3 thoughts on “A Structural Reevaluation of the Collapse of World Trade Center 7

  1. ABSTRACT
    A Structural Reevaluation of the Collapse of World Trade Center 7
    By
    J. Leroy Hulsey, Ph.D., P.E., S.E.,
    University of Alaska Fairbanks
    And
    Zhili Quan, Ph.D., Bridge Engineer
    South Carolina Department of Transportation
    And
    Feng Xiao, Ph.D., Associate Professor
    Nanjing University of Science and Technology
    Department of Civil Engineering
    This report presents the findings and conclusions of a four-year study of the collapse of
    World Trade Center Building 7 (WTC 7) — a 47-story building that suffered a total collapse at
    5:20 PM on September 11, 2001, following the horrible events of that morning.
    The objective of the study was threefold: (1) Examine the structural response of WTC 7
    to fire loads that may have occurred on September 11, 2001; (2) Rule out scenarios that could not
    have caused the observed collapse; and (3) Identify types of failures and their locations that may
    have caused the total collapse to occur as observed.
    The UAF research team utilized three approaches for examining the structural response
    of WTC 7 to the conditions that may have occurred on September 11, 2001. First, we simulated
    the local structural response to fire loading that may have occurred below Floor 13, where most
    of the fires in WTC 7 are reported to have occurred. Second, we supplemented our own
    simulation by examining the collapse initiation hypothesis developed by the National Institute of
    Standards and Technology (NIST). Third, we simulated several scenarios within the overall
    structural system in order to determine what types of local failures and their locations may have
    caused the total collapse to occur as observed.
    The principal conclusion of our study is that fire did not cause the collapse of WTC 7 on
    9/11, contrary to the conclusions of NIST and private engineering firms that studied the collapse.
    The secondary conclusion of our study is that the collapse of WTC 7 was a global failure
    involving the near-simultaneous failure of every column in the building.
    All input data, results data, and simulations that were used or generated during this study
    are available at
    http://ine.uaf.edu/wtc7.

    EXTERNAL PEER REVIEWERS
    Gregory Szuladzinski, Ph.D
    Chartered Consulting Engineer
    Analytical Service Company
    Robert Korol, Ph.D
    Emeritus Professor of Civil Engineering
    McMaster University

    EXECUTIVE SUMMARY
    This report presents the findings and conclusions of a four-year study of the collapse of
    World Trade Center Building 7 (WTC 7) — a 47-story building that suffered a total collapse at
    5:20 PM on September 11, 2001, following the horrible events of that morning. Figure E.1
    shows the layout of WTC 7 in relation to the World Trade Center complex prior to September
    11, 2001. Figure E.2 shows the structural layout of Floor 13. Figure E.3 shows the finite element
    model of WTC 7.
    This study was conducted by a three-person team of researchers at the University of
    Alaska Fairbanks (UAF) Department of Civil and Environmental Engineering with funding
    provided by Architects & Engineers for 9/11 Truth, Inc., a 501(c)(3) nonprofit organization
    whose purpose is to conduct research and educate the public about the World Trade Center
    building collapses on 9/11.
    According to the National Institute of Standards and Technology (NIST) — an agency of
    the U.S. Department of Commerce that investigated the three building failures on 9/11 — the
    collapse of WTC 7 was the first known instance of the total collapse of a tall building primarily
    due to fires. However, many independent researchers have studied the collapse of WTC 7 and
    assembled a body of evidence that raises questions about the validity of NIST’s conclusions.
    The objective of this study, therefore, was threefold: (1) Examine the structural response
    of WTC 7 to fire loads that may have occurred on September 11, 2001; (2) Rule out scenarios
    that could not have caused the observed collapse; and (3) Identify types of failures and their
    locations that may have caused the total collapse to occur as observed.
    The UAF research team utilized three approaches for examining the structural response
    of WTC 7 to the conditions that may have occurred on September 11, 2001.
    First, we simulated the local structural response to fire loading that may have occurred
    below Floor 13, where most of the fires in WTC 7 are reported to have occurred. Second, we
    supplemented our own simulation by examining the collapse initiation hypothesis developed by
    NIST. We also reviewed the collapse initiation hypotheses advanced by private engineering
    firms whose studies were commissioned as part of litigation related to the collapse of WTC 7.
    Third, we simulated several scenarios within the overall structural system in order to determine
    what types of local failures and their locations may have caused the total collapse to occur as
    observed. Before conducting a thorough and detailed evaluation of building response due to fire
    and other issues, we examined the building condition following failure of the World Trade
    Center Buildings 1 and 2. Some debris impact damage is reported to have occurred at the lower
    southwest corner of the building, which we accounted for in simulating the building response.

