(403) - May 2002
World Trade Center
Building Performance Study:
Collection, Preliminary Observations, and Recommendations
5.2 Structural Description
5.2.2 Structural Framing
5.2.3 Transfer Trusses and Girders
5.3 Fire Protection Systems
5.3.1 Egress Systems
5.3.2 Detection and Alarm
5.3.4 Suppression Systems
5.4 Building Loads
5.5 Timeline of Events Affecting WTC 7 on September 11,
5.5.1 Collapse of WTC 2
5.5.2 Collapse of WTC 1
5.5.3 Fires at WTC 7
5.5.4 Sequence of WTC 7 Collapse
5.6 Potential Collapse Mechanism
5.6.1 Probable Collapse Initiation Events
5.6.2 Probable Collapse Sequence
5.7 Observations and Findings
WTC 7 collapsed on September 11, 2001, at 5:20 p.m. There were no known
casualties due to this collapse. The performance of WTC 7 is of
significant interest because it appears the collapse was due primarily
to fire, rather than any impact damage from the collapsing towers. Prior
to September 11, 2001, there was little, if any, record of fire-induced
collapse of large fire-protected steel buildings.
The structural design and construction features of this building, potential fuel loads, fire damage, and the observed sequence of collapse
are presented to provide a better understanding of what may have
happened. However, confirmation will require additional study and
analysis. Information about the structural design and construction
features and the observed sequence of events is based upon a review of
structural drawings, photographs, videos, eyewitness reports, and a 1986
article about the construction features of WTC 7 (Salvarinas 1986). In
addition, the following information and data were obtained from the
Annotated floor plans and riser diagrams of the emergency generators
and related diesel oil tanks and distribution systems (Silverstein
Engineering explanation of the emergency generators and related diesel
oil tanks and distribution systems (Flack and Kurtz, Inc. 2002)
Information on the continuity of power to WTC 7 (Davidowitz 2002)
Summary of diesel oil recovery and spillage (Rommel 2002)
Information on WTC 7 fireproofing (Lombardi 2002)
Information on the New York City Office of Emergency Management (OEM)
tanks at WTC 7 (Odermatt 2002)
The 47-story office building had 1,868,000 square feet of office space.
The top 40 stories of the building (floors 8 to 47) were office type
occupancies. Table 5.1 lists the larger tenants of WTC 7. WTC 7 was
completed in 1987 by a development team composed of the following
Owner/Developer: Seven World Trade Company, Silverstein Development
Corporation, General Partner
Construction Manager: Tishman Construction Corporation of New York
Design Architect: Emery Roth & Sons, P.C.
Structural Consultant: The Office of Irwin G. Cantor, P.C.
Mechanical/Electrical Consultant: Syska & Hennessy, P.C.
Structural Consultant (Con Ed Substation): Leslie E. Robertson
As shown in Figure 1-1 (WTC site map in Chapter 1), WTC 7 was located
north of the main WTC complex, across Vesey Street, and was linked to
the WTC Plaza by two pedestrian bridges: the large Plaza bridge and a
smaller, glass-enclosed pedestrian bridge. The bridges spanned 95 feet
across Vesey Street, connecting the Plaza and the 3rd floor of WTC 7. In
addition to the office occupancies, WTC 7 also contained an electrical
substation, and the WTC Complex shipping ramp, as shown in Figure 5-1.
The substation and shipping ramp occupied major portions of the WTC 7
site. The substation was built prior to the office tower, supplied
electrical power to lower Manhattan, and covered approximately half the
site. The shipping ramp (5,200 square feet in area, approximately 10
percent of the WTC 7 site) was used by the entire WTC complex.
Table 5.1 WTC 7 Tenants
Salomon Smith Barney (SSB)
Standard Chartered Bank
Revenue Service (IRS)
Department of Defense (DOD)
Central Intelligence Agency (CIA)
Revenue Service (IRS)
Office of Emergency Management (OEM)
Federal Home Loan Bank of New York
First State Management Group
ITT Hartford Insurance Groupn
National Association of Insurance
Commissioners (NAIC) Securities Valuation Office
Equal Employment Opportunity
Provident Financial Management
Securities and Exchange Commission
US Secret Service
American Express Bank International
OEM generators and day tank
Upper level of 3rd floor lobby,
Lobby, SSB Conference Center,
rentable space, management offices
Open to 1st floor lobby, transformer
vault upper level, upper level switchgear
Lobby, loading docks,
existing Con Ed
transformer vaults, fuel storage, lower level switchgear
Figure 5-1 Foundation plan - WTC 7.
5.2 Structural Description
With the development of an
office tower in mind, the Port Authority of New York and
New Jersey (hereafter referred
to as the Port Authority) installed caissons intended for future
construction. However, Seven World Trade Company, Silverstein
Development Corporation, General Partner, decided to construct a
building much larger in both height and floor area. The designers
combined the existing caissons inside the substation with new caissons
inside and outside the substation to create the foundation for WTC 7.
Figure 5-1 shows the location of pre-existing caissons built when the
Con Ed substation was constructed along with new caissons that were
installed for the support of the building.
The discrepancy in the column
locations between the substation and the office tower required transfers
to carry loads from the office tower to the substation and finally into
the foundation. Old and new caissons, as well as old and new columns,
also can be seen in the foundation plan shown in Figure 5-1.
5.2.2 Structural Framing
The typical floor framing shown
in Figure 5-2 was used for the 8th through the 45th floors. The gravity
framing consisted of composite beams (typically W16x26 and W24x55) that
spanned from the core to the perimeter. The floor slab was an
electrified composite 3-inch metal deck with 2-1/2-inch normal-weight
concrete fill spanning between the steel beams. Below the 8th floor,
floors generally consisted of formed slabs with some limited areas of
concrete-filled metal decks. There were numerous gravity column
transfers, the more significant of these being three interior gravity
column transfers between floors 5 to 7 and eight cantilever column
transfers in the north elevation at the 7th floor. The column transfers
in the exterior walls are shown in the bracing elevations (Figure 5-3).
The lateral load resisting
system consisted of four perimeter moment frames, one at each exterior
wall, augmented by two-story belt trusses between the 5th and 7th floors
and between the 22nd and 24th floors. There were additional trusses at
the east and west elevations below the 7th floor. An interior braced
core extended from the foundation to the 7th floor. The horizontal shear
was transferred into the core at the 5th and the 7th floors. The 5th
floor diaphragm (plan shown in Figure 5-4) consisted of a reinforced
concrete 14-inch-thick slab with embedded steel T-sections. The 7th
floor was an 8-inch-thick reinforced concrete slab.
The 5th and 7th floors contained
the diaphragm floors, belt trusses, and transfer girders. A 3-D
rendering of Truss 1, Truss 2, Truss 3, and several cantilever transfer
girders is shown in Figure 5-5.
