August 21 2008 NIST WTC7 Report (Draft for Public Comment)
Draft WTC7 Report (Including the Executive Summary):
- NIST NCSTAR 1A *: (115 pages)
- Original PDF (locked): http://wtc.nist.gov/media/NIST_NCSTAR_1A_for_public_comment.pdf
Draft WTC7 Technical Reports:
- Structural Fire Response and Probable Collapse Sequence of World Trade Center Building 7:
- NIST NCSTAR 1-9: Volume 1: Chapters 1 - 8 *: (404 pages)
- Original PDF (locked): http://wtc.nist.gov/media/NIST_NCSTAR_1-9_Vol1_for_public_comment.pdf
- NIST NCSTAR 1-9: Volume 2: Chapters 9 - Appendix E * (382 pages)
- Original PDF (locked): http://wtc.nist.gov/media/NIST_NCSTAR_1-9_vol2_for_public_comment.pdf
- Global Structural Analysis of the Response of World Trade Center Building 7 to Fires and Debris Impact Damage:
- NIST NCSTAR 1-9A *: (170 pages)
- Original PDF (locked): http://wtc.nist.gov/media/NIST_NCSTAR_1-9A_for_public_comment.pdf
Questions and Answers about the NIST WTC 7 Investigation:
(original link): http://www.nist.gov/public_affairs/factsheet/wtc_qa_082108.html
NIST Response to the World Trade Center Investigation Disaster Presentation:
(original link): http://wtc.nist.gov/media/WTC7_News_Briefing_082008.pdf
(original link): http://wtc.nist.gov/media/WTC7_Technical_Briefing_082608.pdf
WTC 7 was unlike the WTC towers in many respects. It was a more typical tall building in the design of its structural system. It was not struck by an airplane. The fires in WTC 7 were quite different from those in the towers. Since WTC 7 was not doused with thousands of gallons of jet fuel, large areas of any floor were not ignited simultaneously. Instead, the fires in WTC 7 were similar to those that have occurred previously in several tall buildings where the sprinklers did not function or were not present. These other buildings did not succumb to their fires and collapse because they were of structural designs that differed from that of WTC 7. The Investigation Team has compiled a list of key factors that enabled ordinary fires to result in an extraordinary outcome. In so doing, the Team recognized that there were additional aspects to be included in the content of some of the 30 earlier recommendations. (NCSTAR_1A_p95)
NIST found no evidence to suggest that WTC 7 was not designed in a manner generally consistent with applicable building codes and standards. (PF) The collapse of WTC 7 represents the first known instance of the total collapse of a tall building primarily due to(PF) fires with characteristics similar to previous fires in tall buildings. The connection, beam, and girder failures in the floor systems, and the resulting structural responses, occurred at temperature below approximately 400°C, well below the temperatures at which structural steel loses significant strength and stiffness.
PRINCIPAL FINDINGS: (NCSTAR_1A_p81)
- WTC 7 withstood debris impact damage that resulted in seven exterior columns being severed and subsequently withstood conventional fires on several floors for almost seven hours.
- The collapse of WTC 7 represents the first known instance of the total collapse of a tall building primarily due to fires. The collapse could not have been prevented without controlling the fires before most of the combustible building contents were consumed.
- WTC 7 collapsed due to uncontrolled fires with characteristics similar to previous fires in tall buildings. The fires in WTC 7 were similar to those that have occurred previously in several tall buildings (One New York Plaza, 1970, First Interstate Bank, 1988, and One Meridian Plaza, 1991) where the automatic sprinklers did not function or were not present. However, because of differences between their structural designs and that of WTC 7, these three buildings did not collapse. Fires for the range of combustible contents in WTC 7 (4.0 lb/ft2 on Floors 7 to 9 and 6.4 lb/ft2 on Floors 11 to 13) persisted in any given location for approximately 20 minutes to 30 minutes. Had a water supply for the automatic sprinkler system been available and had the sprinkler system operated as designed, it is likely that fires in WTC 7 would have been controlled and the collapse prevented.
