Editors: Denis Mitchell and Abdeldjelil Belarbi

This Symposium Volume reports on the latest information related to shear in structural Concrete. The volume contains 14 papers that were presented at the ACI Convention held in Salt Lake City on March 27, 2018. The symposium was sponsored by ACI/ASCE Committee 445 “Shear and Torsion”. This event honored Professor Michael P. Collins (University of Toronto) whose enormous contributions in the development of rational behavioral models for shear and torsion of structural concrete have been paramount.

The papers cover different aspects related to shear in structural concrete including: the size effect in shear for both structural concrete and reinforced masonry; developments of the Modified Compression Field Theory; aspects of shear strengthening using FRP strips; the role of experimental measurements in understanding shear behavior; accounting for shear deformations; sustained loading effects on shear in members without transverse reinforcement; crack-based assessment of shear; key aspects in the design of concrete offshore structures, behavioral models for coupling beams; finite element modeling of punching shear in slabs; and seismic design for shear.

Sincere acknowledgements are extended to all authors, speakers and reviewers as well as to ACI staff for making this symposium a success.

Concrete loaded in tension and compression is prone to the effect of sustained loading. In order to deal with that, building recommendations generally use a sustained loading factor in the range 0.7-0.8. Because the shear capacity of members without shear reinforcement is a function of the concrete strength, it may be wondered whether there is also a sustained loading effect in shear. To answer this question 42 reinforced beams without shear reinforcement have been tested. Amongst them, 28 beams were loaded to failure in about 5 minutes for getting short term reference values for the subsequent sustained loading tests. The other 14 beams were subjected to long-term sustained loading, with a ratio between the applied shear load and the short-term shear resistance between 0,86 and 0,98. In the case that no shear failure occurred during the period of sustained loading, which ranged from three months to three years, the beams where loaded to failure. It was shown that, contrary to concrete loaded in direct compression or tension, the shear capacity is not influenced by a sustained loading effect. On the basis of physical modelling of the beam behaviour it could be demonstrated that the contribution of aggregate interlock in the lower part of the curved bending-shear cracks counteracts crack propagation at the top, so that the development of the crack pattern is arrested.

The behavior of FRP strengthened RC members has not been fully clarified due to lack of accurate constitutive laws for the components of the members. This paper presents experimental and analytical investigations of parameters that affect the various material laws of FRP strengthened RC elements. The material laws of concrete in tension, steel in tension, FRP in tension, softening coefficient, and modified Hsu/Zhu ratios were further developed in this research. To study the behavior and the main affecting parameters of FRP strengthened RC members subjected to shear, experimental tests of panels subjected to pure shear stress fields were conducted. The main variables investigated are steel reinforcement ratio, FRP reinforcement ratio and wrapping schemes of FRP sheets. The results show that the tensile behavior of the concrete and steel is altered because of the externally bonded FRP sheets. Also, the softening coefficient and the Hsu-Zhu ratios for FRP-strengthened RC vary greatly compared to RC elements without FRP reinforcement. Modified constitutive laws are proposed and incorporated in the Softened Membrane Model (SMM) to demonstrate the behavior of FRP-strengthened RC element subjected to pure shear. The newly developed analytical model is referred to as the Softened Membrane Model for FRP strengthened RC members (SMM-FRP).

Nonlinear finite element analysis (FEA), when properly calibrated based on experimental results of reinforced concrete slabs, can be useful to perform parametric studies for the investigation of structural behavior and for the development of future design code provisions. In this paper, a study is presented on how certain modeling decisions influence the FEA results for punching shear analyses of reinforced concrete slabs. The “Concrete Damaged Plasticity” model which is offered in the commercial FEA program ABAQUS is considered to simulate the concrete slabs, where the calibration was conducted based on already tested slab-column connections. The calibrated model is further investigated to discuss the impact of numerous modeling parameters including the boundary and loading conditions, finite element mesh type and size, and the constitutive modeling of concrete. Selected parametric studies are presented and recommendations for each of the investigated parameters are provided to better understand the punching shear behavior of reinforced concrete slabs. The results from the analysis of tested slabs and the subsequent parametric studies using the calibrated model show that that the FEA can be used to predict the punching shear strength, deformation capacity and crack pattern of the slabs.

Two research efforts supervised by the author of this paper and the significance of the findings of those research projects are used to facilitate the discussion to explain a couple of the challenges that the structural engineers faced in the recent past. These research programs are (i) development of the new Strut-and-Tie design provisions of AASHTO LRFD Bridge Design Specifications, and (ii) development of a new prestress loss calculation procedure.

This paper was prepared with the aim of contributing to a session organized to honor Professor Michael Collins of the University of Toronto. The discussion provided within this paper is founded on the teachings of Professor Michael P. Collins and on the experiences of the author as a graduate student. More specifically, the author of the paper had the privilege to learn from Professor Collins (i) the importance of accuracy in calibrating design provisions and (ii) the significance of reducing the design expressions down to the essential parameters. Thus, the research projects summarized in this paper serve to present examples for those important concepts and are from the personal experiences of the author.