Deep Foundations Using Lrfd Method

  • Uploaded by: adnan
  • 0
  • 0
  • April 2020
  • PDF

This document was uploaded by user and they confirmed that they have the permission to share it. If you are author or own the copyright of this book, please report to us by using this DMCA report form. Report DMCA


Overview

Download & View Deep Foundations Using Lrfd Method as PDF for free.

More details

  • Words: 1,566
  • Pages: 52
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

Related Documents


More Documents from ""