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Progressive collapse resistance of an actual six-story reinforced concrete frame structure is evaluated following predefined initial damage. The initial damage was caused by the simultaneous explosion (removal) of two adjacent exterior columns, one of which was a corner column. The mechanism of load redistribution and change in column axial forces (strains) are discussed. In the structure studied, the development of bidirectional Vierendeel (frame) action is identifed as a major mechanism in redistribution of loads. Through careful instrumentation, the change in the direction of beam bending moments in the vicinity of the removed columns is demonstrated. In general, if such a change in the bending moment direction results in high tensile stress in bottom beam reinforcement at the face of a column, brittle local failure in the absence of proper anchorage can occur. This failure did not take place in this building. The nearly century old structure, without satisfying integrity requirements, resisted progressive collapse with a recorded maximum vertical displacement of only 1/4 in. (6.4 mm). DOI: 10.1061/(ASCE)0733-9445(2008)134:3(478) CE Database subject headings: Progressive failure; Structural failures: Collapse; Explosions; Load distribution: Dynamic response: Brittle failure; California.

In order to estimate seismic hazards, there is a need for measures that properly characterize the severity of ground motions. Such measures are used along with the fragility of structures to estimate the probability of failure. Structures subjected to severe seismic ground motions undergo inelastic response. Proper measures of ground motions are required to be well correlated with responses of interest. In this paper, it is shown that modeling a near-source ground motion, by a simple pulse form that matches well with the main velocity pulses of the ground motion, dose not lead to a reliable prediction of the inelastic response of structures having periods of less than about 1 s. An equivalent rectangular acceleration pulse, called the significant peak ground acceleration (SPGA), is defined. Compared to other available measures, the SPGA correlates significantly better with the inelastic response of structures (having periods of less than about 1 s and displacement ductility of at least two). Subsequently, using the main characteristics of structures, a relationship between the SPGA and the response of reinforced concrete (RC) structures is developed. It is shown that the relationship reliably predicts the inelastic response of the structures to pulse-type ground motions. Finally, the efficiency of the SPGA in estimating the fragility of an RC structural wall is compared to that of elastic response spectrum. Given the fact that in the United States about 99.8% of office buildings are less than ten stories, the SPGA and the corresponding demand models for estimating the response of such structures can considerably improve the seismic design and assessment of structures. CE Database subject headings: Bayesian analysis; Earthquake engineering; Ground motion; Nonlinear response; Seismic design.

Initiatives to operationalize the concept of resilience in the building industry are rapidly emerging. The concept of resilience has introduced a way to explore solutions to some important problems in the building industry. However, much of the work that has taken place to date represents activities generally assigned to risk management, which is discussed as being inherently insufficient for sustaining the functions of the built environment under stresses. This commentary considers the opportunities and limitations for mainstreaming resilience into building industry processes and actors. Barriers include indeterminate analytical meaning, event and performance uncertainty, immature regulatory standards setting, and untested enterprise economics. Further, the multiple outcomes of recovery and the relationship between building recovery and adaptation are discussed and, along with economics of resilience investments, a research need highlighted. A simple heuristic is presented to illustrate the complement of resilience to risk management and advance the integration of resilience into existing industry workflows. [ABSTRACT FROM AUTHOR]

An analytical model for the response of a reinforced concrete panel with friction energy dissipators is implemented for different input displacements. For this purpose, a hysteresis element based on an extension of the Bouc-Wen Model is developed. The hysteretic behavior includes the movement of steel anchor bolts inside small holes drilled in the RC panel, as well as the extension of the hole as a result of the impact. The analytical results show good agreement with the experimental results. A simple endochronic model is examined and it is discussed that the Bouc-Wen model could be looked at as a one-parameter endochronic model. These models violate the Il'iushin and Drucker postulates, however it is shown that this has an insignificant effect on the behavior of friction-type systems with small yield displacement compared to the maximum imposed displacement. An equation is developed to calculate the dissipated energy for hysteresis curves that are based on Bouc-Wen model over any time interval of stress or displacement that leads to nondecreasing time history of dissipated energy.

The Bayesian parameter estimation technique is used to develop probabilistic displacement capacity and demand models for reinforced concrete (RC) structural walls. These models are used to assess the seismic fragility of an example RC structural wall. Experimental data are used to develop the capacity model, and nonlinear dynamic analysis is employed to develop the demand model. In the course of developing the capacity model, probabilistic models are also developed for the plastic hinge length, the maximum usable strain of confined concrete including the effect of strain concentration, and the cyclic curvature capacity of the cross section of the wall. The probabilistic models developed in this paper are applicable to a large class of RC structural walls.

