Current Design Methods of Concrete Structures in Fire
When designing a concrete structure, the strength of concrete is neglected in tension, instead the member is designed for the steel re-bars to carry all the tensile force. In regions where the slab is expected to sag (centre of slabs), the steel tensile reinforcement heats up and loses ductility and strength above a certain temperature (400°C when designing in accordance Eurocode 2: Design of Concrete Structures – Part 1-2: General rules – Structural Fire Design). However the concrete provides insulation to the steel from the fire, this insulation can be increased by increasing the amount of concrete cover provided if required. The increase in cover is also required to protect against spalling of the concrete, spalling reduces the distance to the steel and therefore leads to increased temperatures.
However when the slab is hogging (close to the slab-column supports), the underside of the slab is in compression and thus the concrete that carries the load will heat up and lose strength. A common design method of accounting for high temperatures in concrete is the Isotherm method, which neglects the effects of concrete above 500°C whilst assuming full strength is maintained for concrete below 500°C, essentially reducing the thickness of the slab and analysing as if it were at ambient temperature. This method works well for large members, but is more inaccurate for thin members such as flat slabs. Anderberg suggested a refined solution using a two layer approach. Concrete was assumed to have 90% strength for <400°C, 70% for concrete with 400°C<T<600°C and zero strength for concrete >600°C (Buchanan, A.H. "Structural Design for Fire Safety") as shown in the diagrams below, the dotted line represents the Isotherm Method and the dashed line represents Anderbergs Method.
However when the slab is hogging (close to the slab-column supports), the underside of the slab is in compression and thus the concrete that carries the load will heat up and lose strength. A common design method of accounting for high temperatures in concrete is the Isotherm method, which neglects the effects of concrete above 500°C whilst assuming full strength is maintained for concrete below 500°C, essentially reducing the thickness of the slab and analysing as if it were at ambient temperature. This method works well for large members, but is more inaccurate for thin members such as flat slabs. Anderberg suggested a refined solution using a two layer approach. Concrete was assumed to have 90% strength for <400°C, 70% for concrete with 400°C<T<600°C and zero strength for concrete >600°C (Buchanan, A.H. "Structural Design for Fire Safety") as shown in the diagrams below, the dotted line represents the Isotherm Method and the dashed line represents Anderbergs Method.
Images Source: Buchanan, A.H. "Structural Design for Fire Safety"