751.39 Pile Footings: Difference between revisions

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m Bridge has updated this article to show the correct design method (previous method was incorrect).
Smithk (talk | contribs)
Modot, modot
20,986 edits
m Per Bridge, much of the article was updated to conform with LRFD practices.
Line 1: Line 1:
[[Image:751.39 seal course.gif|right|thumb|800px|<center>Note: All pile shall be vertical. </center>]]
[[Image:751.39 seal course.jpg|right|thumb|700px|<center>'''Notes:''' <br>''' 1)''' All pile shall be vertical. <br>
'''2)''' For AASHTO Guide Specification for LRFD, <br>
12” for seismic design category A and 18” for seismic  design category B, C and D.</center> ]]


===General Requirements===
 
==751.39.1 General Requirements==


Water elevation is to be determined for the site conditions by the preliminary design section; generally, less than the average of high and low water.  
Water elevation is to be determined for the site conditions by the preliminary design section; generally, less than the average of high and low water.  


Determine the uplift force per pile by deducting the weight of the seal course and friction between seal course and sheet piling of cofferdam from the uplift force produced by the hydrostatic head "h".  
Determine the factored uplift load per pile by deducting the weight of the seal course and friction between seal course and sheet piling of cofferdam from the uplift force produced by the hydrostatic head "h".  


Use a friction value between the seal course and the sheet piling of 2 lbs./in<sup>2</sup> acting on (perimeter x depth) of seal course.  
Use a friction value between the seal course and the sheet piling of 2 lbs./in<sup>2</sup> acting on (perimeter x depth) of seal course.  


===Pile Pull-out Force===
Factored uplift load per pile =
:Load factor for uplift x (Uplift force of water – Weight of seal course – Friction between seal course & sheet piling) / No. of piles
 
==751.39.2 Pile Pull-out Force==
 
Maximum factored resistance  per pile shall be determined by the minimum of:


Allowable uplift force per pile shall be determined by the minimum of:  
:(1) Factored resistance for skin friction = Geotechnical resistance factor for skin friction x Nominal skin friction resistance


:(1) The allowable friction capacity of pile = skin friction capacity of pile divided by a safety factor of 3.5.  Use "DRIVEN" program to calculate skin friction capacity of pile (without end bearing or tip capacity). (See AASHTO Table 4.5.6.2A.)
::Use "DRIVEN" program to calculate nominal skin friction resistance  of pile (without tip resistance).


:(2) Allowable pull-out capacity of pile due to shear cone failure equal to ultimate pull-out capacity of pile due to shear failure (with maximum shear stress of  <math>2\sqrt{f'c}</math> ) divided by a safety factor of 3. See Figs. 751.39.1, 751.39.2 and the following example.  (See AASHTO 8.16.6.2.)
::Geotechnical resistance factor for skin friction, φ<sub>up</sub>.  from LRFD Table 10.5.5.2.3-1


:(3) Allowable Maximum pull-out capacity = 0.25 * Yield strength of steel * steel area of pile. (See AASHTO 4.5.7.3.)
::Use φ<sub>up</sub> = 0.2 from LRFD table 10.5.5.2.3.-1 for single pile unless higher resistance factor can be justified based on soil type and test method.
:(2) Factored resistance for pile tension  = Structural resistance factor for pile tension  x Yield strength of steel x Steel area of pile.  


Note: Since CIP pile is not filled at this stage, use steel pipe pile area without concrete to determine the allowable maximum pull-out capacity.
::Note: Since CIP pile is not filled at this stage, use steel pipe pile area without concrete to determine the factored resistance for pile tension.  ASTM 252 allows “the wall thickness at any point shall not be more than 12.5% under the specified nominal wall thickness. For steel pipe pile area computation consider 87.5% of wall thickness.
::Structural resistance factor for pile tension, φ<sub>y</sub> = 0.25


{| style="margin: 1em auto 1em auto"
:(3) Factored resistance for 10 psi adhesion = structural resistance factor for 10 psi adhesion x 10 psi x Pile perimeter x Depth of seal course
|-
 
|colspan="2"|[[image:751.39.1.jpg|center|550px]]
::Structural resistance for 10 psi pile adhesion, Φ<sub>3</sub> = 1.0
|-
 
|colspan="2"|[[image:751.39.2.jpg|center|650px]]
::For HP pile perimeter use 2*(Depth + Width) of HP shape.
|-
 
