Page 1 - Open Access - Oct 2019

2065
point of view

Material Modelling of

concrete structures

exposed to elevated

teMperature Dipti Ranjan Sahoo

Abstract (E). The percentage of reduction in strength and modulus
of elasticity depends on the manufacturing process of steel
The recent fire incidences in reinforced concrete (RC) structures rebars. For example, the reduction in modulus of elasticity for
in India warrant for the inclusion of actions of fire effects in the hot-rolled reinforcing steel bars is 20% at 300°C, whereas
the design. The accidental actions of fire effects need to be the corresponding value for the cold-worked rebars is about
considered in the ultimate limit state (ULS) design, rather than 28% . The tensile strain corresponding to the maximum stress
[1]
the serviceability limit (SLS). However, it is not possible to of reinforcing steel, however, increases if the temperature is
include the fire effects directly in the load combinations. Further, increased above the normal threshold value (~ 40°C). The bond
it is necessary to understand the behaviour of materials under strength of both ribbed and plain round bars in reinforced
the influence of elevated temperature. This paper presents a concrete (RC) members is significantly reduced under the
summary of fire actions and their effects on the structure and influence of high temperature.
the fire design principles. The influence of elevated temperature
on material properties has been discussed. The guidelines to 2. FIre ActIons And Its eFFects
model various material parameters of concrete and steel have The action of the fire on structures in the design process
been presented. These parameters can be directly incorporated is often considered by using nominal fire curves. Several
in the design or evaluation of fire resistance of structures. nominal fire curves often used in design codes are the ISO
834 curve, the ASTM E119 curve, the hydrocarbon curve, and
Keywords: Concrete; Design: Fire; Limit states; Reinforcing steel the external fire curve (Figure 1). These nominal fire curves
provide the temperature of gases in the compartment as a
1. IntroductIon function of time elapsed and hence, they represent a fully-
developed fire scenario. In a potential real fire, as in case of a
Durability is an integral aspect of structural design process large compartment, a significant time is elapsed before such
in order to achieve the desired serviceability and safety. a situation is encountered. This initial period of time is not
Serviceability is the ability of a structure and its components considered in the computed fire resistance, which is the most
to perform adequately for the normal use under all possible important period as far as the safety (or evacuation) of people
actions expected during their service life. Structural safety may is considered. Similarly, the cooling down phase of the fire is
be defined as the ability of a structure and its components to not taken into account in the nominal fire curves. Thus, a fire
ensure the sufficient deformability, adequate ultimate resistance, curve for the actual structure should consider the fire risk and
and overall stability under the assumed actions. These actions exposure conditions.
generally include the extreme, frequently-repeated, accidental
or exceptional events. The appropriate levels of reliability are 1200 Hydrocarbon fire 1.2 1.2

targeted in both serviceability and safety of structures for the Stress Stress
ISO 834 fire
specified reference periods. In addition to structural safety, 1000 ASTM E119 fire 1.0 Calcareous 1.0
the design process should consider the aspect of structural fck,f fsy,f aggergates
robustness measured by the ability to sustain only local damage 800 External fire 0.8 Siliceous 0.8
to some structural components without disproportionate failure Temperature (ºC) fck,f / fck 0.6 aggergates 0.6
or collapse of a system subject to accidental loadings (such as 600 fsp,f fct,f / fct
fire, explosions, impact, earthquakes) . 400 0.4 0.4
[1]
In case of fire loadings, the mechanical and physical properties 200 0.2 0.2
are negatively affected due to changes in the microstructure
of concrete under the influence of high temperatures. For 0.0 0.0 0 200 400 600
reinforcing steel as well as prestressing steel, the presence 0 30 60 90 120 150 180 0 400 800 1200
Time (Minutes) co,f cu,f Strain sp,f sy,f st,f su,f Strain T (°C) T (°C)
of high temperature results in the reduction in maximum
tensile strength, yield strength, and modulus of elasticity Figure 1: Comparison of nominal fire curves.

12 2.5 1.2 1.2 1.2
The IndIan ConCreTe Journal | oCToBer 2019 9
10 2.0 1.0 1.0 Hot-rolled 1.0
Hot-rolled
8 0.8 0.8 0.8
1.5 Cold-worked
co,f / co 6 cu,f / cu fsp,f / fsp 0.6 fsy,f / fsy 0.6 Cold-worked Es,f / Es 0.6
1.0
4 0.4 0.4 0.4
Cold-worked
0.5 Hot-rolled
2 0.2 0.2 0.2
0 0.0 0.0 0.0 0.0
0 400 800 1200 0 400 800 1200 0 400 800 1200 0 400 800 1200 0 400 800 1200
T (°C) T (°C) T (°C) T (°C) T (°C)