Use of High-Efficiency Energy Absorbing Device to Arrest Progressive Collapse of Tall Building

The World Trade Center collapse has brought attention to progressive collapse of tall buildings and the study of possible countermeasures. From the viewpoint of energy transfer, this analysis explains why the collapse could not stop by itself once began. By introducing a design parameter called collapse stability index that controls design against progressive collapse, it is found that conventional design of a tall building usually leads to an inherently unstable structure in the event of a progressive collapse. In a subsequent feasibility study in this paper, a heavy-duty metal-based honeycomb energy absorbing structure is proposed. Using a finite element analysis, it is demonstrated that the structure is capable of absorbing potential energy released in a tall building collapse. The added energy absorbing devices will only occupy a small percentage of the total floor space. By properly designing and installing such devices, a progressive collapse, should it happen in a tall building, may be arrested within a few floors, and hence, the building is inherently stable to the progressive collapse. The theory is also elaborated with the example of the World Trade Center collapse.     

What Did and Did Not Cause Collapse of World Trade Center Twin Towers in New York?

Previous analysis of progressive collapse showed that gravity alone suffices to explain the overall collapse of the World Trade Center Towers. However, it remains to be determined whether the recent allegations of controlled demolition have any scientific merit. The present analysis proves that they do not. The video record available for the first few seconds of collapse is shown to agree with the motion history calculated from the differential equation of progressive collapse but, despite uncertain values of some parameters, it is totally out of range of the free fall hypothesis, on which these allegations rest. It is shown that the observed size range (0.01–0.1?mm) of the dust particles of pulverized concrete is consistent with the theory of comminution caused by impact, and that less than 10% of the total gravitational energy, converted to kinetic energy, sufficed to produce this dust (whereas, more than 150?t of TNT per tower would have to be installed, into many small holes drilled into concrete, to produce the same pulverization). The air ejected from the building by gravitational collapse must have attained, near the ground, the speed of almost 500 miles per hour (or 223?m/s, or 803?km/h) on average, and fluctuations must have reached the speed of sound. This explains the loud booms and wide spreading of pulverized concrete and other fragments, and shows that the lower margin of the dust cloud could not have coincided with the crushing front. The resisting upward forces due to pulverization and to ejection of air, dust, and solid fragments, neglected in previous studies, are indeed found to be negligible during the first few seconds of collapse but not insignificant near the end of crush-down. The calculated crush-down duration is found to match a logical interpretation of seismic record, while the free fall duration grossly disagrees with this record.     

Progressive Analysis Procedure for Progressive Collapse

Following the collapse of the World Trade Center towers in September 2001, there has been heightened interest among building owners and government entities in evaluating the progressive collapse potential of existing buildings and in designing new buildings to resist this type of collapse. The General Services Administration and Department of Defense have issued general guidelines for evaluating a building’s progressive collapse potential. However, little detailed information is available to enable engineers to confidently perform a systematic progressive collapse analysis satisfying these guidelines. In this paper, we present four successively more sophisticated analysis procedures for evaluating the progressive collapse hazard: linear-elastic static; nonlinear static; linear-elastic dynamic; and nonlinear dynamic. We discuss the advantages and disadvantages of each method. We conclude that the most effective analysis procedure for progressive collapse evaluation incorporates the advantageous parts of all four procedures by systematically applying increasingly comprehensive analysis procedures to confirm that the possibility of progressive collapse is high.     

Why the Observed Motion History of World Trade Center Towers Is Smooth

The collapse of the World Trade Center towers was initiated by the impact of the upper falling part onto the underlying intact story. At the moment of impact, the velocity of the upper part must have decreased. The fact that no velocity decrease can be discerned in the videos of the early motion of the tower top has been recently exploited to claim that the collapse explanation generally accepted within the structural mechanics community was invalid. This claim is here shown to be groundless. Calculations show that the velocity drop is far too small to be perceptible in amateur video records and is much smaller than the inevitable error of such video records.     