5.2.3 Transfer Trusses
The transfer trusses and
girders, shown in Figure 5-6,
were located between the 5th and 7th
floors. The function and design of each transfer system are described
Truss 1 was situated in the
northeast sector of the core, and spanned in the east-west direction. As
shown in Figure 5-7, this truss was a two-story double transfer
structure that provided load transfers between non-concentric columns
above the 7th floor to an existing column and girder at the 5th floor.
The girder then provided a second load transfer to an additional two
columns. The 7th floor column supported 41 floors and part of the east
mechanical penthouse. Its load was transferred through the triangular
truss into a column located above an existing substation column and
girder at the 5th floor. The 36.5-ton built-up double web girder spanned
in the north-south direction between two new columns that started at the
foundation and terminated at the 7th floor. The truss diagonals were W14
shapes and the horizontal tie was a 22-ton, built-up shape.
Figure 5-2 Plan view of typical floor framing for the 8th
through 45th floors.
Figure 5-3 Elevations of building and core area.
Figure 5-4 Fifth floor diaphragm plan showing T-sections
embedded in 14-inch slab.
Figure 5-5 3-D diagram showing relation of trusses and
Figure 5-6 Seventh floor plan showing locations of
transfer trusses and girders.
Truss 2 was a single transfer located south of Truss 1. As shown in
Figure 5-8, Truss 2 transferred the column load from the 7th floor
through a triangular truss into two existing columns at the 5th floor.
Large gusset plates were provided at the connection between the
diagonals, the columns, and the horizontal tie. The diagonals and the
built-up horizontal tie were field-welded.
Truss 3 was a cantilevered
two-story transfer structure in the north-south direction between the
5th and 7th floors at the western end of the core area. As shown in
Figure 5-9, Truss 3 transferred the loads between columns. The upper
columns carried 41 floors of load and were cantilevered to the north of
the column that went from the foundation to the 7th floor.
The cantilever transfer girders,
shown in Figure 5-10, spanned between the core and the north elevation
at the 7th floor. There were eight transfer girders to redirect the load
of the building above the 7th floor into the columns that went through
the Con Ed substation. These girders cantilevered 6 feet 9 inches
between the substation and the north facade of the building above. The
girders extended an additional 46 feet to the core. The two transfer
girders at the east end of the building were connected to Truss 1,
creating a double transfer. The girders varied in depth from 9 feet at
the north end, to a tapered portion in the middle, and to 4 feet 6
inches at the southern section closest to the core. Each transfer girder
weighed approximately 52 tons. At the north wall, between the 7th and
5th floors, transferred columns were also part of the belt truss that
circled the building as part of the lateral-load-resisting system and
acted as a transfer for the columns above the shipping ramp.
Figure 5-7 Truss 1 detail. (BPM = built-up plate member.)
A variety of framing connections
were used. Seated beam connections were used between the exterior
columns and the floor beams. Single-plate shear connections were
generally used at beam-to-beam connections. Double-angle connections
were provided between some beam and end-plate connections at
beam-to-interior columns. Floor-framing connections used
7/8-inch-diameter ASTM A325 bolts; connections for bracing, moment
frames, and column splices used 1-inch diameter ASTM A490 bolts.
Along the east and west
elevations, center-to-center column spacing was typically less than 10
feet. Column trees were used at these locations. A column tree is a
shop-fabricated column assembly with beam stubs shop-welded to the
column flanges. The field connections were made at the end of the stubs
at the center of the span between columns. One-sided lap plates were
used for both flange and web connections.
Along the north and south
elevations, and within the core up to the 7th floor, the spans were
approximately 28 feet. At these locations, traditional moment frame
construction was used. Top and bottom flange plates, as well as
one-sided web shear plates, were shop-welded to column flanges. The
beams were then field-bolted into the connection.
Figure 5-8 Truss 2 detail. (BPM = built-up plate member.)
The majority of column splices
were bolted according to American Institute of Steel Construction (AISC)
details. They were located 3 feet 6 inches above the floor and were not
designed to accommodate tensile forces.
Columns below the 7th floor were
often "jumbo" shapes (W14x455 to W14x730) or
built-up jumbo box shapes
with plates up to 10 inches thick welded from flange to flange, parallel
to the web, to provide the necessary section properties. For these
massive columns, either the upper shaft was beveled to be field-welded
or side plates were shop-welded to the lower shaft and field-welded to
the upper shaft once the column was erected and plumb.
The majority of the bracing
members were two channels or two T-sections connected to the structure
by a welded gusset plate. A single wide flange cross-section was also
used. These members were connected with web and flange plates, similar
to those used in the moment frames.
Some of the bracing members on the
east and west sides of the building
were as large as the jumbo column
sections. Large connection plates were sandwiched to each side of these
large braces, beams, and columns at their junctions. Bolts attached all
the components to each other at these joints.
The granite façade panels were
manufactured off site and were supported by individual trusses. Each
panel had a single vertical/gravity connection and top and bottom
lateral/wind connections to transmit these forces back to the base
building. Horizontal panel adjustments could be accommodated within the
panel itself. The building columns had welded angles and channels that
provided horizontal and lateral support. The top of the panel was
connected to the angle, and the bottom of the panel was connected to the
channel. These steel-panel connections had vertically slotted holes for
Figure 5-9 Truss 3
5.3 Fire Protection Systems
5.3.1 Egress Systems
There were two main exit
stairways in WTC 7. Stairway 1 was located on the west side, and
Stairway 2 was located on the east side within the central core. Both
exit stairways discharged directly to the exterior at ground level and
were approximately 4 feet 10 inches wide. The stairways were built of
fire-rated construction using gypsum wallboard. Subsequent to the 1993
bombing incident at the WTC, battery operated emergency lighting was
provided in the stairways and photoluminescent paint was placed on the
edge of the stair treads to facilitate emergency egress. In addition to
the battery-powered lighting, the stairs also had emergency system
lighting powered by the generators.
Twenty-eight passenger elevators
and three service elevators served the various levels of WTC 7.
Occupants using the elevators would typically discharge at the third
level and either exit through the Lobby to bridges bringing them over to
the WTC Plaza, or proceed down the escalators to grade level.
5.3.2 Detection and
Smoke detectors were located in
telecommunications, electrical, and communications closets, as well as
inside the HVAC system ducts, in the mechanical rooms, and in all
elevator lobbies. Manual pull stations were provided at the entrances to
stairways and at each of the exits. Speakers for voice evacuation
announcements were located throughout the building and were activated
manually at the Fire Control Center (FCC). Strobes were provided and
were activated automatically upon detection of smoke, water flow, or
initiation of a manual pull station.
Monitoring of the fire-alarm
control panel for WTC 7 was provided independently at a central station.