    Fire Did Not Cause the Collapse of WTC 7
    The principal conclusion of our study is that fire did not cause the collapse of WTC 7 on
    9/11, contrary to the conclusions of NIST and private engineering firms that studied the collapse.
    This conclusion is based upon a number of findings from our different analyses.
    Together, they show that fires could not have caused weakening or displacement of structural
    members capable of initiating any of the hypothetical local failures alleged to have triggered the
    total collapse of the building, nor could any local failures, even if they had occurred, have
    triggered a sequence of failures that would have resulted in the observed total collapse.
    Near-Simultaneous Failure of Every Column Explains the Collapse
    The secondary conclusion of our study is that the collapse of WTC 7 was a global failure
    involving the near-simultaneous failure of every column in the building.
    This conclusion is based primarily upon the finding that the simultaneous failure of all
    core columns over 8 stories followed 1.3 seconds later by the simultaneous failure of all exterior
    columns over 8 stories produces almost exactly the behavior observed in videos of the collapse,
    whereas no other sequence of failures that we simulated produced the observed behavior. We
    cannot completely rule out the possibility that an alternative scenario may have caused the
    observed collapse; however, the near-simultaneous failure of every column is the only scenario
    we identified that was capable of producing the observed behavior.
    Key Findings Upon Which the UAF Team’s Conclusions Are Based
    Approach 1 Findings
    • During our nonlinear connection study (Section 2.1.3.2), we discovered that NIST overestimated the rigidity of the outside frame by not modeling its connections, essentially
    treating the exterior steel framing as thermally fixed, which caused all thermally-induced
    floor expansion to move away from the exterior. The exterior steel framing was actually
    flexible, while the stiffest area resistant to thermal movements, i.e., the point of zero
    thermal movement, was near the elevator shafts.
    • Therefore, during our analysis of WTC 7’s response to fire loading (Section 2.6), we
    found the overall thermal movements at the A2001 base plate support near Column 79
    were not sufficient to displace girder A2001 to the point that it walked off its seat (the
    initiating failure alleged by NIST). Whereas NIST asserted that the differential westward
    displacement of girder A2001 relative to Column 79 was 5.5 inches and later revised its
    calculation to 6.25 inches, we found that the westward displacement of girder A2001
    relative to Column 79 would have been less than 1 inch under the fire conditions reported
    by NIST.

