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[SEAOC] Concrete Pile Requirements in Seismic Zones 3 & 4.

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Dec. 5, 1996

Dear Colleagues:

Over the past several years, the SEAOC Seismology Committee has received
numerous inquiries on the ductility capacity of piles (whether it is
curvature ductility capacity in competent soils, or displacement ductility
capacity in soft medium).  With the 1996 Blue Book finally off-the-ground,
the Committee has had an opportunity to respond to some of these issues.  The
attached file is a synopsis of the consensus opinion of the SEAOC Seismology
Committee, which elaborates on the relevant Provisions of the 1994 UBC as
well as Section 302 of the 1996 Blue Book and other internationally
recognized technical reports.  

This "article" which discusses confinement issues in Cast-in-Place and
Precast Prestressed Concrete Piles was specifically prepared in response to a
letter by Carl Gentry to the Committee.  However, we felt it may be of
interest to other SEAOC on-line members as well.  Please look for future
broadcasts on other issues of interest to our membership.

If you have difficulty downloading the complete document (i.e., the Formulae
are missing, etc.) please do not hesitate to call me to receive a faxed copy.

Ali Sadre, Chairman
SEAOC Seismology Committee 

c/o EsGil Corp.
9320 Chesapeake Dr., # 208
San Diego, CA 92123 
Tel: (619) 560-1468
Fax: (619) 560-1576

Mr. Carl Gentry                                                                                              97-06FD-02
Corollo Engineers                                                                                       Dec. 5, 1996                                                                                                 
2700 Ygnacio Valley Rd., Suite 300
Walnut Creek, CA  94598

Subject:  Concrete Pile Requirements in Seismic Zones 3 and 4

Dear Mr. Gentry:

Thank you for your letter dated June 26, 1996, in which you had expressed concerns about the ductility in concrete piles.  Unfortunately, the earthquake regulations in the 1994 UBC for concrete piles are not yet fully developed.  It was the Seismology Committee's intent to work on all foundation issues, including piles, three years ago.  However, the Committee's time has been recently consumed by the Northridge earthquake of 1994, the 1996 edition of the SEAOC Blue Book and the 1997 Strength Design code change submittal to ICBO.  

Fortunately, the foundation provisions of the 1996 SEAOC Blue Book (Chapter 3) go beyond Chapter 18 of the 1994 UBC and address some of your concerns.  Incidentally, the 1996 Blue Book has been published and should be available from the SEAOC office. However, we would like to address some of the issues you have mentioned.

Spiral Reinforcing for CIP Concrete Piles:  Formula 21-2 for the volumetric ratio of transverse reinforcing is the lower bound for reinforced concrete columns and was taken as the requirement for CIP concrete piles in the UBC.  It should be noted that the other Formula, 10-5, is based on the philosophy of maintaining the axial load strength of the column after spalling of the concrete cover at displacement ductility greater than about 2.

This formula is very conservative for columns having small cross diameters.  The smaller cross sections are required to have a much larger confining pressure on the concrete because of the large effect of Ag/Ac.  This ratio has a much lesser effect on larger sections.  We believe the rational might have been that since Formula 21-2 controls for the larger diameter sections, which are typically used for most drilled pier foundations, there was little need to consider Formula 10-5.  

For the smaller drilled CIP concrete pile diameters, 14" to 20", which you have addressed in your letter and also assuming they are supporting major structural elements, we would be very concerned with using Formula 21-2 as the only basis for determining the volumetric ratio of the spiral transverse reinforcement required.  Here, we would recommend using the 1996 SEAOC Blue Book Section 302.2.2 which applies to Formula 10-5 and gives the following multiplier to assure adequate curvature ductility.

                                   Eq. 302-1

This multiplier will reduce the volumetric ratio of transverse reinforcing required using the typical light pile axial load.  The 1996 SEAOC Blue Book qualifies this adjustment for "competent" soils.  Although not given in the Blue Book, we would tentatively recommend spiral or circular hoop spacing not to exceed one-fifth the diameter of the pile, six times the longitudinal bar diameter, or 8 inches, whichever is smaller for the potential plastic hinge zones or compliance with the special transverse reinforcing requirements of  the 1994 UBC, Section  1921.4.4.

Spiral Reinforcing for Precast Prestressed Concrete Piles:  Research has shown that low volumetric spiral ratios for small section precast prestressed concrete piles result in non-ductile pile behavior.  Formula 21-2 has been shown by testing not to be sufficient for ductile precast prestressed concrete piles.  As a stop gap measure, Section 1809.5.2.3 was placed into the 1994 UBC for precast prestressed concrete piles to alleviate the conservatism of Formula 10-5 for small cross sections, yet provide a moderate amount of confinement reinforcement for ductile behavior.  

What has not been addressed in this section is guidance on maximum spiral pitch based on pile diameter and longitudinal reinforcing size.  Tentatively, we would recommend the spiral pitch not exceed one-fifth the diameter of the pile, six times the longitudinal bar or strand diameter, or 8 inches, whichever is smaller for the potential plastic hinge zones.  

Also, Section 2.5.2 of the PCI Piling Committee Report "Recommended Practice for Design, Manufacture and Installation of Prestressed Concrete Piling" published in the March-April 1993 issue of the PCI Journal provides more details on the horizontal reinforcement requirements for seismic design of piles.  

Pile testing in New Zealand (Park & Hoat Joen) has verified that mild longitudinal reinforcing is not required in precast prestressed concrete piles for ductile performance.  However, non-prestressed longitudinal bar reinforcing will permit greater dissipation of seismic energy, higher ultimate curvatures and increased moment capacity if needed.

Connections of Concrete Piles to Pile Caps:  Some guidance is given in the 1996 SEAOC Blue Book for connecting the pile to the pile cap.  However, prescriptive connections have not been given.  New Zealand has recently tested a variety of pile to pile cap connections with success.  We recommend you look at these tested in "Simulated Seismic Load Tests on Prestressed Concrete Piles and Pile-Pile Cap Connections", PCI Journal, November-December 1990 by Park & Hoat Joen and also recommendations from the previously referenced PCI Piling Committee Report.

Kinematic Interaction:  The effects of potentially large pile curvatures at the interface between soft and stiff soil strata are addressed in the 1996 SEAOC Blue Book Commentary, Section C302.2.3.  We would recommend the use of transverse reinforcing for the potential plastic hinge zone in this interface region.
Ratio of Confined Pile Core to Pile Diameter:  You had mentioned a frequently overlooked detail with regard to the relationship between the outside diameter of the pile and the confined core diameter.  The committee is also concerned that small core diameters are detrimental to the pile performance.  Therefore a 30 % reduction in pile capacity is currently recommended for this condition in the 1996 SEAOC Blue Book, Section 302.2.2.5.  We would not recommend a single large reinforcing bar in the center of the pile regardless of whether the pile was "pinned" or not.  Under this condition the pile must rely on the concrete shear capacity only and there is no additional flexural overstrength or displacement ductility due to the lack of a confined core.  

An excellent reference for further information on the design of foundations is "Seismic Design and Retrofit of Bridges" by Priestley, Seible and Calvi published by John Wiley & Sons, 1996.

Again thank you for your concerns.  We hope the above discussion will be of help to you.  On behalf of the Committee, I would like to acknowledge the efforts of Mr. Tom Hale for preparing the draft of the Committee's consensus. 


Ali Sadre, Chairman
SEAOC Seismology Committee