Buildings: Structures – Day of Reckoning

Contrary to what the general public assumes, structural engineers are rarely involved in the decision-making process on fire safety issues. Undergraduate and post-graduate structural engineering curricula do not usually contain any courses specifically addressing fire resistance, and most structural engineers know little about fire protection. Structural design codes do not contain any requirements with regard to the ability of structures to withstand severe heat exposure, and the usual scope of the structural consultant in a building project is to design the structure for the expected climate temperature range. The prevailing mindset in the design community is that the structural engineer designs the structure and the architect (or sometimes, the fire engineer) specifies the fire protection to comply with building code requirements for prescriptive, fire-resistance rated construction.

The above disposition appeared to be satisfactory. For many decades, multi-storey steel-framed buildings enjoyed an excellent fire performance record in North America and all around the world as they were never reported to have collapsed due to fire. There have been numerous incidents of steel framed buildings surviving severe uncontrolled long-duration fires with the complete burnout of contents on several floors.1 Large-scale fire tests, conducted in the U.S., Australia and the U.K., also clearly demonstrated that steel framed buildings can survive natural fires, even when some structural steel elements are left without any sprayed fire proofing.2

Due to their excellent long track record, multi-storey buildings were perceived by the general public, and among fire fighters, to be invulnerable to collapse during a fire. Unfortunately, these perceptions (and consequent lack of public research funding) translated into an absence of interest in fire resistance issues among structural engineers, and into very limited educational and research activity in the field.

By the early 1980s all university research programs in structural fire resistance were discontinued in North America. Much of the experimental data on structural fire resistance is 30-40 years old, and most of it is out of public access. The true complex behaviour of heated structures is poorly understood even among practising professionals, while prescriptive fire protection regulations over-rely on ad hoc tests and empirical correlations with little scientific basis.

The day that changed all

Surprise and shock swept all over the world on that September morning last year as we watched the attack on the World Trade Center. For 58 minutes the South Tower, and for 104 minutes the North Tower, struggled to survive. Acting in a manner for which they were not designed, transferring forces to the remaining structural members, and resisting the effects of raging fires, the critically damaged frames managed to save the lives of thousands. Holding on until the majority of the occupants had escaped [though tragically 2,800 did not], the towers finally gave up the struggle.

Yet, although the structures performed above and beyond the code requirements, engineers are not willing just to accept the collapse. We are driven to understand how and why, and to provide a solution. This is a trait of our profession in modern progressive societies.

Several investigative teams of engineers are working on the “Ground Zero” site to collect perishable data, record relevant witness accounts and structure the sequence of events. The American Institute of Steel Construction (AISC) is actively participating in this effort. Myriad questions have been posted that would require detailed analysis and comparative studies: Is it possible to design high-rise buildings to withstand such extreme events? Is it feasible? What if the floors had beams in lieu of trusses? What would have been the effect of having additional fire-proofing? Or no fire-proofing at all? Would other types of connections have performed better? What if the tower had a concrete core? These and many other questions are hotly debated, and many conflicting “expert” opinions have already been provided based on the available limited (sometimes, inaccurate) information. The reality, however, is that any methodical study of the building features and their performance in response to the attacks is likely to continue for several years before researchers arrive at solid conclusions.

One of the immediate lessons that could be drawn in the aftermath of these tragic events is that architects, structural and fire engineers must work more closely together when considering extreme events involving fire. Two years ago, AISC initiated a major program to have structural fire protection engineered, and not prescribed. We realized that our biggest hurdle would be convincing a broader segment of structural engineers to become involved. We had to show them that it is worthwhile to become competent in both fire and structural engineering, so as to provide a valued service to the owner.

The World Trade Center tragedy has changed the attitude of structural engineers toward fire safety. Many realize it is time to make structural design for fire an engineered process, as we do for gravity, wind and earthquake, using codified procedures and sophisticated numerical tools. AISC, together with other engineering and standard-setting bodies, is working to establish the basis for engineered fire protection. Along with promoting innovative methods in design specifications and codes, we will continue to facilitate and expand research and educational programs in this area. CCE

Farid Alfawakhiri, Ph.D., P.Eng., is senior engineer, fire design with the American Institute of Steel Construction, Chicago, Illinois.

continued on page 24

1 Dexter, R. J., and Lu, L., “The effects of a severe fire on the steel frame of an office building,” Proceedings of the North American Steel Construction Conference. Las Vegas, NV, May 2000, American Institute of Steel Construction.

2 Gewain, R. G., and Troup E. W. J., “Restrained fire resistance ratings in structural steel buildings.” 2001: Engineering Journal, 2nd Quarter, 78-89.