Complex sentence: After several hours of uncontrolled fire, the steel columns, girders and trusses absorbed enough heat to sag, deform and buckle until the interior structure below the East penthouse was brought down.
Split into simpler sentences: After several hours of uncontrolled fire, the steel columns, girders and trusses absorbed heat and rapidly lost their strength. As they began to sag, deform and buckle, the interior structure below the east penthouse was brought down.