!align="center"|'''For Cast-in-Place Pile||align="2"|For H Pile
'''Plan Reporting'''
|-
 
|align="center"|Surface Area = ''πS(r + R)''||align="center"|Surface Area = ''(2a + 2b+ πR)S''
Seal Course size shall be shown on the plans as designed.  
|-
!colspan="2" align="center|Fig. 751.39.1, Full Shear Cone Failure Surface
|}


Seal Courses are typically designed for one footing only. They may be designed for uplift loads developed under more than one footing or where there is economy of scale in constructing a larger cofferdam, for example:


{| style="margin: 1em auto 1em auto"
:(1) where individual seal courses are proximate and costlier than considering one large seal course under more than one footing, and
|-
|colspan="2"|[[image:751.39.3.jpg|center|650px]]
|-
|colspan="2"|[[image:751.39.4.jpg|center|650px]]
|-
!align="center"|'''For Cast-in-Place Pile||align="2"|For H Pile
|-
|align="center"|Reduced Surface Area = ''πS(r + R)''||align="center"|Reduced Surface Area = ''(2a + 2b+ πR)S''
|-
!colspan="2" align="center|Fig. 751.39.2, Reduced Shear Cone Failure Surface
|}


:(2) where removal of an existing footing requires a larger cofferdam to be constructed enclosing both existing and new locations of footings with the result that a larger than usual seal course size may be less costlier than individually constructed cofferdams.


The reduced surface area defined in Fig. 751.39.2 is a conservative value compared to the actual reduced surface area.  The geometry of the actual surface area is time consuming and complicated to compute, so a standard shape was chosen to ensure efficient use of the designer's time.
Where a seal course is to be designed to resist the uplift loads resulting from the proposed construction of more than one footing, the plans should show or indicate that more than one footing was considered in designing the seal course and the pay item adjusted accordingly.


===Example, Seal Course===
Seal Course size shown on plans should not be allowed to be resized as in combining adjacent seal course(s) or lessening thickness without approval from either the Structural Project Manager or Structural Liaison Engineer and an official change order request from district Construction and Materials Division.


====Problem====
==751.39.3 Example:  LRFD Seal Course Design==


Check if Seal Course design is adequate.  
Check if Seal Course design is adequate.  
Line 62: Line 61:
:* 12 - HP 10 x 42 piles  
:* 12 - HP 10 x 42 piles  
:* Area of an individual HP 10 x 42 Pile = A = 12.4 in<sup>2</sup>  
:* Area of an individual HP 10 x 42 Pile = A = 12.4 in<sup>2</sup>  
:* Depth of HP shape = 9.7 inch
:* Width of HP shape = 10.1 inch
:* Yield strength of steel F<sub>y</sub> = 36 ksi  
:* Yield strength of steel F<sub>y</sub> = 36 ksi  
:* Maximum axial load P<sub>y</sub> = F<sub>y</sub> (A) = 36 ksi (12.4 in<sup>2</sup>) = 446.4 kips  
:* Maximum axial load P<sub>y</sub> = F<sub>y</sub> (A) = 36 ksi (12.4 in<sup>2</sup>) = 446.4 kips  
Line 70: Line 71:
[[image:751.39.5.jpg|center|700px|thumb|<center>'''Fig. 751.39.3, Seal Course Elevation'''</center>]]  
[[image:751.39.5.jpg|center|700px|thumb|<center>'''Fig. 751.39.3, Seal Course Elevation'''</center>]]  


[[image:751.39.6.jpg|center|700px|thumb|<center>'''Fig. 751.39.4, Typical Shear Cone Failure Area</center>]]
===Solution===


[[image:751.39.7.jpg|center|700px|thumb|<center>'''Fig. 751.39.5, Shear Failure Area for Individual Pile</center>]]
Factored uplift load per pile:


 
{| style="margin: 1em auto 1em auto" align=center
====Solution====
 
'''Actual Uplift Force:'''
 
