PIL DESIGN.doc

Design of pile cap General Pile layout pattern: Pile under pile cap should be layout 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 are shown below:

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 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

φvc / $√f c( where φ / '.&* is strength reduction factor, fc( is compressive strength of concrete.

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 d istance, d, from edge of column or pile. The direct shear shrength according to -I is

φvc /'.&*1!.2√f c("*''ρω45ud06u78 ≥ '.&*4"√f c(7 where ρω 4≈ '.''"7 is reinforcement ratio, 5u is factored shear stress, 6u is factored moment at the critical section. +or ρω ≈ '.''" and f c( between 3''' psi and $''' psi,

φvc /'.&*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:

φvc /'.&*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 / % 6inimum spacing of pile: s / !% in x 3 / $ ft

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+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 ! φ5c /'.&*