The need for structural integrity has been recognized ever since the 1968 failure of the Ronan Point Apartment building. Improvements to the ACI code in 1989 required additional reinforcement for structural integrity, however those requirements were based on generally good building practices with little research or analysis to support them. However, since the disproportionate failure of the Murrah Federal building in Oklahoma City, these requirements have received renewed interest and new research conducted. More recently, and primarily due to the aftermath of natural and manmade disasters, the need for designing buildings that are resilient against various hazards has been recognized.

While most of the latest research does not directly analyze the efficacy of the structural integrity requirements, it does consider the overall collapse resistance and robustness of reinforced concrete buildings. Research using field experiments conducted in the last decade indicates that reinforced concrete structures are generally robust against local damage like single column removal. Although structural integrity requirements have been included in ACI 318 since 1989, there still exists areas of improvement. For example, recent laboratory experiments show that flat plate structures may still be vulnerable due to the high likelihood of progressive punching shear failures. Furthermore, for structures designed and built without structural integrity provisions, new research highlights ways to improve their robustness and collapse resistance. Finally, improved analysis models and predictions on the likelihood of collapse lead to better assessment of the risks of collapse.

ACI Committee 377 sponsored two sessions during the Fall 2014 ACI convention in Washington, DC to highlight the importance of structural integrity and resilience of reinforced concrete and precast/prestressed structures subjected to extreme loading conditions. The sessions sought papers on topics including improving the structural integrity of structures, minimum level of required integrity, integrity of precast/prestressed structures, performance-based structural integrity and resilience, infrastructure resilience, issues and new developments in modeling, and assessment of existing structures. Both experimental and analytical investigations were presented. The sessions presented 10 papers covering the design of reinforced concrete buildings against progressive collapse, evaluation of NYC code provisions, analysis and experimental testing of post-tensioned and precast/prestressed structures, methods to improve collapse resistance, and probabilistic analysis of collapse.

This special publication includes eight papers that were presented during the sessions. The papers are alphabetically ordered based on the last names of the first authors.

The possibility of a local structural failure causing global collapse of a 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 prevention 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 reinforced concrete structural model to predict catastrophic failure. The performed analysis indicates that, at the point of incipient failure, uncertainties caused by the variability of the design parameters 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.

This paper presents a two-scale computational model for probabilistic analysis of the collapse behavior of reinforced concrete (RC) buildings subjected to local structural damage. In this model, structural members are modeled as elastic blocks connected by a set of nonlinear cohesive elements, which represents various damage zones that could potentially form during the collapse process. The random constitutive behavior of the cohesive element is determined by the fine-scale stochastic finite element simulations of the corresponding potential damage zone under different loading scenarios. The proposed model is validated through the numerical simulations of recent experiments on a RC frame subassemblage and two flat-slab systems. With the proposed model, a stochastic analysis is performed to investigate the probabilistic collapse behavior of a 10-story RC building under various initial damage scenarios, where the random material properties and gravity loading are sampled by using the Latin Hypercube Sampling (LHS) method. Through stochastic simulations, the occurrence probabilities of various collapse scenarios are calculated and the results are compared with those obtained by using the existing deterministic analysis method.

In the case of a punching shear failure of a slab-column connection, a certain degree of resistance to a progressive collapse in a flat-plate structure can be provided by post-punching capacity. Understanding the behavior of slab-column connections after punching shear failure is critical to understanding if the post-punching capacity could be used to limit the progression of collapse in flat-plate structures. This paper presents test results of eight isolated slab-column specimens with three main variables to investigate post-punching capacity: tensile reinforcement ratio, anchorage of tensile reinforcement, and static or dynamic loading. The results showed that post-punching capacity in slab-column connections with discontinuous compressive reinforcement and no anchorage of the tensile reinforcement was on average 53% of the punching load in static tests. However, this capacity existed only immediately after a punching failure and the residual capacity quickly dropped off as the reinforcing bars were ripped out of slab concrete. If the tensile reinforcing bars were anchored into slab, the post-punching capacity reached 80% of punching capacity with an increasing post-punching capacity with displacement until tensile rupture of the top reinforcement.

This paper explains the physics behind some of the reinforced concrete structural integrity provisions in the 2014 New York City Building Code (NYCBC). These provisions were developed from 2003 to 2007 and adopted in the 2008 NYCBC and have been in effect since then. The provisions studied are the ones related to the mitigation of punching shear failure. This paper shows that the additional bars provide resistance to punching shear failure that covers nearly all dead load and reasonable and different amounts of live load. The provisions are not onerous, having been used in NYC for the last six years and the authors believe they represent an improvement over the prescriptive and non- quantified ACI provisions.