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AASHTO’s LRFD Specifications for Foundation and Earth Retaining Structure Design (Through 2006 Interims and Beyond). Jerry A. DiMaggio, P.E. Principal Bridge/Geotechnical Engineer FHWA, Washington D. C. . Existing Specifications. Standard 17 th Edition. LRFD 3 rd Edition.

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slide1

AASHTO’s LRFD Specifications for Foundation and Earth Retaining Structure Design

(Through 2006 Interims and Beyond)

Jerry A. DiMaggio, P.E.

Principal Bridge/Geotechnical Engineer

FHWA, Washington D. C.

existing specifications
Existing Specifications

Standard

17th Edition

LRFD

3rd Edition

aashto and fhwa have agreed that all state dot s will use lrfd for design of new structures by 2007

“AASHTO and FHWA have agreed that all state DOT’s will use LRFD for design of NEW structures by 2007.”

slide4

AASHTO LRFD Survey

May 2005

AK

AK

95%

WA

WA

MT

MT

ND

ND

100%

MN

MN

35%

ME

ME

40%

100%

SD

SD

OR

OR

VT

VT

ID

ID

WI

WI

10%

5%

100%

100%

NY

NY

MI

MI

WY

WY

50%

NH

IA

IA

0

-

24

-

10

NE

NE

5%

0

0

-

-

2

2

-

-

2

2

MA

PA

PA

60%

IL

IL

100%

OH

OH

IN

IN

RI

UT

UT

5%

NV

NV

CO

CO

CT

MO

MO

75%

KS

KS

WV

WV

90%

NJ

50%

80%

KY

KY

VA

VA

CA

CA

DE

MD

OK

OK

TN

TN

NC

NC

AR

AR

100%

AZ

AZ

NM

NM

SC

SC

5%

50%

MS

MS

AL

AL

GA

GA

TX

TX

LA

LA

13%

HI

FL

FL

100%

Full Implementation

¢

PR

50-90% Partial Implementation

¢

26-50% Partial Implementation

¢

11-25% Partial Implementation

¢

1-10% Partial Implementation

¢

No Implementation

q

slide5

Earthwork and walls: ASD

Superstructure: LRFD

Substructure: LRFD/ASD

Foundations: ASD

reasons for not adopting
Reasons for Not Adopting
  • Human nature.
  • No perceived benefits.
  • Unfamiliarity with LRFD methods.
  • Lack of confidence in the computed results.
  • Perceived errors and inconsistencies.
  • A specification that did not reflect current design practices.
what is fhwa doing
What is FHWA doing?
  • Bridge Design examples.
  • NHI LRFD Training Courses.
  • FHWA Technical Assistance.
  • FHWA/ NCHRP Calibration efforts.
  • AASHTO Section 11 and 10 Revisions.
bridge design examples
Bridge Design Examples

Concrete

Steel

http://www.fhwa.dot.gov/bridge/lrfd/examples.htm

nhi lrfd training courses
NHI LRFD Training Courses

Course 130082A

LRFD for Highway Bridge Substructures and Earth Retaining Structures

fhwa nchrp activities
FHWA/ NCHRP Activities
  • NCHRP Project 12-66, Specifications for Serviceability in the Design of Bridge Foundations
  • NCHRP Report 507, Load and Resistance Factor Design (LRFD) for Deep Foundations
fhwa nchrp activities11
FHWA/ NCHRP Activities
  • Publication No. FHWA-NHI-05-052, Development of Geotechnical Resistance Factors and Downdrag Load Factors for LRFD Foundation Strength Limit State Design
revisions to section 10
Revisions to Section 10
  • Compiled by a Technical Expert Panel
  • Review and input from A Technical Working Group (TWG)
  • Accepted by AASHTO Subcommittee T-15 in June 2005 in Newport, Rhode Island
  • To be published in 2006 Interim

http://bridges.transportation.org/?siteid=34&c=downloads

  • Attachments to Agenda Item 39 Section 3 revisions
  • Attachments to Agenda item 40 Section 10 revisions
topics included
Subsurface investigations

Soil and rock properties

Shallow foundations

Driven piles

Drilled shafts

Rigid and flexible culverts

Abutments

Walls (All types)

Integral abutments

Micropiles

Augercast piles

Soil nails

Reinforced slopes

All soil and rock earthwork features.

Topics Included

Topics NOT Included

section 10 contents

REORGANIZED,

NEW CONTENT

NEW CONTENT

PROPERTY INFO

Section 10 Contents

10.1 SCOPE

10.2 DEFINITIONS

10.3 NOTATION

10.4 SOIL AND ROCK PROPERTIES

10.5 LIMIT STATES AND RESISTANCE FACTORS

10.6 SPREAD FOOTINGS

10.7 DRIVEN PILES

10.8 DRILLED SHAFTS

NO SIGNIFICANT CHANGE

UPDATED

UPDATED, CONSISTANT

section 10 4 soil and rock properties
Section 10.4 Soil and Rock Properties

Subsurface Investigations

Mayne, 2002

GEC 5

Sabatini, 2002

section 10 4 soil and rock properties16
Section 10.4 Soil and Rock Properties

NEW!

