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Pile Cap Design GuidePart No.

1

Timothy W. Mays, Ph.D., P.E.

General overview for:

SUPER PILE 2015

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Pile Cap Design Guide

Introduction to Pile Cap Design

Pile Cap Design Guide - detailed overview of pile cap design, detailing, and

analysis methodologies that represent the current state of practice

2012 International Building Code (IBC) and ACI 318-11/14.

CRSI Design Handbook (2008)

16 inch and 18 inch HP sections with higher allowable loads have been

developed and this guide has an expanded scope that includes pile

allowable loads up to 400 tons (tagged high load piling in this guide) Deeper pile caps with larger edge distances

Finite element study was performed and recommendations for high load

piling details

Lateral loads on pile caps are considered for the first time in a CRSI

publication in this design guide

Tabulated designs are also provided for all CRSI considered pile cap

configurations and a wide range of vertical loading, lateral loading, and

overturning effects.

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Introduction to Pile Cap Design

Pile caps somewhat neglected in handbooks and textbooks

The complex and often misunderstood load path fundamentals warrants a

conservative design approach

Complete nonlinear finite element modeling of pile caps is not practical in

routine design

Strut and tie design models for all pile caps can be unconservative when

certain modes of failure control the pile caps response On the contrary, research performed during the development of this guide

suggests that deeper pile caps associated with larger and stronger piling

than was considered in the CRSI Design Handbook (2008) warrant some

new steel reinforcing details.

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Introduction to Pile Cap Design - Loads

The guide considers pile caps that are loaded by columns supported

directly at the centroid of the pile cap

All loads must be applied to the pile cap at the column-to-pile cap interface

Any combination of gravity loads (i.e., dead, live, or live roof) or

environmental loads (i.e., seismic, wind, rain, or snow)

Tabulated designs and example problems presented in this guide consider

only the effects of dead loads, live loads, wind loads, and seismic loads. In cases where the designer desires to include the effects of other loads,

these additional loads can conservatively be considered as live loads

without changing the methodologies presented herein

ASD is commonly used by geotechnical engineers and LRFD is usedalmost exclusively by engineers designing reinforced concrete pile caps.

Both nominal and factored loads are presented throughout this design

guide.

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Introduction to Pile Cap Design - Behavior

Load Case I was the only case considered in the previous CRSI Design

Handbook (2008).

Piles have a stiffness that is related to (a) the soil t-z or vertical spring

stiffness and (b) the axial stiffness of the pile as defined by the AE/Lpile,where A is the pile cross-sectional area, E is the pile modulus of elasticity,

and Lpile is the overall pile length.

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For the largest pile cap configuration considered in this design guide (i.e.,

30 piles), an assumed pile cap thickness of 59 inches, and reasonable pile

stiffness assumptions (i.e., 40 ft long 10 in square prestressed piles bearing

on rock):

Vertical Pile Stiffness (k/in.) Pcenter(2 piles) Pcorner(4 piles) Pother(24 piles)

100 1/30 1/30 1/30

400 1/28 1/32 1/30.5 - 1/29.5

800 1/27 1/33 1/31 - 1/29

1,200 1/25 1/34 1/32 - 1/28

Rigid 1/7 1/82 (Tension) 1/80 - 1/10

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Load Case II

Axial load, shear, and moment as applied by the supported column (note

that in the figure, all loads contain the subscript u and are factored)

Rigid caps and the top of the piles are modeled as pin connected such that

only axial load and shear are transferred from the pile cap to the top of the

pile.

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Introduction to Pile Cap Design - Detailing

Note that research performed on HP shapes used as piles has consistently

shown (see for Example AISI, 1982) that so long as some minimum

embedment into the pile cap is achieved, the concrete contained in the

overall boundary of the HP shape (i.e., d times bf) adheres to the pile andaids in pile bearing distribution just above the pile.

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Patterns:

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Required Reinforcement:

All tabulated designs are based on the use of Grade 60 reinforcing bars.

Areas of required flexural reinforcement can be based on an average

effective depth, d = Dcap dc, where Dcap = total pile cap depth, and dc isassumed to be 10 inches for structural steel piles, or 8 inches for concrete

and timber piles. The requirements for minimum areas of flexural

reinforcement (ACI 10.5 and 7.12) are satisfied as follows:

(1) if As

bd, use As(2) if As < bd 4/3As , use bd

(3) if 0.0018bDcap 4/3As < bd, use 4/3As(4) if 4/3As < 0.0018bDcap bd, use 0.0018bDcapIn the expressions above, is the maximum of (a) 200/fy = 0.00333 and (b) 3

For 2-pile pile caps only, 0.0018bDcap should be provided as minimum steelfor the short bars.

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Special Details for High Load Piling:

When piles with an allowable load greater than 200 tons (i.e., high load

piles) are used in conjunction with the design procedures presented in this

guide, two additional details are required. Note that the No. 4 hoops at 4inches on center should be placed around all piles in the pile cap. The

continuous No. 6 edge bar should be provided around the entire boundary

of the pile cap, 3 in. from both the pile cap bottom and pile cap edge.