In addition to the emergency generators, the existing uninterruptible
power supply (UPS) provided 4 hours of full operation for the fire-alarm
system and 12 hours of standby operation. The floor contained a
combination of area smoke and heat detectors.
Figure 5-10 Cantilever transfer girder detail.
Concrete floor slabs provided
vertical compartmentalization to limit fire and smoke spread between
floors (see Figure 5-11). Architectural drawings indicate that the space
between the edge of the concrete floor slab and curtain wall, which
ranged from 2 to 10 inches, was to be filled with fire-stopping
A zoned smoke control system was
present in WTC 7. This system was designed to pressurize the floors
above and below the floor of alarm, and exhaust the floor of alarm to
limit smoke and heat spread.
The fireproofing material used
to protect the structural members has been identified by Silverstein
Properties as "Monokote." The Port Authority informed the BPS Team that
New York City Building Code Construction Classification 1B (2-hour
rating for beams, girders, trusses, and 3-hour rating for columns) was
specified for WTC 7 in accordance with the architectural specifications
on the construction notes drawing PA-O. According to the Port Authority,
the construction notes on drawing PA-O also specified the following:
Exterior wall columns (columns engaged in masonry walls)
fireproofed on the exterior side with 2-inch solid gypsum, 3-inch
hollow gypsum, 2-inch concrete or spray-on fireproofing.
Interior columns shall be fireproofed with materials and have rating
conforming with Section C26-313.3 (27-269 current section).
Beams and girders shall be fireproofed with 2-inch grade Portland
cement concrete, Gritcrete, or spray-on fireproofing or other
materials rendering a 2-hour fire rating.
Figure 5-11 Compartmentalization provided by concrete floor slabs.
The Port Authority stated that
it believed the thickness of the spray-on fireproofing was determined by
the fireproofing trade for the specific structural sections used, based
on the Underwriters Laboratories formula for modifications, which were
reviewed by the Architect/Engineer of Record during the shop drawing
submittal. Spray-on fireproofing, as required by the code, was also
listed on the drawing as an item subject to controlled inspections, in
accordance with Section C26-106.3 (27-132 current section). The
Architect/Engineer of Record was responsible for ensuring that the
proper thickness was applied. The Port Authority had extended its
fireproofing inspection program to this building.
The primary water supply appears to have been provided by a dedicated
fire yard main that looped around most of the complex. This yard main
was supplied directly from the municipal water supply. Fire department
connections were located on the south and west sides.
WTC 7 was a sprinklered building. However, on the 5th floor, only the
core spaces were sprinkler protected, and none of the electrical
equipment rooms in the building were sprinkler protected. The sprinkler
protection was of "light hazard" design. The sprinkler system on most
floors was a looped system fed by a riser located in Stairway 2. The
loading dock was protected with a dry-pipe sprinkler system.
The area of
the fuel tank for OEM had a special fire detection and suppression
The Fire Pump Room was located on the ground floor in the southwest
corner of the building and contained an automatic (as well as a manual)
fire pump. There were two Fire Department of New York connections in the
southwest quadrant - one on the south façade and one on the west façade.
Each stairway had standpipes in it. At each floor in each stairway,
there was a 2-1/2-inch outlet with a 1-1/2-inch hose (with a 3/4-inch
nozzle). In addition, the east side of each floor also had a
supplemental fire hose cabinet. Primary water supply to the standpipe
system came from a yard main, which was fed from the municipal water
Power to WTC 7 entered at 13,800 volts (V), was stepped down to 480/277
V by silicone oil-filled transformers in individual masonry vaults
the 5th floor, and was distributed throughout the building. On each
floor, one of the 277 legs was tapped and stepped down to supply
single-phase 120-V branch circuits. The main system had ground fault
protection. Emergency power generators were located on various levels
and provided a secondary power supply to tenants. This equipment
supplied backup power for communications equipment, elevators, emergency
lighting in corridors and stairwells, and fire pumps. Emergency lighting
units in the exit stairways, elevator lobbies, and elevator cabs were
equipped with individual backup batteries.
The tanks that provided fuel for the emergency generators were located
in the building. The Silverstein and Salomon Smith Barney (SSB) fuel
tanks were underground below the loading dock.
The OEM tank was on the
ground floor on a fire-rated steel platform within a 4-hour fire-rated
enclosure. SSB had supply and return piping to the emergency generators
made from a 2-1/2-inch double-wall steel pipe with a 4-inch outside
diameter. The SSB fuel oil riser was single-wall pipe with a masonry
shaft. On the 5th floor, only the horizontal piping was a double-wall
pipe within a pipe. The pumps located at the ground floor could supply
75 gallons per minute (gpm). A 3-gpm fuel supply rate was needed for
each of the nine 1,725-kilowatt (kW)
generators located on the 5th
floor. One gallon would be consumed and the other 2 gallons would
continue to circulate through the system. The SSB fuel oil pumps were
provided with UPS power supported by both base building emergency power
and SSB standby power. The volume between the inner and outer pipes was
designed to contain a leak from the inner pressurized pipe and direct
that fuel oil to a containment vessel.
Upon detection of fuel oil in the
containment vessel, the fuel oil pumps automatically de-energized. The SSB fuel oil pumps and distribution piping were dedicated to the SSB
generator plant. The base building life safety generators and OEM
generators had their own dedicated fuel oil pumps and piping.
Silverstein generators consisted of two 900-kW units, which
located on the 5th floor, and supplied by a 275-gallon day tank. Other
characteristics of the design or controls for the fuel system for the
generators are unknown.
5.4 Building Loads
The degree of impact damage to the south façade could not be documented.
However, damage was evident from review of photographs and video
records. The number of fires observed after the collapse of WTC 1 also
makes it likely that debris impact damage occurred in a number of
An array of fuels typically associated with offices was distributed
throughout much of the building. In addition, WTC 7 contained 10
transformers at street level, 12 transformers on the 5th floor, and 2
dry transformers on the 7th floor. The Con Ed substation contained
(outside the building footprint) eight 30-foot-wide transformers that
supplied 13-kilovoltampere (kVA) power to the 6th floor of the
building. Fuel oil (ranging from diesel to #4) was provided for the
generators serving OEM, SSB, Silverstein Properties, and the U.S. Secret
Service. Table 5.2 shows where the generators, fuel tanks, pumps, and
risers were located for the various occupants. There was also a Con Ed
4-inch-diameter gas line with 0.25 pounds per square inch (psi) (low)
pressure going into WTC 7 for cooking purposes.
Early news reports had
indicated that a high pressure, 24-inch gas main was located in the
vicinity of the building; however, this proved not to be true.
