NIST - Collapse of the World Trade Center

In 2002, the National Institute of Standards & Technology (NIST) announced that they would lead a multi-year investigation into the collapse of the World Trade Center, as well as building #7. The investigation was mandated by the National Construction Safety Team Act passed by Congress in October 2002. The NIST investigation was to look at structural fire protection, life safety, and engineering practice. NIST would build upon the initial work done by FEMA and the American Society of Civil Engineers (ASCE). The investigation was expected result in "lessons learned" and suggest revisions to building codes, standards, and practices. The investigation would be extensive, ranging from heat tests on structural steel members, computer simulation and modeling of smoke and fire movement, to interviews with survivors.

The NIST report, released in September 2005, focuses on how the impact of the aircrafts and subsequent fires led to the collapse of the World Trade Center twin towers, with a related ongoing investigation into the collapse of 7 World Trade Center. NIST determined that the impact of the aircraft caused fireproofing to be knocked off the structural steel, making the columns more vulnerable to the heat from the resulting fires. The heat drastically reduced the structural strength of the steel; That factor, combined with the fact that 33 external columns and 11 core columns were severed and others damaged significantly, means that the remaining columns took on substantial added load to support the building structure above the impact zone. The added load on the remaining, weakened steel columns and floor structure, eventually caused the structure to fail.

Summary

Participants

The NIST investigation was led by S. Shyam Sunder and twelve other experts, with 77 NIST technical staff, 5 NIST experts and consultants, and 51 support staff. NIST also contracted with Applied Research Associates, Inc., Baseline, Inc., Computer Aided Engineeirng Associates, Inc., DataSource, Inc., GeoStats, Inc., Gilsanz Murray Steficek LLP, Hughes Associates, Inc., Isolatek International, Inc., John Jay College, Leslie E. Robertson Associates, R.L.L.P., National Fire Protection Association, National Research Council - Canada, NuStats, Inc., Rolf Jensen & Associates, Inc., Rosenwasser/Grossman Consulting Engineers, P.C., Science Applications International Corporation, Simpson Gumpertz & Heger Inc., S.K. Ghosh Associates, Inc., Skidmore, Owings & Merrill, LLP, Teng & Associates, Inc., Underwriters Laboratories, Inc., University at Buffalo - The State University of New York, University of Chicago Survey Lab, University of Colorado, University of Michigan, Wiss, and Janney, Elstner Associates, Inc.

NIST also consulted with American Airlines, Boeing, Blandford Land Development Corporation, Carr Futures, Inc., City of New York Fire Department, the Federal Bureau of Investigation, Hugo Neu Schnitzer East, Laclede Steel, Marsh & McLennan Companies, Metal Management Northeast, Inc., National Commission on Terrorist Attacks Upon the United States, New York City Law Department, New York City Police Department, the Port Authority of New York and New Jersey, Siemens, Silverstein Properties, Simpson, Thacher & Bartlett LLP, Structural Engineers Association of New York, Wachtell, Lipton, Rosen & Katz, United Airlines, and Williams & Connolly LLP.

Process

Tens of thousands of pages of documents were reviewed, over a thousand people interviewed who were on scene or involved in the design and construction of the World Trade Center, 236 pieces of WTC steel were analyzed, and computer simulations done to model impact of the aircraft on the building structure and subsequent fires. Over 7,000 photographs and 7,000 segments of video (exceeding 150 hours) were also examined to determine the condition of the buildings and progression of the fires. The video and photographs helped investigators to determine the structural damage following impact of the aircraft and progression of the fires. Baseline performance of the structures was also established, as expected under normal design loads and conditions. The effect of the fires on the structural steel was also simulated and analyzed, and events leading to the initiation of the collapse.

The analysis of how the aircraft impacts effected the structures is similar to what is done when analyzing a structure, in order to carry out seismic rehabilitation and retrofitting. Such analysis required establishing knowledge of as-built conditions, including details of the structural design and components, testing of building materials and components, and estimating material degradation. The next step is to prepare a 3D model of the structure, estimate design lateral force, and apply that force on the model to evaluate how the structure responds. It is also necessary to determine the capacity of specific structural members, taking into account their cross-sectional geometry and material properties, and their Demand/Capacity ratios. This analysis allows engineers to identify deficient structural members, or deficiency in lateral stiffness of the structure.^[1]

Report

• NIST NCSTAR 1-1: Design, Construction, and Maintenance of Structural and Life Safety Systems
• NIST NCSTAR 1-2: Baseline Structural Performance and Aircraft Impact Damage Analysis of the World Trade Center Towers
• NIST NCSTAR 1-3: Mechanical and Metallurgical Analysis of Structural Steel
• NIST NCSTAR 1-4: Active Fire Protection Systems
• NIST NCSTAR 1-5: Reconstruction of the Fires in the World Trade Center Towers
• NIST NCSTAR 1-6: Structural Fire Response and Probable Collapse Sequence of the World Trade Center Towers
• NIST NCSTAR 1-7: Occupant Behavior, Egress, and Emergency Communication
• NIST NCSTAR 1-8: The Emergency Response Operations

Structural models

The first part of the investigation involved developing reference structural models of the World Trade Center towers. These were used to establish the baseline performance of each building, under gravity and wind loads. The models were also used to estimate the damage to the buildings caused by aircraft impact and establish the initial conditions for the fire dynamics modeling and the thermal-structural response and collapse initiation analysis. Structural models of the World Trade Center were developed using SAP2000 by Leslie E. Robertson Associates (LERA), under contract to NIST. The models were reviewed by NIST, as well as independently by Skidmore, Owings, and Merrill (SOM).