Addendum to “Why Did the World Trade Center Collapse?—Simple Analysis”

The addendum presents the responses to several questions on a preliminary version (which arrived too late for publication as part of the paper). The questions deal with the aircraft impact at a higher floor of the World Trade Center towers on September 11, 2001 damage to the upper part of the collapsing tower, weakness of connections, plastic cushioning of vertical impact, estimation of the equivalent mass, and the collapse of the adjacent lower building.     

Progressive Collapse of the World Trade Center: Simple Analysis

The collapse behavior of the World Trade Center towers is considered formally as a propagating instability phenomenon. The application of associated concepts enables the residual capacities of both towers after the onset of collapse to be formally estimated. This information is combined into a simplified variable-mass collapse model of the overall dynamical behavior. The resulting, nonlinear governing equation of motion can be solved in closed form, to yield compact information about the overall collapse conditions.     

Dominant Factor in the Collapse of WTC-1

The behavior of the World Trade Center Tower 1 on September 11, 2001 is studied in light of experimental data on the effect of elevated temperature on mechanical properties of structural steel. It is concluded that the damage inflicted by aircraft impact on the insulation of the core framing was the dominant factor in the collapse of the structure.     

Partial Demolition and Extension of Kuwait Medical College Building

This paper presents a simplified approximate analysis of the overall collapse of the towers of World Trade Center in New York on September 11, 2001. The analysis shows that if prolonged heating caused the majority of columns of a single floor to lose their load carrying capacity, the whole tower was doomed.     

Stability of the World Trade Center Twin Towers Structural Frame in Multiple Floor Fires

This technical note presents the results from a nonlinear finite element analysis of a very simple two dimensional model of the World Trade Center Twin-Towers structural frame subjected to fire. The analysis was carried out for a large range of fire scenarios and was reported in detail in a recently published paper. The paper further investigates the results of this analysis to obtain the details of the collapse mechanism found. An interesting series of events leading progressively to overall collapse are discovered and described in detail. The main reason for the failure is found to be the low membrane capacity in compression of the composite steel truss and concrete deck slab floor system.     

Stiffness-Damping Simultaneous Identification under Limited Observation

A more reliable and stable method, compared to the previous one developed by the present writers, of stiffness-damping simultaneous identification of shear-type building structures is proposed using stationary random records under limited observation. It is shown that when stationary random horizontal accelerations are recorded at the floors just above and below a specific story in a shear building model, the story stiffness and the damping ratio can be identified uniquely. The viscous damping coefficient and the material damping ratio can also be identified simultaneously in a numerical model structure. It is also shown that unfavorable effects by extraneous noises, including measurement noises, due to wind excitation, building facilities, and the motion of the occupants can be eliminated partially by using the present method. The accuracy of the present identification method is investigated through the actual records under limited observation in a base-isolated building.     

Scale Modeling of the 96th Floor of World Trade Center Tower 1

This paper presents an experimental investigation of the World Trade Center Tower 1 (WTC1) collapse using a 1/20-scale model. The WTC1 fire on the 96th floor is reconstructed on a small scale, and structural members including the floor trusses and the exterior wall subsystem are built and tested under scaled fire load. Scaling rules are used to determine the values of the insulating material on the structural systems. This experimental study demonstrates the use of scaled models to investigate a real-world fire disaster. Results from the experimental investigation are compared to analytical results and visual evidence compiled in the National Institute of Standards and Technology report on the investigation of the collapse of WTC towers. This study helps engineers and researchers better understand the fire behavior and the associated structural response in WTC1, and a more solidly grounded collapse hypothesis can therefore pursued.     

Engineering Perspective of the Collapse of WTC-I

The writers report on a simulation study of the performance of the North Tower (WTC-I) of the World Trade Center complex during the impact of American Airlines Flight 11 on September 11, 2001. We discuss impact damage that the structural core might have sustained and its possible behavior under structural and thermal loading. Our simulations indicate that the worst damage to the core structure was in stories 95 through 97 of the tower. We estimate that a core collapse mechanism could be initiated if the tower core column temperatures were elevated to about 700°C.     