{| style="margin: 1em auto 1em auto" align=left
|-
|-
|align=left|Uplift force of water = 12'(15')(19')(0.0624 kips/ft<sup>3</sup>) ||= 214 kips  
|align=left|Uplift force of water = 12'(15')(19')(0.0624 kips/ft<sup>3</sup>) ||= 214 kips  
Line 89: Line 85:
|align=left|Net uplift of piles ||= 86 kips
|align=left|Net uplift of piles ||= 86 kips
|-
|-
|colspan="2" align=left|Actual uplift per pile = 86 kips/12 piles = 7.17 kips/pile  
|colspan="2" align=left|Factored uplift load per pile = 86 kips/12 piles = 7.17 kips/pile  
|}
|}




Maximum factored resistance:


Use the minimum of:


(1) Factored resistance for skin friction = Geotechnical resistance factor for skin friction x Nominal skin friction resistance:


:Using "DRIVEN" program, Nominal skin friction resistance of pile is 53.97 kips.


:Geotechnical resistance factor for skin friction = 0.2 for uplift resistance of single pile from LRFD 10.5.5.2.3-1 (Conservatively assumed 0.2),


Factored resistance for skin friction = 0.2 x 53.97 kips = 10.79 kips


(2) Factored resistance for pile tension


:= 0.25 x F<sub>y</sub> x Steel area of pile


:= 0.25 x 36 x 12.4


:=111.6 kips


(3) Factored resistance for 10 psi adhesion = Structural resistance factor for 10 psi pile adhesion x 10 psi x Pile perimeter x Depth of seal course


Structural resistance for 10 psi pile adhesion, Φ<sub>3</sub> = 1.0 


= 1.0 x 10/1000*2 x (9.7+10.1) x 3 x 12


 
= 14.26 kips
'''Allowable Uplift Force:'''
 
Use the minimum of:
 
 
(1) Allowable friction capacity of pile:
: Using "DRIVEN" program, the skin friction capacity of pile is 53.97 kips.
: Allowable friction capacity = 53.97 kips/3.5 = 15.42 kips
 
(2) Allowable pull-out capacity of pile due to shear cone failure:
: (Reference Figs. 751.39.4 and 751.39.5)
 
::* Total Reduced Shear Cone Area for H-pile
:::= ''(2a + 2b + πR)S''
:::= (2 x 9.7" + 2 x 10" + 3.1416 x 13") x 18.38"
:::= 1474 in<sup>2</sup>
::* Ultimate Shear Strength
:::= <math>2 \sqrt{f'c} </math>
:::= <math>2 \sqrt{3000}</math>
:::= 109.5 psi
::* Total Pull-Out Capacity
:::= ''(Total Shear Cone Area) x (Ultimate Shear Strength)    ''
:::= 1474 in<sup>2</sup> x 0.1095 ksi = 161.4 kips ≤ P<sub>y</sub> = 446.4 kips
::* Allowable Pull-Out Capacity
:::= ''Total Pull-Out Capacity / Factor of Safety''
:::= 161.4 kips/3 = 53.8 kips
 
 
(3)  Allowable Maximum Pull-Out Capacity
:='' 0.25 x F<sub>y</sub> x steel area of pile''
:= 0.25 x 36 x 12.4
:=111.6 kips
 


Minimum from (1), (2) & (3):  
Minimum from (1), (2) & (3):  


:Allowable uplift force = 15.42 kips
Maximum factored resistance = 10.79 kips  
:Actual uplift force = 7.17 kips  


::15.42 kips > 7.17 kips O.K.
Factored uplift load per pile  = 7.17 kips  


:Try seal course depth = 2'-6"
7.17 kips ≤ 10.79 kips,    O.K.




'''Actual Uplift Force: '''
'''Try seal course depth = 2'-6" '''


{| style="margin: 1em auto 1em auto" align=left
Factored uplift load per pile  :
{| style="margin: 1em auto 1em auto" align=center
|-
|-
|align=left|Uplift force of water||= 12'(15')(18.5')(0.0624 kips/ft<sup>3</sup>) = 207.8 kips
|align=left|Uplift force of water||= 12'(15')(18.5')(0.0624 kips/ft<sup>3</sup>) = 207.8 kips
Line 167: Line 139:
|Net uplift of piles ||= 101.4 kips
|Net uplift of piles ||= 101.4 kips
|-
|-
|Actual uplift per pile ||= 101.4 kips/12 piles = 8.45 kips/pile
|Factored uplift load per pile ||= 101.4 kips/12 piles = 8.45 kips/pile
|}
|}


