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Application of the Load Resistance Factor Design Platform to Geotechnical Features

(Fact and Fiction)

By

Jerry A. DiMaggio, PE, D.GE, M.ASCE

E-Mail: jdimaggio2@verizon.net

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Real Bio: Jerry A. DiMaggio, PE, D.GE,

M.ASCE

• Pin Ball Machine Repairman – 1yr

• Country Club Maintenance Foreman – 5yrs

• Teamster – 5yrs

• Father – 34yrs

• Grandfather – 2 years, 17 months, 1 month

• Civil Engineer (geotechnical and construction

specialist) - 39yrs

• Husband – 44yrs

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Existing AASHTO Specifications

Standard

18th Edition

LRFD now the

6th Edition 3

3/10/2013

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f (R-Q)

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R-Q (R-Q)

(R-Q) F

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History of AASHTO Code

1931 – First US standard specification for bridges (AASHO)

1973 – LFD for steel and concrete bridge components

(AASHO)

1986 – First year any significant geotechnical guidance

included in the code

1988 to 1993 – Development of LRFD specifications for

design and construction of highway bridges in US modeled

after OHBDC

1991 – Provisions added to AASHTO Standard

Specifications for Highway Bridges for design of drilled

shaft foundations and soldier pile, anchored and MSE walls

1994 – 1st Ed. of AASHTO LRFD Highway Bridge Design

Specifications; 2nd Ed.: 1998, 3rd Ed.: 2004, 6th Ed.: 2012

2002 – Ceased updating AASHTO Standard Specifications

for Highway Bridges as part of LRFD transition

2007 – All Federal-funded structure design must use LRFD

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What Changed with LRFD?

New philosophy of safety

Limit states (strength, service, fatigue, extreme event)

New load models (including new live load)

New load and resistance factors based on reliability

methods and calibrations

Introduce limit state-based provisions for foundation

design and soil and rock mechanics

Develop parallel commentary with design provisions

“Forced” improved communication between structural,

geotechnical and construction disciplines

“Forced geotechnical community to better understand,

loads, performance requirements and conduct better

geomaterial parameter assessments

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The Good with LRFD

Few differences between ASD & LRFD

– Familiar design equations

– Familiar failure/performance criteria

Strive to achieve comparable safety in structure and

substructure components for RB calibration

Knowledge of statistics and reliability theory unnecessary

to use LRFD

Provides platform for rationally integrating performance

data, site characterization and parameter selection into

design

Updated annually; not a “cookbook”, recognizes regional

and local geology

Sets a minimum standard of care for design and monitoring

of geotechnical features

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Geotechnical Features*

A. Shallow Foundations (spread footings and mats)

B. Deep Foundations (drilled shafts, driven piles and micropiles)

C. Earth Retaining Structures (fill and cut)

D. Soil Slopes (engineered fills and cuts)

not addressed

* The guidance in this webinar series is based on the AASHTO LRFD Specifications for bridges and structures but the concepts are applicable to all civil engineering facilities and industries.

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Topics Included

Subsurface investigations

Soil and rock properties

Shallow foundations

Driven piles

Drilled shafts

Microplies

Rigid and flexible culverts

Abutments

Walls (most types)

Integral abutments

Augercast piles

Soil nails

Reinforced slopes

All soil and rock earthwork

features

=====================

There are also AASHTO

LRFD CONSTRUCTION

SPECS (driven, drilled

and micropiles)

Topics NOT Included

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f (R-Q)

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p

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The Not So Good with LRFD

LRFD specification developed by and for bridge

engineers; as such, the spec is based on a

structural framework where -factors are lumped

Geo-specialists work in an ASD world, so LRFD is

unfamiliar and uncomfortable to many

Since 1st edition, numerous revisions have led

users to question LRFD (why can’t the code

writers make up their minds?)

