Fully coupled multi-physics nonlinear analysis of structural space frames subjected to fire using the direct stiffness method

Computational analysis of reinforced concrete structures subjected to fire using a multilayered finite element formulation

Abstract: Fire is a critical risk in reinforced concrete (RC) structures and appropriate structural resistance against it has to be ensured. In this contribution, an approach using corotational layered beam ...

Spalling and pore pressure in HPC at high temperatures

Abstract: High-performance concrete (HPC) are subject to spalling under certain thermal and mechanical conditions. Spalling results mainly from two processes: a “thermo-mechanical” process in which the stress originates in the gradients of thermal deformation within the material, and a thermo-hydral process where spalling is due to the build-up of gas pressure fields in the porous network. This paper deals with the thermo-hydral process. An original device was designed in order to make simultaneous measurements of pressure and temperature at various positions in a concrete specimen (30×30×12 cm3) heated on one face up to 800°C. The specimen was also continuously weighed during the tests, thus, the mass loss, resulting mainly from water transport and loss, was recorded. This campaign was carried out on an ordinary concrete (OC) and a HPC (90 MPa). As expected, the pressure peaks were much higher than in HPC (40 bars) than in OC (20 bars). In HPC, these pressures exceeded the saturated vapor pressure. It is demonstrated that the thermal expansion of liquid water and the transport of water towards the inner part of the specimen play a significant role on the build-up of gas pressure. The experimental correlation between the pressure peaks and the plateau in the temperature curves confirmed the hypothesis that the drying front is preceded by a quasi-saturated layer that acts as a moisture clog.

Numerical analysis of hygro-thermal behaviour and damage of concrete at high temperature

Abstract: A computational analysis of hygro-thermal and mechanical behaviour of concrete structures at high temperature is presented. The evaluation of thermal, hygral and mechanical performance of this material, including damage effects, needs the knowledge of the heat and mass transfer processes. These are simulated within the framework of a coupled model where non-linearities due to high temperatures are accounted for. The constitutive equations are discussed in some detail. The discretization of the governing equations is carried out by Finite Elements in space and Finite Differences in time. Copyright © 1999 John Wiley & Sons, Ltd.

Pore Pressure and Drying of Concrete at High Temperature

Abstract: A mathematical model for water transfer in concrete above 100°C is developed. Drying tests of heated concrete are reported and material parameters of the model are identified from these tests as well as other test data available in the literature. It is found that water transfer is governed principally by the gradient of pore pressure, which represents the pressure in vapor if concrete is not saturated. Permeability is found to increase about 200 times as temperature passes 100°C, which could be explained by a loss of necks on migration passages. The pore volume available to free water increases as dehydration due to heating progresses and as the pore pressure is increased. The temperature effect on pressure-water content (sorption) relations is determined. Thermodynamic properties of water are used to calculate pore pressures. A finite element program for coupled water and heat transfer is developed and validated by fitting test data.

Predicting the fire resistance behaviour of high strength concrete columns

Abstract: A numerical model, in the form of a computer program, for tracing the behaviour of high performance concrete (HPC) columns exposed to fire is presented. The three stages, associated with the thermal and structural analysis, for the calculation of fire resistance of columns are explained. A simplified approach is proposed to account for spalling under fire conditions. The use of the computer program for tracing the response of an HPC column from the initial pre-loading stage to collapse, due to fire, is demonstrated. The validity of the numerical model used in the program is established by comparing the predictions from the computer program with results from full-scale fire resistance tests. Details of fire resistance experiments carried out on HPC columns, together with results, are presented. The computer program can be used to predict the fire resistance of HPC columns for any value of the significant parameters, such as load, section dimensions, fiber reinforcement, column length, concrete strength, aggregate type, and fiber reinforcement.

Hydrothermal model for predicting fire-induced spalling in concrete structural systems

Abstract: A one-dimensional numerical model to predict fire-induced spalling in concrete structures is presented. The model is based on pore pressure calculations in concrete, as a function of time. Principles of mechanics and thermodynamics are applied to predict pore pressure in concrete structures exposed to fire. An assessment of the possibility of tensile fracture is made by comparing the computed pore pressure with temperature-dependent tensile strength. The pore pressure calculations are coupled with heat transfer analysis to ensure that the loss of concrete section, resulting from spalling, is accounted for in subsequent heat transfer analysis. The validity of the numerical model is established by comparing temperature, pore pressure, and concrete spalling predictions with results from fire tests. The computer program is applied to conduct case studies to investigate the influence of concrete permeability, tensile strength of concrete, relative humidity in concrete, and heating rate on fire-induced spalling in concrete members. Through these case studies, it is shown that permeability, tensile strength of concrete, and heating rate have a significant influence on fire-induced spalling in concrete. It is also shown that relative humidity has a marginal influence on fire-induced spalling in concrete.