September 26, 2002

Building design — post 9/11

  • There are techniques that can increase the capacity of structures to resist certain kinds of terrorist attacks, but there is no reliable way to design for the impact of a large airplane.
    Skilling Ward Magnusson Bark

    The horrible events of Sept. 11 have aroused many emotions and created many questions in the minds of the public, the media, political leaders, and design professionals. For officials charged with developing building codes and designers charged with creating buildings the question is very specific: Is there something wrong with our codes and can we change them so that this doesn’t happen again?

    However, even though this appears to be a specific question, it is not. What does “this” mean? Does it mean making buildings more resistant to a terrorist attack, or does it mean giving buildings the ability to resist a direct airplane hit? These are two very different questions.

    To resist the force generated by a direct 767 hit, today’s buildings would need to be 100 times stronger than dictated by code.

    Can we make buildings more resistant to terrorist attack?

    All building codes use requirements based on performance objectives for specific hazards. The primary environmental hazards are gravity, wind and earthquake. When a building is designed, the demand placed on the building by each of these hazards is well understood. The capacity to meet these demands can be supplied.

    Another primary hazard is fire. Even though care is taken to minimize the risk of fires, they do occasionally happen. The fire demand placed on a structure is also understood, and systems can be supplied to meet life safety objectives.

    The problem with trying to develop code requirements for terrorist acts is that the demand is not understood, because it cannot be predicted. Any defined attack “load” could be made inadequate by a larger attack. The “design” attack cannot be determined in the same technical ways that wind or earthquake loads are determined. Rather, the design approach will need to be determined in the political realm, as it is related to societal values rather than a scientific approach.

    Can we make building structures more resistant to terrorist attack? Yes. But, the critical decisions of how big of an attack, how much to spend on hardening building structures, and which buildings should be hardened will need to be made by our legislative representatives. Of course, building officials, architects and engineers will need to assist in this process.

    Can buildings resist direct airplane hits?

    If the “design” terrorist attack is similar to that of Sept. 11, can buildings be given the capacity to meet this demand? To answer this question, it is important to understand the physics at work when a plane in flight is stopped by a building.

    If the performance objective is to “resist” a direct airplane hit and protect people inside the building, the plane cannot be allowed to penetrate the exterior wall. To stop a Boeing 767 traveling in excess of 500 miles per hour in a distance of a few feet would take a deceleration force in excess of 400 million pounds.

    Each tower of the World Trade Center was designed for a total horizontal force (or “design wind load”) of about 15 million pounds. The total design wind load for a more commonly sized high-rise, say, 40 stories tall, would be about 4 million pounds. In other words, to resist the amount of force generated by a direct 767 hit, today’s buildings would need to be 100 times stronger than dictated by code, which is both physically and economically impossible.

    So why did the World Trade Center Towers not collapse immediately due to the impact load on the system? The planes did not stop in a few feet, but had an effective stopping distance of over 100 feet. This would drop the deceleration force down to something close to the capacity of the building.

    Another part of the answer to this question lies in the way that the exterior of the building was structured. The exterior columns were 14-inch square welded steel box columns spaced at 40 inches on center. This means that there was only 26 inches clear between each column. The columns were integral with the steel spandrels beams and formed essentially a solid wall of steel with perforations for windows. This wall construction was able to form a Vierendeel “bridge” over the hole created in one side of each of the towers.

    Both of these facts — that the plane was not stopped at the exterior and that the columns and spandrels were extremely dense — were necessary to prevent the building from collapsing immediately upon impact.

    The final point in answering the question of airplane impact loading is that, even if some miracle technology could be invented to take these kinds of loads from a 767, there are larger planes to be considered. The B-25 that hit the Empire State building was a fraction of the size of a 767. The Rockwell Commander that hit the high-rise in Milan earlier this year was about half the size of the B-25.

    The accompanying diagram gives a comparison of two planes that have actually hit buildings and two planes that would control the design loads if a building structure needed to resist the impact of planes. The larger square in each frame was the floor plate size of the World Trade Center (209 by 209 feet), and the smaller square is a more common size for a major high-rise (140 by 140 feet).