Substructure implementation has lagged

superstructure implementation and has often

been “choppy” and confusing

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f (R-Q)

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(R-Q) F

p

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Reasons for Resisting LRFD Adoption

Human nature

No perceived benefits

Unfamiliarity with LRFD methods

Lack of confidence in the computed results

Perceived errors and inconsistencies

Specification that in some respects did not

reflect current design practices

Geotechnical practice is not as “organized”

as structural practice

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ShigiQi ≤ Rr = Rn

f(g,)

Qn

Rn

Q

R

g Qn

Rn

Q or R

Pro

babili

ty o

f O

ccurr

ence

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Civil Engineering Design Platforms

• Allowable Stress Design (ASD)

– Working Stress Design (WSD)

• Load Factor Design (LFD)

– Ultimate Strength Design (USD)

• Load and Resistance Factor Design (LRFD)

– Limit State Design (LSD)

– Reliability Based Design (RBD)

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Common Goal of ASD, LFD or LRFD

• Designs must be safe

– Capacity > Demand (or Demand < Capacity)

– Resistance > Load (or Load < Resistance)

• In LRFD the terms load and resistance are used

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Resistance Factor

• For geotechnical features the resistance factor, , accounts for uncertainties in: – Extent of subsurface investigation

– Variability of soil or rock properties (parameters)

– Accuracy and reliability of resistance prediction equations

• e.g., Terzaghi vs Meyerhof theories of bearing capacity

– Level and methods of construction monitoring (QC/QA)

– Consequences of failure

Load Factor

• The load factor, g, accounts for uncertainties in – Magnitude and direction of load(s)

– Location of application of load(s)

– Possible load combinations

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Jack Stressing

Anchorage

Dial Gauge

Fixed Base 16

Primary Limit States

1. Strength limit state • Applies to strength

and stability during the design life

2. Service limit state • Applies to stress,

deformation, and cracking under regular operating conditions

AASHTO LRFD Article 1.3.2

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Primary Limit States

3. Extreme event limit state

• Applies to structural survival during once in a design-life events

4. Fatigue limit state

• Applies to restrictions on stress range under repetitive live loading

AASHTO LRFD Article 1.3.2 18

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Definitions

• Extreme Event Limit States—Limit states relating to events such as earthquakes, ice load, and vehicle and vessel collision, scour.

• Extreme event limit states relate to events with return periods in excess of the design life of the bridge or other structure.

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LRFD (or LSD or RBD)

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ShigiQi ≤ Rn

Load Factor (gi) Resistance Factor ()

Load Modifier (hi)

Load Effect (Qi)

Nominal Resistance (Rn)

Factored Load Effect ≤ Factored Resistance

Maintaining separation between Qn and Rn

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Qmean

Qn Rn f(R,Q)

Q,R

gQn Rn

Rmean

g = Load Factor = Resistance Factor

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What is Calibration?

• Calibration is the process of assigning values to resistance factors and load factors to quantify a chosen level of reliability

• LRFD Calibration can be achieved by

– Judgment

– “Fitting” with ASD

– Reliability theory

– Combination of above approaches

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Establish g and by “Fitting” with ASD

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1DL/LLFS

(DL/LL)

LLDLFS

LLDL

QFS

Q LDLD

i

ii

gg

gg

g

)(

• For DL and LL

• For DL only

FSDLFS

DL

QFS

Q DD

i

ii g

g

g

)(

)(

AASHTO Definition of Reliability Index, b

• AASHTO (2007) defines reliability index as

“a quantitative assessment of safety expressed as the ratio of the difference between the mean resistance and mean force effect to the combined standard deviation of resistance and force effect.”

