Just food for thought: How to vacate a big building in case of fire

[Krishna]

Hi

High Rises (say 35+ floors) has stairways which are normal ones but are protected from heat/smoke for 2 hrs. Suppose in a 40 floor building fire happens at say 15th floor. How should evacuation be done. Please note stair ways are safe for 2 hrs. 

One way is all people on all floors immediately vacate.. Can there be better way too?.

Thanks

Krishna