar to the case of the SMM model for shear, this new SMMT model can predict the entire torque–twist curve, including the ranges before and after cracking, as well as the ascending and descending branches. The theoretical predictions from the SMMT compare very well with the test data on torsion available in the literature.     

火灾下混凝土简支板等效火灾荷载的简化计算

根据热传学原理,得出高温下钢筋混凝土板截面温度场的分布,计算出火灾下钢筋混凝土简支板极限抗弯承载力,得出钢筋混凝土简支板的等效火灾荷载,为火灾下钢筋混凝土极限抗弯承载力的计算提供了方便。     

碳纤维材料加固混凝土构件的耐火极限

通过分析CFRP加固钢筋混凝土构件各组成部分耐火性能及相关耐火试验研究数据,结合工程实际,初步提出了对其耐火极限的判定方法,为加固建筑结构防火设计提供了参考,并对今后CFRP加固钢筋混凝土构件耐火极限的研究提出了建议.     

火灾后钢筋混凝土构件材料性能检测的探讨

火灾后的现场检测要根据火灾构件的温度场分布规律和损伤规律,采取符合实际的采样方法,才能准确判断构件的材料性能.本文提出火灾后钢筋混凝土构件材料性能检测方法和注意事项.     

Post-cooling properties of concrete exposed to fire

Concrete structures are able to resist high temperatures due to fire relatively well and they can be repaired afterwards. In order to select appropriate repair strategies, assessment of the condition of a concrete structure after fire is of crucial importance. Previous research has mostly been focusing on the strength of concrete during fire and considering slow cooling of elements to room temperature. Guidelines and models related to these conditions have been incorporated into structural design codes. However, in reality, fast cooling of concrete by means of water occurs frequently and the effect of this cooling method has been much less the subject of research investigations. Nevertheless, the effect of water cooling can be significant. In this article the effect of water cooling on the residual compressive strength, stress-strain diagram and bond strength between concrete and reinforcement is investigated. Two cooling methods are considered, i.e. quenching and spraying of specimens. It is found that the investigated properties are extremely sensitive to heating with subsequent water cooling.     

A quality evaluation method for existing carbonated reinforced concrete members

Objective evaluation of the current quality of existing carbonated reinforced concrete members is important as time may degrade their bearing capacity and rigidity. In this paper, a method in which the weight coefficients of every evaluation factor are determined from the coefficients of variation (COVs) or coefficients of dispersion (CODs) of the standard values of every evaluation factor is proposed in order to evaluate the quality of existing carbonated reinforced concrete members. Because it is unnecessary to gather many experts for evaluating every structure, the method should be more practical and applicable in engineering. Finally, an engineering example shows that this method is feasible to solve the problem of quality evaluation of existing carbonated reinforced concrete members.     

火灾高温作用下钢筋混凝土梁截面温度场的计算与分析

采用有限元—差分相结合的方法编制了钢筋混凝土梁构件截面内部瞬态温度场计算分析的程序,验证表明与精确解符合程度较好。并应用该分析程序进行了三面受火的混凝土矩形梁构件在不同时刻截面内部温度场的计算与分析,对计算结果也进行了对比验证,精确度较高,由此得到混凝土梁构件在各种情况下的截面温度场及其分布的特点与规律,为进一步研究钢筋混凝土梁构件的抗火性能提供了基础。