^{3}/

_{4}times the diagonal dimension of a rectangular pile. Where driven to or penetrating into rock, the spacing shall not be less than 24 inches (610 mm). Where receiving principal support at the end from materials other than rock, or through friction resistance, the spacing shall not be less than 30 inches (762 mm) except that for piles having enlarged bases formed either by compacting concrete or driving a pre-cast base, the minimum center-to-center spacing shall be 4

^{1}/

_{2}feet (1,372 mm). The spacing of piles shall be such that the average load on the supporting strata will not exceed the safe load-bearing value of those strata as determined by test borings or other approved methods.

Longitudinal steel shall be arranged in a symmetrical pattern and be laterally tied with steel ties or wire spiral spaced center to center as follows:

- At not more than 1 inch (25 mm) for the first five ties or spirals at each end; then
- At not more than 4 inches (102 mm), for the remainder of the first 2 feet (610 mm) from each end; and then
- At not more than 6 inches (152 mm) elsewhere.

The size of ties and spirals shall be as follows:

- For piles having a least horizontal dimension of 16 inches (406 mm) or less, wire shall not be smaller than 0.22 inch (5.6 mm) (No. 5 gage).
- For piles having a least horizontal dimension of more than 16 inches (406 mm) and less than 20 inches (508 mm), wire shall not be smaller than 0.238 inch (6 mm) (No. 4 gage).
- For piles having a least horizontal dimension of 20 inches (508 mm) and larger, wire shall not be smaller than
^{1}/_{4}inch (6.4 mm) round or 0.259 inch (6.6 mm) (No. 3 gage).

^{3}/

_{8}inch (9.5 mm) diameter. Spacing of transverse reinforcement shall not exceed the smaller of eight times the diameter of the smallest longitudinal bar or 6 inches (152 mm) within a distance of three times the least pile dimension from the bottom of the pile cap. Spacing of transverse reinforcement shall not exceed 6 inches (152 mm) throughout the remainder of the pile.

The effective prestress in the pile shall be not less than 400 psi (2.76 MPa) for piles up to 30 feet (9144 mm) in length, 550 psi (3.79 MPa) for piles up to 50 feet (15 240 mm) in length and 700 psi (4.83 MPa) for piles greater than 50 feet (15 240 mm) in length.

Effective prestress shall be based on an assumed loss of 30,000 psi (207 MPa) in the prestressing steel. The tensile stress in the prestressing steel shall not exceed the values specified in ACI 318.

_{s}= 0.04(

*f '*/

_{c}*f*)[2.8 + 2.34

_{yh}*P*/

*f '*)]

_{c}A_{g}**(Equation 18-5)**

where: | ||

A_{g} |
= | Pile cross-sectional area square inches (mm^{2}). |

f '_{c} |
= | Specified compressive strength of concrete, psi (MPa). |

f_{yh} |
= | Yield strength of spiral reinforcement ≤ 85,000 psi (586 MPa). |

P | = | Axial load on pile, pounds (kN), as determined from Equations 16-5 and 16-7. |

ρ_{s} |
= | Spiral reinforcement index or volumetric ratio (vol. spiral/vol. core). |

_{0}. In such cases, minimum spiral reinforcement index shall be as specified in Section 1810.3.8.1.

For structures assigned to Seismic Design Category D, E or F, precast prestressed piles shall have transverse reinforcement in accordance with the following:

- Requirements in ACI 318, Chapter 18, need not apply, unless specifically referenced.
- Where the total pile length in the soil is 35 feet (10 668 mm) or less, the lateral transverse reinforcement in the ductile region shall occur through the length of the pile. Where the pile length exceeds 35 feet (10 668 mm), the ductile pile region shall be taken as the greater of 35 feet (10 668 mm) or the distance from the underside of the pile cap to the point of zero curvature plus three times the least pile dimension.
- In the ductile region, the center-to-center spacing of the spirals or hoop reinforcement shall not exceed one-fifth of the least pile dimension, six times the diameter of the longitudinal strand or 8 inches (203 mm), whichever is smallest.
- Circular spiral reinforcement shall be spliced by lapping one full turn and bending the end of each spiral to a 90-degree hook or by use of a mechanical or welded splice complying with Section 25.5.7 of ACI 318.
Where the transverse reinforcement consists of circular spirals, the volumetric ratio of spiral transverse reinforcement in the ductile region shall comply with the following:

**(Equation 18-6)**but not exceed:

**(Equation 18-7)**where:

*A*= Pile cross-sectional area, square inches (mm_{g}^{2}).*f '*= Specified compressive strength of concrete, psi (MPa)._{c}*f*= Yield strength of spiral reinforcement = ≤ 85,000 psi (586 MPa)._{yh}*P*= Axial load on pile, pounds (kN), as determined from Equations 16-5 and 16-7.*ρ*= Volumetric ratio (vol. spiral/vol. core)._{s}This required amount of spiral reinforcement is permitted to be obtained by providing an inner and outer spiral.

**Exception:**The minimum spiral reinforcement required by Equation 18-6 shall not apply in cases where the design includes full consideration of load combinations specified in ASCE 7, Section 2.3.6 and the applicable overstrength factor, Ω_{0}. In such cases, minimum spiral reinforcement shall be as specified in Section 1810.3.8.1.Where transverse reinforcement consists of rectangular hoops and cross ties, the total cross-sectional area of lateral transverse reinforcement in the ductile region with spacing,

^{s}, and perpendicular dimension,*h*, shall conform to:_{c}**(Equation 18-8)**but not less than:

**(Equation 18-9)**where:

^{fyh}= yield strength of transversereinforcement ≤70,000 psi (483 MPa).^{hc}= Cross-sectional dimension of pile core measured center to center of hoop reinforcement, inch (mm).^{s}= Spacing of transverse reinforcement measured along length of pile, inch (mm).*A*= Cross-sectional area of transverse reinforcement, square inches (mm_{sh}^{2}).*f'*= Specified compressive strength of concrete, psi (MPa)._{c}The hoops and cross ties shall be equivalent to deformed bars not less than No. 3 in size. Rectangular hoop ends shall terminate at a corner with seismic hooks.

Outside of the length of the pile requiring transverse confinement reinforcing, the spiral or hoop reinforcing with a volumetric ratio not less than one-half of that required for transverse confinement reinforcing shall be provided.

For structures assigned to Seismic Design Category C, D, E, or F, the maximum factored axial load on precast prestressed piles subjected to a combination of seismic lateral force and axial load shall not exceed the following values:

- 0.2
*f '*for square piles_{c}A_{g} - 0.4
*f '*for circular or octagonal piles_{c}A_{g}