Part 1

World Trade Center 

Building Performance Study

Data Collection, Preliminary 

Observations, and 

Recommendations

Federal Emergency Management Agency

Federal Insurance and Mitigation Administration

Washington, D.C.

FEMA Region II

New York, New York
Purpose of the Study

 Examine the damage caused by these events, including the 

immediate effects of the aircraft impacts on each tower, the 

spread of fires following the crashes, the fire-induced reduction 

of structural strength, and the mechanism that led to the 

collapse of each tower

 Identify additional studies that should be performed 

 Collect data

 Develop an understanding of the response of each affected 

building

 Determine the effects of damage to buildings in the immediate 

vicinity of the towers from falling debris and fires

 Identify the causes of observed behavior

 Identify additional studies that should be performed












































Part 2

World Trade Center 

Building Performance Study

Data Collection, Preliminary 

Observations, and 

Recommendations

Federal Emergency Management Agency

Federal Insurance and Mitigation Administration

Washington, D.C.

FEMA Region II

New York, New York
Recommendation

 Conduct further studies to understand how WTC 3 resisted 

progressive collapse.




















Recommendations

 Conduct further studies to determine the performance of 

connections and their adequacy under real fire exposures.

 Determine the combined structural and fire properties of the 

critical structural connections so that predictions of their 

behavior under overload conditions can be made. This can be 

accomplished with a combination of thermal transfer modeling, 

structural finite element modeling (FEM), and full-scale physical 

testing. 




Truss 3




















Recommendations

 Collect additional data to confirm the extent of the damage to the south face of WTC 7 caused 

by falling debris. 

 Determine the specific fuel loads, especially at the lower levels, 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. Confirm the ability of the diesel fuel pumps to 

continue to operate and send fuel to the upper floors after a fuel line is severed.

 Examine the anticipated temperatures and duration of the fires and the effects of the fires on 

the structure, with an emphasis on the behavior of transfer systems and their connections. 

 Study and confirm suggested mechanisms for a progressive collapse. Determine how the 

collapse of an unknown number of gravity columns brought down the whole building. 

 Explain the role that the axial capacity between the beam-column connection and the 

relatively strong structural diaphragms may have had in the progressive collapse. 

 Study 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 WTC 7. The collapse 

of some structural members and connections may be more detrimental to the overall 

performance of the building than other structural members. Also study the adequacy of 

current design provisions for members whose failure could result in large-scale collapse. 











Recommendations

 Determine whether the observed damage in the column flange, and not at the beam flange, of 

the moment frames top connection plates is due to high restraint in the welds. 

 Conduct further studies as to why the bottom flange welded connection has typically failed at 

the fillet weld to beam interface and not at the fillet weld to seat plate interface.

 Examine the impact response of various moment-connected details. 

 Determine if composite construction would reduce local collapse zones. (There were no shear 

connectors to provide composite action between the floor beams and slab. Composite 

construction would have increased the capacity of the members and may have dissipated 

more of the impact energy; however, it may have also pulled a greater extent of the adjoining 

regions into the collapse zone.) 

 Conduct further studies as to whether perimeter rebar in the slabs could improve the 

structural response by providing catenary action and tensile force resistance in the slabs to 

reduce local collapse zones. 

 Determine whether the partial-strength connections permitted members to break away from 

the structure, thereby limiting the extent of damage. (If the moment connections had been 

designed for the capacity of the sections, as opposed to fully rigid partial strength based on 

design load and stiffness requirements, the building performance is likely to have been 

different.) 

 Determine whether the collapse zone would have been limited if the spandrels on the 16th, 

17th, and 18th floors had not been so grossly distorted through twisting.


















Recommendations

 Utilize the known data and conditions of the perimeter 

structures after the impact damage as a basis for calibration, 

comparison, and verification of existing software intended to 

predict such behavior, and for the development of new software 

for the prediction of the ability of structures to sustain localized 

and global overload conditions. 
Overall Recommendations

 Conduct detailed modeling of the aircraft impacts into WTC 1 and WTC 2 to provide 

understanding of the probable damage state immediately following the impacts. 

 Conduct detailed modeling of the fires and combine with structural modeling to 

develop specific failure modes likely to have occurred. 

 Conduct detailed modeling of the floor framing systems and their connections to 

understand the effects of localized overloads and failures to determine ultimate 

failure modes. Other types of common building framing should also be examined for 

these effects. 

 Conduct studies of the fire-performance of steel trusses with spray-applied fire 

protection, and with end restraint conditions similar to those present in the two 

towers.

 Conduct tests of spray-applied fireproofing materials to understand how well they 

withstand mechanical damage and to determine whether it is appropriate and 

feasible to improve their resistance to such damage.

 Conduct studies to determine, given the great size and weight of the two towers, 

whether there are feasible design and construction features that would permit such 

buildings to arrest or limit a collapse, once it began.