Pile and Pile Cap Connection

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July-August 2001 1Kent A. Harries, Ph.D.Assistant ProfessorDepartment of Civil and Environmental EngineeringUniversity of South CarolinaColumbia, South CarolinaMichael F. Petrou, Ph.D.Associate ProfessorDepartment of Civil and Environmental EngineeringUniversity of South CarolinaColumbia, South CarolinaThis investigation studied the capacity of square pile-to-pile cap connections where the precast, prestressed pile is simply embedded in the cast-in-place pile cap. Both experimental and analytical results are presented. It is shown that the plain embedment can develop the flexural capacity of the pile without distress to the pile cap or connection region provided that a sufficient embedment length is furnished. Equations for determining the required embedment length are provided. For design purposes, it is recommended that a plain embedment length equal to the width of the embedded pile be used.Piles, particularly those embedded in soft soils, may be subjected to large lateral deflections in the event of an earthquake. The lateral deflections can result in high local curvature and moment demands at various loca-tions along the pile length as shown in Fig. 1. Of particular concern is the behavior at the pile-to-pile cap interface.At this location, very high moment demands result from the assumed fixity of the pile-to-pile cap connection. In order for this behavior to occur as assumed, the connection must be able to transmit lateral forces to the pile and remain essentially rigid. For this discussion, it is assumed that the pile cap may translate but not rotate. If rotation is permitted, the demands on the connection are reduced.Behavior of Precast, Prestressed Concrete Pile to Cast-in-Place Pile Cap Connections82 PCI JOURNALJuly-August 2001 83Severe pile damage has been ob-served in past earthquakes.1,2 Pile de-sign for seismic loading assumes that the pile can develop and maintain its moment capacity through large de-formation demands. Indeed, signifi-cant research1,2 has shown that well-detailed precast, prestressed piles can develop and maintain large moments. Although a pile may be detailed to resist large forces, it is also necessary that the pile-to-pile cap connection be able to transfer these forces. There are only a few published investigations which report the behavior of the pile-to-pile cap connection. These stud-ies are summarized further on in this paper. The objective in designing the pile-to-pile cap connection is to provide a connection capable of developing the moment demands on the pile while remaining essentially rigid. Conser-vatively, this requires the connection to be able to develop the theoretical capacity of the pile while remaining elastic. In this paper, the specific case of precast, prestressed piles embedded in cast-in-place pile caps is considered.PILE EMBEDMENT DETAILSThere are a number of options for detailing pile-to-cap connections. Fig. 2 shows the connection detail currently used by the South Carolina Depart-ment of Transportation (SCDOT). It is reported that this detail costs close to $800 per pile to fabricate. The objec-tive of the study presented here was to address this issue and determine if less expensive details could provide ad-equate lateral load resisting capacity.There are a variety of details pro-posed and used in the embedment re-gion of piles in cast-in-place pile caps. Fig. 3 shows a number of these details which are described as follows:A. No treatment; the pile is simply embedded in the pile cap.B. Roughening the exterior of the pile (using a rotary or chipping ham-mer, for instance) to provide additional mechanical bond between the pile and pile cap.C. Grooving (cut or cast in place) the pile surface to provide additional mechanical bond.D. Embedding vertical dowels in the driving head of the pile (after driving).Fig. 1. Bending of long piles due to horizontal ground motion (adapted from Joen and Park,1 1990).Fig. 2. Pile anchorage detail required by SCDOT.E. Drilling horizontal dowels through the pile.F. Confining the immediate embed-ment region with hoop or square spiral reinforcement.G. Confining the immediate em-bedded region with round spiral reinforcement.H. Exposing the strands and em-bedding them in the cast-in-place concrete. Often, the wires will be broomed (separated) or twisted open to form an annular space (a so-called olive anchorage) to im-prove their