Table 5.2 WTC 7 Fuel Distribution Systems
[Office of Emergency Management]
Used Silverstein tank to fill day tanks
Ground floor; 33.3 gpm
Located in shaft in west elevator
275-gallon tank on 7th floor; one
6,000-gallon tank located between low-rise elevators in east
elevator shaft between 2nd and 3rd floors
Three 500-kW on 7th floor on south
Salomon Smith Barney
Two 6,000-gallon tanks under loading
dock on ground level
In Fire Pump Room, west side of ground floor; 75 gpm
Located in shaft in mechanical rooms on southwest corner
None; pressurized recirculating loop
with 2.5-inch-inside-diameter double-wall supply and return steel
pipe on 5th floor
Nine 1,725-kW on 5th floor, six on north
side, three in southwest corner
Two 12,000-gallon tanks under loading
dock on ground level
Between elevator shafts on west side
of ground floor; 4.4 gpm
Located in shaft in west elevator
275-gallon tank on 5th floor
Two 900-kW on 5th floor in southwest
Used Silverstein tank
Used Silverstein pumps
Located in shaft in west elevator
Approximately 50- to 100-gallon tank
under generator on 9th floor
Day tank only
275-gallon tank on 8th floor on west
side next to exterior wall
As described in Section 5.6.2, the sequence of the WTC 7 collapse is
consistent with an initial failure that occurred internally in the lower
floors on the east side of the building. The interest in fuel oil is
therefore directed at the parts of the fuel oil distribution system
having the potential of supporting a fire in the lower floors on the
east side of the building. The risers for the fuel distribution system
were in one of the two utility shafts in the west end of the building.
One exception was the American Express Corporation, which had a
generator with a 275-gallon tank on the west end of the 8th floor. This
tank was the sole supply for the American Express generator and was not
connected to any other fuel oil source. The 275-gallon tank was filled
by bringing containers of fuel oil to the tank and transferring the oil
into the tank. Except for the part of the diesel oil distribution system
serving the SSB generators, all of the generators were located at the
west end, with relatively short horizontal distribution piping.
The SSB system involved three separate generator locations on the 5th
floor: three generator sets in the southwest corner of the building, two
in the northwest section, and four in the northeast section. The
distribution pipe was double-wall welded black iron with
between the pipes. The outer pipe was at least 4 inches in diameter and
the inner pipe at least 2-1/2 inches. The pipe traversed most of the
length of the 5th floor immediately north of a concrete masonry wall
running most of the length of the floor in an east-west direction. At
the east end of the 5th floor and to the south of the wall was a 1- to
2-story mechanical equipment room.
Transfer Trusses 1 and 2 were located
in this room. The east end of Truss 1 was supported by a truss element
that ran perpendicular (i.e., north-south) to the main east-west
portions of the truss. There was a set of double doors opening from the
mechanical room to the area containing the four generator sets
previously mentioned. The fuel oil distribution pipe ran above this door
several feet to the north of the masonry wall. The type, quality, and
hardware on the door set are unknown. The position of the door (i.e.,
open or closed) at the time of the incident is also unknown. Also, no
information was available in regard to the size of the undercut on the
The fuel oil pumps were powered from the generator sets. Fuel oil would
have been pumped from the tanks when the emergency power system sensed a
power interruption. The pump then operated in response to the pressure
difference between the supply and return, and the pump would circulate
oil as long as such a difference existed.
Upon sensing a power
interruption, the system would automatically switch to emergency mode.
This would have been done with a transfer switch that monitored the
building power supply and transferred to the emergency power system if
the power from the Con Ed source was interrupted.
It was also possible
for the transfer to be made manually. Relative to continuity of power to
the building, Con Ed reported that "the feeders supplying power to WTC 7
were de-energized at 9:59 a.m." It is believed that the emergency
generators came on line immediately. It is also believed that some of
them may have stopped operating because of the contamination of the
intake air flowing into the carburetors and radiators. Except for the SSB system, where it is understood that a UPS system provided backup
power to the 75-gpm pump, the flow of oil would stop and, as soon as the
day tanks were empty, the involved generator set would stop running.
The SSB generators did not use day tanks. Instead there was a
pressurized loop system that served all nine generators.
As long as the
75-gpm pump continued to operate, a break in the line could, under some
conditions, have a full or partial break that would not cause the system
to shut down and could discharge up to the 75-gpm capacity of this
positive displacement pump. It is understood that the SSB pump was
supplied power from both the SSB generators and from the UPS.
Engineers from the New York State Department of Environmental
Conservation investigated oil contamination in the debris of WTC 7.
Their principal interest was directed to the various oils involved in
the Con Ed equipment. However, they reported the following findings on
fuel oil: "In addition to Con Ed's oil, there was a maximum loss of
12,000 gallons of diesel from two underground storage tanks registered
as 7WTC." To date, the NY State Environmental Protection Agency (EPA)
and DEC have recovered approximately 20,000 gallons from the other two
intact 11,600-gallon underground fuel oil storage tanks at WTC 7.
Based on the listings in Table 5.2, it is probable that the
20,000 gallons that were recovered were from the Silverstein Properties'
emergency power system. The data obtained from Silverstein indicate that
the pumping rate from their tanks was 4.4 gpm. If the Silverstein pump
had started pumping at 10 a.m., when Con Ed shut down power to the
building immediately following the collapse of WTC 2, and continued
pumping until the collapse of WTC 7 at 5:20 p.m.,
less than 2,000
gallons would have been used. The residual 20,000 gallons found in the
two 12,000-gallon tanks, therefore,
can not be used as an indicator of
whether or not the Silverstein generator sets were on line and running.
Similarly, the SSB pump, which
had a pumping rate of 75 gpm, would have drained the two 6,000-gallon
tanks serving that system in less than 3 hours. This could have
accounted for the lost 12,000 gallons reported by EPA or the tanks could
have been ruptured and the oil spilled into the debris pile. Again,
is not a valid indicator of whether or not the SSB generator sets came
on line. The NY State EPA indicates that the SSB tanks will be pulled
from the debris in the near future. This may or may not give some
indication of the amount of oil still in the tanks when they were
crushed. If there is evidence that the majority of diesel fuel was still
in the tanks, it can be concluded that the SSB system did not discharge
diesel oil as hypothesized in Section 5.6.1. Conversely, evidence that
indicates that the tanks were low on oil at the time of rupture, and
that they were full at the start of the September 11 incident, would
lend support to the hypothesis that the SSB system was operating and
pumping oil from these tanks.
Currently, there are no data available on the post-collapse condition of
the OEM 6,000-gallon tank located between the 2nd and 3rd floors. The
OEM system also included a 275-gallon day tank located on the 7th floor.
The OEM system had a fuel supply system with the capability of
transferring fuel from the Silverstein tanks to the 6,000-gallon OEM
tank. The OEM generator sets were located in the southwest portion of
the 7th floor. OEM also had an 11,000-gallon potable water tank on the
south side of the 7th floor.