Maximum factored resistance :


Use the minimum of:


(1) Factored resistance for skin friction


:= Geotechnical resistance factor for skin friction x Nominal skin friction resistance


:= 0.2 x 53.97 kips = 10.79 kips
(2) Factored resistance for pile tension = 111.6 kips


(3) Factored resistance for 10 psi pile adhesion = Structural resistance factor for 10 psi pile adhesion x 10 psi x Pile perimeter x Depth of seal course


Structural resistance factor for 10 psi pile adhesion, Φ<sub>3</sub> = 1.0 


= 1.0 x 10/1000*2 x (9.7+10.1) x 2.5 x 12


= 10.88 kips


Minimum from (1), (2) & (3):


Maximum factored resistance = 10.79 kips


Factored uplift load per pile  = 8.45 kips


8.45 kips ≤ 10.79 kips,  O.K.


(Using a depth of 2'-6" is an economical design.)




'''Allowable Uplift Force:'''
Use the minimum of:
(1) Allowable friction capacity of pile: Using "DRIVEN" program, the skin friction capacity of pile is 54.37 kips. Allowable friction capacity 54.37 kips/3.5 = 15.53 kips
(2) Allowable pull-out capacity of pile due to shear cone failure = 53.8 kips
(3) Allowable maximum pull-out capacity = 111.6 kips
Minimum from (1), (2) & (3):
:Allowable uplift force = 15.53 kips
:Actual uplift force = 8.45 kips
::15.53 kips > 8.45 kips
:(Thus, using a depth of 2'-6" is an economical design.)




[[Category:751 LRFD Bridge Design Guidelines]]
[[Category:751 LRFD Bridge Design Guidelines]]

Revision as of 14:34, 28 March 2012

Notes:
1) All pile shall be vertical.
2) For AASHTO Guide Specification for LRFD,
12” for seismic design category A and 18” for seismic design category B, C and D.


751.39.1 General Requirements

Water elevation is to be determined for the site conditions by the preliminary design section; generally, less than the average of high and low water.

Determine the factored uplift load per pile by deducting the weight of the seal course and friction between seal course and sheet piling of cofferdam from the uplift force produced by the hydrostatic head "h".

Use a friction value between the seal course and the sheet piling of 2 lbs./in2 acting on (perimeter x depth) of seal course.

Factored uplift load per pile =

Load factor for uplift x (Uplift force of water – Weight of seal course – Friction between seal course & sheet piling) / No. of piles

751.39.2 Pile Pull-out Force

Maximum factored resistance per pile shall be determined by the minimum of:

(1) Factored resistance for skin friction = Geotechnical resistance factor for skin friction x Nominal skin friction resistance
Use "DRIVEN" program to calculate nominal skin friction resistance of pile (without tip resistance).
Geotechnical resistance factor for skin friction, φup. from LRFD Table 10.5.5.2.3-1
Use φup = 0.2 from LRFD table 10.5.5.2.3.-1 for single pile unless higher resistance factor can be justified based on soil type and test method.
(2) Factored resistance for pile tension = Structural resistance factor for pile tension x Yield strength of steel x Steel area of pile.
Note: Since CIP pile is not filled at this stage, use steel pipe pile area without concrete to determine the factored resistance for pile tension. ASTM 252 allows “the wall thickness at any point shall not be more than 12.5% under the specified nominal wall thickness. For steel pipe pile area computation consider 87.5% of wall thickness.
Structural resistance factor for pile tension, φy = 0.25
(3) Factored resistance for 10 psi adhesion = structural resistance factor for 10 psi adhesion x 10 psi x Pile perimeter x Depth of seal course
Structural resistance for 10 psi pile adhesion, Φ3 = 1.0
For HP pile perimeter use 2*(Depth + Width) of HP shape.

Plan Reporting

Seal Course size shall be shown on the plans as designed.

Seal Courses are typically designed for one footing only. They may be designed for uplift loads developed under more than one footing or where there is economy of scale in constructing a larger cofferdam, for example:

(1) where individual seal courses are proximate and costlier than considering one large seal course under more than one footing, and
(2) where removal of an existing footing requires a larger cofferdam to be constructed enclosing both existing and new locations of footings with the result that a larger than usual seal course size may be less costlier than individually constructed cofferdams.

