Excerpts from Pentagon Building Performance Report

		January 2003


		1.2 STUDY TEAM The BPS team included
		specialists in structural, fire, and forensic
		engineering. The following six individuals
		constituted the core group and are the authors
		of this report:

		Paul F. Mlakar, Ph.D., P.E., Lead
		Technical Director
		U.S.Army Corps of Engineers
		Vicksburg, Mississippi
		Specialty: blast-resistant design;
		investigator, Murrah Federal Office Building
		study

		Donald O. Dusenberry, P.E.
		Principal
		Simpson Gumpertz & Heger, Inc.
		Waltham, Massachusetts
		Specialty: blast effects and structural design

		James R. Harris, Ph.D., P.E.
		Principal
		J.R. Harris & Company
		Denver, Colorado
		Specialty: structural engineering

		Gerald Haynes, P.E.
		Fire Protection Engineer
		Bureau of Alcohol,Tobacco, and Firearms
		Washington,D.C.
		Specialty: fire protection
		Long T. Phan, Ph.D., P.E.
		Research Structural Engineer
		National Institute of Standards and Technology
		Gaithersburg, Maryland
		Specialty: concrete structural and fire engineering

		Mete A. Sozen, Ph.D., S.E.
		Kettelhut Distinguished Professor of Structural
		Engineering
		Purdue University
		Lafayette, Indiana
		Specialty: behavior of reinforced-concrete structures



		3.2 EYEWITNESS INTERVIEWS

		On January 8, 2002, BPS team leader Paul Mlakar
		interviewed three eyewitnesses—two of whom
		witnessed the impact of the aircraft and one of
		whom witnessed the subsequent partial collapse
		of the building. All three are professional
		staff members of the Pentagon Renovation
		Program Office and collectively provide a
		coherent and credible account of the events.

		Frank Probst, 58, is a West Point graduate,
		decorated Vietnam veteran, and retired army
		lieutenant colonel who has worked for the
		Pentagon Renovation Program Office on
		information management and telecommunications
		since 1995. At approximately 9:30 A.M. on
		September 11 he left the Wedge 1 construction
		site trailer, where he had been watching live
		television coverage of the second plane strike
		into the World Trade Center towers. He began
		walking to the Modular Office Compound, which
		is located beyond the extreme north end of the
		Pentagon North Parking Lot, for a meeting at 10
		A.M. As he approached the heliport (figure 3.2)
		he noticed a plane flying low over the Annex
		and heading right for him. According to the
		Arlington County after-action report (Arlington
		County, 2002), this occurred at 9:38 a.m. The
		aircraft pulled up, seemingly aiming for the
		first floor of the building, and leveled off.
		Probst hit the ground and observed the right
		wing tip pass through the portable 750 kW
		generator that provides backup power to Wedge
		1.The right engine took out the chainlink fence
		and posts surrounding the generator. The left
		engine struck an external steam vault before
		the fuselage entered the building. As the
		fireball from the crash moved toward him,
		Probst ran toward the South Parking Lot and
		recalls falling down twice. Fine pieces of wing
		debris floated down about him.The diesel fuel
		for the portable generator ignited while he was
		running. He noted only fire and smoke within
		the building at the point of impact. Security
		personnel herded him and others to the south,
		and he did not witness the subsequent partial
		collapse of the building.

		Don Mason, 62, is a communications specialist
		who retired from the United States Air Force
		after 25 years of service. He has worked for
		the Pentagon Renovation Program Office on
		information management and telecommunications
		since 1996. At the time of the crash he was
		stopped in traffic west of the building. The
		plane approached low, flying directly over him
		and possibly clipping the antenna of the
		vehicle immediately behind him, and struck
		three light poles between him and the building.
		He saw his colleague Frank Probst directly in
		the plane’s path, and he witnessed a small
		explosion as the portable generator was struck
		by the right wing.The aircraft struck the
		building between the heliport fire station and
		the generator, its left wing slightly lower
		than its right wing.As the plane entered the
		building, he recalled seeing the tail of the
		plane. The fireball that erupted upon the
		plane’s impact rose above the structure. Mason
		then noticed flames coming from the windows to
		the left of the point of impact and observed
		small pieces of the facade falling to the
		ground. Law enforcement personnel moved Mason’s
		vehicle and other traffic on, and he did not
		witness the subsequent partial collapse of the
		building.

