Simulation of behaviour of floor slabs under fire conditions
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Project data
| Titel | Title Simulierte Brandeinwirkung auf Deckenplatten | Simulation of behaviour of floor slabs under fire conditions Förderer | Funding Staatsbetrieb Sächsisches Immobilien- und Baumanagement (SIB), Niederlassung Dresden I Zeitraum | Period 04.2010 Leiter | Project Manager Dr.-Ing. Harald Michler Bearbeiter | Contributors Dr.-Ing. Harald Michler |
Report in the yearbook 2010
Simulated fire effect on ceiling tiles
Temperature development for system 3, critical cut B
An in-situ test was used to verify the structural safety of a 50-year-old reinforced concrete ribbed floor. In order to also assess the effect of a fire event on the load-bearing capacity of the floor, a thermal run-through calculation was carried out using the unit temperature curve. Of particular interest was the exposure time after which the temperature in the reinforcing steel below becomes critical, since both yield strength and elastic modulus of the steel can decrease significantly with increasing temperature.
Four slab systems were modeled. System 1 modeled the as-built condition with the lower ceiling cladding (HWL panels with plaster layer), insulation of the ceiling cavities, and the concrete spalling in the area of the webs. In system 2, the lowest plaster layer was omitted because it would most likely spall first in the event of a fire. Variant 3 was designed analogously to system 2, but a further increase in heat transport was applied in order to reliably capture the unfavorable effect of the concrete spalling. System 4 was used to simulate the complete failure of the lower ceiling cladding. The undersides of the webs and the thermal insulation in the (now former) cavities between the webs were directly loaded with the fire load. In the area of spalling, the reinforcement was directly heated.
As a result of the simulations, the temperature development was determined in different component axes and layers over a total period of 180 minutes. As expected, the lowest layers in each case experienced a rapid high temperature rise. The decisive factor, however, was that in the presence of the lower HWL cladding in systems 1-3, the temperatures predicted for the reinforcing steel reached a maximum of 200 °C over the entire three-hour period. In this temperature range, however, a maximum 20% reduction in the yield strength of the reinforcement can be expected, which means that the slabs will remain load-bearing even if exposed to fire. As expected, no satisfactory resistance duration could be achieved for System 4 without HWL cladding and ceiling plaster. This was particularly true for the areas where the reinforcement was no longer adequately protected by concrete spalling. For the remaining period of use until the planned refurbishment of the building, it must therefore be ensured that the ceiling cladding is supplemented at missing points and that it is also not removed at any point.