Not infrequently we are asked about the impact of the September 11, 2001, terrorist attacks, and the consequent collapse of three high rise structures at the World Trade Center, on the construction industry in the United States. Our response has been that those horrible events placed the same duty of inspection and introspection upon America's engineering and construction communities that engineers and contractors have faced in the wake of structural failure since the earliest days of construction in the ancient world. This approach to the physical world, and application of the laws of physics in that world, is one of the lessons that engineering students learn in university laboratories.
As a community of professionals first we inspect to determine as best we are able exactly what happened; then we introspect. We assess the risk potential of the event that led to the failure within the context of what risks are considered acceptable; we determine what could have been done to avoid the failure given the risk potential -- and we plan what can be done in the future to prevent or minimize the risk potential of similar failure.
This is exactly the manner in which the 9/11 World Trade Center collapse was addressed. First, two federal agencies, the Federal Emergency Management Administration (FEMA) and the National Institute of Standards & Testing (NIST) conducted completely separate investigations of the collapse of Buildings 1, 2, & 7.
In layperson's terms, both agencies found the engineering mechanics of the collapse of Buildings 1 & 2 (the towers) to be the same. The physical impact of the two airplanes and the breach of primary structural elements on the building perimeter at the point of impact did not cause the collapse. The collapse was caused by the fact that, inside the structures, the physical impact stripped protective fire insulation from secondary structural elements that supported the building's floor systems and provided lateral support for the building's remaining primary structural columns.
Jet fuel carried by the airplanes combined with combustible materials inside the buildings at the point of impact to create the inferno many of us watched in disbelief on live TV. The fire caused the floor elements, stripped of their insulation, to sag, placing unanticipated loading forces on the floor system and the remaining primary structural columns. Global instability followed as the floors failed, progressively, one after the other. As noted above, the floor systems were designed to provide lateral stability to the primary structural columns. When this lateral stability was taken away the columns began to buckle and the towers collapsed.
Make a "T" out of your hands like a football referee. Press your upper hand down forcefully on your lower hand. Watch your knuckles bend outward. This is what happened at the World Trade Center towers when the weight of the upper towers bore down on columns that had lost their lateral support system.
The vast majority of building occupants who lost their lives in the two World Trade Center towers on September 11, 2001, were above the point of impact and unable to gain access to fire exits because all but one stairway in the two towers were cut by the damage from the jet impacts. In the remaining stairway, many occupants found it to be impenetrable because of debris from the impact and smoke from the fires.
This was the inspection. Now what about the introspection?
Buildings in the United States are designed and constructed under standards set by three separate entities that approach the process from three separate perspectives.
These separate entities are:
- The design and construction professionals who actually undertake the work. Their standards are set by their professional responsibility and the contracts they execute for individual projects.
- The owners or developers of the building being designed and constructed. Their standards are established by the projected use of the building, the long term utility of the building for that and future uses, and the degree of risk of future damage to the building that they perceive; and
- The agencies that establish and enforce local building and fire codes.
Building and Fire Code regulation in the United States is a complex subject. The office of these codes is not to completely prevent fires and structural failure under all circumstances. The office of these codes is to protect human life -- to set a common denominator of sorts by category of structures and building uses that will allow human beings sufficient time to evacuate the building prior to structural failure or consumption by fire. Physical design and construction requirements beyond the public safety mandates of the building and fire codes are decisions to be reached, sometimes jointly, by the code official, building owner, designer, and construction contractor.
Building and Fire Codes are enacted by local jurisdictions, normally utilizing standards developed by national code bodies such as the International Code Council. This body and the many standards developers that provide reference documents for its Code, acted upon recommendations for modifications to building and fire codes for high rise structures formulated by NIST as part of its pofst-9/11 investigation. After deliberation, these national code bodies enacted revised regulations for the design and construction of high rise structures that address three generic areas of concern.
Again in layperson terms, the three modifications to high rise building and fire codes involve:
- Revisions to the size, location, and physical attributes of emergency evacuation stairways.
- Revisions to the manner in which fire protection is specified and applied.
- Revisions to the requirements for connecting floor systems to primary structural columns and providing lateral support for primary structural columns.
Owners and developers of buildings (both public and private) that are considered to be at risk of potential terrorist attack have added significant protective systems beyond the requirements of the building and fire codes to minimize the exposure of building occupants.
These advances in design theory and construction practice, and parallel advances in seismic design theory over roughly the same period of time will be the subject of future articles written by Stites & Harbison attorneys.