Deep Foundations Using Lrfd Method

MnDOT Deep Foundation Design Using LRFD Methodology LRFD Bridge Design Workshop June 12, 2007 David Dahlberg, P.E. LRFD Engineer

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

Previous Pile Design Method ƒ Based on Allowable Stress Design (ASD) ∑ Qi ≤ Qult / FS where

Q = service load Qult = ultimate capacity FS = factor of safety

Previous Pile Design Method ƒ Need to consider four things: ƒ ƒ ƒ ƒ

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

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

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

Previous Pile Design Method ƒ Field verification during driving: ƒ MnDOT Modified ENR Formula ƒ CIP piles

3.5E W + 0.1M P= ⋅ S + 0.2 W + M

ƒ H – piles

3 .5 E W + 0 .2 M P= ⋅ S + 0 .2 W+M

ƒ PDA sometimes used

AASHTO LRFD Design Method ƒ 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 ƒ Need to consider four things: ƒ ƒ ƒ ƒ

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

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

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

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

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

AASHTO LRFD Design Method ƒ LRFD Resistance Factor for Driveability ƒ LRFD Table 10.5.5.2.3-1

ƒ LRFD 6.5.4.2

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

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

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

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

New MnDOT LRFD Method

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

Pile Capacity Table

New MnDOT LRFD Method ƒ Field verification during driving ƒ Typically will use MnDOT dynamic formula modified to provide nominal resistance as the output ƒ 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 ƒ Field Verification during driving: ƒ MnDOT Nominal Resistance Pile Driving Formula (for both CIP & H-piles)

10.5E W + 0.1M Rn = ⋅ S + 0.2 W + M ƒ Incorporated by special provision SB2005-2452.2

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

New MnDOT LRFD Method ƒ Resistance factors: ƒ Compare LRFD to ASD LRFD: ∑ γQ ≤ φRn ASD: ∑ Q ≤ Rn /F.S.

Then F.S.= γ / φ

ƒ 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 ƒ Comparisons made with MnDOT Formula, WEAP, Gates Formula, and PDA data

New MnDOT LRFD Method ƒ Field verification ƒ PDA ƒ φdyn = 0.65

ƒ MnDOT Nominal Resistance Pile Driving Formula ƒ φdyn = 0.40

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

ƒ Bridge Plan Load Tables

Implementation for T.H. ƒ MnDOT Foundation Unit (Maplewood Lab) ƒ Providing ultimate capacity estimates

ƒ Regional Bridge Construction Engineers ƒ Provide pile type with maximum resistance ƒ Identify verification method(s) to use

ƒ Designers ƒ ƒ ƒ

Design with LRFD methods and loads Factored loads presented on plans Compare with past ASD designs

Implementation for State Aid ƒ Geotechnical Engineer ƒ Providing ultimate capacity estimates

ƒ Designer ƒ ƒ ƒ ƒ ƒ

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 ƒ Two projects rolled into one: ƒ Development of Resistance Factor for MnDOT Pile Driving Formula ƒ Study of Pile Setup Evaluation Methods

ƒ Research begins this year

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

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

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

Downdrag ƒ Transient loads reduce downdrag, so do not combine live load (or other transient loads) with downdrag ƒ 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 ƒ Discuss with Regional Construction Engineer before using battered piles

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

ƒ Current Practice Using LRFD ƒ Factored loads resisted by: battered pile component + 18 kips/pile resistance

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

Pile Lateral Load Capacity ƒ Piles under combined axial compressive load and moment due to axial and lateral loads at the top of piles ƒ LRFD 6.9.2.2 interaction equation:

Pu 8 ⎛ Mu + ⎜⎜ φ c Pn 9 ⎝ φ f M n

⎞ ⎟⎟ ≤ 1.0 ⎠

Pile Lateral Load Capacity ƒ Inserting known values for Pu, φcPn, φfMn, interaction equation solved for Mu ƒ Lateral load applied at top of pile and increased until the calculated maximum Mu was reached in the pile

Pile Lateral Load Capacity ƒ Results: Fy

Wall t

(ksi)

(in.)

φRnh (kips)

12" CIP 16" CIP

45

all

24

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 HP 14x73

47.8 43.9

NA NA

32 40

Pile Type

Pile Lateral Load Capacity ƒ Results: ƒ Max deflection due to factored loads was approximately 0.5” ƒ Serviceability does not govern

Drilled Shaft Design ƒ Design process is interactive ƒ 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 ƒ Loads & moment for single shafts ƒ Rock sockets ƒ ƒ ƒ ƒ

Drilled Shaft Design

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

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

Questions