Reinforced concrete frame structures built prior to the mid-1970s are susceptible to brittle column failure under seismic action, potentially leading to progressive collapse of the structure. The behavior of columns susceptible to brittle shear-axial failure has been studied previously but rarely has the interaction between damaged columns and the surrounding three-dimensional structure been investigated experimentally and at full scale. In this study, as the second in a series of hybrid simulations, two full-scale reinforced concrete columns of a representative pre-1970s structure were tested at the Multi-axial Full-scale Substructure Testing and Simulation (MUST-SIM) laboratory. Through the use of hybrid simulation, the interaction of the columns with the surrounding structure is studied under a severe seismic motion including vertical excitation. The computational model representing the remainder of the representative 10-story structure is created in the computer program OpenSees. During the hybrid simulation, both physical specimens experience significant loss of shear and axial strength, and the effects of these failures on the surrounding system are described. The three-dimensional computational model in OpenSees allowed for analytical flexural-axial failure of a third column in the structure to occur. The effects of these multiple failures on the response of a full structural system under seismic action are quantified, and the progressive collapse resistance mechanisms are discussed. Copyright © 2016 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Bar slip deformations in reinforced concrete frame structures cause additional flexibility at the boundaries of beams and columns due to penetration of rebar strain into the joints and footings. There are several available analytical models that estimate bar slip deformations, many of which have difficulties capturing the bar slip deformations that occur after flexural-axial damage. A new methodology is proposed to effectively account for bar slip deformations within the existing elements of an analytical model, without increasing the complexity of the system. This new method shows good agreement with previous experimental results and was successfully used in large-scale hybrid simulations. A series of hybrid simulations were conducted involving full-scale concrete columns instrumented to measure the bar slip deformations. The results of these tests, along with previous experimental studies, are used to develop a new model for the slip of rebar embedded in concrete. The proposed bar slip model is developed for use in nonductile columns and is shown to achieve good agreement with experimental results. [ABSTRACT FROM AUTHOR]

Post-punching resistance of a flat slab can help redistribute the gravity loads and resist progressive collapse of a structure following initial damage. One important difficulty with accounting for the post-punching strength of a slab is the discontinuity that develops following punching shear. A numerical simulation technique is proposed here to model and evaluate post-punching resistance of flat slabs. It is demonstrated that the simulation results of punching shear and post-punching response of the model of a slab on a single column are in good agreement with corresponding experimental data. It is also shown that progressive collapse due to a column removal (explosion) can lead to punching failure over an adjacent column. Such failure can propagate throughout the structure leading to the progressive collapse of the structure. Through post-punching modeling of the slab and accounting for the associated discontinuity, it is also demonstrated that the presence of an adequate amount of integrity reinforcement can provide an alternative load path and help resist progressive collapse. [ABSTRACT FROM AUTHOR]

The dynamic gravity-load redistribution of Baptist Memorial Hospital in Memphis, TN, a 20-story reinforced concrete (RC) structure, is evaluated and characterized experimentally and analytically following the removal of an interior ground-floor column. The structure resisted progressive collapse with a measured maximum vertical displacement of only 9.7 mm (0.38 in.). Analytical results using the finite element method are presented, which show good agreement with the experimental data. The propagation of deformation over the height of the structure and its effects on the load redistribution are presented. The effects of the remaining and bent-out steel bars of the removed column on the response of the structure are modeled and evaluated. The response of the structure due to additional dead and live loads and with complete removal of the column (including the steel bars) is analytically evaluated. [ABSTRACT FROM AUTHOR]

Progressive collapse denotes a failure of a major portion of a structure that has been initiated by failure in a relatively small portion of the structure. One approach to evaluate progressive collapse of structures is to study the effects of instantaneous removal of a load-bearing element such as a column. An experimental program is carried out to study the behavior of a 3/8 scaled model of a continuous perimeter beam in a reinforced concrete frame structure following the removal of a supporting column. A detailed finite element model (FEM) is developed and verified to capture the behavior of the beam subjected to large deformation. In order to avoid a detailed FEM of the whole building and to efficiently capture the system response, a three-dimensional nonlinear model of the structure using beam-column and shell elements is also developed. The two models are integrated through hybrid (substructuring) simulations. The potential progressive collapse of the structure and the dynamic load redistributions following column removals are studied. [ABSTRACT FROM AUTHOR]

Through accident or act of terrorism, structures may be subject to conditions that could lead to progressive collapse. Redistribution of loads following an imposed local damage to a structure depends on strength, continuity, redundancy, and deformation and energy dissipation capacities of the structure. For reinforced concrete frame structures, these characteristics depend on the seismic and wind design loads to a great extent. In this paper, using the response of multi-degree-of-freedom and equivalent single-degree-of- freedom systems, it is demonstrated, that the vulnerability of frame structures against progressive collapse caused by man-made hazards depends heavily on their resistance to natural hazards. It is also shown that following the loss of a column and in spite of satisfying the current structural integrity requirements, premature beam bottom bar fracture can occur. Such bar fracture can be avoided if the minimum beam bottom continuous bars are set equal to the minimum flexural reinforcement. [ABSTRACT FROM AUTHOR]

One approach to evaluate progressive collapse of structures is to study the effects of instantaneous removal of a load-bearing element such as a column. In this paper, using experimental and analytical results, potential progressive collapse of an actual 10-story reinforced concrete (RC) structure following the explosion of an exterior column is evaluated. Development of Vierendeel action is identified as the dominant mechanism in redistribution of loads in this structure. The concrete modulus of rupture is identified as an important parameter in limiting the maximum recorded vertical deformation of the system to only 0.25 in. (6.4 mm). The changes in the directions of bending moments in the vicinity of the removed column and their effects such as potential reinforcing bar pullout (bond failure) are studied. Potential failure modes and their consequences are studied. Some shortcomings of integrity requirements in current codes are pointed out and effects of beam reinforcement detail on the development of catenary action are discussed. [ABSTRACT FROM AUTHOR]