Postgrad Research : Linus Lim PhD Project >>

Background | Specimen Details | Parties Involved | Pictures of slab construction

THE BEHAVIOUR OF CONCRETE AND COMPOSITE STEEL-CONCRETE FLOOR SLABS IN FIRE

Objective:

1. To determine the structural behaviour of concrete and composite steel-concrete floor systems in fire and the influence of tensile membrane action on their structural fire resistance.
2. To verify the shell element in SAFIR to model concrete slabs and to use SAFIR as a tool to predict the behaviour of other types of floor systems in fire

Scope of tests:

The primary objective of these tests is to determine the structural behaviour of concrete floor slabs in a fire. The second objective is to validate the finite element programme, SAFIR, as a modelling tool for slab analysis. This will allow the internal stresses and strains to be determined more comprehensively than obtained through experimental testing alone.

 The floor slabs will be tested in the BRANZ furnace. Six  floor slabs will be tested, comprising three reinforced concrete flat slabs and three proprietary composite steel-concrete floor systems. The floor systems will be simply supported on all four edges, supported vertically on the steel frame around the furnace and will be unrestrained against thermal expansion. The furnace aperture measure 3.0m by 4.0m and the floor systems will be built so that they can be simply supported over the furnace opening.

Theoretical background and impetus for the tests :

A series of full-scale fire tests conducted at Cardington on an eight storey building have shown that unprotected reinforced concrete floor slabs on steel decking do not collapse after a compartment burnout, despite very high measured steel temperatures, and suffering considerable deformations (Newman et al., 2000). It is postulated that the load capacity is sustained by means of tensile membrane action within the floor slab.

While a composite steel-concrete slab resists loads through bending and shear under ambient conditions, a different load resisting mechanism is mobilised to resist the loads when it is subjected to elevated temperatures. Observations in the Cardington tests have shown that the composite slabs do not collapse when subjected to a fire. Rather, the slabs act as a membrane, on the colder perimeter beams and protected columns.

Tensile membrane action can occur in slabs with fixed or simply supported edge conditions and occurs when the slabs undergo large deflections.  If a simply supported  slab undergoes relatively large deflections, the regions of the slab on the supports start to move inwards but are restrained by the adjacent outer regions. This creates a compressive ring around the outside edges of the slabs balanced by tensile membrane forces in the middle of the slab. If the reinforcing mesh fractures, the slab will fail as the tensile field cannot be sustained.

In another series of tests conducted under ambient temperatures at the Building Research Establishment (BRE), a two way spanning simply supported composite slab with no in-plane horizontal restraint at its edges has carried loads considerably in excess of those predicted by conventional yield line design principles (Bailey, 2000). This is a fairly recent discovery in the rapidly evolving fire research industry.

Based on these tests, Bailey (2000) has developed a method for determining the ultimate load carrying capacity of slabs incorporating the effects of tensile membrane enhancement. This method has been extrapolated to tentatively predict the response of composite slabs in fire and applied to the fire tests undertaken on the 8 storey steel framed test building at the Cardington Large Building Test Facility in 1995/1996. The postulated membrane behaviour is not yet fully understood, especially at elevated temperatures. Newman et al.(2000) have applied the tensile membrane procedure to a limited range of conditions involving low to moderate fire severity, in which the influence of elevated temperatures on the components is expected to be minor. Clifton et al. (2001) have extended this design method for application in New Zealand, taking account the effect of elevated temperatures in the load resisting components and the strength contribution of the secondary beams to the slabs. These proposed design methods have yet to be verified with a fire test, which is one of the principal objectives of these tests.

References

Bailey, C.G. (2000) Design of Steel Structures with Composite Slabs at the Fire Limit State, Final Report. Building Research Establishment. U.K.

Clifton, G.C., Hinderhofer, M.D., and Schmid, R. (2001) Design of Multi-Storey Steel Framed Buildings with Unprotected Secondary beams or joists for Dependable Inelastic Response in Severe Fires. HERA Steel Design and Construction Bulletin No. 60, HERA, Manukau City, New Zealand.

Concrete Structures Standard (1995) NZS 3101: Part1: 1995. The Design of Concrete Structures, Standards New Zealand, Wellington.

Newman, G.M., Robinson, J.T. and Bailey, C.G. (2000) Fire Safe Design: A New Approach to Multi-Story Steel-Framed Buildings. The Steel Construction Institute, Berkshire, U.K.