Pile Cap Design Guide Part No.
1
General overview for: SUPER PILE 2015
Timothy W. Mays, Ph.D., P.E.
Pile Cap Design Guide Introduction to Pile Cap Design • Pile Pile Cap Cap Design Design Guid Guide e - detail detailed ed overv overview iew of of pile pile cap cap desig design, n, detai detailin ling, g, and and analysis methodologies that represent the current state of practice • 2012 Interna Internationa tionall Building Building Code Code (IBC) (IBC) and and ACI 318-11/14. 318-11/14. • CRSI CRSI Desi Design gn Hand Handbo book ok (200 (2008) 8) • 16 inch inch and 18 inch inch HP sections sections with with higher higher allowab allowable le loads 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 Deeper pile pile caps caps with with larg larger er edge edge distan distances ces • Finite element element study study was was perform performed ed and and recommen recommendation dations s for high load load piling details • Lateral Lateral loads loads on pile pile caps caps are considered considered for for the the first first time time in in a CRSI publication in this design guide • Tabulated abulated designs designs are also provide provided d for all all CRSI considered considered pile 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 Desig 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 elastic 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 30 piles), an assumed pile cap thickness of 59 inches, and reasonable stiffness assumptions (i.e., 40 ft long 10 in square prestressed piles be 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.
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 (no 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 only axial load and shear are transferred from the pile cap to the top of pile.
Pile Cap Design Guide Introduction to Pile Cap Design - Detailing
• Note that research performed on HP shapes used as piles has consis 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 a 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 bar Areas of required flexural reinforcement can be based on an average effective depth, d = Dcap – dc, where Dcap = total pile cap depth, and d assumed to be 10 inches for structural steel piles, or 8 inches for concr 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/f y = 0.00333 and (b) 3 • For 2-pile pile caps only, 0.0018bDcap should be provided as minimum 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 guide, two additional details are required. Note that the No. 4 hoops at 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 bound 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 iden 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 (includ an adverse 3 in. tolerance effect) is located on the opposite side of the 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 loads ranging from 40 tons to 400 tons in varying increments are includ • Two separate spreadsheets are also available to the design engineer
• The first spreadsheet was used to generate the tabulated pile cap desi but can also be used to design other pile caps with allowable pile loads vary from the increments presented in the tables or when pile shapes o types vary. • The first spreadsheet also helps the designer customize the solution a preferred reinforcing arrangement is desired. • The second spreadsheet allows the designer significant freedom to v from many of the requirements, recommendations, and assumptions presented in the guide.
• For example, the designer may need to minimize pile cap edge distanc when pile caps are adjacent to a property line or use less than the
Pile Cap Design Guide Pile Cap Design for Vertical Forces - Examples
Example 1: 16 Pile Cap – This example is a symmetrical cap (i.e., squar plan) with multiple rows of piles on all 4 sides of the column. The larger p 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 in the example.
Example 2: 5 Pile Cap – This example is also a symmetrical cap (i.e., sq in plan) but it has only 1 row of piles on each side of the column. The sm pile cap plan dimensions result in hooked bars and it has a unique pile la 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 specia 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 roun columns (rather than equivalent square columns).
Example 5: 5 Pile Cap – This example was selected as a comparison de with Example 2 and it utilizes high load piles.
Example 6: 16 Pile Cap – This example was selected as a comparison d 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 concentra moments (Mx and My), shears (Vx and Vy), and axial load (P – tension o 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 piles) and pinned connections between the top of the pile and the pile c
• Once the pile actions are known, the actual pile cap design procedure presented in Chapter 5 for column axial loading is still applicable with o minor modifications necessary.
• Practical tabulated gravity plus lateral load designs are presented that the designer to quickly determine the adequacy of the tabulated gravity pile cap designs to resist combinations with column applied shear and bending moment in cases (or load combinations) where the full factore 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 case the designer should consider other assumptions when pile axial forces result in net tension, particularly when seismic demands are considere
Pile Cap Design Guide Pile Cap Design for Lateral Forces Eight Pile Cap
3 √ 2 2 2 3 9 √ 6 2 2 √2 3 √ 3 92 9 2 2 2 2 9 2 22 4 2 2 92 9 2
Pile Cap Design Guide Pile Cap Design for Lateral Forces
Table 6.1. Pile cap moments of inertia I x and Iy for pile cap configurations 2 th 30 assuming all piles have an equivalent cross sectional area of A = 1.0 ft 2. Number of Piles - Configuration 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Ix (ft4 ) NA
0.522 2 2 1.3522 4.6522 4.6522 8722 13.16222
Iy (ft4
0.0.5522 222 4322 4.6522 2 9 2 12 2 15 2 21 2 14 182
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 ft 2 not required since the areas cancel out when solving for the actual pile force). Number of Piles - Configuration 2 3 4 5 6 7 8 9 10
Maximum Force (k) in Edge Pile Caused Moment Mx (k-ft) NA
1.15 0.5 0.35 0.33 0.29 0.19 0.17 0.19
Maximum Force (k) in Edge Pile Caused Moment My (k-ft)
0.58 0.5 0.35 0.25 0.33 0.22 0.17 0.17
Pile Cap Design Guide Pile Cap Design – Other Topics
Pile Cap Design Guide Pile Cap Design – Other Topics