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AASHTO Definition of Reliability Index, b

• Assuming uncorrelated normally distributed probability distributed functions for R, Q and g, the Reliability Index, β, is as follows:

• Reliability Index is also known as Safety Index because it is based on Safety Margin, i.e., R-Q

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COV

1

DevStd

Mean

ss

QR

2Q

2R

meanmean b

b

• β increases as Pf reduces

• Need relationship between β and Pf

• F-1(1-Pf) is the value of the standard normal variate at the probability level 1-Pf

)P(1ss

QRf2

Q2R

meanmean F

b 1

Calibration of b with Pf

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Calibration of b with Pf

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AASHTO Table 3.4.1-1

Load

Combination

Limit State

DC

DD

DW

EH

EV

ES

EL

PS

CR

SH

LL

IM

CE

BR

PL

LS WA WS WL FR TU TG SE

Use One of These at

a Time

EQ IC CT CV

STRENGTH

LIMIT

I γp 1.75 1.00 — — 1.00 0.50/1.20 γTG γSE — — — —

II γp 1.35 1.00 — — 1.00 0.50/1.20 γTG γSE — — — —

III γp — 1.00 1.40 — 1.00 0.50/1.20 γTG γSE — — — —

IV γp — 1.00 — — 1.00 0.50/1.20 — — — — — —

V γp 1.35 1.00 0.40 1.0 1.00 0.50/1.20 γTG γSE — — — —

EXTREME

EVENT

I γp γEQ 1.00 — — 1.00 — — — 1.00 — — —

II γp 0.50 1.00 — — 1.00 — — — — 1.00 1.00 1.00

SERVICE LIMIT

I 1.00 1.00 1.00 0.30 1.0 1.00 1.00/1.20 γTG γSE — — — —

II 1.00 1.30 1.00 — — 1.00 1.00/1.20 — — — — — —

III 1.00 0.80 1.00 — — 1.00 1.00/1.20 γTG γSE — — — —

IV 1.00 — 1.00 0.70 — 1.00 1.00/1.20 — 1.0 — — — —

FATIGUE - LL,

IM & CE only

I — 1.50 — — — — — — — — — — —

II — 0.75 — — — — — — — — — — —

Selecting a Load Combination

Limit State Load

Combination Primary Application

Strength

I Normal vehicles, no wind

II Special or permit vehicles

III Locations where wind

exceeds 55 mph

IV Very high DL to LL ratios

V Normal vehicles with wind

Service

I Crack width in concrete, etc.

II Steel structures only

III P/S concrete structures only

IV P/S concrete structures only

Extreme

Event

I Includes earthquake

II Includes ice and collision

Fatigue I/ II Fatigue vehicle only

AASHTO LRFD Article 3.4.1 29

Load Factors for Permanent Loads, gp

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Type of Load, Foundation Type, and

Method Used to Calculate Downdrag

Load Factor

Maximum Minimum

DC: Component and Attachments

DC: Strength IV only

1.25

1.50

0.90

0.90

DD: Downdrag Piles, Tomlinson Method

Piles, Method

Drilled shafts, O’Neill and Reese (1999) Method

1.4

1.05

1.25

0.25

0.30

0.35

DW: Wearing Surfaces and Utilities 1.50 0.65

EH: Horizontal Earth Pressure

Active

At-Rest

AEP for anchored walls

1.50

1.35

1.35

0.90

0.90

N/A

EL: Locked-in Construction Stresses 1.00 1.00

EV: Vertical Earth Pressure

Overall Stability

Retaining Walls and Abutments

Rigid Buried Structure

Rigid Frames

Flexible Buried Structures other than Metal Box Culverts

Flexible Metal Box Culverts and Structural Plate Culverts with Deep

Corrugations

1.00

1.35

1.30

1.35

1.95

1.50

N/A

1.00

0.90

0.90

0.90

0.90

ES: Earth Surcharge 1.50 0.75

AASHTO Table 3.4.1-2

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Loads Factors for Permanent Loads

• Selected to produce max./min. total extreme force effects

• For maximum force effects, loads that reduce maximum force effects should be factored by minimum load factor

AASHTO 3.4.1

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Identify Controlling Load Combinations and Factors

• Sliding at Strength I Limit State

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AASHTO Section 10.4 Soil and Rock Properties

Article Topic

10.4.1 Informational Needs

10.4.2 Subsurface Exploration

10.4.3 Laboratory Tests

10.4.4 In Situ Tests

10.4.5 Geophysical Tests

10.4.6 Selection of Design Properties

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AASHTO - Section 11 Outline Article Topic

