1 . 2 ▁× ▁ 3 . 4 ▁m , ▁ 2 2 7   kg ) ▁window pan es ▁det ached ▁from ▁the ▁building ▁and ▁crash ed ▁to ▁the ▁side walk ▁hundreds ▁of ▁feet ▁below . ▁Police ▁closed ▁off ▁surrounding ▁streets ▁whenever ▁wind s ▁reached ▁ 4 5   m ph ▁( 7 2   km / h ). ▁Under ▁the ▁direction ▁of ▁Frank ▁H . ▁D urg in ▁of ▁M IT ' s ▁Wright ▁Brothers ▁Wind ▁T unnel ▁a ▁scale ▁model ▁of ▁the ▁entire ▁Back ▁Bay ▁and ▁an ▁a ero el astic ▁model ▁of ▁the ▁John ▁Han cock ▁Tower ▁were ▁built ▁and ▁tested ▁in ▁the ▁wind ▁tunnel ▁to ▁identify ▁the ▁problem . ▁The ▁research ▁raised ▁questions ▁about ▁the ▁struct ural ▁integrity ▁of ▁the ▁entire ▁building ▁( due ▁to ▁un antic ip ated ▁tw isting ▁of ▁the ▁structure ), ▁but ▁did ▁not ▁account ▁for ▁the ▁loss ▁of ▁the ▁glass ▁pan els . ▁An ▁independent ▁labor atory ▁eventually ▁confirmed ▁that ▁the ▁failure ▁of ▁the ▁glass ▁was ▁due ▁to ▁oscill ations ▁and ▁repeated ▁thermal ▁str esses ▁caused ▁by ▁the ▁expansion ▁and ▁contra ction ▁of ▁the ▁air ▁between ▁the ▁inner ▁and ▁outer ▁glass ▁pan els ▁which ▁formed ▁each ▁window ; ▁the ▁res il ient ▁bond ing ▁between ▁the ▁inner ▁glass , ▁reflect ive ▁material , ▁and ▁outer ▁glass ▁was ▁so ▁st iff ▁that ▁it ▁was ▁transm itting ▁the ▁force ▁to ▁the ▁outer ▁glass ▁( inst ead ▁of ▁absor bing ▁it ), ▁thus ▁causing ▁the ▁glass ▁to ▁fail . ▁ ▁In ▁October ▁ 1 9 7