Structural strength saved Pentagon lives on 9/11

13 FEB, 2003 BY EDITOR

https://www.newcivilengineer.com/latest/structural-strength-saved-pentagon-lives-on-9-11-13-02-2003/

HELICALLY WOUND column reinforcement helped limit the death toll during the terrorist attack on the Pentagon on 11 September 2001, according to the first official report into the disaster. Energy absorbing helical reinforcement, continuity of beam reinforcement through supports and the insitu concrete outer wall combined to prevent a more extensive collapse of the building, the report says. The concrete outer wall acted as a transfer girder, creating substantial residual load capacity and alternative load paths, says the report.

During the attack 125 Pentagon personnel and 64 passengers and crew on a hijacked Boeing 757 died when the aircraft flew into the building (see box). The Pentagon Building Performance Report, published by the American Society of Civil Engineers (ASCE), concludes that fire was the cause of the limited collapse that did occur 20 minutes after impact. The blaze acted directly on the main steel reinforcement bars of badly damaged columns, weakening them (see box).

The report commends the performance of recently installed anti-blast measures, which helped keep most of the jet fuel and the post impact fireball outside the building. Some damaged columns suffered dramatic deformations yet still supported the floor above. The report recommends more research into the residual performance of such extremely deformed columns, and into the energy-absorbing capacity of reinforced concrete elements. It acknowledges that ‘much has been written on means to prevent progressive collapse, but little detailed guidance’ has been incorporated into US building codes. A ‘focused effort to accumulate research and practical experience’ into structural robustness is also recommended.

British engineers said the report confirms European opinions on the need for robustness and resistance to progressive collapse in structures. Babtie director Gordon Masterton said the report contained nothing ‘particularly surprising - we already knew about the benefits of spirally reinforced columns, and of continuity at joints and alternative load paths.’ He added: ‘However, I am surprised that the investigators didn’t recommend more work on fire performance.

‘After all, one third of those in the Pentagon who died did so as a result of the fire, and fire was a contributing factor in the deaths of another third.’ US codes are still trying to absorb the lessons of the 1995 Oklahoma bomb disaster, said ASCE World Trade Center investigation leader Gene Corley. ‘This latest report will reinforce the need for change, especially for buildings seen as potential terrorist targets, ’ he added.

Built to last

The Pentagon was a typical US concrete framed building of its time. It was built in record time during the early 1940s in response to wartime needs and designed to become a document store once hostilities were over. By modern standards spans were short, column centres close together and partitions were heavy and dense. Typically on the ground floor the columns would measure 530mm by 530mm at maximum centres of little more than 6m. So when the Boeing smashed through the insitu concrete outer wall it entered a very different environment to that of the World Trade Center twin towers in New York which were struck by hijacked aircraft on the same day.

Instead of a large clear floorplate and a drywall core the Pentagon offered defences in depth: rows of sturdy columns with spirally wound shear reinforcement, good continuity at joints and over designed floors. Had the hijackers aimed at almost any other wall of the Pentagon the results might have been very different, says the ASCE report. After more than 50 years of high intensity use the building was ageing. Plans for refurbishment were hastily modified in the light of the 1995 Oklahoma bomb.

This meant protection against external explosions was rapidly incorporated: Kevlar mesh reinforced the masonry infill wall panels and modern blast resistant windows were installed. But only one ‘wedge’ of the building had been refurbished before the terrorists struck, and it was the reinforced outer walls and window frames of this refurbished section that took the initial impact.

Columns in a wedge shaped area extending some 75m from the point of impact took severe damage, with many blasted out of the way completely. As the fuselage began to disintegrate, the area of damage narrowed. Many columns on the fringe of this wedge shaped area suffered severe displacements, but kept standing.

At least a dozen columns at the point of impact or immediately behind it were wiped out. The first floor slab sagged an estimated 600mm at an external expansion joint, but only collapsed 20 minutes after impact. And the new windows seem to have performed well, keeping much of the external fireball out of the building.

Generally, damage from the ensuing internal fire was typical of serious office fires. But close to the impact point, columns still struggling to support the first floor stood little chance. With cover to the reinforcement stripped off over large areas and the main steel reinforcement exposed directly to the fire, the columns could only hold up for a vital 20 minutes.

9/11: What happened

At 9.38am local time on Tuesday 11 September 2001 a Boeing 757-200 with 64 people on board struck the west wall of the Pentagon’s outer ring just below first floor level, at an angle of about 42infinity. Investigators concluded that on impact the Boeing weighed around 82,000kg and was travelling close to 250m/sec. Its inner wing and fuselage tanks probably contained 16,500kg of fuel. At the moment of impact it appears the aircraft was flying almost level with the ground but with a slight bank to the left. The left engine hit the ground at almost the same moment the nose struck the outer wall.

As the fuselage penetrated the dense forest of columns inside the concrete framed building its nose section disintegrated almost instantaneously, spilling the cockpit crew and hijackers. Massive kinetic energy kept the rest of the fuselage moving forward, decelerating at 30G or more.

Most of the wings were ripped off as they hit the facade, rupturing the fuel tanks and causing an external fireball. An avalanche of building debris and shredding aluminium roared through the ground floor offices, as far as the opposite wall. When it subsided, all on board were dead and scattered around. Among their bodies were 125 dead and dying Pentagon workers. Then the fire began to spread.

Holloway's Commentary:

Was 911 a worst case scenario? 911 was a case where American spooks knew the attack was coming and were completely unable to do anything about it. America's pants were up and we still got hit. DIA spooks bungle two assassination attempts on Putin. The Russians then declare they will use nuclear weapons in response to America's spook war in Chechnya. Americans had already experienced two tactical nuke attacks in CONUS. WTC-93 and OKC Bombing were both tactical nuclear attacks. In response the DIA/NOC offices, the Pentagon's most important offices are reinforced. 

Russia declares they will respond with nuclear weapons to America's actions in Chechnya.

The NSA publicizes the NORTHWOODS forgeries to cover their ass in anticipation of the attack. After a cover story is released to the press claiming he lost a briefcase with classified material, FBI Special Agent John P. O'Neil is put in charge of WTC security knowing that an attack was about to occur. Spooks in the know start making stock bets. America's spook agencies were looking in all the right places but were unable to remotely image a nuclear weapon the size of a baseball. 911 was a combined cyber warfare and commando attack.