MnDOT’s LRFD Bridge Design Manual

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MnDOT’s LRFD Bridge Design Manual Powered By Docstoc
					MnDOT Deep Foundation
  Design Using LRFD
    Methodology
   LRFD Bridge Design Workshop
          June 12, 2007

       David Dahlberg, P.E.
         LRFD Engineer
Presentation Overview

  Previous Pile Design Method
  AASHTO LRFD Pile Design
  Method
  New MnDOT LRFD Method
  Pile Downdrag
  Pile Lateral Load Capacity
  Drilled Shaft Design
Previous Pile Design Method

   Based on Allowable Stress Design (ASD)
          ∑ Qi ≤ Qult / FS
     where
        Q = service load
        Qult = ultimate capacity
        FS = factor of safety
Previous Pile Design Method

  Need to consider four things:
    Capacity of soil
    Structural capacity of pile
    Driveability of pile (max driving stresses)
    Field verification during driving operation to
    ensure required resistance is obtained
Previous Pile Design Method

  Design soil allowable capacity determination
  based on combination of:
    Static analysis w/ F.S (done by geotechs)
    Correlation of borings with field verification
    method (done by Regional Construction
    Engineer)
Previous Pile Design Method

  Typical pile was 12” dia. CIP w/0.25” wall
    60 to 75 ton allowable maximum load
    (based on considering past practice,
     AASHTO, experience, and driveability
     of the pile)
Previous Pile Design Method

   Majority of pile capacities based on field
   measured initial drive capacity
   Soil/pile setup used when warranted by
   soil profile
     Only in low initial capacity situations
Previous Pile Design Method

  Field verification during driving:
    MnDOT Modified ENR Formula

                        3.5E W + 0.1M
      CIP piles     P=        ⋅
                       S + 0.2 W + M


                        3 .5 E W + 0 .2 M
      H – piles     P=          ⋅
                       S + 0 .2   W+M

    PDA sometimes used
AASHTO LRFD Design Method

 Requires use of factored loads & nominal
 resistance
     ∑ ηi ⋅ γi ⋅Qi ≤ φ⋅Rn
    where
       η = load modifier
       γ = load factor
       Q = service load
       φ = resistance factor
       Rn = nominal (ultimate) resistance
AASHTO LRFD Design Method

 Need to consider four things:
   Capacity of soil
   Structural capacity of pile
   Driveability of pile (max driving stresses)
   Field verification during driving operation to
   ensure required resistance is obtained
AASHTO LRFD Design Method

  Capacity of soil:
    Estimated by geotechnical engineer using static
    pile analysis
    Resistance factors φstat from LRFD
    Table 10.5.5.2.3-1
AASHTO LRFD Design Method

 LRFD Resistance Factors for Piles
 LRFD Table 10.5.5.2.3-1
AASHTO LRFD Design Method

  Structural capacity of
  pile:
     CIP piles per LRFD
     6.9.5.1
       φc ·(Asffy+0.85f’c·Ac)
     H piles per LRFD 6.9.4.1
       φc ·Asfy
     Resistance factors for
     axial resistance per LRFD
     6.15.2 and 6.5.4.2
AASHTO LRFD Design Method

LRFD Resistance Factors for Steel Piles
found in LRFD 6.5.4.2
AASHTO LRFD Design Method

 Driveability (max driving resistance):
   Per LRFD 10.7.8:
         0.9· φda·fy
   Resistance factor per LRFD
   Table 10.5.5.2.3-1 and LRFD 6.5.4.2
AASHTO LRFD Design Method


LRFD Resistance Factor for Driveability
   LRFD Table 10.5.5.2.3-1




   LRFD 6.5.4.2
AASHTO LRFD Design Method

 Field verification during driving
 operation to ensure required resistance
 is obtained:
   Verification by static load test, dynamic
   testing (PDA), wave equation, or dynamic
   formula
   Uses resistance factor φdyn from
   LRFD Table 10.5.5.2.3-1
  AASHTO LRFD Design Method

LRFD
Resistance
Factors for
Piles
LRFD Table
10.5.5.2.3-1
New MnDOT LRFD Method

Capacity of soil:
  Look in the Foundation Report
  Typical Foundation Report should include:
    Project description
    Field investigation and foundation conditions
    Foundation analysis
    Recommendations
    Additional sections as needed
New MnDOT LRFD Method

 Foundation analysis should include:
   Nominal Resistance (ultimate capacity)
   estimates provided by Foundations Unit
   Initial drive and set-up graph which shows
   resistance as a function of depth
New MnDOT LRFD Method
New MnDOT LRFD Method

 Pile Resistance φRn for design
   Determined considering LRFD structural
   capacity of pile, maximum LRFD driving
   resistance, and past experience



         Pile Capacity Table
New MnDOT LRFD Method

 Field verification during driving
   Typically will use MnDOT dynamic formula
   modified to provide nominal resistance as
   the output

   Will use PDA on larger projects by running
   a PDA on the test piles to calibrate the
   MnDOT dynamic formula for other piles
New MnDOT LRFD Method

  Field Verification during driving:
    MnDOT Nominal Resistance Pile Driving
    Formula (for both CIP & H-piles)

              10.5E W + 0.1M
         Rn =        ⋅
              S + 0.2 W + M
    Incorporated by special provision
    SB2005-2452.2
New MnDOT LRFD Method
LRFD
Resistance
Factors for
Piles
LRFD Table
10.5.5.2.3-1
New MnDOT LRFD Method

