Re-evaluation of the World Trade Centre 7 collapse

Materials World magazine
,
29 Oct 2019

A four-year American study looking at the collapse of the World Trade Centre Building 7 has been conducted using software tools and simulations to re-evaluate its architectural failures. Shardell Joseph investigates.

The 47-storey World Trade Centre Building 7 (WTC7) was the third structure to collapse after the terrorist attack where two aeroplanes flew into the Twin Towers in New York, USA, on 11 September 2001. Last month, the study, A structural re-evaluation of the collapse of World Trade Center 7, used a variety of modelling techniques, simulations and software tools to re-examine the structural integrity of the building.

Architects and Engineers for 9/11 Truth – a non-profit organisation representing more than 3,000 architects and engineers with the aim to compile research and evidence regarding the destruction of the three World Trade Centre towers – funded the study. The organisation collaborated with researchers from the University of Alaska Fairbanks (UAF) who conducted the study, and found some key differences to the initial USA government’s National Institute of Standards and Technology (NIST) investigation.

The study stated that, ‘According to NIST – an agency of the USA Department of Commerce that investigated the three building failures on 9/11 – the collapse of WTC7 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 WTC7 and assembled a body of evidence that raises questions about the validity of NIST’s conclusions.’

Architects and Engineers for 9/11 Truth Director of Strategy, Ted Walter, told Materials World that the team thought the next step was to use computer modelling ‘to really look at it from a different vantage point’. Walter added that the researchers tried to reproduce NIST’s results of the collapse, following the scrutiny from engineers and architects since the incident.

‘We started looking for researchers, engineers and professors who were interested in taking on this project, and we found Dr J Leroy Hulsey at the University of Alaska Fairbanks. His goal was very simple, which was to model this building as faithfully as you could possibly do it, and not be swayed by any arguments from either side of this issue – just model the building based on the best data that we have, about how the building was constructed,’ said Walter.

‘It was a pretty straightforward scientific process. What they found was what you would anticipate, which is that even in the worst case, scenario, fire was not going to create the kind of thermal movement or displacement of the structural members that could trigger the series of failures that NIST alleges took place.’

The aim of the study was to determine three different factors – to look at the structural response of WTC7 to fire loads that may have occurred, to rule out scenarios that could have caused the collapse, and identify types of failures and their locations that may have caused the collapse to occur. The primary finding from the study was that fire did not cause the collapse of WTC7, along with other factors.

‘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,’ the report read.

Examining mechanical and thermal properties

One part of the process plan was to examine the mechanical and thermal properties of the building materials. This included the evaluation of steel’s mechanical properties at moderately high temperatures, the thermal properties of steel, the probable aggregate type for concrete floors at the building site, and the thermal properties of concrete, once the aggregate type was established.

This examination of building materials was taken further by analysing the influence of structural steel installation on the structural response, which included:

  • Connections for beams, girders and columns
  • Thermal expansion of the floor system
  • A non-composite floor system with no interface friction between the concrete floor and supporting members
  • A non-composite floor system with friction between concrete floor and supporting members
  • A partially composite floor system
  • A composite floor system, and
  • Effectiveness of the installed shear connectors at girders and beams, as well as the resistance of them without shear connectors when subjected to a fire load.
  • In regards to studying the steel within the building’s framework, Hulsey said, ‘We spent time to scientifically evaluate the material properties for the steel members and we carefully examined their mechanical properties.’ This established a confidence level that the simulated elements closely approximated the actual steel members.

As for the aggregate concrete elements, ‘consider that as a building experiences an increase in temperature due to fire, the materials will expand and the magnitude of the expansion is a function of the thermal properties of the material,’ UAF Department of Civil and Environmental Engineering Professor, J Leroy Hulsey, told Materials World.

‘So concrete expansion will be different than steel and if these two materials are attached together – then the overall behaviour gets complicated and will cause thermal stresses which can be significant. We accounted for these phenomena. It is important to evaluate performance based on an accurate representation of the geometry and the material properties of both the steel members and the concrete floor slab.

Simulating and modelling

The primary manner in which the researchers analysed the collapse was through testing hypothetical scenarios to determine what type of local failures could and could not produce the key features described in other investigations.

Three approaches were used to examine the structural response of WTC7 to the conditions that may have occurred. These were modelling the structural framing and simulating the local structural response to fire loading, evaluating NIST’s collapse hypothesis against UAF’s own simulation, and the third was simulating various scenarios within the overall structure to determine the types of local failures and their locations.

To do this, the research team used three different software tools. First, AUTOCAD was used to simulate a virtual geometry for each floor and generate a three-dimensional depiction of the building, based on drawings. This data was then imported into ABAQUS to perform fire analysis, structural response to windblasts and simulation of a progressive structural collapse, modeling the floor framing at the 12th and 13th floors separately in each programme. Data was also imported into SAP2000 – a thermal deformation analysis tool – which was later used to model all the floors that the team had floor plans for.

The two-storey modelling results for both ABAQUS and SAP2000 were compared by applying horizontal structural heat loads. The team then used ABAQUS in order to develop nonlinear models of the connections. Using these software tools meant the fire damage to the floor framing on floors 12 and 13 was evaluated. Wire elements for beams and grinders and shell elements for concrete slabs were then used to simulate the interaction between the concrete floors, beams and girders.

‘In total, the 47-storey SAP2000 model had 39,978 joints, 44,507 frame elements, and 7,832 area elements. The number of SAP2000 frame elements for floors 12 and 13 alone was 2,026 and the number of area ones was 4,390. In addition to using wire elements for the steel framing, we examined the floor expansion characteristics and response using ABAQUS solid elements,’ the report notes.

‘Once we were sure that our 3D model of the building was accurate, the steel member sections, material properties, orientation, and connections that were used in the building were accurately simulated to evaluate their response to load during a fire,’ said Hulsey.

‘This was thoroughly examined as the members and floor system expanded as the system heated up. It was our findings that response due to fire did not cause the structure to collapse.’

The outcomes

After four years of hypothetical simulation and modelling analysis, the research team came to many different conclusions regarding the architectural structure of the building, the materials used and their response to fire loading, and the collapse itself. Hulsey rounded up the main conclusions taken from the study, stating that fire did not cause the building destruction, the structure did not suffer a progressive collapse, that it was global and not a progressive failure. Essentially, a failure sequence of local failures lead to a large-scale collapse.

‘It is our conclusion that the collapse of WTC7 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,’ the report read.

‘Despite simulating a number of hypothetical scenarios, we were unable to identify any progressive sequence of failures that could have taken place on September 11, 2001, and caused a total collapse of the building, let alone the observed straight-down collapse with approximately 2.5 seconds of free fall and minimal differential movement of the exterior.’

Read the full draft report here: bit.ly/2ly3aPZ, and the NIST final report on the collapse of WTC7 here: bit.ly/2WejpQr.