International Concrete Abstracts Portal

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.

The cross-section and reinforcement in a concrete beam must be selected to provide sufficient strength at the ultimate limit state while limiting the service deflection to an acceptable magnitude. ACI 318 analytical models for flexural capacity and deflection of slender beams assume that plane sections remain plane after bending and perpendicular to the longitudinal axis, but this hypothesis ignores the presence of diagonal cracking and related deformations associated with the imposed shear. This paper reports on an analytical deflection model developed using simplifications to the Modified Compression Field Theory that superimposes contributions from the flexural deformations arising from member curvatures and the shear deformations arising from diagonal cracking. The model is shown to be in better agreement with test data than the ACI 318 deflection model that only accounts for curvatures. A parametric study was completed using the model to gain insight into the influence of beam span-to-height ratio and the longitudinal and transverse reinforcement ratios on beam deflection. Recommendations are made on using a holistic design approach to satisfy both strength and serviceability requirements for a given span-to-height ratio.

Short coupling beams are susceptible to brittle shear failures that need to be suppressed with dense transverse and diagonal reinforcement. To reduce the amount of shear reinforcement and improve the service behavior, researchers have proposed a solution with steel fiber-reinforced concrete (FRC). However, while this solution is promising, there are no sufficiently simple mechanical models capable of describing the complete shear behavior of short FRC coupling beams. This paper proposes such a model based on first principles: kinematics, equilibrium, and constitutive relationships for the mechanism of shear resistance. The model is compared with tests from the literature and with a significantly more complex finite element model (FEM). It is shown that, while the proposed kinematic approach requires a straightforward input and negligible time for computations, it also provides a similar (or better) accuracy as the FEM with excellent shear strength predictions.

The research described in this paper studies the effect of the effective depth, d, on the shear behavior of large reinforced masonry beams. Five fully grouted shear critical reinforced masonry beams ranging in effective depth from 300 mm to 1400 mm were tested to failure under three point loading to investigate their cracking behavior and ultimate shear strengths. The experimental shear strengths were compared with the failure shear stresses predicted using three different design codes: the TMS 402 code, the CSA S304.1-2004 code and the CSA A23.3-14 code for reinforced concrete. The test results show that the size effect in reinforced masonry is real and very significant, in that failure shear stresses decreased as the effective depth increased. It is shown that as the effective depth increases, the longitudinal crack width and spacing at mid-depth increase as well. These wider cracks initiate shear failure at a lower shear stress due to reduced aggregate interlock capacity. It is shown that the TMS masonry design code gives non-conservative predictions of the shear strength of large masonry beams. The most accurate prediction of the size effect in masonry is given by the CSA A23.3 -2014 code which is based on the General Method of shear design used extensively to design reinforced concrete. The paper highlights the necessity to revise masonry design codes to address the size effect.

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.