THE CONCRETE BRIDGE MAGAZINE

FALL 2018

ASPIRE is a quarterly magazine published by PCI in cooperation with the associations of the National Concrete Bridge Council. The editorial content focuses on the latest technology and key issues in the Concrete Bridge Industry.

Issue link: http://www.aspiremagazinebyengineers.com/i/1027820

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0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.20 0.20 0.40 0.50 0.60 0.70 s (Volumetric Ratio) P u / f' c A g Spiral Requirements for Piles f' c = 8000 psi, f yh = 60 ksi, 2 in. Cover ACI 318-02 NZS NZS UBC & IBC ATC-32 PCI RECOMMENDED C O N C R E T E B R I D G E T E C H N O L O G Y 42 | ASPIRE Fall 2018 The origins of confinement reinforcement requirements—specifically, circular spiral for square, octagonal, or circular concrete piles—have long been somewhat of a mystery. The outcome has been a myriad of prescriptive rules, which vary widely depending on the code, standard, or specification applicable to a project. The graph below shows the historical variability of some of these prescriptive requirements for 24-in.-octagonal prestressed concrete piles. Inexplicably, the requirements also vary significantly depending on whether the piles are cast-in-place or precast concrete and driven. These provisions affect the size, spacing, and depth below the pile head where varying quantities of spiral reinforcement are required. Clearly, these provisions significantly affect the cost of piles. This article reviews the status of recent research and its applicability to concrete piles. Background Th e p r im a r y p u r p o s e s o f s p i r a l reinforcement are to provide confinement to the pile concrete core so that it behaves in a duc tile manner under combinations of axial and lateral loads, to provide support to restrain bucking of nonprestressed longitudinal reinforcement, and to provide adequate shear strength. The lateral loads imposed by earthquakes are of great concern. Therefore, the spiral reinforcement requirements naturally escalate as the Seismic Design Category (SDC) increases. Spiral reinforcement in high SDCs, sized in accordance with current requirements, can become very heavy, and, in some cases, may be unconstructable, particularly for smaller pile sizes. For driven piles, the spiral also confines the concrete at the head and tip to mitigate bursting during driving. Mild steel driving rings have also been used for this purpose. I n 1 9 9 3 , t h e P r e c a s t / P r e s t r e s s e d Concrete Institute (PCI) published its "Recommended Practice for Design, M a n u f a c t u r e, a n d I n s t a l l a t io n o f Prestressed Concrete Piling" (RP). 1 This document provided equations for determining spiral volumetric ratios in moderate and high seismic regions, based primarily on research performed in New Zealand by Joen and Park. 2 However, the equation in the PCI RP for high seismic regions provided roughly half of the spiral volumetric ratio recommended by the New Zealand research shown in the graph. This PCI equation was proposed in the apparent belief that half of the target ductility sought by the New Zealand researchers would be sufficient for high seismic regions in the United States, although the reason for this conclusion is not clear. The PCI RP equation was adopted in the 2000 edition of the International Building Code (IBC); however, IBC 2000 maintained the upper limits on volumetric ratio from previous editions. 3 Chapter 20 of the PCI Bridge Design Manual 4 recommends the full volumetric ratio of spiral resulting from the New Zealand research, although this recommendation has not been adopted into American Association of State Highway and Transportation Officials' AASHTO LRFD Bridge Design Specifications . 5 Iowa State University Research In light of the uncertainty surrounding prescriptive requirements for spiral reinforcement, PCI funded a research project in 2006 at Iowa State University to develop a rational means of determining spiral volumetric ratios in prestressed concrete piles. The results of this research were detailed in a final report, 6 and a summary was published in the PCI Journal . 7 A single equation is proposed to quantify spiral volumetric ratios depending on target curvature ductility. A value of target curvature ductility is suggested for high seismic regions based on a review of literature published on pile testing and a Confinement Reinforcement Requirements for Concrete Piles by Stephen J. Seguirant, Concrete Technology Corporation Variability of spiral reinforcement requirements based on axial load level for 24-in.-octagonal prestressed concrete piles. Note: ATC = Applied Technology Council; NZS = New Zealand Standard. All Figures and Photos: Concrete Technology Corporation.

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