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Behavior of Earthquake-Resistant Reinforced Concrete Column-Foundation Connections

Neupane, Utsav
Lequesne, Rémy D.
Lepage, Andrés
Darwin, David
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Abstract
Reversed-cyclic tests of large-scale reinforced concrete column-foundation connections with hooked column longitudinal bars were performed to investigate the mechanisms of moment-transfer and breakout failure. Two tests were conducted and then analyzed in combination with results from four tests reported in Niyonyungu (2024) and one test reported in Chen (2021). The columns were 2-ft (605 mm) square and longitudinally-reinforced with 12 or 16 No. 8 (25 mm) bars. Other variables included column hooked-bar embedment length (15 or 19 times the column bar diameter), foundation thickness (18 or 30 in. (460 or 760 mm)), the presence or absence of foundation shear reinforcement, and foundation flexural reinforcement (No. 6 (19 mm) bars spaced at 5.5, 9, or 12 in. (140, 225, or 300 mm) in two directions). The nominal concrete compressive strength was 6000 psi (42 MPa) and reinforcement was Grade 60 (420). Test results show that specimen strength was limited by column bar yielding and drift ratio capacity was governed by breakout. Results show that, for connections like those tested, drift ratio increases when either: a) column longitudinal bar yield strain penetration from the top of the foundation to the onset of the bends is delayed, or b) breakout resistance is increased. Increased foundation longitudinal reinforcement ratio and longer column bar embedment both delayed yielding at the onset of the column-bar bend and correlated with greater drift ratio capacity. Providing foundation shear reinforcement with shear strength exceeding 40% of the transfer moment based on the eccentric shear stress model, ignoring the concrete contribution, was shown to delay breakout and increase drift ratio capacity. The top mat of foundation reinforcement participated more in resisting transfer moment than implied by a strength model based on equal participation of top and bottom mats, indicating the top mat of reinforcement should be designed for the full transfer moment instead of splitting the reinforcement between top and bottom mats. Top mat strains, which were much more sensitive to column bar embedment length than to foundation depth, were largest within the column width and decreased as the distance from the column face increased up to approximately 1.5 times the column hooked-bar embedment length, suggesting that the concept of a transfer width is useful for designing foundations for column moment transfer. Taken together with the observation that top mat strains were more affected by column hooked-bar embedment length than foundation thickness, the definition of transfer width for moment transfer calculations should based on column bar embedment length.
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2024-10
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University of Kansas Center for Research, Inc.
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Neupane, U., Lequesne, R. D., Lepage, A., and Darwin, D., “Behavior of Earthquake-Resistant Reinforced Concrete Column-Foundation Connections,” SM Report No. 162, The University of Kansas Center for Research, Inc., Lawrence, KS, October, 2024, 250 pp.
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