Memoria

Design of pile cap

General

Pile layout pattern:

Pile under pile cap should be layout symmetrically in both directions. The column or wall on pile cap should be centered at the geometric center of the pile cap in order to transferred load evenly to each pile. Example of pile layout pattern are shown below:
 
[pic]

Pile spacing, edge distance, and pile cap thickness: 

In general,piles should be spacing at 3 times of pile diameter in order to transfer load effectively to soil. If the spacing is less than 3 times of diameter, pile group settlement and bearing capacity should be checked.
Pile diameter
12”
14”
16”
18”
20”
22”
24”

Pile spacing
3’-0”
3’-6”
4’-0”
4’-6”
5’-0”
5’-6”
6’-0”

Pile cap thickness is normal determined by shear strength. Forsmaller pile cap, the thickness is normally governed by deep beam shear. For large pile cap, the thickness is governed by direct shear. When necessary, shear reinforcement may be used to reduced thickness pile cap.
[pic]
The edge distance is normally governed by punching shear capacity of corner piles.

Theory:

Punching shear

The punching shear strength according to ACI is
(vc = 4(fc’where ( = 0.85 is strength reduction factor, fc’ is compressive strength of concrete.
The critical section of punching shear stress is at a distance, d/2, from edge of pile, d is the effective depth of pile cap. For corner pile, the critical section normally extends to the corner edge of pile cap since it gives less shear area.

Direct shear or beam shear

The critical section of directshear is at a distance, d, from edge of column or pile.
The direct shear shrength according to ACI is
(vc =0.85[1.9(fc’+2500(((Vud/Mu)] ( 0.85(2(fc’)
where (( (( 0.002) is reinforcement ratio, Vu is factored shear stress, Mu is factored moment at the critical section. For (( ( 0.002 and fc’ between 3000 psi and 4000 psi,
(vc =0.85[1.9(fc’+0.1(fc’(Vud/Mu)] ( 0.85(2(fc’)

Deep beamshear

Deep beam shear is evaluated at face of column when ( < d and Vu*d/Mu ( 1
The shear strength is calculated as follows:
(vc =0.85{(d/()[3.5-2.5(Mu/Vud)][1.9(fc’+2500(((Vu*d/Mu)]} ( 0.85(10(fc’)
where ( is the distance from face of column to the nearest pile. For (( ( 0.002 and fc’ between 3000 psi and 4000 psi,
(vc =0.85{(d/()[3.5-2.5(Mu/Vud)][1.9(fc’+0.1(fc’ (Vu*d/Mu)]} ( 0.85(10(fc’)Flexural reinforcement

Design of flexural reinforcement is the same as spread footing design. The critical section is at face of column.

Pile load calculation

Pile load can be calculated as
pi = P/n+Mx*dx/Iy+ My*dy/Ix
where pi is axial load for individual pile, P is column load, M is moment from column moment and/or from eccentricity between center of column and center ofpile group, n is total number of piles, dx and dy are x and y distance from center of pile group, Ix and Iy are moment of inertia of pile group in x and y directions. Ix and Iy are calculated as
Ix = ( dy2, Iy = ( dx2.

Design procedure

1. Estimate number of pile needed. Selection pile layout pattern. Calculate individual pile load. The maximum pile load shall not exceed allowablepile capacity.
2. Calculate factored pile load. Assume a depth of pile cap, calculate factored moment and shear at critical section, check direct shear
3. Calculate moment and shear at face of column, check deep beam shear.
4. Check punching shear and edge distance.
5. Design flexural reinforcement.

Design Examples

Pile cap design example:
Design Data:
Column dead load: PD =300 kip
Column live load: PL = 350 kip
Column dead load moment: MDX = 40 ft-kip, MDY = 80 ft-kip
Column Live load moment: MLX = 35 ft-kip, MLY = 65 ft-kip
Column size: 18"x18" concrete column
Type of pile: 16 in diameter concrete pile
Allowable pile compression capacity: Pc = 125 kip
Allowable pile tension capacity: Pt = 50 kip
Compressive strength of concrete: fc’ = 3000 psi
Tensile...