Global models

Floor truss and perimeter wall structure, under construction

The structural models included two global models (one of each tower) of the major structural components and systems of the towers. Sub-models that were part of the global models included the exterior walls (exterior wall, foundation to floor 4; exterior wall trees (floors 4 to 9); exterior wall, floors 9 to 106; exterior wall, floors 107 to 110), core columns, and hat truss. These sub-models were assembled into a unified model. Rigid and flexible diaphragms represented the floor systems, boundary conditions, gravity and wind loads, and masses were added to the unified model.

Floor models

The structural models developed also included floor models of a typical truss-framed floor and typical beam-framed floor. Truss-framed floor structures were used on the tenant floors, while mechanical floors (7, 41, 75, and 108) and near mechanical floors (9, 43, 77, 107, 110, and roof) had typical beam-framed floors. The truss-frame floor models included all primary structural members of the floor system, including the primary trusses, bridging trusses, spandrel beams, columns above and below the floor level, concrete slabs, dampers, strap anchors, and beams in the core. The model of the typical beam-framed floor contained all primary structural members of the floor system, including the primary composite beams, horizontal trusses, spandrel beams, columns above and below the floor level, concrete slab, dampers, and beams in the core.

Floor trusses connected to the viscoelastic damper with bolts, which were in turn connected with bolts to the perimeter wall structure. The floor truss structure was also connected with bolts to the truss seat angle, and welded to the gusset plate.

Baseline performance

Demand/capacity ratios (DCR)

A demand/capacity ratio less than 1.0 is considered acceptable when evaluating the "health" of a structure and its structural components. The ratio is an overall measure of the building's resistance to shaking (due to an earthquake) or other lateral forces. In the design of the World Trade Center, DCRs were estimated for the "design load case", along with a lower estimate for the "state-of-the-practice" case. For both estimates, a fraction of components had DCRs that exceeded 1.0 - these were mainly found at the exterior wall columns around the corners, where the hat truss connected to the exterior walls, and below floor 9. A few core columns also had DCRs that exceeded 1.0. While ideal for DCRs to be less than 1.0, the performance of the towers on 9/11 was most likely not affected by the fraction of members for which the demand exceeded capacity due to the following:

• the inherent factor of safety in the allowable stress design method
• the load redistribution capability of ductile steel structures
• on September 11, the towers were subjected to in-service live loads (a fraction of the design live loads) and minimal wind loads.

The floor truss models were subjected to gravity loads in order to determine DCRs. Under original WTC design loads, all floor trusses had DCRs less than 1.14, while floor truss components were leas than 1.0 for 99.4 percent of components. Inside the core, DCRs for all floor beams were less than 1.08, and more than 99 percent of the floor beams had a DCR of less than 1.0.

Damage from aircraft impact

Damage to the exterior of the structures due to the aircraft impacts could be identified from photograph and video evidence. However, no visible information could be obtained for the extent of damage to the interior of the towers, including the structural system (floors and core columns), partition walls, and interior building contents. This information was required for fire dynamics simulations and post-impact structural analysis. As well, information on dispersion of jet fuel and location of combustible aircraft debris, along with extent of damage to the fireproofing on structural steel were crucial to the thermal and structural analysis. Aircraft impact damage analyses were done to estimate the tower damage.

Tower and aircraft impact models

Highly detailed tower and aircraft impact models were required to estimate the damage caused by the plane impacts. The level of detail was such that SAP2000 is unable to handle a model of this size and complexity. Instead, the modeling was done using LS-DYNA, which is a non-linear finite element analysis package. A total of three models was developed for this analysis: one for the impact region in each tower (floors 92-100 of the North Tower and floors 77-85 of the South Tower), along with a comprehensive model of a Boeing 767 aircraft.

The Piecewise Linear Plasticity model in LS-DYNA was used to model the nonlinear rate-dependent deformation and failure of the steel structures. Model parameters for each grade of steel were based on engineering stress-strain data obtained through mechanical and metallurgical analysis of structural steels. Finite element analysis of the test specimens were conducted with a fine and a medium mesh to capture the nonlinear material behavior up to failure.

Recommendations

The NIST investigation resulted in 30 recommendations for:^[2]

• Increased structural integrity
• Enhanced fire resistance of structures
• New methods for fire resistant design of structures
• Improved active fire protection
• Improved building evacuation
• Improved emergency response
• Improved procedures and practices
• Education and training

Review

The International Code Council (ICC) commended "NIST and its subcontractors on the quality and thoroughness of the reports. The level of effort, complexity of the issues, organization and presentation, thoroughness and professionalism associated with and exhibited by the reports is commendable and well represents to others in the U.S. and the world the technical and management capabilities and expertise of the U.S. public and private sectors.^[3] The ICC has established an Ad Hoc Committee on Terrorism Resistant Buildings, consisting of code officials, engineers, architects, and other building professionals to review the NIST report and recommendations.^[4]

See also

• World Trade Center Building Performance Study - FEMA

References

Other references

• NIST and the World Trade Center
• SAP2000 models - computer models of the WTC, from NIST