		Rich Fitzharris, 52, is an electrical engineer
		and a former residential contractor. He has
		been the operations group chief of the Pentagon
		Renovation Program Office since 1996. He was in
		the Modular Office Compound at the time of the
		crash and rushed to the site on foot, arriving
		before the partial collapse. He recalls that
		the building—near the area of impact—was in
		flames, and he remembers seeing small pieces of
		debris, the largest of which might have been
		part of an engine shroud. He was at the
		heliport when a portion of the structure
		collapsed. The collapse initiated at the fifth
		floor along the building expansion joint,
		proceeded continuously and was completed within
		a few seconds. According to the Arlington
		County after-action report, this occurred at
		9:57 a.m., or 19 minutes after impact.





		3.7 SUMMARY OF THE IMPACT

		The Boeing 757 approached the west wall of the
		Pentagon from the southwest at approximately
		780 ft/s.As it approached the Pentagon site it
		was so low to the ground that it reportedly
		clipped an antenna on a vehicle on an adjacent
		road and severed light posts. When it was
		approximately 320 ft from the west wall of the
		building (0.42 second before impact), it was
		flying nearly level, only a few feet above the
		ground (figures 3.2 and 3.13, the latter an
		aerial photograph modified graphically to show
		the approaching aircraft). The aircraft flew
		over the grassy area next to the Pentagon until
		its right wing struck a piece of construction
		equipment that was approximately 100 to 110 ft
		from the face of the building (0.10 second
		before impact (figure 3.14). At that time the
		aircraft had rolled slightly to the left, its
		right wing elevated.After the plane had
		traveled approximately another 75 ft, the left
		engine struck the ground at nearly the same
		instant that the nose of the aircraft struck
		the west wall of the Pentagon (figure 3.15).
		Impact of the fuselage was at column line 14,
		at or slightly below the secondfloor slab.The
		left wing passed below the second-floor slab,
		and the right wing crossed at a shallow angle
		from below the second floor slab to above the
		second-floor slab (figure 3.16)

		A large fireball engulfed the exterior of the
		building in the impact area. Interior fires
		began immediately.

		The impact upon the west facade removed
		first-floor columns from column lines 10 to 14.
		First-floor exterior columns on column lines 9,
		15, 16, and 17 were severely damaged, perhaps
		to the point of losing all capacity. The
		secondfloor exterior column on column line 14
		and its adjacent spandrel beams were destroyed
		or seriously damaged. Additionally, there was
		facade damage on both sides of the impact area,
		including damage as high as the fourth floor.
		However, in the area of the impact of the
		fuselage and the tail, severe impact damage did
		not extend above the third-floor slab.

		Immediately upon impact, the Ring E structure
		deflected downward over the region from an
		expansion joint on column line 11 south to the
		west exterior column on column line 18 (figures
		3.8–3.10).The deformation was the most severe
		at the expansion joint, where the deflection
		was approximately 18 in. to 2 ft.

		The structure was able to maintain this
		deformed shape for approximately 20 minutes, at
		which point all five levels of Ring E collapsed
		from column line 11 to approximately column
		line 18 (figure 3.12).




		5. BPS SITE INSPECTIONS

		Members of the BPS team inspected the site on
		two occasions. Between September 14 and
		September 21, 2001, team leader Paul Mlakar had
		limited access to the site while rescue and
		recovery operations were still in progress. On
		this early inspection visit, he examined the
		exterior of the building and portions of the
		building interior.

		Controlled access to the site was granted to
		the full team after rescue and recovery
		operations were complete. On October 4, 2001,
		the Pentagon team, together with John Durrant,
		the executive director of ASCE’s institutes,
		and W. Gene Corley, the BPS team leader at the
		World Trade Center, inspected the interior and
		exterior of the damaged area of the Pentagon
		for approximately four hours.

		The inspection of the BPS team focused on
		obvious physical damage, primarily in the
		region of the impact.This inspection was not
		comprehensive. It did not address fire damage
		to concrete as a material, and it did not
		result in full documentation of all physical
		damage or as-built construction.

		***** By the time the full Pentagon BPS team
		visited the site, all debris from the aircraft
		and structural collapse had been removed
		(figure 5.1) and shoring was in place wherever
		there was severe structural damage. The design
		team charged with reconstructing the Pentagon
		was assessing the building and preparations
		were being made to demolish the areas for
		reconstruction. Consequently, the Pentagon BPS
		team never had direct access to the structural
		debris as it existed immediately after the
		aircraft impact and subsequent fire. . . .