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Pile Cap Design for Vertical Forces - Shear

26 pile cap patterns

In order to determine the demand associated with all 6 limit states identified

in the figure (i.e., 1 through 6) the number of piles applying shear to the

critical section must first be determined.

Piles are considered shear inducing members if their centerline (including

an adverse 3 in. tolerance effect) is located on the opposite side of the pile

cap critical section relative to the column.

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Pile Cap Design for Vertical Forces - Tabulated

Tabulated pile cap designs for the 26 pile cap patterns using allowable pile

loads ranging from 40 tons to 400 tons in varying increments are included

Two separate spreadsheets are also available to the design engineer.

The first spreadsheet was used to generate the tabulated pile cap designs,

but can also be used to design other pile caps with allowable pile loads that

vary from the increments presented in the tables or when pile shapes or

types vary.

The first spreadsheet also helps the designer customize the solution when

a preferred reinforcing arrangement is desired.

The second spreadsheet allows the designer significant freedom to vary

from many of the requirements, recommendations, and assumptions

presented in the guide.

For example, the designer may need to minimize pile cap edge distances

when pile caps are adjacent to a property line or use less than the

recommend pile spacing in some cases.

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Pile Cap Design for Vertical Forces - Examples

Example 1: 16 Pile Cap This example is a symmetrical cap (i.e., square in

plan) with multiple rows of piles on all 4 sides of the column. The larger pile

cap plan dimensions result in straight bars and it is one of the easiest pile

configurations to work with calculation wise. Low pile service loads are usedin the example.

Example 2: 5 Pile Cap This example is also a symmetrical cap (i.e., square

in plan) but it has only 1 row of piles on each side of the column. The smaller

pile cap plan dimensions result in hooked bars and it has a unique pile layout.

It is the only cap that utilizes 45 degree angles in the pile plan geometry.

Moderate pile service loads are used in the example.

Example 3: 6 Pile Cap This example is an unsymmetrical cap (i.e.,

rectangular in plan). It was also chosen since it is also one of the special

caps where Limit State 4 calculations require an average width w inorthogonal directions.

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Pile Cap Design for Vertical Forces - Examples

Example 4: 7 Pile Cap This example is an unsymmetrical cap. It was

chosen since it is one of only two caps that are uniquely detailed for round

columns (rather than equivalent square columns).

Example 5: 5 Pile Cap This example was selected as a comparison designwith Example 2 and it utilizes high load piles.

Example 6: 16 Pile Cap This example was selected as a comparison design

with Example 1 but it is designed for combined gravity and lateral loading.

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Pile Cap Design for Lateral Forces

Design, and detail pile caps to resist the combined effects of concentrated

moments (Mx and My), shears (Vx and Vy), and axial load (P tension or

compression)

Applied at the centroid of the pile cap and by the supported column.

The procedure assumes a rigid pile cap (relative to the axial stiffness of the

piles) and pinned connections between the top of the pile and the pile cap

Once the pile actions are known, the actual pile cap design procedure

presented in Chapter 5 for column axial loading is still applicable with only

minor modifications necessary.

Practical tabulated gravity plus lateral load designs are presented that allow

the designer to quickly determine the adequacy of the tabulated gravity only

pile cap designs to resist combinations with column applied shear andbending moment in cases (or load combinations) where the full factored

axial load is not applied.

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Principle of superposition

The piles resist overturning via increased and decreased axial forces

depending on their position relative to the pile cap centroid.

The shear demand in each pile may be assumed equal in many cases, but

the designer should consider other assumptions when pile axial forces

result in net tension, particularly when seismic demands are considered.

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Eight Pile Cap

32

6 32 2

92

2

3292

2

39

2 2 2

2 42922

922 2

9

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Table 6.1. Pile cap moments of inertia Ix and Iy for pile cap configurations 2 through30 assuming all piles have an equivalent cross sectional area of A = 1.0 ft2.

Number of Piles - Configuration Ix (ft4) Iy(ft4

2 NA 0.523 0.52 0.524 2 25 22 226 1.52 427 32 328 4.52 4.529 62 6210 4.52 9211 62 12212 82 15213 72 21214 13.22 14215 162 182

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Table 6.2. Maximum pile forces in edge piles for pile cap configurations 2 through 30assuming all piles have an equivalent cross sectional area (note A = 1.0 ft2not requiredsince the areas cancel out when solving for the actual pile force).

Number of Piles - Configuration Maximum Force (k) in Edge PileCaused Moment Mx(k-ft)Maximum Force (k) in Edge PileCaused Moment My(k-ft)

2 NA 3 1.15

0.58 4

0.5

0.5

5 0.35 0.35 6 0.33

0.25 7 0.29

0.33

8 0.19 0.22 9 0.17

0.17 10 0.19

0.17

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Pile Cap Design Other Topics

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Pile Cap Design Other Topics

02 Mays Final

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