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This Symposium Volume reports on the latest advancements related to the various facets of modeling and performance assessment of concrete structures. The volume contains 10 papers that were presented at the ACI Convention held in Toronto on April 1st, 2025. The symposium was dedicated to celebrate Prof. Frank J. Vecchio’s extraordinary research contributions and accomplishments in the development of behavioral models and analytical tools for the assessment of concrete structures. The papers cover different aspects related to modeling and performance assessment of concrete structures including developments of the Modified Compression Field Theory, finite element modeling of punching shear in slabs, behavior and modeling of steel fiber reinforced concrete members subjected to torsion, modeling of concrete structures subjected to impact loading, behavior and modeling of slender walls, modeling of concrete frame elements, behavior and modeling of GFRP reinforced members, crack-based assessment of concrete structures, and advancements in modeling deterioration mechanisms and repaired concrete structures. Sincere acknowledgements are extended to all authors, speakers and reviewers as well as to ACI staff for making this symposium a success. Anca-Cristina Ferche, Editor Vahid Sadeghian, Editor

The modified compression field theory (MCFT) is a general model for the behavior of diagonally cracked reinforced concrete. When applied to beams and columns, a number of very significant simplifications can be made to make it easier to apply in practice for day-to-day design and strength assessment. While the methods used to generate the simplified MCFT have been explained in previous papers, the actual historical and technical process that led to the work was somewhat different than the technical arguments made in the previous publications. This paper explains the procedures that actually occurred in 1999 to 2002 to generate the equations of the simplified MCFT for members with stirrups. The process shows the importance of working backwards from solutions and engineering intuition in the generation of technical theories. While the analysis method ended up being practical and technically sound, its creation was a people-based process and learning about it can hopefully be helpful to future researchers who want to know how “it really happened”.

Punching shear failure of RC flat slab connections cause loss of slab’s supports. The detached slab is falling and impacting the slab below. That problem requires thorough investigation and appropriate design guidelines. This paper presents research results on various aspects of this impact scenario. The analysis is based on an advanced numerical model that has been formulated, and the impact analyses follow the damage evolution in the concrete and reinforcement until complete connections failure of the impacted slab is developed, and a progressive collapse scenario starts. The effects of slab geometry and material properties were examined, and the contribution of special shear reinforcement and integrity rebars were investigated. The potential contribution of added drop panels to enhance slab resistance were examined. The slabs impact effect on the supporting columns has been investigated as well. The suitability of current static loading design-criteria to provide safe design against dynamic/impact punching shear is assessed. It shows that the current static-loading based design standards cannot ensure resilience of flat slab connections to impact loading and therefore cannot prevent a progressive collapse scenario. Analyses results are compared with inspected failure details of a collapsed RC flat slabs parking garage building, and excellent agreement is obtained.

Experimental testing of a reinforced concrete shear wall subjected to combined axial load and reverse cyclic lateral displacements was conducted to investigate rocking and sliding observed in a companion wall tested without axial loading, and to assess the effect of axial load on residual drifts. The application of 10% axial load resulted in greater lateral load capacity and stiffness, as well as increased ductility. The presence of axial load contributed to satisfying lower residual drift limits at higher transient drifts. Further analysis was conducted to disaggregate the total lateral displacement into sliding, rocking, shear, and flexure mechanisms. Comparison to the companion wall demonstrated that the present wall had significantly greater contribution from flexural effects with the axial load delaying the influence of rocking until crushing of the concrete. A complementary numerical study of the wall with axial load was conducted, and a modelling methodology was presented to better capture the fracture phenomena of steel reinforcement. This methodology accounted for local fracture of reinforcement and a reduction of reinforcement area due to the presence of strain gauges. The simulation of failure and the predicted lateral displacement capacity were significantly improved compared to a model that did not consider these phenomena.

Current nonlinear modeling software for concrete frames typically employs line elements with plastic hinges defined at user-selected locations. While this is a simple and computationally efficient approach, a number of drawbacks limit its application. They include the challenges with defining the interacting shear and moment hinge curves, uncertainties with hinge locations and lengths, and difficulties in capturing the post-peak response. Two-dimensional continuum methods address these limitations, but their computational cost limits their applicability. This study presents an alternative modeling method, and associated computer software, with the objective of combining the simplicity of frame elements with the accuracy and result visualization capabilities of continuum methods. The method, developed in the last two decades, employs a distributed-plasticity, layered-section approach based on the Disturbed Stress Field Model (DSFM). The distributed-plasticity approach eliminates the need for defining plastic hinges while the DSFM enables capturing the shear, moment, and axial force interaction. The total-load and secant-stiffness formulation provides numerically stable solutions, even in the post-peak region. This paper presents an overview of the theoretical approach, unique aspects, and capabilities of this method. The validation studies undertaken for 148 experimental specimens, subjected to static (monotonic and cyclic) and dynamic (impact, blast, and seismic) load conditions, are also presented.