Shear Strength of Continuous Lightly Reinforced Concrete Joist Systems

Abstract

The objective of this research is to study the shear strength of continuous lightly reinforced concrete joist systems. Six two span joists, with and without web reinforcement, and two multiple web joists without web reinforcement were tested. The main focus of this study was to determine the shear cracking capacity and to investigate load sharing between joists. Shear cracking loads are determined using crack pattern and stirrup strain analyses. Behavior is evaluated in both the .o,.irive and the negative moment regions. The primary variables in this research are the longitudinal reinforcement ratio, p,.. (0.76% and 1.04% for negative moment regions and from 0.79% to 2.43% for positive moment regions), and nominal stirrup strength, Pvfvy (0 to 70 psi) for single web joists and placement of the load in multiple web joists. Stirrup effectiveness in joists is analyzed based upon ACI provisions and the number of stirrups intercepted by the critical shear crack. Nominal shear stresses and load sharing between the joists are compared with current ACI design pro , The tests indicate that ACI 318-89 overestimates the shear cracking load and shear capacity of lightly reinforced concrete joists in negative moment regions, and under estimates the shear cracking load but not the shear capacity in positive moment regiOns. In the study, the stirrup contribution in both the negative and positive moment regions equaled or exceeded the value predicted by ACI 318-89. In the positive moment regions of members with stirrups, the concrete contribution to shear capacity was often below the shear cracking load, contrary to the usual assumption. The study indicates that significant load sharing occurs between the joists, but that the load sharing is adequate only to distribute local overloads. The additional I 0% in the concrete contribution to shear capacity, as allowed by ACI 318-89, is not available for joist systems as a whole.