- The probable collapse sequence that caused the global collapse of WTC 7 was initiated by the buckling of Column 79, which was unsupported over nine stories, after local fire-induced damage led to a cascade of floor failures. The buckling of Column 79 led to a vertical progression of floor failures up to the east penthouse and to the buckling of Columns 80 and 81. An east-to-west horizontal progression of interior column buckling followed, due to loss of lateral support to adjacent columns, forces exerted by falling debris, and load redistribution from other buckled columns. The exterior columns then buckled as the failed building core moved downward, redistributing its loads to the exterior columns. Global collapse occurred as the entire building above the buckled region moved downward as a single unit.
- The collapse of WTC 7 was a progressive collapse. The American Society of Civil Engineers defines progressive collapse—also known as disproportionate collapse—as the spread of local damage, from an initiating event, from element to element, eventually resulting in the collapse of an entire structure or a disproportionately large part of it (ASCE 7-05). Despite extensive thermal weakening of connections and buckled floor beams, fire-induced damage in the floor framing surrounding Column 79 over nine stories was the determining factor causing the buckling of Column 79 and, thereby, initiating progressive collapse. This is the first known instance where fire-induced local damage (i.e., buckling failure of Column 79; one of 82 columns in WTC 7) led to the collapse of an entire tall building.
- The transfer elements (trusses, girders, and cantilever overhangs) did not play a significant role in the collapse of WTC 7. Neither did the Con Edison substation play a significant role in the collapse of WTC 7.
- Prior to the collapse, there had been no damage to the SFRM that was applied to the steel columns, girders, and beams, except in the vicinity of the structural damage from the collapse of WTC 1, which was near the west side of the south face of the building.
- Even without the initial structural damage caused by debris impact from the collapse of WTC 1, WTC 7 would have collapsed from fires having the same characteristics as those experienced on September 11, 2001.
- Early fires in the southwest region of the building did not play a role in the collapse of WTC 7. The fires in this region were not severe enough to heat the structure significantly; and, unlike the northeast region, where collapse initiated, there were no columns supporting long span floors in the southwest region.
- The collapse time of the upper 18 stories of the north face of WTC 7 (the floors clearly visible in the video evidence) was 40 percent greater than the computed free fall time. This is consistent with physical principles.
- Diesel fuel fires did not play a role in the collapse of WTC 7. The worst-case scenarios associated with fires being fed by the ruptured fuel lines (a) could not have been sustained long enough, or could not have generated sufficient heat, to raise the temperature of a critical column (i.e., Column 79) to the point of significant loss of strength or stiffness, or (b) would have produced large amounts of visible smoke that would have emanated from the exhaust louvers. No such smoke discharge was observed.
- Hypothetical blast events did not play a role in the collapse of WTC 7. NIST concluded that blast events could not have occurred, and found no evidence whose explanation required invocation of a blast event. Blast from the smallest charge capable of failing a critical column (i.e., Column 79) would have resulted in a sound level of 130 dB to 140 dB at a distance of at least half a mile if unobstructed by surrounding buildings (such as along Greenwich Street and West Broadway). This sound level is comparable to a gunshot blast, standing next to a jet plane engine, and more than 10 times louder than being in front of the speakers at a rock concert. The sound from such a blast in an urban setting would have been reflected and channeled down streets with minimum attenuation. However, the soundtracks from videos being recorded at the time of the collapse did not contain any sound as intense as would have accompanied such a blast
- Calculated fire-elevated temperatures in the interior columns, including Columns 79, 80, and 81, stayed below 200 ºC on all of the floors. The exterior column temperatures were below 150 ºC, except on Floors 12 and 13, where the east and south exterior columns reached 300 ºC. At these temperatures, structural steel experiences relatively little loss of strength or stiffness. Thus, WTC 7 did not collapse due to fire-induced weakening of critical columns.
- The simulated fires on Floors 7, 12, and 13 heated portions of the tops of the floor slabs to over 900 ºC. The temperatures of some sections of the beams supporting Floors 8, 12, 13, and 14 exceeded 600 ºC. The temperatures of some sections of the floor beams at Floors 9 and 10 reached 400 ºC.
- The buckling failure of Column 79 between Floor 5 and Floor 14 was the initiating event that led to the global collapse of WTC 7. This resulted from thermal expansion and failures of connections, beams, and girders in the adjacent floor systems.