development.Typically, embedments will include a combination of these details. For example, the SCDOT detail shown in Fig. 2 incorporates Details B, D, and G. Each additional detail has an as-sociated cost in terms of both money and time. 84 PCI JOURNALStudies of Pile Embedment DetailsJoen and Park1 reported tests of six pile-to-pile cap connection types. The piles were tested under combined axial load and reversed cyclic lateral loads. The axial load was kept constant at 0.2Ag f c for all tests. All piles tested were 16 in. (406 mm) octagonal piles. Two specimens were provided with a 32 in. (813 mm) embedment having Details B and G (see Fig. 3). Another two specimens had 24 in. (610 mm) embedments with Details G and H (exposed strand left straight). A fifth specimen had a 36 in. (914 mm) em-bedment with Details G and H (ex-posed strand provided with an olive anchorage). The final specimen was provided with only a 2 in. (51 mm) embedment and Details D and G.The theoretical capacity of the pile was obtained in each test and only the sixth detail showed significant distress to the pile cap and the embedment re-gion, which led to a significant decay of the load-deflection response of the pile.1 Sheppard2 summarized the results of both experimental and post-earthquake field investigations. Two embedment details were presented as being ad-equate for the pile-to-pile cap connec-tion to behave in a desirable manner. The first suitable detail is H; the sec-ond is D. It is implied that confining Detail G is also provided.Curiously, the details presented by Sheppard show minimal embedment of the pile, similar to the sixth speci-men described by Joen and Park.1 As such, it would appear that the details recommended by Sheppard may be inadequate for severe seis-mic loading. No experimental results concerning this aspect are reported by Sheppard.PLAIN PILE-TO-PILE CAP EMBEDMENTThe objective of this study is to in-vestigate the behavior of a plain em-bedment (Detail A in Fig. 3). In this detail, the capacity of the pile is de-veloped along the length of the em-bedment. The pile-to-pile cap connec-tion should be designed such that it is adequate to develop the theoretical moment capacity of the pile. Note that the scope of this study is restricted to driven precast, prestressed piles em-bedded in cast-in-place pile caps.There are two proposed models, namely, Mattock and Gaafar3 and Marcakis and Mitchell,4 for determin-ing the capacity of the pile-to-pile cap connection. Both models assume that a rigid body (pile) is embedded in a cast-in-place concrete monolith (pile cap). Both models are based on the mobilization of an internal moment arm between bearing forces Cf and Cb as shown in Fig. 4.Mattock and Gaafar3A parabolic distribution of bearing stresses is assumed for Cb, and Cf is computed by a uniform stress equal to 0.85f c. The bearing stresses are distrib-uted over the width of the embedded pile, b. Following these assumptions and calibrating the calculated stresses against experimental data, the required embedment length, Le, may be deter-mined from:Fig. 3. Proposed pile embedment details.July-August 2001 85V f bb bLaL cV f bb bLaLu c eeu c ee j(, \,(+,,,,,]]]]]] j(, \,(+540 58 0 220 884 50 58 0 220 880 66110 6611... ..( ).. .. psi units (1a) MPa units (1b)c,,,,,]]]]]]( )V f bb bLaL cV f bb bLaLu c eeu c ee j(, \,(+,,,,,]]]]]] j(, \,(+540 58 0 220 884 50 58 0 220 880 66110 6611... ..( ).. .. psi units (1a) MPa units (1b)c,,,,,]]]]]]( )where a is the shear span of the pile (distance from pile cap to assumed point of zero moment) and 1 is the concrete stress block factor defined in ACI 319-99, Section It is suggested that the shear span be increased by an amount equal to the concrete cover, c, to account for pos-sible spalling of the soffit of the pile Fig. 4. Analytical methods for determining capacity of embedment.cap as shown in Fig. 4. The value of b is given by Mattock and Gaafar as the width of the element into which (in this case) the pile is embedded.This value is intended to account for the spreading of the compressive stresses away from the embedment as indicated in Fig. 4(b). For a single pile in a pile cap, this value is taken as the width of the pile cap. For a pile group, this value may be conservatively taken as the pile spacing.Marcakis and Mitchell4 Using slightly different assumed stress distributions shown in Fig. 4(b), Marca-kis and Mitchell4 proposed the following expression for determining the