The Secret Service diesel distribution system, like the OEM system, was
designed to refurbish its supply from the Silverstein tanks. This
appears to have been pumped directly to a day tank having an estimated
capacity of 50 to 100 gallons located near the northwest corner of the
9th floor. The generator set was also in the same location.
The 275-gallon tank associated with the American Express generator was
located at the west end of the 8th floor. If full, the 275 gallons
represent a potential of about 600 MegaJoules , which would be enough to
cause a serious fire that could spread to other fuels,
but not felt to be
enough to threaten the stability of the building's structural elements.
5.5 Timeline of Events
Affecting WTC 7 on
September 11, 2001
The effects of the collapse of WTC 1 and WTC 2, the ensuing fires in WTC
7, and the collapse of WTC 7 are discussed below. Figure 5-12 shows the
vantage points of the photographs taken illustrating these effects, as
well as the extent of the debris generated by each of the collapses.
5.5.1 Collapse of WTC 2
9:59 a.m., WTC 2 (the south tower) collapsed. The approximate extent
of its debris is shown in Figure 5-12(A). It appears that the collapse
of WTC 2 did not significantly affect the roof, or the east, west, and
north elevations of WTC 7. It is unknown if there was any damage to the
south elevation after WTC 2 collapsed, but both the covered, tubular
pedestrian bridge (see Figure 5-13) and the Plaza bridge were still
standing after the collapse of WTC 2.
5.5.2 Collapse of WTC 1
10:29 a.m., WTC 1 (the north
tower) collapsed, sending its debris into the streets below.
and severity of the resulting damage to WTC 7 are currently unknown.
However, from photographic evidence and eyewitness accounts discussed
below, it was assumed that the south side of the building was damaged to
some degree and that fires in WTC 7 started at approximately this time.
Figure 5-14 is an aerial photograph
that shows the debris clouds spreading around WTC 7 just after the
collapse of WTC 1. Figure 5-15 is a photograph of WTC 1 debris between
the west elevation of WTC 7 and the Verizon building.
Figure 5-12(B) shows a plan-view diagram approximating the extent of
this debris just after the collapse of WTC 1.
It does not appear that
the collapse of WTC 1 affected the roof, or the east, west, and north
elevations of WTC 7 in any significant way. However, there was damage to
the southwest corner of WTC 7 at approximately floors 8 to 20, 24, 25,
and 39 to 46, as shown in Figure 5-16, a photograph taken from West Street.
Figure 5-12 Sequence of debris generated by collapses of WTC 2, 1, and
Figure 5-13 Pedestrian bridge (bottom center) still standing after WTC 2
has collapsed, sending substantial dust and debris onto the street, but
before WTC 1 (top center) has collapsed.
Figure 5-14 View from the north of the WTC 1 collapse and spread of
debris around WTC 7. Note the two mechanical penthouses of WTC 7 are
Figure 5-15 Debris from the collapse of WTC 1 located between WTC 7
(left) and the Verizon building (right).
Figure 5-16 Damage to southwest corner of WTC 7 (see box), looking from
Figure 5-17, a photograph taken across from the World
Financial Center (WFC), shows the west elevation and indicates damage at
the southwest corner of WTC 7 at the 24th, 25th, and 39th through 46th
According to the account of a firefighter who walked the 9th floor along
the south side following the collapse of WTC 1, the only damage to the
9th floor façade occurred at the southwest corner. According to
firefighters' eyewitness accounts from outside of the building,
approximately floors 8-18 were damaged to some degree. Other eyewitness
accounts relate that there was additional damage to the south elevation.
5.5.3 Fires at WTC 7
Currently, there is limited information about the ignition and
development of fires at WTC 7, as well as about the specific fuels that
may have been involved during the course of the fire. It is likely that
fires started as a result of debris from the collapse of WTC 1.
According to fire service personnel,
fires were initially seen to be
present on non-contiguous floors on the south side of WTC 7 at
approximately floors 6, 7, 8, 10, 11, and 19.
The presence of fire and smoke on lower floors is also confirmed by the
early television news coverage of WTC 7, which indicated light-colored
smoke rising from the lower floors of WTC 7.
Figure 5-17 Building damage to the southwest corner and smoke plume from
south face of WTC 7, looking from the World Financial Plaza.
Note damage to WFC 3 in the foreground.
Video footage indicated that the majority of the smoke appeared to be
coming from the south side of the building at
that time as opposed to
the other sides of the building. This is corroborated by Figure 5-17,
photograph taken at 3:36 p.m. that shows the south face of WTC 7 covered
with a thick cloud of smoke, and only small amounts of smoke emanating
from the 27th and 28th floors of the west face of WTC 7.
News coverage after 1:30 p.m. showed light-colored smoke flowing out of
openings on the upper floors of the south side of the building.
photograph (Figure 5-18) of the skyline at 3:25 p.m., taken from the
southwest, shows a large volume of dark smoke coming from all but the
lowest levels of WTC 7, where white smoke is emanating.
The mode of fire and smoke spread was unclear; however, it may have been propagated through interior shafts, between floors along the south
façade that may have been damaged, or other internal openings, as well
as the floor slab/exterior façade connections.
It appeared that water on site was limited due to a 20-inch broken water
Vesey Street. Although WTC 7 was sprinklered, it did not appear
that there would have been a sufficient quantity of water to control the
growth and spread of the fires on multiple floors. In addition,
firefighters made the decision fairly early on not to attempt to fight
the fires, due in part to the damage to WTC 7 from the collapsing
the fire progressed
throughout the day fairly unimpeded by automatic or
A review of photos and videos indicates that there were limited fires on
the north, east, and west faces of the building. One eyewitness who saw
the building from a 30th floor apartment approximately 4 blocks away to
the northwest noted that fires in the building were not visible from
that perspective. On some of the lower floors, where the firefighters
saw fires for extended periods of time from the south side, there
appeared to be walls running in an east to west direction, at least on
floors 5 and 6, that would have compartmentalized the north side from
the south side. There were also air plenums along the east and west
walls and partially along the north walls of these floors instead of
windows that may have further limited fires from extending out of these
floors and, therefore, were not visible from sides other than the south.
Figure 5-18 WTC 7, with a large volume of dark smoke rising from it,
just visible behind WFC 1 (left). A much smaller volume of white smoke
is seen rising from the base of WTC 7. Note that the lower,
lighter-colored smoke (to right) is thought to be from the two collapsed
As the day progressed, fires were observed on the east face of the 11th,
12th, and 28th floors (see Figure 5-19).
The Securities and Exchange
Commission occupied floors 11 through 13. Prior to collapse, fire was
seen to have broken out windows on at least the north and east faces of
WTC 7 on some of the lower levels.
On the north face, photographs and videos show that the
located on approximately the 7th, 8th, 11th, 12th, and 13th floors.
