Page 7 - Open-Access-Oct-2019

point of view

Hydrocarbon fire

1200
1.2 1.2
Hydrocarbon fire

Stress
1200 Hydrocarbon fire 1200 1200 Hydrocarbon fire 1.2 1.2 Stress Stress 1.2 1.2 1.2 1.2
ISO 834 fire
ISO 834 fire
ISO 834 fire Stress Stress Stress Stress 1000 1000 ISO 834 fire Stress 1.0 1.0 1.0 1.0
Calcareous
1000
1000 1.0 1.0 Calcareous ASTM E119 fire 1.0 1.0 Calcareous
ASTM E119 fire
Calcareous
ASTM E119 fire
ASTM E119 fire fck,f fsy,f fsy,f 800 aggergates External fire fck,f 0.8 0.8 fsy,f fsy,f 0.8 0.8 aggergates 0.8 0.8
aggergates
fck,f
aggergates
Temperature (ºC)
fck,f
Temperature (ºC) Temperature (ºC) 600 External fire fsp,f Temperature (ºC) 400 aggergates fct,f / fct fct,f / fct 0.6 0.6 fsp,f fsp,f fck,f / fck 0.6 fck,f / fck 0.6 aggergates fct,f / fct 0.6 fct,f / fct 0.6
0.8 0.8 800
External fire
800
Siliceous
800

External fire
Siliceous

Siliceous
Siliceous
aggergates
fck,f / fck
fck,f / fck
aggergates
0.6 0.6 600
600
600
fsp,f
0.4
0.4
0.4
0.4
0.4 0.4
0.4 0.4 400
400
400
0.2 0.2 200
200
200 200 0.2 0.2 0.2 0.2 0.2 0.2
0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0 200 200 400 400 600 600
0
800
600
180
120
400
200
800 150
90120
600
400 90
0 0 30 30 60 60 90 90 120 150 180 0 0 0 0 30 30 60 400 60 Time (Minutes) 150 180 1200 0 0 co,f 200 400 cu,f Strain Strain Strain 0 0 400 400 T (°C) T (°C) 800 1200 1200 T (°C) T (°C)
150
800
1200
180
120
T (°C)
Strain
T (°C)
Strain
Time (Minutes)
T (°C)
Time (Minutes) co,f co,f cu,f cu,f Strain sp,f sp,f sy,f sy,f st,f st,f su,f su,f Strain Time (Minutes) co,f T (°C) cu,f sp,f sp,f sy,f sy,f st,f st,f su,f su,f Strain
(a) (b)
12 2.5 2.5 1.2 1.2 1.2 1.2 1.2 1.2
1.2 1.2
12 12 2.5 2.5 1.2 1.2 12 1.2 1.2
Hot-rolled
1.0 1.0 2.0
1.0 1.0
10 10 2.0 2.0 1.0 1.0 10 10 Hot-rolled 2.0 1.0 1.0 1.0 1.0 Hot-rolled 1.0 1.0
Hot-rolled
Hot-rolled
Hot-rolled
8
0.8 0.8
8 8 0.8 0.8 Hot-rolled 0.8 0.8 8 1.5 1.5 0.8 0.8 Hot-rolled 0.8 0.8 0.8 0.8 Cold-worked
Cold-worked
Cold-worked
1.5 1.5 Cold-worked Cold-worked
Cold-worked
co,f / co co,f / co 6 6 cu,f / cu cu,f / cu fsp,f / fsp fsp,f / fsp 0.6 0.6 fsy,f / fsy co,f / co fsy,f / fsy 0.6 0.6 co,f / co 6 Cold-worked cu,f / cu 1.0 Es,f / Es cu,f / cu 1.0 fsp,f / fsp 0.6 fsp,f / fsp 0.6 fsy,f / fsy 0.6 fsy,f / fsy 0.6 Es,f / Es 0.6 Es,f / Es 0.6
Es,f / Es
Cold-worked
6
0.6 0.6
4
4 4 1.0 1.0 0.4 0.4 0.4 0.4 4 0.4 0.4 0.4 0.4 Cold-worked 0.4 0.4 0.4 0.4
Cold-worked
Cold-worked
Cold-worked Hot-rolled
Hot-rolled
0.5 0.5 2 2 0.5 0.5 Hot-rolled 0.2 0.2 0.2 0.2 0.2 Hot-rolled
2 2 0.2 0.2 0.2 0.2 0.2 0.2 0.2
0
0.0 0.0
0 0 0.0 0.0 0.0 0.0 0.0 0.0 0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0 400 800 1200
800
1200
1200
1200
1200
400
1200
800
400
0 0 400 800 1200 0 0 400 800 1200 0 0 400 800 1200 0 0 0 0 400 400 400 800 800 1200 1200 0 0 0 0 400 400 400 800 800 800 1200 1200 0 0 400 400 800 800 1200 1200 0 0 400 400 800 800 1200 1200 0 400 800 1200
400
800
800
1200
800
400
400
800
1200
T (°C)
T (°C) T (°C) T (°C) T (°C) T (°C) T (°C) T (°C) T (°C) T (°C) T (°C) T (°C) T (°C) T (°C)
T (°C)
T (°C)
T (°C)
T (°C)
T (°C)
T (°C)
(c) (d)
Figure 3: Design stress-strain parameters of concrete under fire (a) Peak compressive strength, (b) Tensile strength, (c) Strain corresponding to the peak
compressive stress, (d) Ultimate strain.
As expected, the magnitude of strain in concrete is increased and (iii) modulus of elasticity. Figure 4 shows the variation of
with the increase in temperature. Figures 3(c) and (d) show these parameters with temperature for the hot-rolled and
the variation of strain corresponding to peak strength as well cold-worked steel. The tensile stress corresponding to limit
as ultimate strain. An exponential variation in the increase in of proportionality remains constant for temperature less than
concrete strain corresponding to peak strength is assumed 600°C, beyond which a gradual reduction is expected for both
till the temperature reaches 600°C, beyond which the types of steel. At temperature near 700°C, this stress value is
corresponding increase is considered to be constant. The about 10% of the value at the room temperature. Hot-rolled
maximum value of ɛ co,f is considered as the 10 times of the value steel shows the higher proportionality stress as compared to the
at the room temperature (ɛ co). However, such a large increase is cold-worked steel. Figure 4(b) shows the variation of yield stress
not expected in the ultimate strain (ɛ cu) of concrete at elevated of steel with temperature. The yield stress values for both steel
temperature. As shown in Figure 3(d), the rate of increase types remain unchanged till a temperature of 400°C. A steep
in ultimate strain is linear with the varying temperature. The reduction in yield stress is noted for temperature range of 400-
maximum value of ɛ cu,f is recommended as about 2.4 times of the 700°C. Hot-rolled steel has a marginally higher yield strength
value corresponding to normal temperature (ɛ cu). under elevated temperature. Figure 4(c) shows the variation in
the modulus of elasticity (E s) of steel with the temperature. Cold-
4.1.2 Steel worked steel shows a mild degradation in E s with the elevated
temperature as compared to the hot-rolled steel in which the
As discussed earlier, the production process of reinforcing steel
plays an important role in their behaviour under fire loading. reduction in E s started at much lower value of temperature
Figure 2(b) shows the design stress-strain relationships for the (~100°C). It is, therefore, extremely important to consider the
reinforcing steel under elevated temperature. The main design steel type in the computation of deflection of a RC member
parameters are (i) the limit of proportionality, (ii) the yield stress, exposed to high temperature.
12 The IndIan ConCreTe Journal | oCToBer 2019