Experimental fire tests of two-way concrete slabs

Abstract

A series of full-scale fire tests conducted at the Cardington steel framed test building in the
\nUnited Kingdom have shown that unprotected composite floor slabs do not collapse after a
\ncompartment burnout, despite suffering considerable deformations and very high measured
\nsteel temperatures. When the unprotected steel beams weakened, the load resistance
\nprogressively transferred from the beams to the slabs which resisted the loads by tensile
\nmembrane action. These tests have prompted extensive computer modelling by many
\nresearchers to simulate the behaviour of steel framed buildings and tensile membrane action
\nof the slabs under fire conditions. However, there are no published fire tests of two-way
\nconcrete slabs in a controlled furnace environment
\nThis report describes the fire tests of two-way concrete slabs conducted at the BRANZ fire
\nresistance furnace. The tests were conducted to investigate the behaviour of unrestrained
\nsimply suppmted slabs in a controlled furnace environment. The test data can be used to
\nverify current simple design methods and sophisticated computer models.
\nSix slabs were tested, comprising three reinforced concrete flat slabs and three composite
\nsteel-concrete slabs. The slabs measured 3.3m by 4.3m and had thicknesses ranging from
\n90mm to 130mm. The three flat slabs had different quantities of reinforcing steel to
\ninvestigate their effect on controlling crack widths to prevent integrity failure. The slabs were
\nsimply supported on all four sides over the furnace with no horizontal restraint. The slabs
\nwere subjected to a live load of 3.0kPa and were heated on the underside with the gas time
\ntemperature curve following the ISO 834 standard fire for three hours.
\nThe slabs petformed very well in the fire tests, supporting the loads for the full duration of
\nthree hours without collapse. By three hours, the gas temperatures had reached 1100°C and
\nhigh temperatures were measured across the depth of the slab. The temperatures of the
\nreinforcing steel exceeded 700°C. All the slabs suffered extensive surface cracking and loss of
\nmoisture. Some of the slabs suffered large midspan deflections (up to 270mm) and full depth
\ncracks which were associated with the yield line crack pattern. The slabs with higher steel
\ncontents and closer bar spacings suffered only surface cracking, while the slabs with the lower
\nsteel content suffered full-depth cracks. The cold-drawn mesh used in the tests performed well
\nand did not fracture as might have been expected if such large strains had been imposed at
\nambient temperature; this is due to the increased ductility of the steel caused by the elevated
\ntemperatures.
\nThe slabs resisted collapse even though the calculated ultimate load capacities had dropped
\nsignificantly below the level of the applied loads. The structural fire resistance of the slabs in
\nthe tests exceeded the predictions of code recommendations. The tests illustrated the
\nsignificant effect of tensile membrane action on the structural fire resistance of two-way slabs
\nwhich resisted collapse despite significant loss of flexural strength.