American Express Bank International occupied the 7th and 8th floors.
7th floor also held the OEM generators and day tank. Photographs of the
west face show fire and smoke on the 29th and 30th floors.
Figure 5-19 Fires on the 11th and 12th floors of the east
face of WTC 7.
It is important to note that floors 5 through 7 contained structural
elements that were important to supporting the structure of the overall
building. The 5th and 7th floors were diaphragm floors that contained
transfer girders and trusses. These floors transferred loads from the
upper floors to the structural members and foundation system that was
built prior to the WTC 7 office tower.
Fire damage in the 5th to 7th
floors of the building could, therefore, have damaged essential
With the limited information currently available,
fire development in
this building needs additional study. Fires were observed to be located
on the lower levels for the majority of
the time from the collapse of WTC 1 to the collapse of WTC 7. It appears
that the sprinklers may not have been effective due to the limited water
on site, and that the development of the fires was not significantly
impeded by the firefighters because manual firefighting efforts were
stopped fairly early in the day.
Available information indicates that fires spread horizontally and
vertically throughout the building during the course of the day. The
mode of spread was most likely either along the south façade that was
damaged, or internally through shafts or the gap between the floor slab
and the exterior wall. It is currently unclear what fuel may have been
present to permit the fires to burn on these lower floors for
approximately 7 hours. The change in the color and buoyancy of the smoke as the day progressed
may indicate a change in the behavior of the fires. The darker color may
be indicative of different fuels becoming involved, such as fuel oil, or
the fire becoming ventilation limited. The increased buoyancy of the
fires suggests that the heat release rate (or "fire size") may have also
The mechanisms behind these apparent changes in behavior are currently
unknown and therefore various scenarios need to be investigated further.
These include gathering additional information regarding storage of
materials on various levels, the quantity and combustibility of
materials, and the presence of dense storage, including file rooms, tape
vaults, etc. In addition, further analysis is needed on the specific
locations of the fuel tanks, supply lines, fuel pumps, and generators to
determine whether it may have been possible for a fuel line to be
severed by the falling debris, allowing the pumps to run and pump fuel
out of the broken pipes.
5.5.4 Sequence of WTC 7
Approximately 7 hours after fires initiated in WTC 7, the building
collapsed. The start of a timed collapse sequence was based on
17:20:33, the time
registered by seismic recordings described in Table 1.1 (in Chapter 1).
The time difference between each of the figures was approximated from
time given on the videotape. Figures 5-20 to 5-25 illustrate the
observed sequence of events related to the collapse.
Timeline of Major Events1
Chapter 1 [pg 10])
10:28:31 EDT (14:28 UTC)
WTC 1 began collapsing
after 102 minutes, 5 seconds.
Large debris from the
collapse fell on WTC 3, 5, 6, and 7; the Winter Garden; and the
American Express (World Financial Center 2) building. WTC 3
collapsed to the 3rd floor, and fires were initiated in WTC 5, 6,
17:20:33 EDT (21:20 UTC)
WTC 7 began collapsing
1 Based on
seismic recordings made by the Lamont-Doherty Earth Observatory of
Columbia University, 34 kilometers north of the WTC site.
2 EDT = Eastern
Daylight Time; UTC = Coordinated Universal Time. Times cited in
this report and based on these times, rounded to the nearest minute.
~5:20:33 p.m. WTC 7 begins to collapse. Note the two mechanical
penthouses at the roof on the east and west sides in Figure 5-20.
~5:21:03 p.m. Approximately 30 seconds later, Figure 5-21 shows the east
mechanical penthouse disappearing into the building. It takes a few
seconds for the east penthouse to "disappear" completely.
~5:21:08 p.m. Approximately 5 seconds later, the west mechanical penthouse
disappears (Figure 5-22) or sinks into WTC 7.
~5:21:09 p.m. Approximately 1 or 2 seconds after the west penthouse
sinks into WTC 7, the whole building starts to collapse. A north-south
"kink" or fault line develops along the eastern side as the building
begins to come down at what appears to be the location of the collapse
initiation (see Figures 5-23 and 5-24).
~5:21:10 p.m. WTC 7 collapses completely after burning for approximately 7
hours (Figure 5-25). The collapse appeared to initiate at the lower
floors, allowing the upper portion of the structure to fall.
Figure 5-20 View from the north of WTC 7 with both mechanical penthouses
The debris generated by the collapse of WTC 7 spread mainly westward
toward the Verizon building, and to the south. The debris significantly
damaged 30 West Broadway to the north, but did not appear to have
structurally damaged the Irving Trust building at 101 Barclay Street
to the north or the Post Office at 90 Church Street to the east.
average debris field radius was approximately 70 feet. Figures 5-12(C)
and 5-26 show an approximation of the extent of the debris after the
collapse of WTC 7.
5.6 Potential Collapse
5.6.1 Probable Collapse
WTC 7 collapsed approximately 7 hours after the collapse of WTC 1.
Preliminary indications were that, due to lack of water,
firefighting actions were taken by FDNY.
Section 5.5.4 describes the sequence of the WTC 7 collapse.
described sequence is consistent with building collapse resulting from
an initial (triggering) failure that occurred internally in the east
portion of a lower floor in the building.
There is no clear evidence of
exactly where or on which floor the initiating failure occurred.
Possibilities can be divided into three potential scenarios based on
floor. In each case, the concern is the failure of either a truss or one
or more columns in the lower floors of the east portion of the building.
Each of the scenarios is a hypothesis based on the facts known and the
unknown conditions that would be required for the hypothesis to be
valid. The cases are presented not as conclusions, but as a basis for
Figure 5-21 East mechanical penthouse collapsed. (From video.)
Figure 5-22 East and now west mechanical penthouses gone. (From video.)
Figure 5-23 View from the north of the "kink" or fault developing in WTC
4th Floor Scenarios.
The bottom cords of the transfer trusses were part of the support of the
5th floor slab and, as such, were located below the slab and above the
ceiling of the 4th floor in a position exposed to fire from below. The
bottom cord members were massive members weighing slightly over 1,000
pounds per foot. Such members are slow to heat up in a fire. It was
reported that these bottom cords were fireproofed. The space below was
the cafeteria dining room. The best information available indicates that
the dining room was furnished with tables and chairs. The intensity and
duration of a fire involving these furnishings would not be expected to
sufficiently weaken either the trusses or the columns supporting the
trusses. Member collapse as a result of a fire on the 4th floor would
require either that there was significant additional fuel or that the
fireproofing on the trusses or columns was defective. Fuel oil leakage
from the 5th floor is also a possibility; however, no evidence of
leakage paths in the east end of the second floor was reported.
Figure 5-24 Areas of potential transfer truss failure.
Figure 5-25 Debris cloud from collapse of WTC 7.
