PSI - Issue 64

Piero Colajanni et al. / Procedia Structural Integrity 64 (2024) 1815–1823 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

1820

6

Figure 6 shows instead the geometric properties of the case-study bridge for which the procedure of fire analysis was applied. In this case two tendons of 4 strands with diameter 15 mm are applied for each beam of the deck.

50

85 25

160

110

200

200

25

75

150

150

150

150

75

25

1000

50

800 700

50

a

b

Fig. 6 Properties of case-study bridge

Figure 7 shows the isotherms in the section of the bridge beam for the standard curve at 30 (Figure 7a) and 60 minutes (Figure 7b), and for the hydrocarbon curve, at 30 (Figure 7c) and 60 minutes (Figure 7d).

b

a

d

c

Fig. 7 Isothermal curves for the bridge beam. a) Standard curve at 30 min. b) Standard curve at 60 min. c) Hydrocarbon curve at 30 min. d) Hydrocarbon curve at 60 min.

Figure 8 shows the corresponding diagrams of useful ultimate moment M u,L with time for the RC bridge with external prestressing subjected to the standard and hydrocarbon curves, with a protective sheath of 5 mm for each tendon. The trend for the actual bridge is similar to the one of the experimental beam, considering the difference between the tendon diameter and cross-section properties. The characteristic and frequent combinations of loads are evaluated according to the moving loads defined by Eurocode 1 (CEN, 2005a), transversely distributed to the outermost beam, considered as the one most loaded, through the rule of Courbon-Engesser. The failure under the characteristic combination of loads is achieved at about 7 minutes from the beginning of the fire event when the standard curve is considered, while less than 5 minutes are sufficient for the hydrocarbon fire. This result does not change between the experimental model and the actual bridge. The curve of the RC section without prestressing, coinciding with the curves represented in the figure starting from the beginning of the horizontal branch, shows instead a better behavior in the actual bridge due to the greater number of bars that are placed at the bottom chord, and are involved in the heat transfer in time. For this reason, the standard curve shows