11.1 Scope

11.2 Definitions

11.3 Notation

11.4 Soil Properties and Materials

11.5 Limit States and Resistance Factors

11.6 Abutments and Conventional Retaining Walls

11.7 Piers

11.8 Non-gravity Cantilevered Walls

11.9 Anchored Walls

11.10 Mechanically Stabilized Earth Walls

Refer to Section 3 for Loads and Load Factors

AASHTO Section 10.7 Driven Piles

Article Topic

10.7.1 General

10.7.2 Service Limit State Design

10.7.3 Strength Limit State Design

10.7.4 Extreme Event Limit State Design

10.7.5 Corrosion and Deterioration

10.7.6 Minimum Pile Penetration

10.7.7 Driving Criteria for Bearing

10.7.8 Drivability Analysis

10.7.9 Test Piles

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Comparison of LRFD and ASD Geotechnical approaches for Structural Foundations and

Earth Retaining Structures

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Same Different

• Determining resistance • Comparison of load and resistance

• Determining deflection • Separation of resistance and deflection

NEW IN LRFD

• Additional design equations

• New load computation methods

• Deformation based analysis for extreme events

• Significantly expanded commentary and guidance for designers

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Loads

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Structural Geotechnical

New

Fill

Bridge Deck

Soft Soil Consolidating

Due to Fill Weight

Bearing Stratum

New

Fill

New

Fill

Bridge Deck

Soft Soil Consolidating

Due to Fill Weight

Bearing Stratum

Lateral Squeeze

Downdrag

– See FHWA Soils and Foundation Manual (2006) for more information on geotechnical loads

– Refer to AASHTO Section 3 and Tables 3.4.1-1 and 3.4.1-2 for loads and load factors

• Axial compression resistance for single piles

• Pile group compression resistance

• Uplift resistance of single piles

• Uplift resistance of pile groups

• Pile punching failure in weaker stratum

• Single pile and pile group lateral resistance

• Constructability, including pile driveability

Strength Limit State Driven Piles ARTICLE 10.5.3.3

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• Negative shaft resistance (downdrag)

• Lateral squeeze

• Scour

• Pile and soil heave

• Seismic considerations

SPECIAL DESIGN CONSIDERATIONS

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40

STRENGTH LIMIT STATES

Structural

Axial

Driven Assess Driveability

Flexure

Shear

Geotechnical Axial

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YOU KNOW YOU HAVE PILE DAMAGE WHEN:

The Pile Falls Over

After Driving !

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Concrete (5.5.4.2) Axial Comp. = 0.75 Flexure = 0.9 (strain dependent) Shear = 0.9

Steel (6.5.4.2) Axial = 0.5-0.7 Combined Axial= 0.7-0.8 Flexure = 1.0 Shear = 1.0

Timber (8.5.2.2 and .3) Compression = 0.9 Tension = 0.8 Flexure = 0.85 Shear = 0.75

LRFD

Specifications

Structural Resistance Factors 10.7.3.13 Pile Structural Resistance

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Service Limit State Checks

Global Stability Vertical and Horizontal

Displacements

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10.5 LIMIT STATES AND RESISTANCE

• Strength Limit State (will be discussed later)

– Structural Resistance

– Geotechnical Resistance

– Driven Resistance

• Service Limit State

– Resistance Factor = 1.0 (except for global stability)

• Extreme Event Limit State

– Seismic, superflood, vessel, vehicle, ice

– Use nominal resistance (except for uplift)

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Field methods

Static load test

Dynamic load test (PDA)

Driving Formulae

Wave Equation Analysis

Static analysis methods

Determining Nominal Axial Geotechnical Resistance of Piles

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Geotechnical Safety Factors for Piles (ASD)

