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37 Figure 21. Comparison of longitudinal strains. Summary gate interlock, interface shear transfer, or shear friction. The last of these terms will be used here. The interface on which The current provisions, in which the yield strength of A1035 shear acts is referred to as the shear or slip plane. A schematic stirrups is taken as 100 ksi, were used to design reinforced- representation of the shear friction mechanism is shown in concrete and prestressed beams. These specimens performed Figure 23. The shear friction mechanism arises by virtue of well in terms of crack patterns, crack widths, and capacity. the roughness of concrete crack interfaces. As a rough inter- The experimental data do not suggest any unusual attributes face displaces in a shear mode (slipping, resulting in a defor- insofar as using A1035 as shear reinforcement. mation as shown in Figure 23), a "wedging action" develops forcing the crack to open in a direction perpendicular to the interface (crack width, w). This crack opening or "dilation of 2.6 Shear Friction the shear crack" engages the reinforcement (having area Avf) The shear-carrying mechanism present when shear is crossing the crack resulting in a "clamping" force, Avf fs being transferred across a concrete interface subject to Mode II generated. The clamping force attributed to the interface (sliding mode) displacement is commonly known as aggre- reinforcing steel, Avf fs , is engaged as the crack opens. Thus, Figure 22. Load--average stirrup forces (Specimen SR2).