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
LRFD
17th Edition
3rd Edition
“AASHTO and FHWA
have agreed that all
state DOT’s will use
LRFD for design of
NEW structures by
2007.”
AK
95%
AK
AASHTO LRFD Survey
May 2005
WA
100%
WA
MT
35%
ND
MT
OR
100%
MN
40%
MN
WI
WI
WY
WY
UT
CO
90%
CO
KS
50%
KS
MO
IL
5%
IL ININ
MO
AZ
NM
NM
OK
100%
OK
WV
TN
NC
VT
5%
VT
0- 24 - 10
0- 2-2
NJ
DE
MD
SC
50%
SC
MS
MS
TX
VA
VA
NC
TN
AR
AR
5%
TX
13%
PA
WV
80%
KY
CA
PA
100%
OH
OH
KY
CA
AZ
NY
IA
NE
UT
75%
NY
50%
MI
MI
IA
5%
NE
60%
NV
ME
SD
ID
NV
ME
100%
SD
10%
ID
100%
OR
ND
LA
LA
HI
 Full Implementation
 50-90% Partial Implementation
 26-50% Partial Implementation
 11-25% Partial Implementation
 1-10% Partial Implementation
q No Implementation
AL
AL
GA
GA
FL
100%
FL
PR
NH
MA
RI
CT
Earthwork and walls: ASD
Superstructure: LRFD
Substructure: LRFD/ASD
Foundations: ASD
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?
•
•
•
•
•
Bridge Design examples.
NHI LRFD Training Courses.
FHWA Technical Assistance.
FHWA/ NCHRP Calibration efforts.
AASHTO Section 11 and 10
Revisions.
Bridge Design Examples
Concrete
Steel
http://www.fhwa.dot.gov/bridge/lrfd/examples.htm
NHI LRFD Training Courses
Course 130082A
LRFD for Highway
Bridge Substructures
and Earth Retaining
Structures
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 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
•
•
•
•
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)
Topics NOT Included
•
•
•
•
•
•
Integral abutments
Micropiles
Augercast piles
Soil nails
Reinforced slopes
All soil and rock
earthwork features.
Section 10 Contents
10.1 SCOPE
NO SIGNIFICANT CHANGE
10.2 DEFINITIONS UPDATED
UPDATED, CONSISTANT
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
REORGANIZED,
NEW CONTENT
NEW CONTENT
PROPERTY INFO
Section 10.4 Soil and Rock Properties
GEC 5
Sabatini, 2002
Subsurface
Investigations
Mayne, 2002
Section 10.4 Soil and Rock Properties
10.4.6 SELECTION OF DESIGN PROPERTIES
•
•
•
•
•
Soil Strength
Soil Deformation
Rock Mass Strength
Rock Mass Deformation
Erodibility of rock
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
Downdrag
• Methods for computing
• Load Factors
• Use of minimum load factors clarified
Piles, -method
Piles, -method
Drilled Shafts, O’neill
and Reese (1999)
Maximum
1.4
Minimum
0.25
1.05
1.25
0.30
0.35
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
L
B
P
ML
MB
e
eL
B
B’
q
L’
Section 10.6 Spread Footings
Hough method
Elastic Settlement of
cohesionless soils
 σ'vo Δσ v 
1

ΔH  H c log 
C'  σ'vo 
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
Shear
through
overburden
Inclination
Factors
Bearing
Capacity
Factors
Shape
Correction
Factors
correction
factor
Settlement
correction
factors removed
Section 10.7 Driven Piles
Settlement of pile groups
4 new diagrams
From:
Hannigan (2005)
Section 10.7 Driven Piles
Qt
Ht
Mt
The P-y method specified
for horizontal deflection
P
y
Section 10.7 Driven Piles
S
P
P
Pm * P
y
D
P-multiplier (Pm)
Spacing (S) Row 1 Row 2
3D
0.7
0.5
5D
1.0
0.85
Row 3
0.35
0.7
Section 10.7 Driven Piles
Field determination of nominal resistance
Static load test
Dynamic load test
Section 10.7 Driven Piles
Static analysis methods
•
Nordlund –
Thurman method
added
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 Piles
Comp Str
ksi
Tens Str
ksi
30
Requirements for
driveability analysis
have been added and
clarified
20
10
Ult Cap
kips
Stroke
800
ft
16.0
600
12.0
400
8.0
200
4.0
0
160
320
480 Blows/ft
Section 10.7 Driven Piles
10.7.3.2
PILE LENGTH ESTIMATES FOR
CONTRACT DOCUMENTS
10.7.6
Determination of minimum pile
penetration
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 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 shafts
A+B
A+D
QS
Resistance
Total Resistance
B+C
A
Side Resistance
D
Tip Resistance
QP
Displacement
B
C
Conclusion
Future Enhancements
Overall stability
• Weight is both a load and a resistance
• Service limit state (should be strength
limit state)
+
WT
N tan f
cl
T
N
T
WT
l
WT
l
WT
N
T
N tan f
cl
T
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 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 Enhancements
V
HH
Pile head fixity
• Connection details
• Effects of axial loads
Future Enhancements
Dx
Dz
Serviceability limits
NCHRP 12-66
Due April 2006
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 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 Do?
•
Communicate your findings to
AASHTO’s SubCommitteee members
AASHTO Section 11
•
Design specifications for:
•
•
•
•
•
Conventional
gravity/semigravity walls
Non-gravity cantilevered walls
Anchored walls
Mechanically Stabilized Earth
(MSE) walls
Prefabricated modular walls
LRFD Specifications for
Foundation/ Earth Retaining
Structure Design
Questions?