Structures Design Guidelines | July 2006
3 - Substructure and Retaining Walls |

## 3.11 Pier, Column, and Footing Design (07/05) July 2005 - Per TDB C05-04, added paragraph N. for restrained bars and ties.

A. For tall piers or columns, detail construction joints to limit concrete lifts to 25 feet. When approved by the Department, a maximum lift of 30 feet may be allowed to avoid successive small lifts (less than approximately 16 feet) which could result in vertical bar splice conflicts or unnecessary splice length penalties.

B. Detail splices for vertical reinforcing at every horizontal construction joint; except that the splice requirement may be disregarded for any lift of 10 feet or less.

C. Coordinate the lift heights and construction joint locations with the concrete placement requirements of the specifications.

D. On structures over water, vertical post-tensioning (except in cylinder piles) cannot extend below an elevation that is 12 feet above MHW, regardless of the Environmental Classification. On structures over land, vertical post-tensioning strand cannot extend below 5 feet above finished grade (post-tensioning bars are excluded from this restriction for structures over land). This restriction excludes the use of post-tensioning from the submerged substructure (fresh or salt) to an air-dry condition within the substructure component. This applies to both CIP and/or precast substructures.

Commentary: Precast pier segments traditionally have horizontal joints every 7 to 10 feet and at least 4 tendons vertically. Even with the highest level of attention to field inspection, many joints are not watertight. Water and chloride intrusion has been documented in superstructure and substructure post-tensioned tendons in Florida. The interstice area within the 7-wire strand and the space between strands allows water and oxygen intrusion. Tendon geometry and duct placement (wobble) are factors that contribute to how much grout surrounds the tendon bundle. Forensic examinations of corroded tendons led to the conclusion that duct material must offer the primary protection for the strand material.

E. Precast pier sections with spliced sleeve connections are allowed.

F. The minimum wall thickness for segmental piers is 12-inches.

G. Where external tendons exit the bottom of pier caps, provide a one half-inch by one half-inch drip recess around the tendon duct.

H. For bridges designed for vessel collision, design pier columns to be solid concrete from 15 feet above MHW to 2 feet below MLW. Voided sections that are filled after the column is constructed may be used.

I. For all land projects, voided substructure piers and columns located within the clear zone, regardless of the presence of guardrail or barriers, must be filled with concrete to 15 feet above the finished grade. For precast voided piers, the fill section may be accommodated with a secondary pour. Show mass concrete fill section to be cast against two layers of roofing paper.

J. Locate the bottom of all footings, excluding seals, a minimum of 12-inches below MLW. Consider lowering the footing further when the tides will consistently expose piles for extended periods.

K. All voided substructures must be sealed from possible sources of leaks and contain free-exiting drains or weep-holes to drain away water that may collect from any source including condensation.

L. Drains in voided piers may be formed using 2-inch diameter permanent plastic pipes set flush with the top of the bottom slab or solid section. Slope interior top of solid base toward drains or weep-holes. Provide weep-holes with vermin guards. Show in the Contract Drawings, locations and details for drains taking into account bridge grade and cross-slope.

M. Provide inspection access for all hollow piers. See Other Box Sections in SDG 4.6
.

N. Detail ties in solid, reinforced-concrete compression members so that no longitudinal bar shall be more than 24-inches1, measured along the tie from a restrained2 bar. A restrained bar is one which has lateral support provided by corner of a tie having an included angle of not more than 135 degrees. Where the column design is based on plastic hinging capability, no longitudinal bars shall be farther than 6-inches clear on each side along the tie from such a laterally supported bar and the tie reinforcement shall meet the requirements of AASHTO LRFD 5.10.11.4.1. d through f. Where the bars are located around the periphery of a circle, complete circular ties may be used if the splices in the ties are staggered. See Figure 3-20 for an illustration of auxiliary ties in compression members which are not designed for plastic hinging.

O. For additional post-tensioning requirements see SDG 4.5.

Commentary:

1. This addition brings the LRFD specification back into conformity with the wording in the Standard Specifications. Although ACI requires closer restraining tie bars, ACI Commentary states, “Limited tests on full-size, axially-loaded, tied columns containing full-length bars (without splices) showed no appreciable difference between the ultimate strength of columns with full tie requirements and no ties at all.”

### Table 3.7 Required tendons for post-tensioned substructure elements:

Post-Tensioned Bridge Element | Minimum Number of Elements
Hammerhead Pier Straddle Beams C-Pier Column (Bars Only) C-Pier Cap C-Pier Footings (Bars only) | 6
Hollow cast Piers I-Section Piers | 8