Figure 5-26 Debris generated after collapse of WTC 7.
5th Floor Scenarios.
From a structural standpoint, the most likely event would have been the
collapse of Truss 1 and/or Truss 2 located in the east end of the 5th
and 6th floors. These floors are believed to have contained little if
any fuel other than the diesel fuel for the emergency generators, making
diesel oil a potential source of fire. As noted in Section 5.4, the fuel
distribution system for the emergency generators pumped oil from tanks
on the lower floors to the generators through a pipe distribution
system. The SSB fuel oil system was a more likely source of fire around
the transfer trusses. The SSB pump is reported as a positive
displacement pump having a capacity of 75 gpm at 50 psi. Fuel oil was
distributed through the 5th floor in a double-wall iron pipe. A portion
of the piping ran in close proximity to Truss 1. However, there is no
physical, photographic, or other evidence to substantiate or refute the
discharge of fuel oil from the piping system.
The following is, therefore, a hypothesis based on potential rather than
demonstrated fact. Assume that the distribution piping was severed and
discharged up to 75 gpm onto the 5th floor in the vicinity of Truss 1.
Seventy-five gpm of diesel fuel have the potential of approximately 160
megawatts (MW) of energy. If this burning diesel fuel formed pools
around Truss 1, it could have subjected members of that truss to
temperatures significantly in excess of those experienced in standard
fire resistance test furnaces (see Appendix A).
If the supply tanks were
full at the start of the discharge, there was enough fuel to sustain
this flow for approximately 3 hours. If the assumed pipe rupture were
incomplete and the flow less, the potential burning rate of the
discharged oil would be less, but the duration would be longer. At even
a 30-gpm flow rate (about 60 MW potential), the exposed members in the
truss could still be subjected to high temperatures that would
progressively weaken the steel. For the above reasons,
it is felt that
burning of discharged diesel fuel oil in a pool encompassing Truss 1
and/or Truss 2 needs to be further evaluated as a possible cause of the
In evaluating the potential that a fire fed by fuel oil caused the
collapse, it is necessary to determine
whether the following events
The SSB generators called for fuel. This would occur
once the generators came on line.
The pumps came on, sending fuel through the distribution
There was a breach in the fuel distribution piping and
fuel oil was discharged from the distribution system.
Although there is no physical evidence available, this hypothesis
assumes that it is possible that both the inner and outer pipes were
severed, presumably by debris from the collapse of WTC 1. Depending on
ventilation sources for air, this is sufficient to flashover the space
along the north wall of this floor.
The temperature of the fire gases
would be governed to a large extent by the availability of air for
combustion. The hot gases generated would be blocked from impacting
Trusses 1 and 2 by the masonry wall separating the generation area
from the mechanical equipment room, assuming that this wall was still
intact after collapse of the tower and there were no other significant
penetrations of walls.
The discharged fuel must be
ignited. For diesel oil to be ignited, there must be both an ignition
source and the oil must be raised to its flash point temperature of
about 60 ˚C (140 ˚F). Because there were fires on other floors of WTC
7, an assumption of ignition at this level in the building is
reasonable, but without proof.
There is sufficient air for combustion of the discharged
The air required for combustion of 75-gpm (160 MW potential) diesel
fuel is approximately 100,000 cubic feet per minute (cfm). If less air
is available for combustion, the burning rate will decrease
proportionally. As the engine generator sets come on line, automatic
louvers open and 80,000 cfm are provided for each of the nine SSB
engines. A portion is used as combustion air for the drive engines;
the rest is for cooling, but could supply air to an accidental fire.
Given open louvers and other sources for entry of air, it is,
therefore, probable that a fuel oil spill fire would have found
sufficient air for combustion.
The hot fire gases reach and heat the critical member(s).
For this to happen, the fire must have propagated either fuel or hot
gases to the members in the truss in the mechanical equipment room. If
the double door to the mechanical equipment room was either open or
fell from its frame at some point, or if the door was undercut, the
spilled fuel oil might have flowed into the mechanical equipment room,
enveloping truss members in the main (hottest) portion of the flame.
Such a situation could produce an exposure possibly exceeding that in
the standard furnace test producing localized heat fluxes approaching
the 200 kW/m2 used by Underwriters Laboratories to simulate a
hydrocarbon pool fire, with exposure temperatures in the range of
1,200 degrees ˚C (2,200 ˚F). If such intense
exposure existed, the steel would be weakened more rapidly than
normally expected. If the door was of superior construction (as with a
fire door), it is unlikely that the fire would have reached the
trusses in the mechanical equipment room until such time that the door
further hypothesis that would help explain the long time lapse between
the collapse of WTC 1 and the collapse of WTC 7 would be that the
masonry wall and door resisted the fire for a number of hours, but
eventually failed. The new opening then allowed the fire (still supplied
with a continuous discharge of fuel oil) to flow into the mechanical
equipment room, envelope elements of the fireproofed trusses, and
eventually cause a buckling collapse of one or both of them. For the
fire to last long enough for this to occur, the flow rate would have to
be around 30 gpm. At a rate of 30 gpm, the fuel would last for about 7
hours and would produce a fire of about 60 MW. The possibility that such
a scenario could occur would be dependent on the specific construction
details of the wall, the door, and the fireproofing on the truss.
Another hypothesis that has been advanced is that the pipe was
penetrated by debris at a point near the southwest corner where there
was more damage caused by debris from the collapse of the towers. This
would have resulted in fuel oil spilling onto the 5th floor, but not
being immediately ignited. However, a major portion of the 12,000
gallons in the SSB tanks would pump out onto the 5th floor, forming a
large pool. At some point, this would have ignited and produced the
required fire. This hypothesis has the advantage of assuming a pipe
break in the area most severely impacted by the tower debris and
accounts for the long delay from the initial incident to the collapse of
WTC 7. The principal challenge is that such a fire would have more
severely exposed Truss 3. If Truss 3 had been the point of collapse
initiation, it is not expected that the first apparent sign of collapse
would be the subsidence of the east penthouse.
Evaluation of fires on the 3rd to 6th floors is complicated by the fact
that these floors were windowless with louvers, generally in a plenum
space separating any direct line of sight between the open floor space
and the louvers. None of the photographic records found so far show
fires on these floors.
Further investigation is required to determine whether the preceding
scenarios did or could have actually occurred.
Other Involved Floors Scenarios. Fire was known to have occurred on
other floors. If a fire on one of these floors involved a large
concentration of combustible material encasing several columns in the
east portion of the floor, it might have been of sufficient severity to
cause the structural members to weaken. Such fuel concentrations might
have been computer media vaults, archives and records storage, stock or
storage rooms, or other collections. It is possible that the failure of
at least two or possibly more columns on the same floor would have been
enough to cause collapse.
