Shalva Marjanishvili

The possibility of a local structural failure to propagate into a global collapse of the structural system has fueled the continued development of improved computational methods to model building behavior, as well as “best practices” engineering standards. In spite of these efforts, recent events are bringing the issue of collapse mitigation to the forefront and highlighting the shortcomings of existing design practices. The catastrophic nature of structural collapse dictates the need for more reliable methodologies to quantify the likelihood of structural failures and strategies to minimize potential consequences. This paper presents the results of a stochastic nonlinear dynamic analysis study of a simple structural model to predict catastrophic failure. The performed analysis indicates that, at the point of incipient failure, uncertainties associated with the analysis become increasingly large. Consequently, it may not be possible to accurately predict when (and if) failure may occur. Recognizing the need to understand uncertainties associated with risk and probabilities of unlikely events (low probability and high consequence events), this paper sets the stage to better understand the limitations of current numerical analysis methods and discuss innovative alternatives.

The possibility of local structural failure to propagate into global collapse of the structural system has fueled the continued development of improved computational methods to model building behavior, as well as "best practices" engineering standards. In spite of these efforts, recent events are bringing the issue of collapse mitigation to the forefront and highlighting the shortcomings of existing design practices. The catastrophic nature of structural collapse dictates the need for more reliable methodologies to quantify the likelihood of structural failures and strategies to minimize potential consequences. This paper presents the results of a stochastic nonlinear dynamic analysis study of a simple structural model to predict catastrophic failure. The performed analysis indicates that, at the point of incipient failure, uncertainties associated with the analysis become increasingly large. Consequently, it is not be possible to accurately predict when (and if) failure may occur. Recognizing the need to understand uncertainties associated with risk and occurrence of low-probability- high-consequence events, this paper sets the stage to better understand the limitations of current numerical analysis methods and discuss innovative alternatives.

In current practice, progressive collapse analysis typically includes two types of hazards: the initial hazard that causes localized damage and the subsequent response of the structure to bridge loads across the damaged areas (analysis referred to as alternate path method). However, little detailed information is available on a third type of hazard, such as fire that typically follows the initial hazard. Prolonged exposure of a damaged structure to fire could be detrimental to the short-term stability of that structure and may pose a significant threat to the safe evacuation of building occupants. This paper presents a study of the effects of fire following an extreme event (i.e., blast or impact) that causes failure of one column on the perimeter of a common steel building frame. The approach focuses on a steel structure that is designed to satisfy new U.S. Dept. of Defense (DoD) guidelines and assumes that the extreme event not only damages one column but also damages active fire protection (i.e...

Design for progressive collapse resistance requires that the structure remains stable in the event that a primary structural member is removed following an extreme event such as blast or impact. However, these methodologies do not take into account the potential hazard of fire that is often a byproduct of an extreme event. This paper examines the fire resistance of a prototype steel high-rise building, which meets progressive collapse requirements per GSA. The building is modeled with the removal of a perimeter column thus simulating a consequence immediately following an extreme event. The building is then subjected to uncoupled thermal - structural analyses representing various fire scenarios based on the following parameters: (1) fire protection, (2) fire type, (3) location, and (4) fire spreading. The results show that even though the building meets progressive collapse design requirements, if the building sustains localized damage to the extent where nearby fire protection is removed or damaged, a subsequent fire may quickly lead to building collapse. While this finding is for one particular building prototype, it is a potential concern particularly for high-rise buildings, which must remain stable for a substantial time to completely evacuate building occupants.

In this paper we present a comparative study of precast prestressed concrete façade panel analysis using single degree of freedom (SDOF) and finite element methodologies. LS-Dyna was used to develop the finite element formulation. The goal of the study is to identify simple, easy and economical analytical procedure to determine the reaction loads for the precast panels. This study was motivated by a real project, with the requirements dictated by UFC 4-010-01 DoD Minimum Antiterrorism Standards for Buildings. The reaction forces obtained from the two analysis methods are juxtaposed and the differences are discussed. This paper presents a summary of our findings, conclusions and recommendations.

This paper proposes a quantitative measure and mathematically reproducible definition of structural robustness as it pertains to a structure's ability to minimize the potential for structural progressive collapse. Structural robustness is defined by the structure's ability to withstand any unforeseen loading as well as initiating damage scenarios without a disproportionate response. Presented here is a study that finds that in the absence of the specific progressive collapse load scenarios and unknown location and extent of initiating damage, progressive collapse potential can be minimized by employing the concept of structural robustness as well as safety and reliability, which are all interdependent qualities of a structural system. This study also concludes that structural robustness represents a structure's ability to absorb small structural failures (perturbations) without affecting the overall structural integrity, and can be measured as a standard deviation of the progressive collapse dynamic resistance probability density function.

Hydrocarbon vapor-phase explosions have long been known to create destructive overpressure and impulse loads when ignited both inside and outside buildings. This creates a hazard to both building structural elements and personnel. We have recently implemented the Baker-Strehlow and Baker-Strehlow-Tang curves into our Blast Analysis Module (BAM) code which computes the performance of structural members exposed to air-blast to determine component level of damage. A case study on the design of an internal laboratory cell to an accidental vapor-phase explosion of jet-fuel will be presented. The lab is 3150 sq. ft. with a 31 ft. high roof, and it is internal to a larger multi-story structure (approx. 35,000 sq. ft.). This cell must be designed to survive an explosion with a level of damage that prohibits collapse and personnel fatality, and it may be vented to the atmosphere to reduce the internal gas pressure as described by ASCE. The case-study will document the overpressure and impulse time history inside the lab cell, demonstrate the implications of structural hardening of the walls and roof with deflagration venting, and the final design with the response of the system.

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.

Loss of a column can cause catastrophic collapse of a building. In air-blast analysis, consideration of column design is critical, particularly for ground floor columns as they are often subjected to substantial axial loads and high blast pressures due to their proximity to unsecured public streets. While there are extensive studies of columns subjected to static out-of-plane loads, relatively little information is available regarding the dynamic behaviour of such elements. As a result, column response is evaluated using simplified dynamic methods and conservative flexural deformation performance limits are set in order to account for column behaviour not explicitly included in the analysis, such as the possibility of buckling. Structural behaviour of columns subjected to air-blast load can be analyzed using advanced dynamic methods, such as explicit and implicit finite element methods. These methods take into account the complexity of the beam-column behaviour including buckling and axial-bending-shear interaction. Additionally, non-uniform loading and arrival time effects can be considered. Although these methods provide enhanced modeling of column behaviour, existing failure criterion is still based on conservative flexural deformation performance limits developed for more simplified analysis methods. In this paper, the authors examine available criteria for columns. Column response to air-blast loads using explicit finite element methods is determined and compared with results determined using single-degree-of-freedom (SDoF) analyses. Column performance is evaluated based on current design ductility and rotation limits and failure criterion is examined.

Impact loads in structural design are often related to vehicular impact, partial structural collapse, construction accidents or blast fragmentation. For design purposes, impact loads are often treated as static loads multiplied by a dynamic increase factor; however, this method can overly simplify the event. More sophisticated analyses use energy methods, contact elements and explicit finite element method to determine the impact behavior; however, the complexity of these methods is unattractive for most structural engineers. These analytical methods are either overly simplified or excessive complex, therefore, simplified methods must be developed that account for the complexity of the phenomenon while still providing an easy tool for structural engineers. The purpose of this paper is to identify parameters in an impact scenario associated with free falling particles impacting a stationary object. These parameters are used to determine a time-dependent load and a time-dependent mass for simplification to a single degree of freedom system.