- The connection, beam, and girder failures in the floor systems, and the resulting structural responses, occurred at temperature below approximately 400 °C, well below the temperatures at which structural steel loses significant strength and stiffness.
- Thermal expansion was particularly significant in causing the connection, beam, and girder failures, since the floor beams had long spans on the north and east sides (approximately 15 m, 50 ft).
- Heating of the long beams resulted in proportionately large thermal elongation relative to the other components of the floor system, in effect, compressing the beams along their length. This led to distortion of the beams and breaking of the connections of the beams to the floor slabs. Furthermore, the simple shear connections used in the typical floor framing were not able to resist these axial compressive forces that developed as the floor framing was heated.
- At Column 79, heating and expansion of the floor beams in the northeast corner caused the loss of connection between the column and the key girder. Additional factors that contributed to the failure of the critical north-south girder were (1) the absence of shear studs that would have provided lateral restraint and (2) the one-sided framing of the east floor beams that allowed the beams to push laterally on the girders, due to thermal expansion of the beams.
- The fires thermally weakened Floors 8 to 14. As Floor 13 fell onto the floor below, a cascade of floor failures continued until the damage reached the massive Floor 5 slab, leaving Column 79 without lateral support for nine floors. The long unsupported length of Column 79 led to its buckling failure.
Vertical Progression of Collapse
- Once Column 79 buckled, there was a vertical progression of floor system failures up to the east penthouse, followed by the buckling of Columns 80 and 81.
- The buckling of Column 79 at the lower floors led to downward movement of the upper section of Column 79. The adjacent floor framing was pulled downward, leading to the observed kink in the east penthouse roof framing.
- As the lower floors surrounding Column 79 fell downward, Column 80 and Column 81 had increased unsupported lengths as well as falling debris impacts and loads being redistributed from adjacent columns. This led to buckling of Columns 80 and 81, and resulted in a vertical progression of failure of the floor systems up to the roof level across the entire east side of WTC 7.
- Columns 79, 80, and 81 were the only interior support for the gravity loads in the eastern region of the building. Once these three columns buckled and their upper sections began to descend, there was insufficient support for the floors, up to the east penthouse.
- None of these columns were significantly weakened by elevated temperatures; temperatures did not exceed 300 °C in the core or perimeter columns in WTC 7.
Horizontal Progression of Collapse
- Columns 76 through 78 were the next line of columns to buckle, due to loss of lateral support, impact by falling debris, and load redistribution from Columns 79 through 81. The failure of Truss 2 was not essential to the failure of Columns 77 and 78, as they would have buckled like the other columns.
- The remaining interior columns buckled in succession from east to west in the lower floors due to loss of lateral support from floor system failures, forces exerted by falling debris impact, and load redistributed to them from other buckled columns.
- The initial westward progression and the overall speed of the collapse was not sensitive to the extent of the estimated structural damage to WTC 7 due to the debris from the collapse of WTC 1. When the global collapse was nearly complete, there was some small sensitivity to the extent of the initial damage in the southwest portion of the building.
- The exterior columns buckled at the lower floors (between Floors 7 and 14) due to load redistribution to the exterior columns from the building core as the interior columns buckled and the building core moved downward. The entire building above the buckled-column region then moved downward in a single unit, as observed, completing the global collapse sequence.
- Computer simulations of the fires, the thermal heating of the structure, the thermally induced damage to the structure, and the structural collapse can be used to predict a complex degradation and collapse of a building. The overall features and timing of the prediction were consistent with the videographic evidence.
- The uncertainties in predicting the precise progression of the collapse sequence increased as the analysis proceeded due to the random nature of the interaction, break up, disintegration, and falling of the debris. The uncertainties deriving from these random processes increasingly influence the deterministic physics-based collapse process. Thus, the details of the progression of horizontal failure and final global collapse were sensitive to the uncertainties in how the building materials (steel, concrete) and building systems and contents interacted, broke up, and disintegrated.
- These computational models comprise a set of research tools that can take months (eight months in this case) for a complete simulation. Their adaptation for engineering practice would forestall future disasters, while reducing the potential for structural overdesign.
4.5.2 Building Design and Structural Safety
- NIST found no evidence to suggest that WTC 7 was not designed in a manner generally consistent with applicable building codes and standards.
