Providing External Confinement to Flexural Members to Improve Ductility – The Good, The Bad, And The Ugly

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Title: Providing External Confinement to Flexural Members to Improve Ductility – The Good, The Bad, And The Ugly

Author(s): Andrew Budek and Gianmario Benzoni

Publication: Special Publication

Volume: 258

Issue:

Appears on pages(s): 69-84

Keywords: FRP, flexure, piles, experimental, development length

Date: 12/1/2008

Abstract:
Development of the full inelastic lateral capacity of a reinforced concrete pile shaft is likely to require the formation of a plastic hinge below grade level. It has been shown through analytical and experimental investigation that the soil around the pile has a significant confining effect on the pile shaft, allowing the development of larger plastic strains in the compression zone than would be predicted based on the amount of transverse reinforcement provided. It was postulated that this confining effect could be built into precast prestressed piles by the addition of a GFRP jacket in the potential plastic hinge region during the construction process. Two large-scale prestressed pile specimens were thus fitted and tested in flexure to simulate a typical subgrade moment pattern. The piles exhibited higher flexural strength and significantly lower ductility capacity than a control specimen which did not have a GFRP jacket. Failure was through complete tendon rupture at a wide flexural crack which opened at the point of maximum moment. High clamping pressures from the jacket upon the tendons were caused by dilation of the compression zone. This pressure ‘anchored’ the tendons under the jacket, preventing bond slip over a wide region and forcing large inelastic strains into the short tendon length exposed at the major flexural crack. The ACI 318 equation for development length was found to give a reasonable quantitative prediction of the enhanced bond strength, expressed as reduced flexural transfer (i.e., development) length of the tendons by considering active confining pressure.