Pile Cap Design

Part No.

Pile Cap Design Guide

1

General overview for: SUPER PILE 2015

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

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.

Pile Cap Design Guide 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 cap’s 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.

Pile Cap Design Guide 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 used almost exclusively by engineers designing reinforced concrete pile caps. Both nominal and factored loads are presented throughout this design guide.

Pile Cap Design Guide 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.

Pile Cap Design Guide Introduction to Pile Cap Design - Behavior • 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.) 100

Pcenter (2 piles) 1/30

Pcorner (4 piles) 1/30

Pother (24 piles) 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

Pile Cap Design Guide Introduction to Pile Cap Design - Behavior • 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.

Pile Cap Design Guide 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 and aids in pile bearing distribution just above the pile.

Pile Cap Design Guide Introduction to Pile Cap Design - Detailing

Pile Cap Design Guide Introduction to Pile Cap Design - Detailing

Pile Cap Design Guide Introduction to Pile Cap Design - Detailing

Pile Cap Design Guide Introduction to Pile Cap Design - Detailing Patterns:

Pile Cap Design Guide Introduction to Pile Cap Design - Detailing 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 is assumed 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 0.0018bDcap In 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 steel for the short bars.

Pile Cap Design Guide Introduction to Pile Cap Design - Detailing 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 4 inches 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.

Pile Cap Design Guide 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.

Pile Cap Design Guide 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.

Pile Cap Design Guide 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 used in 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” in orthogonal directions.

Pile Cap Design Guide 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 design with 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.

Pile Cap Design Guide 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 and bending moment in cases (or load combinations) where the full factored axial load is not applied.

Pile Cap Design Guide Pile Cap Design for Lateral Forces • 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.

Pile Cap Design Guide Pile Cap Design for Lateral Forces Eight Pile Cap

√3 2

2

√3 6 2

9 2 2







2

2

4

2



2



9 2

2



√3 2

√3 9

9 2 2

2

9 2 2

2 9

Pile Cap Design Guide Pile Cap Design for Lateral Forces Table 6.1. Pile cap moments of inertia Ix and Iy for pile cap configurations 2 through 30 assuming all piles have an equivalent cross sectional area of A = 1.0 ft2. Number of Piles - Configuration 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Ix (ft4) NA 0.5 2 2

2 2 1.5 2 3 2 4.5 2 6 2 4.5 2 6 2 8 2 7 2 13.2 2 16 2

Iy (ft4) 0.5 2 0.5 2 2

2 2 4 2 3 2 4.5 2 6 2 9 2 12 2 15 2 21 2 14 2 18 2

Pile Cap Design Guide Pile Cap Design for Lateral Forces Table 6.2. Maximum pile forces in edge piles for pile cap configurations 2 through 30 assuming all piles have an equivalent cross sectional area (note A = 1.0 ft2 not required since the areas cancel out when solving for the actual pile force). Number of Piles - Configuration 2

Maximum Force (k) in Edge Pile Caused Moment Mx (k-ft) NA

Maximum Force (k) in Edge Pile Caused Moment My (k-ft)

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

Pile Cap Design Guide Pile Cap Design – Other Topics

Pile Cap Design Guide Pile Cap Design – Other Topics