forth, while a great many trees fell under the 

 force of a strong, sustained high wind. The 

 trees with snapped tops were generally less 

 than 2 ft. d.b.h., whereas those that fell were 

 somewhat larger. Tops were snapped off at a 

 nearly uniform height of 60 to 75 feet at a 

 diameter of approximately 1.0 foot. Across 

 open reaches along Pack River below the 

 bridge the trees feU to the north or northeast, 

 but at the mouth of McCormick Creek trees 

 were blown to the east and southeast. Above 

 McCormick Creek some blowdown was to the 

 south and southwest. - 



All of these clues suggest strong erratic 

 winds just before the fire and the presence of 

 strong fire whirls building and collapsing ran- 

 domly in the area surrounding the Pack River 

 bridge (fig. 17). 



The sequence of events leading to the 

 destruction of the bridge was also examined. 

 The fire burned out both approaches and left 



the center span intact with some of its paint 

 hardly scorched. The metal plate treads on 

 the approaches were found draped from the 

 concrete piers to the ground, indicating that 

 as the wooden stringers and decking col- 

 lapsed, temperatures had reached 1,200° F. 

 This temperature level is not unusual in a fire. 



Laboratory tests showed the design of the 

 approaches provided sustained combustion in 

 the space between stringers, and the ap- 

 proaches could have continued burning after 

 the main fire had passed. The burning period 

 probably extended over many hours until the 

 decking broke up and the stringers collapsed. 

 Then an increase in heat occurred as the mate- 

 rial piled up on the ground, resulting in a peri- 

 od of flaming combustion followed by a long 

 period of glowing combustion. Both types of 

 burning contributed to the spalling of the con- 

 crete piers. Additional information on the 

 Pack River bridge is provided in Appendix 3. 



Figure 17. — Fire destruction at Pack River Bridge. 



21 



