Yes / The validity of the modification factor specified in the ACI 318-11 shear provision for concrete members to account for the reduced frictional properties along crack interfaces is examined using a comprehensive database comprised of 1716 normalweight concrete (NWC) beam specimens, 73 all-lightweight concrete (ALWC) beam specimens, and 54 sand-lightweight concrete (SLWC) beam specimens without shear reinforcement. Comparisons of measured and predicted shear capacities of concrete beams in the database show that ACI 318-11 provisions for shear-transfer capacity of concrete are less conservative for lightweight concrete (LWC) beams than NWC beams. A rational approach based on the upper-bound theorem of concrete plasticity has been developed to assess the reduced aggregate interlock along the crack interfaces and predict the shear-transfer capacity of concrete. A simplified model for the modification factor is then proposed as a function of the compressive strength and dry density of concrete and maximum aggregate size on the basis of analytical parametric studies on the ratios of shear-transfer capacity of LWC to that of the companion NWC. The proposed modification factor decreases with the decrease in the dry density of concrete, gives closer predictions to experimental results than does the ACI 318-11 shear provision and, overall, improves the safety of shear capacity of LWC beams.