Keywords: masonry in fire, finite element, slenderness ratio, eccentricity, boundary conditions.

A finite element model called MasSET has been developed which is capable of predicting the structural behaviour of single leaf masonry walls subject to elevated temperatures. The analysis models a slice through the wall as a column strip in plane stress, and also includes material with geometric non-linearity. The model has been previously validated by comparison with experimental results[1] and is used in this paper to conduct a parametric study investigating the effects of slenderness ratio, load eccentricity and boundary conditions. The results of the investigation are presented by way of failure temperatures for each condition, and show conclusive findings to the effects of each parameter investigated.

In the event of a fire, masonry walls in building serves as a fire barrier in addition to architectural and structural functions. Often the subdivision of a building into fire- isolated compartments is achieved by the use of masonry walls in conjunction with fire-resisting floors above and below. The dominant factors, which affect the performance of a wall exposed to fire, are the thermal, physical and chemical properties of the constituent masonry units and the joint mortar, the dimensions of the wall, the end restraints and the load conditions. The role of a single leaf masonry wall in a fire situation is generally seen to be three folds:

• Firstly a load-bearing wall must provide structural stability to prevent collapse. This provision is termed structural adequacy.
• The wall is required to maintain integrity, i.e. preventing the passage of flame from one side of the wall to the other, in an attempt to compartmentise a fire.
• The wall must provide adequate insulation properties to prevent excessive temperature on the unexposed face creating further spread of flame.

The increasing emphasis on the use of masonry walls as a load bearing element has lead the primary interest of this study to the first criterion of structural adequacy. The maintenance of structural stability may also in part fulfil the requirements of the second criterion, while also providing safe egress for any building occupants.

In fire separating elements, such as masonry walls, heat is usually exposed to one side of the element only. This is particularly important in the case of brick masonry due to its low thermal conductivity, producing high thermal gradients over the cross section [2]. Thermal bowing is therefore produced due to differential thermal expansion. With the hot face of the wall expanding more rapidly than the cool one, the wall will tend to bow towards the fire. The fire-exposed face of the wall will also experience a considerable reduction in mechanical material properties, which effectively can be represented as a reduction in thickness of the hot face[3]. As a result of the change in thickness any applied load will have moved towards the fire exposed face and must be recognised as having the advantageous effect of maintaining structural stability by counteracting thermal bow.

Current design code does not provide concise calculation models for the determination of fire resistance times. Some design guidance is available based entirely on isolated standard fire tests, which related minimum periods of fire resistance to required wall thickness[4]. Important parameters such as load, load eccentricity, material type and slenderness ratios are not accounted for.

The finite element model called MasSET (Masonry Subject to Elevated Temperatures) presented in this paper has been employed to conduct a parametric study investigating several influential factors on the behaviour of single leaf compartment walls in fire.

MasSET is a two-dimensional finite element model written in displacement formulation. It models a slice through a masonry wall as a column strip using eight noded isoparametric elements, and is capable of predicting thermal response under various restraint and loading conditions. The effects of both material and geometric non-linearity have been accounted for and a smeared cracking model was incorporated to simulate the brittle behaviour of a concrete type material. The model assumes the structure to firstly receive an applied load, followed by successive temperature increments. At present MasSET is not coupled with a thermal analysis program and explicitly defined temperature distributions are directly assigned to the nodal points by the user as input data. The standard or modified non-linear Newton Raphson iteration procedure is adopted to reach a converged solution, whereby convergence is based on the norm ratio of the external and internal nodal force vectors.

1

O' Connor, D.J., Nadjai, A., O'Gara, M.E., "A Numerical Model for the Behaviour of Masonry under Elevated Temperatures", Proc. Fourth Int. Conf. Comp. Struct, Edinburgh, 229-237, 1998

2

Cooke, G.M.E., "Thermal bowing and how it affects the design of fire separating constructions", Interflam '88 - Fourth Int. Fire Conf. 22-24, March, Church College Cambridge, 230-236,1988.

3

Lawerence, S.J., Gnankrishnan, N., "The fire resistance of masonry walls", National Building Technology Centre, Technical Record No. 531.

4

CP 121. Code of Practice for Walling, Pt 1: Brick and Block Masonry. British Standards Institution, London, 1973.

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