5.6.2 Probable Collapse
The collapse of WTC 7 appears to have initiated on the east side of the
building on the interior, as indicated by the disappearance of the east
penthouse into the building. This was followed by the disappearance of
the west penthouse, and the development of a fault or "kink" on the east
half of WTC 7 (see Figures 5-23 and 5-24).
The collapse then began at
the lower floor levels, and the building completely collapsed to the
ground. From this sequence,
it appears that the collapse initiated at
the lower levels on the inside and progressed up, as seen by the
extension of the fault from the lower levels to the top.
During the course of the day, fires may have exposed various structural
elements to high temperatures for a sufficient period of time to reduce
their strength to the point of causing collapse.
The structural elements
most likely to have initiated the observed collapse are the transfer
trusses between floors 5 to 7, located on lower floors under the east
mechanical penthouse close to the fault/kink location.
If the collapse initiated at these transfer trusses, this would explain
why the building imploded, producing a limited debris field as the
exterior walls were pulled downward. The collapse may have then spread
to the west. The building at this point may have had extensive interior
structural failures that then led to the collapse of the overall
building. The cantilever transfer girders along the north elevation, the
strong diaphragms at the 5th and 7th floors, and the seat connections
between the beams and columns at the building perimeter may have become
overloaded after the collapse of the transfer trusses and caused the
interior collapse to propagate to the whole floor and to the exterior
frame. The structural system between floors 5 and 7 appears to be
critical to the structural performance of the entire building.
An alternative scenario was considered in which the collapse started at
horizontal or inclined members. The horizontal members include truss
tension ties and the transfer girder of the T-1 truss at the east side
of the 5th floor. Inclined members spanned between the 5th and 7th
floors and were located in a two-story open mechanical room. The
horizontal haunched back span of the eastern cantilever transfer
girders, located roughly along the kink, rested on a horizontal girder
at the 7th floor supported by the T-1 transfer truss. Even if the
cantilever transfer girder had initiated the collapse sequence, the back
span failure would most likely have not caused the observed submergence
of the east mechanical penthouse.
The collapse of WTC 7 was different from that of WTC 1 and WTC 2, which
showered debris in a wide radius as their frames essentially "peeled"
outward. The collapse of WTC 7 had a small debris field as the façade
was pulled downward, suggesting an internal failure and implosion.
To confirm proposed failure
mechanisms, structural analysis and fire
modeling of fuels and anticipated temperatures and durations
to be performed. Further study of the interaction of the fire and steel,
particularly on the lower levels (i.e., 1st-12th floors)
undertaken to determine specific fuel loads, location, potential for
impact from falling debris, etc. Further research is needed into
location of storage and file room combustible materials and fuel lines,
and the probability of pumps feeding fuel to severed lines.
5.7 Observations and Findings
This office building was built over an electrical substation and a power
plant, comparable in size to that operated by a small commercial
utility. It also stored a significant amount of diesel oil and had a
structural system with numerous horizontal transfers for gravity and
The loss of the east penthouse on the videotape suggests that the
collapse event was initiated by the loss of structural integrity in one
of the transfer systems. Loss of structural integrity was likely a
result of weakening caused by fires on the 5th to 7th floors.
specifics of the fires in WTC 7 and how they caused the building to
collapse remain unknown at this time. Although the total diesel fuel on
the premises contained massive potential energy,
the best hypothesis has
only a low probability of occurrence. Further research, investigation,
and analyses are needed to resolve this issue.
The collapse of WTC 7 was different from that of WTC 1 and WTC 2. The
towers showered debris in a wide radius as their external frames
essentially "peeled" outward and fell from the top to the bottom. In
contrast, the collapse of WTC 7 had a relatively small debris field
because the façade came straight down, suggesting an internal collapse.
Review of video footage indicates that
the collapse began at the lower
floors on the east side. Studies of WTC 7 indicate that
began in the lower stories, either through failure of major load
transfer members located above an electrical substation structure or in
columns in the stories above the transfer structure. Loss of strength
due to the transfer trusses could explain why the building imploded,
with collapse initiating at an interior location. The collapse may have
then spread to the west, causing interior members to continue
collapsing. The building at this point may have had extensive interior
structural failures that then led to the collapse of the overall
building, including the cantilever transfer girders along the north
elevation, the strong diaphragms at the 5th and 7th floors, and the seat
connections between the interior beams and columns at the building
Certain issues should be explored before final conclusions are reached
and additional studies of the performance of WTC 7, and related building
performance issues should be conducted. These include the following:
Additional data should be collected to confirm the
extent of the damage to the south face of the building caused by
Determination of the specific fuel loads, especially at
the lower levels, is important to identify possible fuel supplied to
sustain the fires for a substantial duration. Areas of interest
include storage rooms, file rooms, spaces with high-density
combustible materials, and locations of fuel lines. The control and
operation of the emergency power system, including generators and
storage tanks, needs to be thoroughly understood. Specifically, the
ability of the diesel fuel pumps to continue to operate and send fuel
to the upper floors after a fuel line is severed should be confirmed.
Modeling and analysis of the interaction between the
fires and structural members are important. Specifically, the
anticipated temperatures and duration of the fires and the effects of
the fires on the structure need to be examined, with an emphasis on
the behavior of transfer systems and their connections.
Suggested mechanisms for a
progressive collapse should be studied and confirmed.
collapse of an unknown number of gravity columns brought down the
whole building must be explained.
The role of the axial capacity between the beam-column
connection and the relatively strong structural diaphragms may have
had in the progressive collapse should be explained.
The level of fire resistance and the ratio of
capacity-to-demand required for structural members and connections
deemed to be critical to the performance of the building should be
studied. The collapse of some structural members and connections may
be more detrimental to the overall performance of the building than
other structural members. The adequacy of current design provisions
for members whose failure could result in large-scale collapse should
also be studied.
Davidowitz, David (Consolidated Edison). 2002. Personal communication on the continuity of power to
WTC 7. April.
Flack and Kurtz, Inc. 2002. Oral communication providing engineering
explanation of the emergency generators and related diesel oil tanks and
distribution systems. April.
Lombardi, Francis J. (Port Authority of New York and New
Jersey). 2002. Letter concerning WTC 7 fireproofing. April 25.
Odermatt, John T. (New York City
Office of Emergency Management). 2002. Letter regarding OEM tanks at WTC
Rommel, Jennifer (New York State Department of Environmental
Conservation). 2002. Oral communication regarding a November 12, 2001,
letter about diesel oil recovery and spillage. April.
Salvarinas, John J. 1986. "Seven World Trade Center, New York,
Fabrication and Construction Aspects," Canadian Structural Engineering
Silverstein Properties. 2002. Annotated floor plans and riser diagrams
of the emergency generators and related diesel oil tanks and
distribution systems. March.