# Pile Analysis and Design Overview

June 4, 2024

Piles are structural elements that transfer the applied loads from the superstructure through weak soil strata to a competent deeper dense soil. This article discusses the required steps in the pile analysis and design, particularly concrete piles such as Drilled Piers and Auger Cast Piles. The software ASDIP FOUNDATION will be used to support the discussion.

## What typical loads act on a pile?

Piles are members mostly loaded axially, but often they support lateral loads and moments as well. The typical loads transferred to the underlying soil through piles are gravity loads such as dead, live, roof live, snow, etc., as well as lateral loads such as wind and seismic loads.

Bridge structures supported on piles may transfer other loads such as earth pressure, collision, and surcharges. For piles exposed to water and stream pressures, these loads should also be considered.

## The soil-structure interaction

Piles typically transfer the axial loads in two ways: skin friction, and end bearing. If the supporting soil media is primarily granular soils, then end bearing will govern. For cohesive soils, a larger portion of the support will come from skin friction along the pile. The soils report should provide the pile diameter and length, based on the soil strata information. To increase the end bearing capacity, a drilled pier can have a bell at the tip.

On the other hand, the lateral load and moment applied at the pile head are resisted by the soil surrounding the pile. This can be modelled as a series of soil springs conveniently spaced along the pile, called p-y springs, as in the beam on elastic foundation approach. The lateral displacement (y) of a pile causes a soil reaction (p) in the opposite direction.

As shown in the image above, the soil springs are non-linear, reflecting the actual behavior of the soil. Furthermore, the structural properties of the springs vary with the depth as well. If we consider that a typical pile will be embedded in several soil layers with different soil properties, all this greatly complicates the mathematical solution.

The appropriate upper and lower bound calculations of the soil properties have been considered in the software with the use of published p-y curves for different soil conditions. ASDIP FOUNDATION performs the lateral analysis as described above, and shows the results in both graphical and tabular formats, as shown below. Note that the results can be conveniently sorted by load combination, for granular checking.

## How do you design the reinforcement?

The minimum prescriptive requirements in the design codes are more stringent in areas of high seismicity due to uncertain ductility demands that may be required. Drilled piers require longitudinal rebars when designed to resist uplift loads, or when the maximum factored moment exceeds the design cracking moment. ACI Table 18.13.5.7.1 summarizes the minimum reinforcement requirements in concrete piles, as shown below.

Where structures are classified as Seismic Design Category SDC C, the minimum steel area ratio is 0.0025 and the minimum cage length is the larger of L/3, 10 ft, 3*D, depth where Mu=0.4*Mcr. Spiral spacing for the top 3*D is the smaller of 6" and 8 db.

Special provisions apply for SDC D and higher. For Site Class E and F, the minimum steel area ratio is 0.005 and a full cage length is required with more stringent spiral spacing. For Site Class D and lower, moderate minimum requirements between the two cases above are applicable. ASDIP FOUNDATION checks all the code provisions regarding reinforcement design in concrete piles.

## How do you check the pile capacity?

The pile strength is best represented by the φPn-φMn interaction diagram, similar to a concrete column. The diagram is generated by varying the location of the neutral axis and calculating the internal forces required to maintain the equilibrium and strain compatibility. The process is repeated multiple times for different neutral axis locations until the diagram is completed.

ASDIP FOUNDATION generates the interaction diagram of the concrete pile, and shows the maximum factored axial load and bending moment along the pile for comparison purposes. The pile capacity is adequate if the point representing the loads falls inside the usable area of the interaction diagram, as shown below.

## Takeaway

The analysis and design of piles may be difficult and time-consuming. The lateral analysis involves complex calculations, and multiple code provisions must be checked during the design phase. ASDIP FOUNDATION includes pile analysis and design, with multiple options to save you time and effort.

Detailed information is available about this structural engineering software by visiting ASDIP FOUNDATION. For a design example see the post Pile Design Example Using ASDIP FOUNDATION. For our collection of blog posts about footing design please visit Footing Design.