ON THE MECHANISM OF PROGRESSIVE COLLAPSE OF THE HIGH-RISE BUILDING WTC-7 DURING EVENTS OF 11 SEPTEMBER 2001 IN NEW YORK

The collapse of 47-storeyed WTC-7 skyscraper which occurred during 7 hours after terrorist attacks on theTwinTowers of theWorldTrade Center became the main riddle of the events of September 11, 2001. The WTC-7 skyscraper wasn't a direct object of terrorist attack therefore circumstances of progressing collapse of this building caused fair amount of serious research, and also was followed by numerous myths and falsifications of events of September 11, 2001. The version of the mechanism of WTC-7 progressing collapse, presented in this article, is based on the theory of fire resistance of objects in case of exposure to combined hazardous effects (CHE) taking into account the crucial role of creep strain of steel constructions under such conditions. It is noted that propagation of creep strain of the steel constructions under described conditions was so considerable that could cause partial damage of their fire protection and eventually lead to the fire resistance exhaustion and to buckling of the steel columns in east part of the stiffening core during a fire. As a result of loss of the bearing capacity of stiffening core columns, due to the redistribution of load on the external envelope of the building, the critical warm-up temperature of external envelope columns (in a fire development zone) decreased to the level of their warm-up temperatures during a fire. It led to loss of bearing capacity of the external envelope columns in a fire zone and to total destruction of WTC-7.