		***** The aircraft had entered the building at
		an angle, traveling in a northeasterly
		direction. With the possible exception of the
		immediate vicinity of the fuselage’s entry
		point at column line 14, essentially all
		interior impact damage was inflicted in the
		first story:The aircraft seems for the most
		part to have slipped between the firstfloor
		slab on grade and the second floor. The path of
		damage extended from the west exterior wall of
		the building in a northeasterly direction
		completely through Ring E, Ring D, Ring C, and
		their connecting lower floors.There was a hole
		in the east wall of Ring C, emerging into AE
		Drive, between column lines 5 and 7 in Wedge 2
		(figure 5.16).The wall failure was
		approximately 310 ft from where the fuselage of
		the aircraft entered the west wall of the
		building.The path of the aircraft debris passed
		approximately 225 ft diagonally through Wedge 1
		and approximately 85 ft diagonally through a
		portion of Ring C in Wedge 2. . . .

		***** Most of the serious structural damage was
		within a swath that was approximately 75 to 80
		ft wide and extended approximately 230 ft into
		the first floor of the building.This swath was
		oriented at approximately 35 to 40 degrees to
		the perpendicular to the exterior wall of the
		Pentagon.Within the swath of serious damage was
		a narrower, tapering area that contained most
		of the very severe structural damage. This
		tapering area approximated a triangle in plan
		and had a width of approximately 90 ft at the
		aircraft’s entry point and a length of
		approximately 230 ft along the trajectory of
		the aircraft through the building.





		6. DISCUSSION

		6.1 IMPACT DAMAGE

		The site data indicate that the aircraft
		fuselage impacted the building at column line
		14 at an angle of approximately 42 degrees to
		the normal to the face of the building, at or
		slightly below the second- story slab.
		Eyewitness accounts and photographs taken by a
		security camera suggest that the aircraft was
		flying on nearly a level path essentially at
		grade level for several hundred feet
		immediately prior to impact. Gashes in the
		facade above the second-floor slab between
		column lines 18 and 20 to the south of the
		collapse area suggest that the aircraft had
		rolled slightly to the left as it entered the
		building.The right wing was below the
		second-floor slab at the fuselage but above the
		second-floor slab at the tip, and the left wing
		struck the building entirely below the
		second-floor slab, to the north of column line
		14.

		***** The width of the severe damage to the
		west facade of the Pentagon was approximately
		120 ft (from column lines 8 to 20).The
		projected width, perpendicular to the path of
		the aircraft, was approximately 90 ft, which is
		substantially less than the 125 ft wingspan of
		the aircraft (figure 6.1). An examination of
		the area encompassed by extending the line of
		travel of the aircraft to the face of the
		building shows that there are no discrete marks
		on the building corresponding to the positions
		of the outer third of the right wing. The size
		and position of the actual opening in the
		facade of the building (from column line 8 to
		column line 18) indicate that no portion of the
		outer two-thirds of the right wing and no
		portion of the outer one-third of the left wing
		actually entered the building.

		***** It is possible that less of the right
		wing than the left wing entered the building
		because the right wing struck the facade
		crossing the level of the second-floor slab.The
		strength of the second- floor slab in its own
		plane would have severed the right wing
		approximately at the location of the right
		engine. The left wing did not encounter a slab,
		so it penetrated more easily. In any event, the
		evidence suggests that the tips of both wings
		did not make direct contact with the facade of
		the building and that portions of the wings
		might have been separated from the fuselage
		before the aircraft struck the building. This
		is consistent with eyewitness statements that
		the right wing struck a large generator before
		the aircraft struck the building and that the
		left engine struck a ground-level, external
		vent structure. It is possible that these
		impacts, which occurred not more than 100 ft
		before the nose of the aircraft struck the
		building, may have damaged the wings and caused
		debris to strike the Pentagon facade and the
		heliport control building.

		The wing fuel tanks are located primarily
		within the inner half of the wings.The center
		of gravity of these tanks is approximately
		one-third of the wing length from the fuselage.
		Considering this tank position and the physical
		evidence of the length of each wing that could
		not have entered the building, it appears
		likely that not more than half of the fuel in
		the right wing could have entered the
		building.While the full volume of the left wing
		tank was within the portion of the wing that
		might have entered the building, some of the
		fuel from all tanks rebounded upon impact and
		contributed to the fireball. Only a portion of
		the fuel from the left and right wing tanks and
		the center fuselage tank actually entered the
		building.

		The height of the damage to the facade of the
		building was much less than the height of the
		aircraft’s tail. At approximately 45 ft, the
		tail height was nearly as tall as the first
		four floors of the building. Obvious visible
		damage extended only over the lowest two
		floors, to approximately 25 ft above grade.


	
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