Figure 29. — View of Pack River Bridge looking southeast. 



Figure 30. — Burnout of north approach ^an. Pack 

 River Bridge, showing distortion of steel surface 

 plates. 



out of one of the approach spans and the de- 

 formed steel plates. The other span is identi- 

 cal in appearance. It is apparent from the 

 photograph that the steel plates had been suf- 

 ficiently softened by the heat to conform to 

 the shape of the surface they fell upon when 

 the span gave way. 



Hypotheses of Ignition 

 and Burning 



The probability of ignition of the bridge is 

 of course dependent on the residence time of 

 the fire in the surrounding area. Anderson^ 

 gives us a relationship between 0, the resi- 

 dence time in minutes, and D, the particle 

 diameter in inches. 



e = 8D 



From evidence of burned limb stumps, 2 

 inches in diameter, we estimate a residence 

 time in the surrounding fuels area of 16 

 minutes. Limbs of larger diameter would help 

 sustain the residence time past 16 minutes; 

 this would certainly be enough time to ignite 

 the brush near the approach spans (the re- 

 mains of this brush may be seen in figure 17). 

 The burning brush would then act as an igni- 

 tion source for the bridge. 



The- importance of closely positioned kin- 

 dling fuels is demonstrated by the fact that 

 fire damage to the central span was limited, 

 owing to the absence of a nearby source of 

 ignition. Except for occasional hot flames the 

 region just above the creek bottom would be 

 relatively cool compared to the surrounding 

 area. We conclude that fire activity in the 

 central span near the concrete piers was 

 simply an extension of the fire in the ap- 

 proach spans. During the period of burning, 

 winds near the bridge were predominantly up- 

 stream or downstream, that is, normal to the 

 bridge, as evidenced by the creosote smoke 

 deposited on the steel beams on either side of 

 the central span. Consequently, we believe the 

 fire traveled crosswise to the wind and simply 

 burned out near the concrete piers. If there 

 had been ignition on the windward side of the 

 central span, more extensive damage would 

 have occurred in that section, 



^Anderson, Hal E. Heat transfer and fire spread. U.S.D.A. 

 Forest Serv., Inter mountain Forest and Range Exp. Sta., 

 Ogden, Utah. (In preparation.) 



34 



