: The Pentagon had already undergone the first phases of a new anti-terrorist protection upgrade when the airliner hit it a year ago, and these improvements limited the damage and saved lives. Some of those included: 

@ A new heavier outside wall with reinforced concrete and a kevlar lining to stop fragments of concrete being spread out by any explosion.

@ Armored glass windows two inches thick. Such windows directly adjacent to the blast zone survived intact while older windows hundreds of feet away shattered.

@ A new fire sprinkler system kept the fire from spreading, and put out secondary fires. Only the fires directly fed by jet fuel continued to burn.

Now, even more improvements are being added:

@ All of the stairwells now go to the top floor; previously, about half of them did not. (The 5th floor was added late during construction.) These stairwells give people on that floor a better chance to escape any fire or major attack.

@ New cross-corridors (called "half-corridors" as they are narrower) connect the rings. Previously, there were ten major corridors; now there will be 15. The added corridors provide more escape routes in the event on an attack, but also make it easier to move around the building during a normal work day.

@ More pipes have been added to the fire sprinkler system so that damage to one part of the building doesn't cut off the water to other areas, allowing a fire to spread. 

@ New hardened walls have been added inside the building, surrounding the corridors, half-corridors, stairwells, and elevators. These are fastened to the concrete floors with steel plates. These should keep blast damage from spreading through the relatively soft wallboard previously used.

@ New glow-in-the-dark signs work without electricity, and are lower on the wall so they can be seen in rooms filling with smoke.--Stephen V Cole