- WTC 7 was adequately designed for vertical loads due to gravity and lateral loads due to wind.
- The vertical (gravity) load resisting system comprised the core and exterior columns, which received gravity loads from the floor framing. Of particular note were the three core columns on the east side of the building (Columns 79, 80, and 81), which supported large span floor areas with approximately 15 m (50 ft) spans on at least one side.
- Above Floor 7, the lateral load resisting system comprised the exterior moment frame with a perimeter belt truss at Floors 22 through 24. There were also a perimeter belt truss between Floors 5 and 7, diaphragms at Floors 5 and 7 that transferred lateral wind loads to the core columns, and bracing in the core below Floor 7 that transferred lateral loads to the foundation. However, this system was not able to provide a secondary load path for gravity loads.
- Above Floor 7, there was no bracing, or other load redistribution mechanism, to transfer gravity loads between interior columns. The floor framing, which was the only load path between columns, could not redistribute loads between columns because the shear connections from the interior floor beams to columns were only designed for transferring vertical shear loads.
- Transfer girders, trusses, and cantilever overlays were used to transfer column loads above Floor 7 to a different column layout below Floor 5.
- The structural design did not explicitly evaluate fire effects, which was typical for engineering practice at that time and continues to remain so today. Many of the shear connections in WTC 7 were not capable of resisting lateral loads resulting from thermal expansion effects in the steel floor framing when the floor beams were heated. (NCSTAR_1A_p91)
4.5.3 SFRM Requirements and Application
• It is likely that the Monokote MK-5 SFRM, applied to the steel framing and metal decks, was undamaged by the impact of the debris from the collapse of WTC 1, except in the area where direct structural damage to WTC 7 occurred.
• NIST simulations showed that, for the heaviest columns in WTC 7, when properly insulated, it would have taken an exposure of about 7 h at post-flashover upper layer gas temperatures to raise the steel temperature to 600 ºC, the point at which the steel strength would have been reduced by half. A similar calculation indicated it would have taken about 4 h to reach this temperature for an insulated lighter column. These times are both far longer than the time over which postflashover gas temperatures were sustained in the computed WTC 7 fires. For comparison, this steel temperature would have been reached in under one-half hour if the insulation were not applied.
• It is unlikely that the collapse of WTC 7 would have been prevented had the insulation thickness on the floor beams been increased by 50 percent (from ½ in. to ¾ in.). NIST calculations indicated that the time to reach the steel temperature of 649 °C (1200 °F) would have increased by about 10 min to 20 min.
• The ASTM E119 test does not capture critical behavior of structural systems, e.g., the effect of thermal expansion or sagging of floor beams on girders, connections, and/or columns. The thermal expansion of the WTC 7 floor beams that initiated the probable collapse sequence occurred at temperatures below approximately 400 °C. Thus, to the extent that thermal expansion, rather than loss of structural strength, precipitates an unsafe condition, thermal expansion effects need to be evaluated. The current fire resistance rating system, which does not include thermal expansion effects, is not conservative.
• The architectural drawings indicated that there were fire-rated walls between tenant spaces on the same floor and between tenant spaces and the building core. Spaces housing mechanical equipment, power transformers, emergency power generators, and other such equipment were enclosed in fire-rated partitions. While the partitions between offices, conferences rooms, etc. were not required to be fire-rated, there was evidence from both the visual evidence and the fire simulations that some of these partitions did retard the spread of the fires.
• There was no evidence of floor-to-floor fire spread until perhaps just before the WTC 7 collapse. Thus, the fire-rated floors were successful as fire penetration barriers.
WTC7 collapse compared with NIST's no-impact-damage model: http://www.youtube.com/watch?v=QpvejJCrhwM
A substantial portion of the evidence collected by NIST in the course of the Investigation has been provided to NIST under nondisclosure agreements. (NCSTAR_1A_p4)
NIST could not verify the actual (or as-built) construction, the properties and condition of the materials used, or changes to the original construction made over the life of the buildings. (NCSTAR_1A_p4)
This report describes how the fires that followed the impact of debris from the collapse of WTC 1 (the north tower) led to the collapse of WTC 7. (NCSTAR_1A_p15)