    Approach 2 Findings
    Under our second approach, we used a solid element model to evaluate the validity of
    NIST’s collapse initiation hypothesis, introducing a number of assumptions made by NIST that
    we considered to be invalid or, at best, questionable (Section 3.1). These assumptions included
    assuming the east exterior wall to be rigid and thermally fixed, assuming shear studs on several
    beams were broken due to differential thermal movement, assuming no shear studs were installed
    on girder A2001, and assuming that the bolts fastening girder A2001 to its seats at Columns 44
    and 79 were broken (Section 3.1.1). Allowing for these overly generous assumptions, we found
    the following:
    • When girder A2001 is heated to the temperatures assumed by NIST, it expands such that
    it becomes trapped behind the side plate on the western side of Column 79 as it is pushed
    to the west by thermally expanding floor beams. This prevents the girder’s web from
    traveling beyond the bearing seat, thus preventing the girder from walking off its seat
    (Section 3.2.1).
    • NIST, by its own admission, did not include the partial height web stiffeners known to be
    on girder A2001. In addition to stiffening the web, these stiffeners significantly increase
    the bending resistance of the flange. In a subsequent analysis where we removed the side
    plate described in the previous paragraph in order to allow for further westward travel of
    girder A2001, we found that the stresses in the girder flange and stiffener would not be
    sufficient to cause the flange to fail, thus preventing the girder from walking off its seat
    (Section 3.2.2).
    • In a preliminary collapse initiation hypothesis, NIST posited that beam G3005 buckled
    because its thermal expansion was restrained by girder A2001. We found that this can
    happen only when the three lateral support beams S3007, G3007, and K3007 spanning
    from beam G3005 to the north exterior wall are not included in the model. While these
    short beams are observed in some of the figures in the NIST report, they are missing from
    the model(s) used in the thermal and structural analysis shown in the report (Section
    3.2.3).
    Separate from the NIST investigation, two studies of WTC 7’s collapse were
    commissioned by opposing sides in the lawsuit “Aegis Insurance Services, Inc. v. 7 World Trade
    Center Company, L.P.” Experts working in connection with engineering firms Ove Arup &
    Partners (Arup) and Guy Nordenson and Associates (Nordenson) were retained by the plaintiffs.
    The engineering firm Weidlinger Associates Inc. (Weidlinger) was retained by the defendants.
    After evaluating NIST’s collapse initiation hypothesis, we reviewed the Arup, Nordenson, and
    Weidlinger reports and found the following:
    • Arup’s finite element analysis corroborates our finding that girder A2001 would become
    trapped behind the western side plate of Column 79. However, Arup’s analysis then goes
    on to contend that the five beams to the east of girder A2001 were heated enough to sag
    and pull the girder to the east and off its seat. Putting aside whether this initiating
    mechanism is valid, we found that Nordenson incorrectly calculated the impact force of
    the falling girder by considering it as a point load, thus implying an infinite stiffness and
    no deflection. Calculating the impact force correctly, we found that it is less than 10% of
    the 632,000 lb. force required to shear the girder bearing seat support welds at Floor 12.
    Therefore, the northeast corner of Floor 12 would not have collapsed if the Floor 13
    girder came off its seat at Column 79, and a cascade of floor failures would not ensue.
    • The Weidlinger report was prepared as a rebuttal to the Arup and Nordenson reports.
    Among its points of rebuttal, it corroborates our finding that the falling Floor 13 beam
    and girder assembly could not break through Floor 12. The Weidlinger report contends
    instead that Floors 9 and 10 were simultaneously heated to between 750° and 800°C in
    the exact same area of each floor, eventually causing those floors to fail and triggering a
    cascade of floor failures down to Floor 5. However, the details of the thermal analysis are
    not shown in the Weidlinger report, and the thermal analysis has not been made public. It
    is important to understand that steel structural members reaching temperatures of 750°C
    due to office fires can be considered extraordinary. Without any analysis provided to
    substantiate such temperatures, Weidlinger’s collapse initiation hypothesis must be
    viewed skeptically and can be assumed to have a very low probability of occurrence
    (Section 3.4.1).

    Approach 3 Findings
    Under our third approach, we simulated a number of hypothetical scenarios in order to
    determine what types of local failures and their locations may have caused the total collapse to
    occur as observed. Based upon a series of analyses, we found the following:
    • Columns 79, 80, and 81 did not fail at the lower floors of the building, as asserted by
    NIST. In order to allow for the observed collapse of the east penthouse approximately 7
    seconds prior to the collapse of the rest of the structure, these columns needed to have
    failed at the upper floors of the building all the way to the penthouse. Yet there were no
    documented fires above Floor 30. Therefore, fire did not cause the collapse of Columns
    79, 80, and 81 nor the collapse of the east penthouse (Section 4.3).
    • The hypothetical failure of Columns 79, 80, and 81 — the three easternmost core
    columns — would not trigger a horizontal progression of core column failures. Therefore,
    the hypotheses of NIST, Arup/Nordenson, and Weidlinger that the buckling of Column
    79 could trigger a progressive collapse of the entire building are invalid, and the collapse
    of Columns 79, 80, and 81 high in the building was a separate and distinct event (Section
    4.5).
    • Even if we assume the failure of Columns 79, 80, and 81 could lead to the failure of the
    next row of core columns, the hypothetical failure of Columns 76 to 81 would overload
    the exterior columns around the southeast of the building, rather than overloading the
    next row of core columns to the west, which would result in the building tipping to the
    southeast and not in a straight-down collapse (Section 4.5).
    • The simultaneous failure of all core columns over 8 stories followed 1.3 seconds later by
    the simultaneous failure of all exterior columns over 8 stories produces almost exactly the
    behavior observed in videos of the collapse. The collapse could have started at Floor 16
    and below and produced the same behavior (Section 4.6).
    It is our conclusion based upon these findings that the collapse of WTC 7 was a global
    failure involving the near-simultaneous failure of all columns in the building and not a
    progressive collapse involving the sequential failure of columns throughout the building
    Figure E.3 Finite Element Model of WTC 7 in SAP2000, as viewed from the south

    \\][//

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