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. |
