International Concrete Abstracts Portal

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.

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.

A simple debonding technique was proposed to reduce strain localization in reinforcing bars in the region of wide flexural cracks in reinforced concrete (R/C) beams, in order to enhance the resistance of R/C buildings to disproportionate collapse. Debonding was achieved by heat-shrinking polyolefin tube over the reinforcing bar. Results from testing of a No. 8 reinforcing bar showed that with an 8 in (203 mm) debonding length on both sides of a ¼ in (6.35 mm) wide gap, simulating a wide flexural crack, the elongation of the reinforcing bar prior to fracture was about 38 % more than for the case without the debonding technique. This observation demonstrated that the debonding method could effectively reduce strain localization, thereby delaying the fracture of reinforcing bars. To analyze the effects of debonding, detailed finite-element models of the test specimens were developed, which adequately captured the experimental results. R/C frame structures were analyzed by applying the debonding model under a column removal scenario. The results indicated that the debonding method could enhance the development of catenary action in the beams of R/C frame structures.

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.

Structural response of post-tensioned floors following a severe event such as a sudden column loss is an important topic, which has not been extensively studied in the past. In this paper, the system-level response and collapse resistance of an actual post-tensioned parking garage is evaluated analytically and verified by experimental data following the sudden loss of a column. The field data demonstrated that the slab resisted collapse with a permanent maximum vertical displacement of about 2.4 in (61 mm). The development of the membrane forces and their effects on the response of the slab, particularly the region in the neighborhood of the lost column which experienced cracks at the bottom of slab, are studied analytically. According to analytical results, which are in good agreement with experimental data, slab flexural strength is increased due to formation of additional compressive membrane forces. The floor system resists the additional compressive membrane force by formation of a peripheral tensile ring.