May 15, 2002—A report conducted following the September 11, 2001 attacks on New York City’s World Trade Center (WTC), by the Federal Emergency Management Agency (FEMA), engineering agencies, and others, concluded that the buildings impacted by the aircraft could have remained standing in their damaged states if the severe fires had not also broken out. The heat produced by this burning jet fuel was apparently not sufficient in itself to cause the structural collapses. However, as the burning jet fuel spread, it ignited much of the buildings’ contents, causing simultaneous fires across several floors, the heat output of which was comparable to the power produced by a large commercial power generating station. The resulting damage was sufficient to induce the collapse of both structures.
The study did not reveal any substandard structural features, and, in fact, many structural and fire protection features of the design and construction were superior to the minimum code requirements.
Several building design features identified as key to the buildings’ ability to remain standing as long as they did and to allow the evacuation of most building occupants included: the robustness and redundancy of the steel framing system; adequate egress stairways that were well marked and lighted; and conscientious implementation of emergency exiting training programs for building tenants.
The engineers also identified several design features that should undergo more detailed evaluation, in order to understand their contribution to the performance of these buildings and how they may perform in other buildings, including:
- the type of steel floor truss system present in these buildings and their structural robustness and redundancy when compared to other structural systems;
- use of impact-resistant enclosures around egress paths;
- resistance of passive fire protection to blasts and impacts in buildings designed to provide resistance to such hazards; and
- grouping emergency egress stairways in the central building core, as opposed to dispersing them throughout the structure.
Although the attacks on the World Trade Center are a reason to question design philosophies, the evaluation team noted that it may not be technically or financially feasible to design all structures to withstand the effects of impacts by rapidly moving aircraft, and the ensuing fires, without collapse. They also believe there are insufficient data to determine whether there is a reasonable threat of attacks on specific buildings to recommend inclusion of such requirements in building codes.
The engineers recommend the following points to be considered for buildings that are being evaluated or designed for extreme events:
- Structural framing systems need redundancy and/or robustness, so that alternative paths or additional capacity are available for transmitting loads when building damage occurs.
- Fireproofing needs to adhere under impact and fire conditions that deform steel members, so that the coatings remain on the steel and provide the intended protection.
- Connection performance under impact loads and during fire loads needs to be analytically understood and quantified for improved design capabilities and performance as critical components in structural frames.
- Fire protection ratings that include the use of sprinklers in buildings require a reliable and redundant water supply. If the water supply is interrupted, the assumed fire protection is greatly reduced.
- Egress systems currently in use should be evaluated for redundancy and robustness in providing egress when building damage occurs, including the issues of transfer floors, stair spacing and locations, and stairwell enclosure impact resistance.
- Fire protection ratings and safety factors for structural transfer systems should be evaluated for their adequacy relative to the role of transfer systems in building stability.
The team also encouraged an improved level of interaction between structural and fire protection engineers. For fireproofing recommendations and the complete executive summary, visit FEMA.