Where a seal course is to be designed to resist the uplift loads resulting from the proposed construction of more than one footing, the plans should show or indicate that more than one footing was considered in designing the seal course and the pay item adjusted accordingly.

Seal Course size shown on plans should not be allowed to be resized as in combining adjacent seal course(s) or lessening thickness without approval from either the Structural Project Manager or Structural Liaison Engineer and an official change order request from district Construction and Materials Division.

751.39.3 Example: LRFD Seal Course Design

Check if Seal Course design is adequate.

Given

  • Concrete Strength f'c= 3000 psi
  • Pile spacing = 3'-0"
  • 12 - HP 10 x 42 piles
  • Area of an individual HP 10 x 42 Pile = A = 12.4 in2
  • Depth of HP shape = 9.7 inch
  • Width of HP shape = 10.1 inch
  • Yield strength of steel Fy = 36 ksi
  • Maximum axial load Py = Fy (A) = 36 ksi (12.4 in2) = 446.4 kips
  • Hydrostatic head h = 19'
  • Seal Course = 12' x 15' x 3'
  • Pile embedment below seal course = 15'
Fig. 751.39.3, Seal Course Elevation

Solution

Factored uplift load per pile:

Uplift force of water = 12'(15')(19')(0.0624 kips/ft3) = 214 kips
Weight of seal course = 12'(15')(3')(0.15 kips/ft3) = -81 kips
Friction of sheet pile = (15'+12')(2)(3')(144)(0.002kips/in2) = -47 kips
Net uplift of piles = 86 kips
Factored uplift load per pile = 86 kips/12 piles = 7.17 kips/pile


Maximum factored resistance:

Use the minimum of:

(1) Factored resistance for skin friction = Geotechnical resistance factor for skin friction x Nominal skin friction resistance:

Using "DRIVEN" program, Nominal skin friction resistance of pile is 53.97 kips.
Geotechnical resistance factor for skin friction = 0.2 for uplift resistance of single pile from LRFD 10.5.5.2.3-1 (Conservatively assumed 0.2),

Factored resistance for skin friction = 0.2 x 53.97 kips = 10.79 kips

(2) Factored resistance for pile tension

= 0.25 x Fy x Steel area of pile
= 0.25 x 36 x 12.4
=111.6 kips

(3) Factored resistance for 10 psi adhesion = Structural resistance factor for 10 psi pile adhesion x 10 psi x Pile perimeter x Depth of seal course

Structural resistance for 10 psi pile adhesion, Φ3 = 1.0

= 1.0 x 10/1000*2 x (9.7+10.1) x 3 x 12

= 14.26 kips

Minimum from (1), (2) & (3):

Maximum factored resistance = 10.79 kips

Factored uplift load per pile = 7.17 kips

7.17 kips ≤ 10.79 kips, O.K.


Try seal course depth = 2'-6"

Factored uplift load per pile  :

Uplift force of water = 12'(15')(18.5')(0.0624 kips/ft3) = 207.8 kips
Weight of seal course = 12'(15')(2.5')(0.15 kips/ft3) = -67.5 kips
Friction of sheet pile = (15'+12')(2)(2.5')(144)(0.002 kips/in2) = -38.9 kips
Net uplift of piles = 101.4 kips
Factored uplift load per pile = 101.4 kips/12 piles = 8.45 kips/pile

Maximum factored resistance :

Use the minimum of:

(1) Factored resistance for skin friction

= Geotechnical resistance factor for skin friction x Nominal skin friction resistance
= 0.2 x 53.97 kips = 10.79 kips

(2) Factored resistance for pile tension = 111.6 kips

(3) Factored resistance for 10 psi pile adhesion = Structural resistance factor for 10 psi pile adhesion x 10 psi x Pile perimeter x Depth of seal course

Structural resistance factor for 10 psi pile adhesion, Φ3 = 1.0

= 1.0 x 10/1000*2 x (9.7+10.1) x 2.5 x 12

= 10.88 kips

Minimum from (1), (2) & (3):

Maximum factored resistance = 10.79 kips

Factored uplift load per pile = 8.45 kips

8.45 kips ≤ 10.79 kips, O.K.

(Using a depth of 2'-6" is an economical design.)

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