Basis for Design and Type of Construction Control

Increasing Design/Construction Control

Subsurface exploration X X X X X

Static analysis X X X X X

Dynamic formula X

Wave equation X X X X

CAPWAP analysis X X

Static load test X X

Factor of Safety (FS) 3.50 2.75 2.25 2.00 1.90

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AASHTO STANDARD SPECIFICATIONS

Pile Testing Methods

Analysis Method Resistance Factor

() (AASHTO 2012)

Est. Measure

Cap

acity

Stress

Energy

Cap

acity

Stress

Energy

Dynamic formula 0.10 (EOD) or 0.40 (EOD) X

Wave equation 0.50 (w field confirmation of hammer) X X X

Dynamic testing* 0.65 (2%) or 0.75 (100%) (0.5 uplift) X X X

Static load test** 0.75 to 0.80 (wo/w dynamic) (0.6 UPLIFT)

X

47 * Dynamic Test requires signal matching

**Static Test requires one test pile per site

Resistance Factors Static Analysis Methods AASHTO Table 10.5.5.2.3-1

Method Resistance Factor,

Compression Tension

- method 0.35 0.25

b- method 0.25 0.20

- method 0.40 0.30

Nordlund- Thurman 0.45 0.35

SPT 0.30 0.25

CPT 0.50 0.40

Group 0.60 0.50 48

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Driven Pile Time Dependent Effects on Axial Geotechnical Resistance

Article 10.7.3.4 Setup Relaxation

RP

RS

RP

RS

RP

RS

RP

RS

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Point Bearing on Rock

Article 10.7.3.2 • Soft rock that can be penetrated by pile driving may be

treated similar to soils.

• Steel piles driven into soft rock may not require tip reinforcement.

• On hard rock the nominal resistance is controlled by the structural capacity. See Article 6.9.4.1 and the driving resistances in 6.5.4.2 and 6.15 for severe driving.

• Dynamic testing should be used when the nominal resistance exceeds 600 kips.

• C10.7.3.2.3 Provides qualitative guidance to minimize pile damage when driving piles on hard rock.

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Super Coastal Extreme Event

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• Scour

• Vessel and Vehicle collision

• Seismic loading and site specific situations.

(Uplift Resistance should be 0.80 rather than 1.00 for all extreme checks.)

EXTREME EVENT LIMIT STATES

AASHTO 10.5.5.3

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ASCE LRFD Webinar Series

* Check ASCE website for latest information

# Topic 2010 2011 2012 2013

1 Fundamentals of LRFD

Part 1 6/30 1/18, 10/13 4/2, 1/7 , 8/5 (Mon)

2 Part 2 7/15 2/4, 10/21 4/26 1/24; 8/19 (Mon)

3 Subsurface Explorations 4/15 2/17, 8/18 2/3, 11/6 6/27 (Th)

4 Shallow Foundations 1/6, 5/7, 11/8 5/20, 12/12 10/16

5 Deep Foundations

Driven Piles 1/25, 6/1, 12/14 6/21, 11/7 6/8 2/13

6 Drilled Shafts 2/8, 6/11 1/7, 7/8 1/23, 7/9 4/15

7 Micropiles 9/10 3/3, 7/29 1/12, 8/9 6/10 (Mon)

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Earth Retaining Structures

Fill Walls 8/20 3/11, 9/12 9/10

9 Cut Walls 10/21 9/30 2/28, 9/28

10 MSE Walls 4/4, 12/2 8/27 5/13 (Mon)

11 Ground Anchors 5/2 3/29, 12/11 4/30 (Tu)

12 Deep Foundations – Lateral Analysis 5/7 3/4 , 9/16 (Mon)

13 Extreme Events 5/21

3/28 , 9/30 (Mon)

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SINCE THE 1500’s THE

NAME and FAMILY

DIMAGGIO HAS

REPRESENTED THE

VERY BEST IN COFFEE,

BASEBALL AND

GEOENGINEERING AND

GEO-CONSTRUCTION

EXCELLENCE!

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Thank You for Your Attention!

Jerry D. “Alias Joe D’ Cousin”

jdimaggio2@verizon.net

Any Questions?

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