PIL DESIGN.doc

(1)

Design of pile cap

General

Pile layout pattern:

Pile under pile cap should be l

Pile under pile cap should be layout symmetrically in both ayout symmetrically in both directions. directions. The column or wall onThe column or wall on pile cap should be centered at the geometric center of the pile cap in order to transferred load pile cap should be centered at the geometric center of the pile cap in order to transferred load

evenly to each pile.

evenly to each pile. Example of pile Example of pile layout pattern arlayout pattern are shown below:e shown below:

Pile spacing, edge distance, and pile cap thickness:

(2)

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 checed.

Pile diameter !"# !$# !%# !&# "'# ""# "$#

Pile spacing 3()'# 3()%# $()'# $()%# *()'# *()%# %()'#

Pile cap thicness is normal determined by shear strength. +or smaller pile cap, the thicness is normally governed by deep beam shear. +or large pile cap, the thicness is governed by direct shear. hen necessary, shear reinforcement may be used to reduced thicness pile cap.

The edge distance is normally governed by punching shear capacity of corner piles.

Theory:

Punching shear

The punching shear strength according to -I is

φ

vc / $

√

f c(

(3)

The critical section of punching shear stress is at a distance, d0", from edge of pile, d is the effective depth of pile cap. +or 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 direct shear is at a distance, d, from edge of column or pile. The direct shear shrength according to -I is

φ

vc /'.&*1!.2

√

f c("*''

ρ

ω45ud06u78

≥

'.&*4"

√

f c(7

where

ρ

ω 4

≈

'.''"7 is reinforcement ratio, 5u is factored shear stress, 6u is factored moment at

the critical section. +or

ρ

ω

≈

'.''" and f c( between 3''' psi and $''' psi,

φ

vc /'.&*1!.2

√

f c('.!

√

f c(45ud06u78

≥

'.&*4"

√

f c(7

Deep beam shear

9eep beam shear is evaluated at face of column when

ω

 d and 5u;d06u

≥

!

The shear strength is calculated as follows:

φ

vc /'.&*<4d0

ω

713.*)".*46u05ud781!.2

√

f c("*''

ρ

ω45u;d06u78=

≥

'.&*4!'

√

f c(7

where

ω

is the distance from face of column to the nearest pile. +or

ρ

ω

≈

'.''" and f c( between

3''' psi and $''' psi,

φ

vc /'.&*<4d0

ω

713.*)".*46u05ud781!.2

√

f c('.!

√

f c( 45u;d06u78=

≥

'.&*4!'

√

f c(7

Flexural reinforcement

9esign 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 / P0n6x;dx0Iy 6y;dy0Ix

where pi is axial load for individual pile, P is column load, 6 is mo ment from column moment

(4)

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 /

Σ

dy", Iy /

Σ

dx".

Design procedure

!. Estimate number of pile needed. >election pile layout pattern. alculate individual pile load. The maximum pile load shall not exceed allowable pile capacity.

". alculate factored pile load. -ssume a depth of pile cap, calculate factored moment and shear at critical section, chec direct shear

3. alculate moment and shear at face of column, chec deep beam shear. $. hec punching shear and edge distance.

*. 9esign flexural reinforcement.

Design Examples

Pile cap design example: 9esign 9ata:

olumn dead load: P9 / 3'' ip

olumn live load: P? / 3*' ip

olumn dead load moment: 69@ / $' ft)ip, 69A / &' ft)ip

olumn ?ive load moment: 6?@ / 3* ft)ip, 6?A / %* ft)ip

olumn siBe: !&Cx!&C concrete column Type of pile: !% in diameter concrete pile

-llowable pile compression capacity: Pc / !"* ip

-llowable pile tension capacity: Pt / *' ip

ompressive strength of concrete: f c( / 3''' psi

Tensile strength of reinforcing steel: f y / %' si

Reuirement: Design pile group and pile cap

>olution:

!. Estimate number of pile and select pile layout pattern

Total service pile vertical load: P / P9P? / %*' ip

Estimate number of pile: n / P0Pc / *."

Try a six)pile layout pattern, n / %

(5)

You're Reading a Preview Unlock full access with a free trial.

(6)

+actored shear force: 5u / Pu"Pu$Pu% / *3%.3 ip

The factored moment face of column is

6u / 4Pu" Pu$Pu%74" ft F 2 in7 / %D'.$ ft)ip

The deep beam shear strength of concrete is as follows:

The distance from pile to face of column,

ω

/ "$ in F 2 in / !* in

The length of pile cap is b / !! ft The ratio, 5u;d06u / "."% G !

φ

5c /'.&*<4d0

ω

713.*)".*46u05ud781!.2

√

f c('.!

√

f c( 45u;d06u78=bd/"*"$ ip

φ

5c /'.&*4!'

√

f c(7bd / "!&$ ip G *3%.3 ip H..

$. 9esign reinforcement in short direction: 6u / %D'.* ft)ip

+actor: J n / 6u04'.2;b;d"7 / *% si, m / f y0'.&*f c( / "3.*

Jeinforcement ratio:

ρ

ω/ 4!0m71!)

√

4!)"mJ n0f y78 / '.'''2$

hec minimum reinforcement:

ρ

min/

ρ

ω;$03 / '.''!" or

ρ

min/ '.''"

-rea of reinforcement: -s / '.''";b;d / 2.$ in"_.

Kse !'L2 bar, -s / !' in".

9esign reinforcement in longitudinal direction: b / D.* ft 6u / 4Pu*Pu%74$ ft F 2 in7 / !!D$ ft)ip

+actor: J n / 6u04'.2;b;d"7 / !*'.$ si

Jeinforcement ratio:

ρ

ω/ 4!0m71!)

√

4!)"mJ n0f y78 / '.''"%

hec minimum reinforcement:

ρ

min/

ρ

ω;$03 / '.''3$

-rea of reinforcement: -s / '.''3$;b;d / !'.* in".