«R. SHANKAR NAIR R. Shankar Nair R. Shankar Nair, Ph.D., P.E., S.E. is a principal and senior vice president of Teng & Associates, Inc. in Chicago. In ...»
Department of Defense and the U.S. Department of Energy, Third Edition, 1977, reprinted by the Federal Emergency Management Agency Longinow, A. and Alfawakhiri, F., “Blast Resistant Design with Structural Steel – Common Questions Answered,” Modern Steel Construction, AISC, October 2003 Longinow, A., “Survivability in a Nuclear Weapon Environment,” DCPA Contract DCPA01-77-0229, for Defense Civil Preparedness Agency, Washington, D.C. 20301, May 1979 Pickering, E. E., and Bockholt, J. L., “Probabilistic Air Blast Failure Criteria for Urban Structures,” Contract DAHC20-67-C-0136, for Office of Civil Defense, Office of the Secretary of the Army, Washington, D.C., 20310, November 1971 Newmark, N. M., “An Engineering Approach to Blast Resistant Design,” University of Illinois Engineering Experiment Station, Reprint Series No. 56, 1953 TM 5-855-1, “Fundamentals of Protective Design for Conventional Weapons,” U.S. Department of the Army, November 1986 (http://www.military-info.com/MPHOTO/p021c.htm) TM 5-1300 Structures to Resist the Effects of Accidental Explosions,” U.S. Departments of the Army, Navy and Air (http://www.military-info.com/MPHOTO/p021c.htm) Force, November 1990
Jon Magnusson is Chairman/CEO of Magnusson Klemencic Associates, a Seattle/Chicago structural/civil engineering firm, with completed projects in 43 states and 35 countries. He is a licensed engineer in 24 states. He has worked on the designs for projects that have won national structural engineering excellence awards from the American Council of Engineering Companies (ACEC) twelve times in the last 17 years. His projects have won three awards from the American Institute of Steel Construction in the last 5 years. During that period alone, he has been structural engineer of record for over $2 billion worth of construction.
An important part of building structural design is strategically meeting certain performance objectives for a set of defined hazards. Understanding these concepts and communicating them to building owners, and even the general public, is becoming increasingly important.
Many buildings have been subjected to loads greatly in excess of their design criteria and have not collapsed. Lessons learned from several of these buildings are shared, including a “submarine” concept for building construction.
Structures are designed for certain loads and hazards. Structural engineers need to communicate clearly with the building owner, architect, and building officials about what loadings may have been considered, or possibly more importantly, not considered in a project design.
Many things can be learned from investigating structures that have been subjected to loads beyond what was contemplated in their design. The overloads may have been due to intentional malicious acts or accidental hazards. The damage patterns and behavior of members and connections can give hints into how to make structures more resistant to these overloads.
WHAT ARE THE OJBECTIVES OF “DESIGN”?
Building designers can not possibly design for every extremely remote hazard that their project may be subjected to in its life. Commercial buildings are not designed for meteorite impact, or for nuclear blasts, or for other kinds of military attacks. However, the design process does
include looking at four major hazards:
1. Gravity 2. Wind 3. Earthquake 4. Fire
Each of these must be defined. Gravity is well-defined and extremely predictable! Fire is typically dealt with by mitigating the hazard through event control such as sprinklers, fireproofing, and active firefighting so that the structure does not need to take the fire load.
Wind and earthquake are defined on a probabilistic basis that, while not as precise as gravity, is quite reliable. Figure 1 shows examples of this approach.
Fig. 1. Wind Speed and Direction Probabilities for Houston, TX and Seismic Hazard for Tacoma, WA For each hazard, performance objectives are developed. Examples of performance objectives for wind and seismic are shown in Figure 2.
Fig. 2. Wind Acceleration and Seismic Ground Motion Performance Objectives Once the design hazards and corresponding performance objectives are defined the design can proceed to bring these into conformance. For rational design, these steps must be repeated over
and over for each element of the building system:
1. Hazard Definition
2. Performance Objectives
3. Conformance Strategies It is critically important that all design disciplines have consistent performance objectives for the different design hazards. For example, if a sprinkler system is part of a conformance strategy for the structure, it had better have performance objectives that it be operational under the same hazard.
Usually the magnitude and probability of extreme loadings are not predictable. Unfortunately, many of the extreme loadings being considered now in designs are blast loadings due to intentional detonations intended by the perpetrators to cause damage and injury.
When the Murrah Federal Building in Oklahoma City was attacked the blast was equivalent to 4,000 lbs. of TNT. The hazard associated with a truck bomb could be 60,000 lbs. of TNT, or 15 times greater than the Murrah attack. And again, this is not an upper limit because it is always possible to postulate multiple trucks bombs in an attack.
The terrorists in the attack of September 11, 2001 ultimately had control of three planes (temporarily four) and could have used them all to attack one target. If “plane attacks” are to be considered as a design hazard, then much larger planes need to be considered. Figure 3 compares two planes that have already hit buildings, with two planes that are even larger.
Fig. 3. B-25 hit Empire State Building, 767-200 used in WTC attacks.
Clearly, many of these hazards are beyond the realm of cost effective resistance, and in many cases beyond the ability to overcome the physics of the hazard.
One of the most common strategies to resist progressive collapse is to use a notional removal of one exterior element at a time and creating alternate load paths. This does not relate to any specific hazard and therefore does not create a performance objective for a “real” threat. It is simply meant to increase the redundancy of the structure. Many structures that have not been designed for this criterion actually have shown some capacity to lose a column without global collapse.
