Assessment of Existing Structures and Repairs - Offshore Structures: Design, Construction and Maintenance - page 478

300

20°C

250

200°C
300°C
400°C
500°C

200

150

600°C

100

700°C

50

800°C

0.5 1.0 1.5 2.0

0

Strain (%)

FIGURE 7.7 Stress-strain relationship at different temperatures.

Figure 7.8 shows that steel softens progressively from100 ° C to 200 ° C and above.

Only 23% of ambient-temperature strength remains at 700 ° C. At 800 ° C, strength

reduced to 11% and at 900 C.

Degradation of steel strength and stiffness is presented in Figure 7.8 .
Strength and stiffness reductions are very similar for S235, S275 and S355
structural steels and hot-rolled reinforcing bars.

° C to 6%. In general, steel melts at about 1500 °

7.7.1 Degree of Utilization
Based on European code EC3 for steel, the degree of utilization can be deter-
mined from the following equation:

E f : d
R f : d : 0

μ 0 =

(7.23)

where E fi.d is the design loading of a member in fire and R fi.d.0 is as a proportion
of its resistance at ambient temperature but including the material safety factor
for the fire limit state.
A degree of utilization can be used if there is no chance of overall or lateral-
torsional buckling,

η f calculated as proportion of design loading at ambient

temperature

γ M : f
γ M1

μ 0 = η f (7.24)

where η f is the reduction factor for the design load level for the fire situation.