At 9:37 a.m. on September 11, 2001, a 62-year-old Pentagon employee and retired Air Force communications specialist was sitting in traffic west of the Pentagon when a roaring jet engine passed so low overhead that it clipped the radio antenna of the car behind him.

The airplane, the hijacked American Airlines Flight 77, sliced through three light poles in the Pentagon parking lot before slamming into the first floor of the building and exploding in a fireball, instantly killing 125 people inside the Pentagon plus all 64 passengers onboard, including the five hijackers.

While the act was horrific and all the losses on that day were devastating, structural damage analysis revealed that the death toll at the Pentagon could have been far worse, if not for some critical engineering decisions made 60 years earlier.

Construction of the Pentagon began, ironically enough, on September 11, 1941. While America had not yet entered World War II, President Franklin D. Roosevelt knew he needed a home base for impending military operations near the nation’s capital. Wartime urgency meant that the Pentagon was completed in record time—just 16 months using 15,000 construction workers.

Steel was rationed for the war effort, so the Pentagon was built almost entirely of reinforced concrete, including 41,000 concrete pilings and concrete ramps instead of stairs connecting the building’s five floors. Completed in 1943, the Pentagon remains the world’s largest low-rise office building with 6.5 million square feet of office space containing up to 26,000 workers.

When the Pentagon was built, no one knew that it would become an iconic monument to U.S. military power—or a target. In fact, the architects thought it would be abandoned after the war and turned into a massive record storage depot. Their prediction was wrong, but fortuitous.

Thinking the Pentagon would need to store heavy caches of records for the long haul, the U.S. Army Corps of Engineers built in excess strength and structural redundancies that would end up saving hundreds and potentially thousands of lives on 9/11.

Donald Dusenberry is a structural engineer who co-authored a landmark report for the American Society of Civil Engineers on the damage sustained by the Pentagon on 9/11 and the lessons learned from its resilience. Dusenberry was at Ground Zero in New York City just days after the Twin Towers fell and toured the Pentagon crash site a few weeks later.

What he and his colleagues discovered after carefully documenting and analyzing the damage inflicted on the Pentagon, was that even though 26 first-floor cement columns were completely destroyed and 15 others severely damaged by the fiery crash, the upper floors of the Pentagon didn’t immediately collapse. In fact, it was a full 30 minutes before a portion of the building directly above the crash site collapsed, allowing more than enough time for survivors to escape.

Incredibly, not one Pentagon worker was killed during the partial collapse of the second through fifth floors. Compare that to the tragic fate of thousands of people trapped inside the World Trade who couldn’t escape before the towers fell.

An estimated 800 people were working in the section or “wedge” of the Pentagon where the impact occurred on the morning of September 11, which is far fewer than normal. In an incredible stroke of luck, that wedge had recently undergone a major renovation and only a fraction of the workers had moved back into their offices. If the plane had hit any other section of the building that day, there could have been as many as 4,500 Pentagon employees in the flight path.

It’s nearly impossible to imagine the force with which Flight 77 slammed into the broadside of the Pentagon. The Boeing 757 weighed an estimated 82.4 metric tons and was traveling at speeds exceeding 530 mph, according to flight recorder data.

The brunt of the damage to the building was inflicted by the airplane’s fuel reserves in the wings and the fuselage. Flight 77 took off from Washington D.C.’s Dulles International Airport at 8:20 AM on its way to Los Angeles with heavy tanks of fuel for the cross-country journey. Most of that fuel was still unspent when the jetliner struck the Pentagon.

According to Dusenberry and his team’s analysis, the lightweight wings and non-fuel sections of the plane sheared off almost immediately upon impact, but the heavy fuel tanks barreled through the first floor, creating a flow of debris that tore through the building like an avalanche, leaving a path of destruction twice the length of the aircraft.

Dusenberry compares the Boeing 757 aircraft to a water balloon.

“The balloon itself is not very strong, but if you fill it up with water and you throw it at something, it hits and exerts a fairly high force,” says Dusenberry. “It’s that moving mass that needs to be stopped by the object it strikes that results in the generation of force.”

What Dusenberry and his colleagues were there to figure out was how the second floor of the Pentagon remained standing after dozens of first-floor columns were destroyed or severely damaged. The upper sections only collapsed after sustaining severe damage from a raging fire.

The team’s expert conclusion was that two strategic design decisions made 60 years earlier had kept the Pentagon upright. The first had to do with the way the concrete columns holding up the floors and ceilings were reinforced. Spiral rebar, they discovered, had saved the day.

When building with concrete, steel reinforcement rods are embedded in the structure to give it added strength. In modern construction, a concrete column would likely be supported with widely spaced horizontal hoops of rebar running vertically up its core. But back in the 1940s, the standard was to use a continuous loop of tightly spiraling rebar.

The advantage of the spiral reinforcements was immediately obvious to Dusenberry’s team. Inside the blackened and hollowed-out first-floor office space, they found severely bent columns where the exterior layer of concrete had been sheared off, but the concrete core inside the spiral rebar remained intact. Incredibly, those deformed, sheared off columns were still standing.

“If they had used hoops instead of spirals, I expect the performance wouldn’t have been as good,” says Dusenberry. “They certainly weren’t anticipating terrorist attacks or explosions or anything. This is a happy outcome of what they were doing for other reasons at the time.”

The second thing that held the Pentagon together after the attack was the way the flooring and ceiling concrete was reinforced. First of all, the support columns on each floor were spaced relatively tightly, with maximum distances of 20 feet. So the concrete beams and girders above them only had to span short distances.

And inside those concrete beams, the engineers had run long stretches of rebar that overlapped from one beam to the next. Dusenberry says that it was precisely those overlapping steel supports that held up heavy sections of damaged concrete ceiling even when the underlying columns had collapsed.

“That steel rebar can act as a suspender that holds the crushed concrete in the floor above,” says Dusenberry. “Even though it doesn't operate as beam anymore, it hangs it like a clothesline hangs clothes.”

The Pentagon is a one-of-a-kind building, a five-sided fortress of concrete whose architectural style has long since gone out of fashion, if it ever was in fashion. But Dusenberry says that modern architects and engineers can learn a lot from the lessons taught by 9/11, the most important being the critical importance of redundancy and ductility.

Redundancy is the planning for alternative load paths if the primary structural elements are lost or destroyed. The Pentagon did this through tight column configurations and overlapping rebar in the beams. Ductility is the ability of structural elements to bend under extreme loads, but not break, as exhibited by the spiral rebar in the surviving Pentagon columns.

“There are buildings being designed today with the consideration that there could be an event,” says Dusenberry. “Not necessarily a malevolent event, but one that damages a critical structural element. For example, you can design a building such that an upper floor, should you lose a column below, will actually hang the building below it.”