Preventing Disproportionate Collapse

“Disproportionate collapse” is structural collapse disproportionate to the cause; it is often, though not always, progressive, where “progressive collapse” is the collapse of all or a large part of a structure precipitated by damage or failure of a relatively small part of it. There have been many attempts to develop design guidelines and criteria that would reduce or eliminate the susceptibility of buildings to this form of failure. In recent years, the particular focus has been on the prevention of progressive collapse due to deliberate attack. The present study suggests, however, that these guidelines and criteria may be of limited value. Arguably the most important deficiency in the state of the art of design to prevent disproportionate or progressive collapse is uncertainty about the design event: We have the technology now to design for almost anything, but most recent building failures due to explosions and terrorist attacks have involved insults to the building not anticipated in design guidelines and criteria.     

Mitigating Risk from Abnormal Loads and Progressive Collapse

A progressive collapse initiates as a result of local structural damage and develops, in a chain reaction mechanism, into a failure that is disproportionate to the initiating local damage. Such collapses can be initiated by many causes. Changes in building practices to address low probability/high consequence events and to lessen building vulnerability to progressive collapse currently are receiving considerable attention in the professional engineering community and in standard-writing groups in the United States, Canada, and Western Europe. Procedures for identifying and screening specific threat scenarios, for assessing the capability of a building to withstand local damage without a general structural collapse developing, and for assessing and mitigating the risk of progressive collapse can be developed using concepts of probabilistic risk assessment. This paper provides a framework for addressing issues related to low probability/high consequence events in building practice, summarizes strategies for progressive collapse risk mitigation, and identifies challenges for implementing general provisions in national standards such as ASCE Standard 7, Minimum design loads for buildings and other structures.     

第三系砂砾岩的性质与桩基承载力评价

通过对南昌世界贸易中心等工程实践的分析，就该区内砂砾岩的性质及桩基承载力提出了新的评价方法。     

Behavior and Design of Commercial Multistory Buildings Subjected to Blast

The behavior of nonmilitary buildings subjected to blast is considered. Case studies from World War II are described, as well as more recent events from the detonation of large vehicle borne devices in the Middle East, North America, and Europe. Conventional methods for nonseismic design are shown to lead to frames with overstrong beams connected together by relatively weak connections. This may explain much of the evidence from bomb damaged buildings in which building connections have been observed to fracture in a brittle manner when subjected to blast. The risk of progressive collapse may be minimized by strengthening beam to column connections located at close proximity to potential vehicle borne devices and a capacity design method for such strengthening is advocated.     

Young children in the aftermath of the World Trade Center attacks.

The attacks of September 11, 2001, on the World Trade Center were unprecedented acts of terrorism on U.S. soil. The disaster provides an opportunity to understand the responses of young children to a traumatic event of this proportion. This retrospective study took place within a year of the attacks and examined the relationship of levels of exposure to the World Trade Center disaster and family level predictors to trauma symptoms in a highly exposed sample of 180 young children in New York City. Data were collected through interviews with parents of children five years or younger at the time of the attacks. Primary variables included direct exposure and post 9/11 child and parent functioning, including trauma symptoms. Child trauma symptoms were related to direct exposure to the disaster, previous trauma, negative changes in parenting, and increased couple tension. Findings support the conceptualization that children's responses to traumatic events must be addressed within the caregiving context of family relationships. Clinical and preventive intervention for young children should be aimed at multiple levels of the social ecology. (PsycINFO Database Record (c) 2011 APA, all rights reserved)     

纽约世贸中心用厚板钢剖析及对宝钢建筑用厚板开发的启示

采集了一组来自纽约世贸中心废墟的不同厚度的废钢样块，分析了该大厦建筑用厚板钢的化学成分、机械性能及金相组织，并结合国内高层用厚板钢市场的调研，探讨了宝钢建筑用宽厚板开发中应注意的相关事宜。