Mehrdad Sasani

Experimental and Analytical Progressive Collapse Evaluation of Actual Reinforced Concrete 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. 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.

Progressive Collapse of Reinforced Concrete Structures : A Multihazard Perspective

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.

Gravity Load Redistribution and Progressive Collapse Resistance of 20-Story Reinforced Concrete Structure following Loss of Interior Column

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.

Progressive Collapse-Resisting Mechanisms of Reinforced Concrete Structures and Effects of Initial Damage Locations

AbstractComputational simulations for analyzing progressive collapse resistance of structures following initial damage require specific attention to structural modeling of floor systems. In collapse analysis of RC structures, it is shown that the degrees of freedom of nonlinear beam, joist, and slab sections must include flexural and axial deformations. It is also shown that ignoring torsional cracking of beams can lead to a significant overestimation of the progressive collapse resistance of structures. Evaluating the response of a seven-story RC structure following 15 simulated single column removal scenarios, it is shown that a top floor column removal is more likely to cause structural collapse than failure on a lower floor. This is in part due to the lack of Vierendeel frame action after a top floor column removal. For the simulated scenarios in which the structure resists progressive collapse without experiencing large vertical displacements, the resistance is primarily provided by Vierendeel frame ...

Seismic fragility of short period reinforced concrete structural walls under near-source ground motions

The Bayesian parameter estimation technique is used to develop probabilistic displacement and strength capacity and demand models for reinforced concrete structural walls. Experimental data are used to develop the capacity models, and nonlinear dynamic analysis is employed to develop the demand models. Both flexural and shear failures are accounted for. These models are used to assess the seismic fragility of an example RC structural wall. As a new measure of the ground motion intensity, the significant peak ground acceleration is defined and incorporated in the probabilistic demand models and fragility assessment. It is shown that, for short period structures, this measure better correlates with the inelastic response than the elastic response spectrum.

Life-Safety and Near-Collapse Capacity Models for Seismic Shear Behavior of Reinforced Concrete Columns

In seismic assessment of existing reinforced concrete (RC) structures, it is important to reliably predict potential column shear failure. Shear strength and deformation capacities can be used to evaluate this failure. This paper evaluates the reliability of three available strength models for estimating the shear strength of RC columns using existing experimental test results. Deformation capacity models are developed for life-safety and near-collapse performance levels. The models account for the difference between the deformation of double-curvature and double-ended columns, in which damage can concentrate on one end of the specimen, and that of cantilever columns. The models include the effects of the transverse reinforcement, axial load, and the aspect ratio of columns on their deformation capacities. Using shear force transfer mechanics, a shear strength capacity model is also developed for RC columns that can estimate column shear strength significantly better than the currently available models. In addition to the section and material characteristics, the shear strength capacity model accounts for the effects of the aspect ratio, compressive load, and displacement ductility on the column strength.

Disproportionate collapse research needs

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.

Collapse Resistance of a Seven-Story Structure with Multiple Shear-Axial Column Failures Using Hybrid Simulation

AbstractThe postfailure behavior of nonductile reinforced concrete columns under seismic loadings, and the system-level behaviors that can resist collapse after such failures, are not well understo...

Experimental and Analytical Evaluation of Progressive Collapse Resistance of a Full-Scale Structure Following Sever Loss of Load Bearing Elements

Experimental and analytical studies are carried out to evaluate the response of an actual reinforced concrete structure following severe initial damage, which was caused in part by simultaneous explosion (removal) of four adjacent columns. In addition, two beam segments in the vicinity of the removed columns were also exploded. The structure resisted progressive collapse following such severe initial damage. The progressive collapse resisting mechanisms primarily included the axial-flexural action of the second floor deep beams and Vierendeel action of the flat plate system in floors above. In the analytical studies, a nonlinear model of the structure is used, which accounts for axial-flexural interaction of beams and slabs. The analytical results show good agreement with experimental data.

Defining resilience for the US building industry

ABSTRACT 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.