Study on the Smoke and Toxic Gas from Vehicle Fires on the Lower Lane of a Double Deck Steel Bridge

Abstract

This study investigates smoke dispersion and toxic gas distribution in lower-deck vehicle fires on double-decker bridges through 27 sets of scaled experiments (1:8 model). Key findings reveal: windless conditions form stable ceiling jets, while crosswinds (> 3.3 m/s inward) disrupt stratification, creating hazardous turbulent smoke layers that reduce visibility to critical levels (e.g., 1.2 m thickness at 200 MW). CO₂ concentrations peak at 0.82% (16 × safe limit), CO reaches 13,352 ppm (267 × threshold), with vertical concentration gradients showing 3 × higher toxicity near upper decks. Gas concentrations scale proportionally with heat release rate (200 MW fires yield 1.5 × 100 MW levels). Most adverse conditions occur at 2 m/s outward and 4.4 m/s inward winds, where flame attachment to structural members exacerbates gas accumulation. The empirical evidence substantiates restricting tanker trucks from lower decks to mitigate catastrophic fire risks.