    The figure reveals that the design loads for an Airbus A380 (now being manufactured) would be 4.5 times greater than that of the 767’s used in the attack on each of the Trade Center Towers, with more than eight times the amount of fuel.

    Can buildings be designed for direct airplane hits? Yes and no. Yes, for small aircraft. A definite no, for large commercial aircraft.

    How can we minimize the chance of progressive collapse?

    This is still one more question that some people are asking. Because the towers ultimately collapsed with one floor crashing down upon the next, it has been called a progressive collapse.

    Again, it is important to think carefully about the question. Aren’t all collapses progressive? Something breaks, then something else breaks, and so on. Normally, when the term progressive collapse is used, it specifically refers to the loss of one or two columns or bearing walls that cause a collapse to propagate vertically.

    In the case of the World Trade Center there were about 40 columns lost on one face of each of the towers and there was no propagation of collapse from this loss. So did the World Trade Center have good resistance to progressive collapse? By normal use of the term “progressive collapse” it did. The collapse that did ultimately occur was progressive, like all collapses, but was not “progressive collapse” that some international codes address.

    The difficulty in understanding this concept is illustrated with the following story.

    A New York fire chief wrote that experienced firefighters know that the buildings that are most susceptible to progressive collapse are buildings that are well-tied together (i.e., able to transfer building loads from one element to another, such as a column). Yet, virtually every structural engineer will advise that one of the best ways to prevent progressive collapse is to tie the building together. How can there be this kind of a contradiction?

    The difference is that the engineer is thinking about losing a column or two and the fire chief is talking about losing a whole part of a building. As the event that initiates the progressive collapse becomes larger than losing a column, the risk becomes that the strong horizontal ties of a building will cause the collapse to propagate horizontally.

    Any discussion of code provisions with respect to progressive collapse must recognize that both the engineer and the fire chief, are right depending on the kind of hazard that is defined.

    What about other code issues?

    Although this article has focused primarily structural design, caution should also be used when examining proposed code changes for other disciplines. The urge to “do something” should not predicate reactive code revisions. Instead, changes should be made only when a direct and realistic benefit to building or occupant safety can be attained.

    At least six safety systems present in the World Trade Center towers were completely and immediately disabled or destroyed upon impact: fireproofing, automatic sprinklers, compartmentalization and pressurization, lighting, structure and exit stairs.

    No building can be expected to perform with the total destruction of multiple safety systems. Additionally, not all buildings are subject to the same level or type of potential threat. For instance, a 100-story office building in a downtown environment is subjected to significantly different risks than a distribution warehouse located in the suburbs. Likewise, the performance we expect from a hospital or school is different than the performance we expect from a shopping center.

    However, threats do exist of a nature and scale that perhaps some portion of our built environment should address directly. Car bombs, bio-chemical attacks, and deliberately set fires fall into this category.

    Today, much of what we “know” about enhanced safety design is qualitative rather than quantitative. Research is needed in areas such as blast protection, indoor air quality management, fire protection, and building egress to understand the effectiveness of various enhancement proposals. The National Institute of Standards and Technology, for one, is embarking on a multi-year research program to determine some answers. Moving forward, clear and quantitative information will allow the development of appropriate and effective design solutions.


    There are structural techniques that can increase the capacity of building structures to resist certain kinds of terrorist attacks. However, there is absolutely no reliable way to design for the impact of a large scale commercial airliner. Any code changes to address these kinds of hazards will depend on decisions within the political realm. However, building officials and designers must assist to help arrive at rational solutions.There will be great difficulty in trying to legislate which new building structures have any risk of terrorist attack.

    Every building professional must be careful to think through these new sets of problems in order to get to the right questions so that the right solutions can be found for society and real increases in safety can be achieved.

    Jon Magnusson, P.E., is chairman and CEO of Skilling Ward Magnusson Barkshire.

    Other Stories:

    Copyright ©2009 Seattle Daily Journal and DJC.COM.
    Comments? Questions? Contact us.