International Concrete Abstracts Portal

Six large-scale reinforced concrete column-foundation connections, designed to represent interior connections of a continuous foundation slab, were subjected to reversed-cyclic displacements to investigate the effects of foundation longitudinal reinforcement ratio (ρ), foundation thickness (h), column hooked-bar embedment length (ℓe), and foundation shear reinforcement on connection deformation capacity. The specimens were designed to be nominally similar to a specimen from an earlier study, except for the variables of interest. In all specimens, strength was limited by column bar yielding and deformation capacity was governed by concrete breakout, even though the embedment length was adequate to yield the column bars. ACI CODE-318-25 was published late in the course of this study, so specimens did not satisfy ACI CODE‑318-25 Section 25.4.11 requirements for anchorage of bar groups in tension. Test results indicate that improved drift ratio capacity can be achieved by either: a) delaying column-bar yield strain penetration into the foundation with increased foundation longitudinal reinforcement ratio ρ or column longitudinal bar embedment length ℓe; or b) increasing concrete breakout resistance with longer ℓe or foundation shear reinforcement that has a shear strength exceeding the shear demand associated with 40% of the transfer moment based on the eccentric shear stress model assuming vc = 0. The concept of transfer width (based on ℓe instead of h) is useful for designing foundations to resist column moment transfer, but foundation flexural reinforcement for moment transfer should be placed near the top of the foundation instead of divided between the top and bottom mats. Column longitudinal bars should be extended to the bottom mat of foundation reinforcement because terminating the bars near foundation middepth resulted in earlier breakout and lower deformation capacity.

In this study, the behavior of diaphragm-to-wall connections with collector reinforcement and construction joints was investigated. Four slab-to-wall connection specimens were tested under cyclic loading. Diaphragm connection details, such as shear friction reinforcement (that is, slab dowel bars anchored by 90-degree hooks within the wall) and the use of spandrel beams as collectors, were considered as test variables. When fabricating the specimens, concrete was consecutively cast for the wall and slab, and construction joints were placed on the sides of the wall and spandrel beams. The tests showed that the diaphragm connections exhibited the typical ductile behavior characterized by the robust initial stiffness and subsequent post-yield plastic behavior. Before concrete failure on the front of the wall, the load transfer from the diaphragm to the wall was governed by a nodal zone action; then, the subsequent connection behavior was dominated by shear friction as sliding failure occurred on the side of the wall along the slab construction joints. The diaphragm-to-wall connection strengths were evaluated using the strut-and-tie model and shear friction theory. The calculated strengths were in good agreement with the test strengths. Based on the investigation results, design considerations of the diaphragm-to-wall connection were proposed.

Recent tests showed that anchorage failure could be the primary mechanism that limits the strength and deformation capacity of column-footing connections. An experimental program consisting of the reversed cyclic load testing of 16 approximately full-scale column-footing subassemblages was thus conducted to investigate the effect of various reinforcement details on connection strength, drift capacity, and failure mode. The main parameters evaluated were type of anchorage for the column longitudinal bars (either hooks or heads), extension of column transverse reinforcement into the footing, and longitudinal and transverse reinforcement ratios in the footing. Test results indicate that even when column longitudinal reinforcement extends into the joint with a development length in accordance with ACI 318-19, a cone-shaped concrete breakout failure may occur, limiting connection strength and deformation capacity. The use of transverse reinforcement in the connection over a region extending up to one footing effective depth away from each column face proved effective in preventing a concrete breakout failure. However, for the specimens with column headed bars, extensive concrete crushing adjacent to the bearing side of the heads and spalling beyond the back side of the heads led to significant bar slip and “pinching” in the load versus drift hysteresis loops at drift ratios greater than 3%. The use of U-shaped bars in the joint between the column and the footing or slab, as recommended in ACI 352R-02, led to improved behavior in terms of strength and deformation capacity, although it did not prevent the propagation of a cone-shaped failure surface outside the joint region. Based on the test results, the basic concrete breakout strength, Nb, corresponding to a 50% fractile, in combination with a cracking factor ψc,N = 1.25, is recommended when using Section 17.6.2. of ACI 318-19 for calculation of concrete breakout strength in connections similar to those tested in this investigation.

The present study investigated the contribution of slabs to the lateral load-carrying capacity of shear walls coupled with slabs. Cyclic lateral load tests were conducted on five two-story wall specimens at half scale. The test parameters included the thickness of the slab, the wall opening length, the use of punching shear reinforcement, and the use of parallel walls. The test results showed that, due to the slab effect, the strengths of the coupled wall specimens were 38 to 88% greater than the strength of walls without the slab effect. Furthermore, the initial stiffness of the specimens was significantly increased by the slab effect. During early loading, local failure of the slabs occurred at the wall-slab connection. However, the coupled walls exhibited ductile behavior up to a 2% drift ratio, without significant degradation of strength. Nonlinear finite element analysis was performed on the test specimens. Based on the results, the initial stiffness and effective stiffness of the walls and coupling slabs were evaluated for the seismic design of coupled walls.

This study presents an analytical model for estimating the effective compressive strength of a reinforced concrete (RC) column intersected by a floor slab made of relatively low-grade concrete. The proposed model is based on a theoretically sound background and considers the force equilibrium and strain compatibility conditions in the vicinity of the interface between the upper and lower columns and an intervening slab. By using the parametric study results obtained from finite element analyses, the effects of the confinement provided by the surrounding slab and the secondary stresses induced by the Poisson effect in the columns and slab panel zone were formulated in detail. A simple design expression was then derived for the better applicability of the proposed method for the estimation of the effective compressive strength of the exterior and interior columns with an intervening slab, in a unified manner. To verify the proposed model, the test results of 81 exterior columns and 24 interior columns were collected from the literature. It was found that the effective compressive strengths obtained by the proposed methods were in good agreement with these test results.