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Computational Science, Engineering & Technology Series
ISSN 17593158 CSETS: 5
CIVIL AND STRUCTURAL ENGINEERING COMPUTING: 2001 Edited by: B.H.V. Topping
Chapter 16
Behaviour of Compartment Masonry Walls In Fire Situations A. Nadjai, D. Laverty and M. O'Gara
School of the Built Environment, University of Ulster, Belfast, United Kingdom A. Nadjai, D. Laverty, M. O'Gara, "Behaviour of Compartment Masonry Walls In Fire Situations", in B.H.V. Topping, (Editor), "Civil and Structural Engineering Computing: 2001", SaxeCoburg Publications, Stirlingshire, UK, Chapter 16, pp 407431, 2001. doi:10.4203/csets.5.16
Keywords: masonry, walls, fire, thermal bowing, deflection, load, strain.
Summary
A mathematical model is developed to simulate the lateral deflection response of
axially loaded masonry walls subjected to elevated temperatures. The developed
model is based on the method of slice approach dividing the wall into a series of
discrete slices and generates 'through the thickness' changes in material properties
and stress strain relationships. The transient nature of the fire process is modelled
using a time step analysis applying thermal profiles derived from Lagrangian shape
functions acting on temperatures at specific node points, the latter determined
experimentally. To verify the model an experimental programme was conducted on
scale model walls of different height and subjected to different loads.
Use of theoretically based methods has gathered momentum in the last two decades, especially with the advances in computer technology. Computing power enables detailed evaluation of a dynamic transient process. The aim is to generate the fire process and develop a representative computer model. With sound knowledge of material properties and the effects of temperature, then computer models are capable of recreating the effects of material degradation and subsequent thermostructural response. Analysis can be very complex and specialised, and designers may be unable to understand the intricacies of tools such as finite element. Unless thermal effects on material properties are thoroughly understood then computer models will be flawed. For example before Anderberg [1] no one understood the significant influence of transient strain on concrete response, theoretical models prior to this time therefore did not truly represent total strain response. Even today there is still very limited data available on material property response with temperature. One must also understand the effects of interaction of various parameters. The simultaneous combined effects of load and temperature acting in concrete specimens produces significantly different response than the additive effects applied separately. The present work describes the development of one such approach, a mathematical model to predict the deflection response of axially loaded concrete masonry walls in fire situations. To validate the theoretical model a comprehensive suite of experimental tests was conducted on walls of varying slenderness ratio and different load. The methodology used for the experimental testing was unique in that scale walls were used, requiring an enhanced fire curve to generate thermal similitude in model and prototype. The theoretical model considers internal equilibrium of axial forces and moments and generates an equivalent linear strain and associated strain gradient (curvature) from total strains through the wall cross section. The effect of load and load eccentricity are incorporated by computing linear stress distribution though the section using conventional structural mechanics. The combined effects of temperature and stress are considered at discrete points through the wall and are integrated through the section to compute the resultant wall curvature and subsequent deflection. The model under pins the significant influence of Load Induced Thermal Strain (LITS) on deflection response. In fire separating elements such as firewalls, heat is usually exposed to one face. This is particularly important in the case of masonry materials due to its generally low thermal conductivity, producing high thermal gradients over the cross section [2]. If the wall is unrestrained this leads to differential thermal expansion of the material. With the hot face expanding more rapidly than the cool one, the unrestrained wall will tend to bow towards the fire. It has been the experience of may experimental programs [1,4] that in cases of high slenderness ratio thermal displacements may be of a magnitude to cause instability buckling. There has been little attempt to mathematically describe the thermostructural behaviour of masonry walls. Free thermal bowing equations have been derived based on simplified conditions, which will be discussed herein. The present work attempts to develop these equations to situations of fixed end conditions, and applied axial loading. References
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