 Resistance factors:
   Compare LRFD to ASD
   LRFD: ∑ γQ ≤ φRn
   ASD: ∑ Q ≤ Rn /F.S. Then F.S.= γ / φ

   Average γ ≈ 1.4
   For MnDOT formula, φdyn = 1.4/3.0 ≈ 0.45
   For PDA, φdyn = 1.4/2.25 ≈ 0.60
New MnDOT LRFD Method

  Comparisons made with MnDOT Formula,
  WEAP, Gates Formula, and PDA data
New MnDOT LRFD Method

  Field verification
    PDA
      φdyn = 0.65

    MnDOT Nominal
    Resistance Pile
    Driving Formula
      φdyn = 0.40
New MnDOT LRFD Method

   Monitoring method determines required
   driving resistance for the Contractor
   For example, assume a factored design
   load of 100 tons/pile:
     PDA verification
       Rn = Qu/ φdyn = 100/0.65 = 154 tons

     MnDOT Ultimate formula
       Rn = Qu/ φdyn = 100/0.40 = 250 tons
New MnDOT LRFD Method




   Example
New MnDOT LRFD Method
New MnDOT LRFD Method


      Pile Capacity Table
New MnDOT LRFD Method
New MnDOT LRFD Method



 Bridge Plan
 Load Tables
Implementation for T.H.
 MnDOT Foundation Unit (Maplewood Lab)
   Providing ultimate capacity estimates
 Regional Bridge Construction Engineers
   Provide pile type with maximum resistance
   Identify verification method(s) to use
 Designers
   Design with LRFD methods and loads
   Factored loads presented on plans
   Compare with past ASD designs
Implementation for State Aid

  Geotechnical Engineer
    Providing ultimate capacity estimates
  Designer
    Provide pile type with maximum resistance
    Identify verification method(s) to use
    Design with LRFD methods and loads
    Factored loads presented on plans
    Compare with past ASD designs
Research

 Two projects rolled into one:
   Development of Resistance Factor for
   MnDOT Pile Driving Formula

   Study of Pile Setup Evaluation Methods

 Research begins this year
Downdrag

 Downdrag is the
 downward load induced
 in the pile by the settling
 soil as it grips the pile
 due to negative side
 friction
 Covered in LRFD 3.11.8,
 10.7.1.6.2, 10.7.2.5, and
 10.7.3.7
Downdrag

 Estimated downdrag load will be given in
 the Foundation Report
 For piles driven to rock or a dense layer
 (end bearing piles), nominal pile
 resistance should be based on pile
 structural capacity
Downdrag

 For piles controlled by side friction,
 downdrag may cause pile settlement,
 which will result in reduction of the
 downdrag load
 Amount of pile settlement difficult to
 calculate, so downdrag on friction piles to
 be considered on a case by case basis
Downdrag

 Transient loads reduce downdrag, so do not
 combine live load (or other transient loads) with
 downdrag
 Consider a load combination with DC + LL and
 also a load combination that includes DC + DD,
 but do not consider LL and DD within the same
 load combination
 Discuss with Regional Construction Engineer
 before using battered piles
Pile Lateral Load Capacity

  Past Practice Using ASD
    Service loads resisted by:
          battered pile component
                         +
          12 kips/pile resistance
  Current Practice Using LRFD
    Factored loads resisted by:
          battered pile component
                         +
          18 kips/pile resistance
Pile Lateral Load Capacity

  Parametric study conducted:
    12” & 16” diameter CIP piles
    HP10x42, HP12x53 and HP14x73
    Single layer of noncohesive soil with
    varied friction angles of 30˚, 32˚, 34˚,
    36˚, and 38˚
    ENSOFT program L-Pile 5.0.30 used for
    this study
Pile Lateral Load Capacity

  Piles under combined axial compressive
  load and moment due to axial and lateral
  loads at the top of piles

  LRFD 6.9.2.2 interaction equation:


       Pu    8 ⎛ Mu        ⎞
            + ⎜            ⎟ ≤ 1.0
      φ c Pn 9 ⎜ φ f M n
               ⎝
                           ⎟
                           ⎠
Pile Lateral Load Capacity

  Inserting known values for Pu, φcPn, φfMn,
  interaction equation solved for Mu

  Lateral load applied at top of pile and
  increased until the calculated maximum
  Mu was reached in the pile
Pile Lateral Load Capacity

  Results:
                   Fy     Wall t    φRnh
      Pile Type
                  (ksi)    (in.)   (kips)
      12" CIP      45      all      24
      16" CIP      45      1/4      28
      16" CIP      45      5/16     40
      16" CIP      45      3/8      40
      16" CIP      45      1/2      40
      HP 10x42     50      NA       24
      HP 12x53    47.8     NA       32
      HP 14x73    43.9     NA       40
Pile Lateral Load Capacity

  Results:
    Max deflection due to factored loads
    was approximately 0.5”

    Serviceability does not govern
Drilled Shaft Design

  Design process is interactive
  Designer, Regional Construction Engineer,
  and geotechnical engineer need to discuss:
       Proposed construction method
       Permanent vs. temporary casing
       Shaft diameter
       Vertical & horizontal loads for multiple row
       shaft foundation
       Loads & moment for single shafts
       Rock sockets
Drilled Shaft Design
Drilled Shaft Design

  Resistance factors
  vary:
     Tip/side
     resistance
     Load tests
     Base grouting
Drilled Shaft Design

   Existing foundation load tables given in
   MnDOT Bridge Design Manual
   Appendix 2-H do not include drilled
   shafts
   Spread footing load tables were used in
   the past
   New load tables to be created for drilled
   shafts
Questions

				
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