Page 2 - Open Access - Oct 2019

point of view

Both direct and indirect effects of fire influence the structural 3.1 Material Properties under elevated
performance. The direct effects of fire on the structure is the temperature
increase in temperature resulting in the introduction of heat in
the components due to the combined convection and radiation. This section provides the basic features of concrete and steel
During a fire, the concrete elements are generally subjected under elevated temperature as follows:
to non-uniform temperature. Since the material properties are
strongly influence by the temperature, this results in the restraint 3.1.1 Concrete
forces, internal stresses, and deformations. Indirect effects Concrete is combination of various materials and admixtures.
arise from the restrained expansion of members under thermal The progressive thermal damage in concrete under fire
loading due to fire. This results in the increased compressive change the material properties. As a result, cross-section of a
forces in columns and the modified bending moments in the member consists of several concrete layers of different material
continuous beams or slabs. properties that keep changing during and after the fire. Since
the stress-strain relationships are modified due to the change in
3. FIre desIgn PrIncIPles temperature, it is quite complex to establish the strength and
stiffness characteristics with temperature. The modified material
Fire design of a structure is dependent on various parameters,
such as, the fire, the temperature, the material properties in properties are discussed below:
interaction with exposure, and the thermal characteristics. In the The type of cement, maximum aggregate size, the water-
fire design of concrete structures, the behaviour of the entire to-cement ratio and the rate of heating have no significant
structure as well as the interaction between the deformations influence on the percentage reduction in the compressive
and reduced strength in the damaged components should be strength of concrete under fire. The main important parameters
considered. In many cases, the redistribution of stresses and affecting the compressive strengths of concrete are the type
deformations in the part of structures not directly exposed to fire of aggregate, the aggregate-to-cement ratio, and the test
may represent the critical load combinations in the design. procedure. The change in the modulus of elasticity of concrete
under fire is similar to the compressive strength. However, the
The accidental nature of fire loading requires its effects to be reduction in the modulus of elasticity is much higher due to the
considered in the limit state of collapse (i.e., ultimate limit state) reduction in the compressive strength in presence of increased
rather than the serviceability limit state. In accordance with limit strain. The reduction in the tensile strength of concrete under
state approach, the resistance (R d,f) of the structure computed fire is much faster than the compressive strength but slower
based on characteristic design values of material properties than the modulus of elasticity. In addition to the aggregate
in case of fire should be higher than the characteristic design type and the aggregate-to-cement ratio, the curing condition
actions (Q d,f). Mathematically, greatly influence the deformation in concrete under first heating
under the applied loading. The thermal conductivity is generally
R d,f > Q d,f (1)
reduced, whereas the specific heat is increased due to the
elevated temperature. This results in the decrease in the thermal
It is not possible to include the accidental fire action in the form
of the amplified loading in the above equation as it consists of diffusivity of concrete with increasing temperatures.
indirect effects of actions induced due to differential/restrained
thermal expansion. Considering the conditional probabilities for 3.1.2 Steel
such accidental actions, the design values of material properties Both strength and stiffness of steel are reduced due to the
(f df) in case of fire are expressed as follows: increased temperature. The reduction in yield strength of steel
strongly depend on the production process. Hot-rolled steel is
f d,f = d (T) ⁄ γ m,f (2) relatively less temperature sensitive, whereas the cold-worked
f
prestressing steel are highly sensitive to temperature. The
where, f d(T) is the material properties under fire temperature (T) reduction in the modulus of elasticity is much faster than the
and γ m,f is the material partial safety factor considered as 1. In yield strength under elevated temperature. Though thermal
case of fire, the uncertainty is entirely accounted in the actions properties of steel are influenced by temperature increase, they
by means of very severe fire (standard fire) or more realistic are not relevant in concrete structures as they do not affect
fire (natural fire), unlike in many other accidental actions (e.g., the temperature distribution. The reduction in bond strength
earthquakes) where the uncertainty is taken care of in both between concrete and steel is very critical in prestressed
materials and actions. Further, it is well known that uncertainty in concrete members. Spalling of concrete is crucial in high-
material properties of concrete under high-temperature is very strength concrete as they have more dispersed porosity helping
small. in the formation of vapour pressure in the pores.

10 The IndIan ConCreTe Journal | oCToBer 2019