Paul F. Mlakar

Disproportionate collapse research needs

Mechanisms of progressive collapse resistance are described which help identify new design requirements that effectively improve collapse resistance of structures. Criteria must be established to identify structures for which progressive collapse resistance may need to be enhanced through the implementation of additional design requirements. Enforcing disproportionate collapse-resistant design criteria beyond current practice for all structures may not be justified because most buildings are not at significant risk. Simplified approaches may be sufficient for buildings that require explicit evaluation of their progressive collapse resistance following column removal. One such approach is based on a comparison between the strength of the structure under a downward force at the location of removed column and the axial compressive force in the column. If the structure is identified to be susceptible to collapse, more advanced analysis is justified. Such analyses may be required more often in evaluation of existing structures than in design of new structures. The results of such analyses involve significant uncertainty and their reliability heavily depends on the modeling and analysis assumptions. One of the more important assumptions that require further investigation is the deformation capacity of elements subjected to interacting actions (such as tension combined with flexure and/or shear). Features that are important in modeling structures for collapse analysis as well as parameters that are important in developing progressive collapse resistant mechanisms require further studies.

The Pentagon Building Performance in the 9/11 Crash

Following the 9/11 crash of an airliner at the Pentagon, the American Society of Civil Engineers established a team to study the response of the structure. The team reviewed available information on the structure, the crash loading, and the resulting damage. The team then analyzed the essential features of column response to impact, the residual frame capacity, and the structural response to the fire. Plausible mechanisms for the response of the structure to the crash were determined and recommendations were offered for the future design and construction of all buildings.

Structural Damage Induced by a Terrorist Attack on the Pentagon

On September 11, 2001, a large commercial aircraft traveling at high speed crashed into the Pentagon. The aircraft struck essentially at the second story slab, damaging columns on the first and second story and the slab of the second story in the outer ring of the structure. Then the aircraft debris slid under the second floor, 94.5 m (310 ft) into the building, inducing significant damage to the three outer rings of the west side of the Pentagon. Numerous columns were removed or significantly damaged by the aircraft debris as it moved through the building. As a result, a portion of the outer ring collapsed approximately 20 minutes after the crash. This paper summarizes the condition of the building after the attack. 1 Technical Director, U.S. Army Corps of Engineers, 3909 Halls Ferry Road, Vicksburg, Mississippi 39180; phone 601-634-3251; e-mail Paul.F.Mlakar@erdc.usace.army.mil 2 Principal, Simpson Gumpertz & Heger Inc., 41 Seyon Street, Building 1, Suite 500, Waltham, Massachusetts 02453; phone 781-907-9000; e-mail dodusenberry@sgh.com 3 Principal, J.R. Harris & Company, 1580 Lincoln Street, Suite 770, Denver, Colorado 80203-1509; phone 303-860-9021; e-mail Jim.Harris@JRHarrisAndCo.com 4 Fire Protection Engineer, Bureau of Alcohol, Tobacco, and Firearms, Arson & Explosives Division, 800 K Street, NW, Room 710, Washington, District of Columbia 20001; phone 202-927-88 t 4; e-mail GAHaynes@atfhq.atf.treas.gov 5 Research Structural Engineer, National Institute of Standards and Technology, Building and Fire Research Laboratory, Building 226, Room B-158, Gaithersburg, Maryland 20899-8611: phone 301-975-6077; e-mail long.phan@nist.gov 6 Kettelhut Distinguished Professor of Structural Engineering, Purdue University, School of Civil Engineering, 129G Civil Engineering Building, West Lafayette, Indiana 47907-1284; phone 765-494-2186; e-mail sozen@cerico.ecn.purdue.edu 10 FORENSIC ENGINEERING