10.4.6 SELECTION OF DESIGN PROPERTIES

  • Soil Strength
  • Soil Deformation
  • Rock Mass Strength
  • Rock Mass Deformation
  • Erodibility of rock

NEW!

section 10 5 limit states and resistance factors
Section 10.5 Limit States and Resistance Factors
  • Resistance factors revised
  • Additional discussion on the basis for resistance factors
  • Additional discussion of extreme event considerations
articles 3 4 1 and 3 11 8
Articles 3.4.1 and 3.11.8

Downdrag

  • Methods for computing
  • Load Factors
  • Use of minimum load factors clarified
section 10 6 spread footings
Section 10.6 Spread Footings

Eccentricity provisions clarified

B′ = B – 2eB

L′ = L – 2eL

Q = P/(B’ L’)

Applies to geotechnical design for settlement and bearing resistance

section 10 6 spread footings20
Section 10.6 Spread Footings

Hough method

Elastic Settlement of cohesionless soils

section 10 6 spread footings21

Shear through overburden correction factor

Shape Correction Factors

Inclination Factors

Bearing Capacity Factors

Section 10.6 Spread Footings

NOMINAL RESISTANCE

COHESION

UNIT WEIGHT

DEPTH

WIDTH

qn = c Ncm +  Df Nqm Cwq + 0.5  B Nm Cw 

Nc sc ic

Nq sq dq iq

N  s  i 

Water table correction

Settlement correction factors removed

section 10 7 driven piles
Section 10.7 Driven Piles

Settlement of pile groups

4 new diagrams

From:

Hannigan (2005)

section 10 7 driven piles23

P

y

Section 10.7 Driven Piles

Qt

The P-y method specified for horizontal deflection

Mt

Ht

section 10 7 driven piles24
Section 10.7 Driven Piles

S

P

P

Original curve

Modified curve

Pm * P

y

D

P-multiplier (Pm)

section 10 7 driven piles25
Section 10.7 Driven Piles

Field determination of nominal resistance

Static load test

Dynamic load test

section 10 7 driven piles26
Section 10.7 Driven Piles

Static analysis methods

  • Nordlund – Thurman method added
section 10 7 driven piles27
Section 10.7 Driven Piles

Static analysis methods

  • Primary use is for pile length estimation for contract drawings
  • Secondary use for estimation of downdrag, uplift resistance and scour effects
  • Should rarely be used as sole means of determining pile resistance
section 10 7 driven piles28
Section 10.7 Driven Piles

Requirements for driveability analysis have been added and clarified

section 10 7 driven piles29
Section 10.7 Driven Piles

NEW!

NEW!

10.7.3.2 PILE LENGTH ESTIMATES FOR CONTRACT DOCUMENTS

10.7.6 Determination of minimum pile penetration

section 10 8 drilled shafts
Section 10.8 Drilled shafts

Refers to driven piles section where possible

  • Downdrag
  • Group settlement
  • Horizontal displacement (single and group)
  • Lateral squeeze
  • Water table and buoyancy
  • Scour
  • Group resistance (cohesive soil only)
  • Uplift (group and load test sections)
  • Buckling
  • Extreme event limit state
section 10 8 drilled shafts31
Section 10.8 Drilled shafts
  • Static analysis methods for soil and rock have been updated
  • Consideration of both base and side resistance in rock is now included

O’Neill and Reese (1999)

section 10 8 drilled shafts32
Section 10.8 Drilled shafts

A + B

A + D

Total Resistance

B + C

A

Side Resistance

B

Resistance

C

D

QS

Tip Resistance

QP

Displacement

future enhancements
Future Enhancements

Overall stability

  • Weight is both a load and a resistance
  • Service limit state (should be strength limit state)

WT

WT

N tan f

N tan f

cl

l

cl

l

T

N

T

WT

N

WT

T

T

future enhancements35
Future Enhancements

Inclination Factors

  • Ignored by many practicing engineers
  • Based on small scale tests and theory
  • Effect of embedment (Df)
  • Resistance factors are for vertical load

Q

Df

future enhancements36
Future Enhancements

Nominal bearing resistance of rock

  • Very little guidance available
  • CSIR Rock Mass Rating System proposed
  • CSIR developed for tunnel design
  • Includes life safety considerations and therefore, margin of safety
  • May be conservative
future enhancements37

H

H

Future Enhancements

V

Pile head fixity

  • Connection details
  • Effects of axial loads
future enhancements38

Dx

Dz

Future Enhancements

Serviceability limits

NCHRP 12-66

Due April 2006

what should i know and do
What Should I Know and Do?
  • Become familiar with BOTH the AASHTO standard specifications and LRFD specs.
  • Develop an understanding of your agency’s current design practice
what should i know and do40
What Should I Know and Do?
  • Develop and compare results for SEVERAL example problems with LRFD and YOUR standard design practice
  • Translate your current practice to an LRFD format
what should i know and do41
What Should I Know and Do?
  • Communicate your findings to AASHTO’s SubCommitteee members
aashto section 11
AASHTO Section 11
  • Design specifications for:
    • Conventional gravity/semigravity walls
    • Non-gravity cantilevered walls
    • Anchored walls
    • Mechanically Stabilized Earth (MSE) walls
    • Prefabricated modular walls
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