Advanced Numerical Method for Estimate Fire Resistance of Partially Encased Beams
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abstract
The ultimate design goal for fire safety engineering is the conception of safe structures. To
achieve this goal, advanced calculation methods may be used for the assessment of safety in
structures and in particular of structural elements. In order to ensure that ultimate limit
requirements are fulfilled, it is necessary to predict failure of each type of material and
element during the design process of buildings.
This paper deals with the numerical modelling of partially encased beams, which are
composed structural elements, widely used in tall buildings, with two or more different
materials and types of construction. Normally are used with reinforcement rebars and with or
without structural link to slabs that may use concrete to increase fire resistance.
Instability problems may occur because concrete may not have the age to resist and also
because the concrete may slip over steel, crack or crush. Lateral torsional buckling (TLB) is
an instability phenomenon that should be considered.
This paper presents Ansys material and geometric non-linear finite element model for
determining lateral torsional buckling resistance of partially encased beam with and without
encasement reinforcement in fire conditions. The steel part of the composed section will be
modelled by finite shell element, concrete part by three-dimensional finite solid elements, the
bond slip contact with finite non-linear spring elements and the rebar reinforcement will be
considered in perfect contact with concrete, using finite bar element.
Elevated temperatures will be applied in both four sides of the cross section along the beam
length, based on ISO834 standard fire. Beams will also be subjected to uniform bending
corresponding to a specific degree of load bearing capacity. Failure of concrete will be
predicted and based on smeared band approach through modification of the stress - strain
fields. Fire resistance will be determined for the last time increment in which it is possible to
sustain the equilibrium.