This approach generally results in much stronger horizontal framing systems with significant axial capacity. It is important to consider what happens when an unexpected hazard occurs that removes two or more columns. Does this strong horizontal construction then cause a horizontal propagation of the collapse? A New York City Fire Chief reported to the World Trade Center Building Performance Assessment Team that the structures that are most susceptible to progressive collapse are the ones that are well tied together. Mark Loizeaux of Controlled Demolition, Inc., whose occupation is taking down buildings, has also said that the easiest buildings to take down are the ones with high levels of continuity.
Designers should consider the possibility of negative impacts of excessive horizontal ties under more extreme loading when using the notional removal technique.
Ronan Point – United Kingdom This is the most famous case of “pure” progressive collapse. There were five deaths. There was extensive vertical propagation of the collapse, but almost no horizontal propagation. If the building had been well tied together and the initiating event was larger, would the entire structure have collapsed?
Murrah Federal Building – Oklahoma City Complete vertical and some horizontal propagation of the collapse. The blast was the equivalent of 4,000 lbs. of TNT.
600 California – San Francisco Crane accident demonstrated tremendous ductility of concrete filled steel pipes.
World Trade Center 1 and 2 – New York The highly redundant steel exterior moment frame was able to bridge about 140 feet of missing columns. Intense fires ultimately brought down both buildings.
Bankers Trust – New York Debris from collapse of WTC 2 removed an exterior column over a partial height of the building. The redundancy of the structure above provided the necessary bridge to transfer loads from the missing column.
World Financial Center 3, American Express – New York Sections of the corner column were destroyed. The corner bay was supported by cantilevered structure above and stiffening provided by the exterior wall system.
World Trade Center 3, Marriott Hotel – New York The Marriott was crushed by debris from both WTC 1 and WTC 2. WTC 2 hit it first and, even though hundreds of tons of debris partially collapsed the southern part of the building, the collapse did not propagate to the north. The floor connections were not strong enough to allow the propagation.
AN ALTERNATIVE STRATEGY
Based on observations of these buildings, the concept of structural compartments seems to have merit. Within each compartment, strong horizontal ties could be used to prevent vertical propagation of a collapse from a relatively small overload. In the event of a massive overload, the collapse would propagate horizontally until it hit an extra strong bulkhead wall (or one with weak connections) to arrest the collapse. This dual level protection concept is similar to the way that a submarine design deals with military hazards.
Regardless of the strategies employed it is critical to identify the design hazards, performance objectives, and conformance strategies and discuss these with the building owner, architect, and building officials so that all parties have appropriate expectations and understanding of risk.
The purpose of this paper is to review the inherent resistance of conventionally designed buildings for which specific blast or progressive collapse requirements have not been included in the design. Current blast and progressive collapse design guidelines are outlined and assessment of ordinary structural design vulnerabilities are discussed. Simple concepts are introduced to provide a more robust structure with respect to blast and progressive collapse threat.
BLAST AND PROGRESSIVE COLLAPSE
The loadings produced by blast events are typically much higher than the design loadings for which an ordinary structure is designed. These loadings, referred to as overpressures in the technical literature, have been extensively studied by the Department of Defense and other military agencies for their effect on various structures. Important parameters are the size of charge, distance of explosion to structure, ground or air burst, etc. The overpressures are usually measured in PSI – pounds per square inch on the element being impacted. This is noteworthy since most code design loads for ordinary structures are given in pounds per square foot. Relationships for the overpressure have been developed and given the size of the charge and distance from the structure a value can be calculated. As noted, these overpressures are usually well beyond the capacity of the structure. Local failures of structural elements in the region of the explosion is likely.
Since the risk or threat level is highly variable and local capacities are easily exceeded, more detailed analysis is unnecessary and it is commonly assumed the element impacted will fail. The effect of the blast is then studied by removing the impacted element (or elements) from the structure and then analyzing the modified structure.
The effect of the blast can be in the opposite direction for which the design loads were considered, resulting in existing capacities to resist blast being further reduced. Upward pressures from blast effects on the lower floors can put beams and girders into a reverse bending mode resulting in bottom flanges becoming compression elements with large unbraced lengths. The overpressures can easily overcome the downward design loads. The Oklahoma City Federal building is an example of the blast loading resulting in loading the concrete structure in a direction opposite to the main design resistance. Also, blast can produce significant side loading on elements that had no original design loads in that direction. Local floor failure can result in significant unbraced lengths on columns as witnessed in the first attack on the World Trade Center.
For ordinary buildings the best preventive measures are to keep the blast away from the structure with barriers, etc.
(i.e. Defensive Design). The GSA (1) and DOD (2) have developed stand-off distances, barrier designs etc. for various threats. These documents address requirements for new and existing government buildings. ASCE has published a state of the art reference (3).
Progressive collapse is the disproportionate collapse of a structure due to a failure of a much smaller (albeit important) element. Obviously this includes but is not limited to blast effects. Since progressive collapse can encompass a much larger portion of the structure (or the entire structure) with many different collapse possibilities, a specific assessment approach is not possible. It is best to look at the specific guidelines outlined in the above documents and comment on what is missing in ordinary buildings to provide an evaluation of a design. Progressive collapse is a global assessment of the structure whereas blast is usually a local element assessment.
The GSA document provides an insight to current thinking related to mitigating progressive collapse. Basic to this approach is the concept of multiple load paths and structural redundancy which will produce a robust structure.