the following is a quote from 911 In Plane Sight:

"How does a Boeing 757, constructed from lightweight aluminum, penetrate over 9 ft. of steel reinforced concrete? Recently discovered photographs shed light on this unexplained feat of physics."

I have an explanation. The plane was traveling at max speed when it hit the Pentagon. It's bound to do some damage. Example- a pressurized stream of water can cut through anything including metal and diamond.