Punching Shear Strength and Deformation Capacity of Fiber Reinforced Concrete Slab-Column Connections under Earthquake-Type Loading.

Abstract

The use of discontinuous steel fiber reinforcement to increase punching shear strength and deformation capacity of slab-column connections was investigated. Both gravity-type and earthquake-type loadings were evaluated. For comparison purposes, the effectiveness of headed stud reinforcement in connections subjected to lateral displacement reversals was also investigated.

The experimental research was conducted in three stages. First, ten slabs with different fiber reinforced concretes and steel reinforcement ratios were tested under monotonically increased concentrated load. Specimens with a 1.5% volume fraction of either regular strength (160 ksi) or high-strength (335 ksi) hooked steel fibers exhibited the best performance in terms of punching shear strength and deformation capacity. These two materials were therefore selected for further investigation.

In the second research stage, two approximately 1/2-scale slab-column subassemblies reinforced with the materials selected from the first testing phase were tested under combined gravity load and uni-axial lateral displacement reversals. Both specimens were able to sustain drift cycles up to 5% with connection rotations exceeding 0.05 rad under a gravity shear ratio between 0.19 and 0.45.

In the last research phase, three approximately full-scale slab-column connections, two reinforced with fiber reinforced concrete and the other with headed shear studs, were tested under combined gravity load and bi-axial lateral displacements. The two fiber reinforced concrete subassemblies showed a drift capacity of approximately 2.3% in each principal direction, with average peak connection rotations on the order of 0.04 rad under a gravity shear ratio of approximately 0.4. The specimen with shear stud reinforcement, on the other hand, failed by punching shear during the cycle at 1.15% drift in each principal direction. Average peak connection rotation for this specimen was 0.026 rad.

Based on experimental results, it was concluded that the use of fiber reinforced concrete with either regular strength or high-strength hooked steel fibers in a 1.5% volume fraction is effective in increasing punching shear resistance and deformation capacity of slab-column connections subjected to combined gravity load and lateral displacement reversals. The limited test results indicate that design criteria for headed shear studs in the 2008 ACI Building Code may not be conservative.