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LRFD for MSE Walls

PW - Lesson 8

RM - Chapter 4, Sections 4.1 & 4.2

Lesson 8 – Learning OutcomesParticipants will be able to:• Discuss the basic principles of

LRFD• Identify and discuss strength,

service, and extreme event limit states

• Discuss the application of various load and resistance factors as well as load combinations

Goals of Design• Constructability

• Safety

• Serviceability

• Inspectability

• Economy

• Aesthetics

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Key Goal of Design

• Safety

• Expression of Safety• Resistance > Load• Load < Resistance

• Capacity > Demand• Demand < Capacity

Design Platforms• Several available design platforms

• Traditional • ASD – Allowable Stress Design

• Recent• LRFD – Load and Resistance

Factor Design

LRFD vs. ASD• Regardless of the method, the

following condition must be satisfied

Load Effects ≤ Resistance

• Difference in methods is based on how uncertainties in loads and resistances are accounted for

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Uncertainties

• Magnitude

• Direction

• Location

• Frequency

• Combinations

• Soil variability

• Prediction model

• Construction QC

• Extent of exploration

• Failure consequences

ResistancesLoads

How do you account for uncertainties?

Load ≤ Resistance

Allowable Stress Design (ASD)

• Uncertainty in loads and resistances is combined in a single FS applied to nominal resistance

Q ≤R

n

FS

Load and Resistance Factor Design (LRFD)

Load modifier, , accounts for ductility, redundancy and operational importance; ≈ 1 omitted from equation

Load Effects ≤ Resistance

i

iQ

i ≤ R

n

Load Factor

1.0

Resistance factor

1.0

iQ

i ≤ R

n

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Notations

• (phi) is used for both the soil frictionangle and LRFD resistance factor– Friction angle is subscripted to

differentiate from load factor, e.g., r

• (gamma) is used for both soil unitweight and LRFD load factor– Unit weight is subscripted to differentiate

from load factor, e.g., r

LRFD vs. ASD

• Load and resistance factors for MSE walls are currently calibrated by fittingto ASD results.

• Therefore, designs using LRFD procedure should not significantly vary from past, expected ASD designs.

Concept of Limit State

• A limit state can be thought of as a boundary between desired and undesired performance of a design feature

• A limit state is alternatively known as “performance criterion”

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LRFD Limit States iQ

i ≤ R

n

Extreme Event Fatigue

Strength Limit

Service Limit

Load Combinations (Limit States)PW

RM-A

• Permanent Loads

• Transient Loads

Identify Loads and Basic Equations

Load Designations

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Common Permanent Loads

• DC

• DW

• EH

• EV

• ES

• DD

Typical Applicable Permanent Loads for MSE walls

Permanent Loads

EH = Lateral earth loads

EV = Vertical pressure from DL of earth fill

ES = Earth surcharge load

EH – Lateral Earth PressureLateral earth pressure (p)

Stiffness of structure

Characteristics of retained earth

p = ksz

k = ko, ka, or kp

s = soil weight p

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Horizontal Earth PressureMSE Walls - EXTERNAL

• Resultant force, Fa = (1/2) ka s H2

ka = active coefficient

s = unit weight of soil

H = height of Pressure Diagram

EV and EH – MSE Walls

EV

EH

EV and EH – MSE Walls

EV

EV

EH

EHEH

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EV and EH – MSE Walls

EV

EV

EH

EHEH

ES – MSE Walls

EV ES

FootingES

Load FactorsPW

RM-A

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Permanent-Load Load Factors PW

Load Factors, P Typically used with MSE Walls

Type of LoadLoad Factor

Max. Min.

DC: Component & Attachments 1.25 0.90

EH: Active Horizontal Earth Pressure 1.50 0.90

EV: Vertical Earth PressureRetaining Walls and Abutments 1.35 1.00

ES: Earth Surcharge 1.50 0.75

MINI MsE ExerciseWill use

throughout Lessons 8, 9 & 10

Not MINI ME!

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MINI MsE ExerciseWill use

throughout Lessons 8,

9 & 10

Individual Exercise- What are the nominal and factored

weights of this reinforced mass?

Nominal Wt = _______ lb/ft

Factored Wt =

(maximum)_______ lb/ft

Factored Wt =

(minimum)_______ lb/ft

Individual Exercise- What are the nominal and factored

weights of this reinforced mass?

Nominal Wt = 10,000 lb/ft

Factored Wt =

maximum13,500 lb/ft

Factored Wt =

minimum10,000 lb/ft

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Maximum & Minimum Load Factors

• Two load factors, a maximum and a minimum, are listed for permanent loads

• For each load combination, both maximum and minimum permanent loads shall be investigated.

• For permanent force effects, the load factors that produces the more critical combination shall be selected.

Note

• For External Stability, the typicalapplication of load factors to produce the extreme factored force effect is illustrated in the following figures

External Failure Mechanisms

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Typical Sliding & Eccentricity Load Factors

EH-MAX

= 1.50

EV-MIN

= 1.00

EVMIN = 1.00EVMAX = 1.35

EHMIN = 0.90EHMAX = 1.50

Typical Bearing Resistance Load Factors

EH-MAX

= 1.50

EV-MAX

= 1.35

EVMIN = 1.00EVMAX = 1.35

EHMIN = 0.90EHMAX = 1.50

Transient Loads

• LL

• PL

• IM

• BR

• CE

• CT

• CV

• CR

• SE

• SH

• TG

• TU

• EQ

• FR

• IC

• LS

• WA

• WS

• WL

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Typical Applicable Transient Loads for MSE Walls

Transient Loads

CT = Vehicular collision force Extreme EventEQ = Earthquake load

LL = Vehicular live load Design EventLS = Live load surcharge

• There are no max or min values. Either apply or don’t apply the transient load with the single load factor

Live Load Surcharge (LS)

• A live load surcharge (LS) is applied where vehicular load (LL) is expected to act on the surface of the backfill within a distance equal to one-half of the wall height behind the back face of the wall

• LS is a uniform surcharge

LS – MSE Walls

EV

EH

LS

LSBearing, global stabilitySliding

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Complex Geometries• Bridge abutment

• Spread footing on top of MSE wall

• Deep foundations through MSE wall

• See example problems

External Stability

Determine Applicable Loads and Load Combinations, Qi

Check Settlement for Wall Geometry using Unfactored Loads (i = 1.0): i < tol

Determine Ultimate (Nominal) Resistance of Foundation, Rn

Determine Allowable Geotechnical Capacity of

Foundation, Rn / FS

Determine Factored Geotechnical Resistance of

Foundation, Rn

Factor Loads for Each Combination, i Qi

ASD LRFD

Note:

• For most designs, strength limit states generally controls.

• Service limit states may control aspects such as joint width openings, etc.

• Extreme event limit states may affect both the member sizes as well as deformations.

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Note:

• Complete lists of Load Notations, Load Combinations, and Load Factors for Permanent Loads are in Reference Manual – Appendix B.

• Only a few of these loads and load combinations are applicable to MSE walls on a routine basis. These are:

Capacity:Demand Ratio

CDR can be used to compare min and max load factors, to compare controlling case between different limit states, and to compare what aspect (eg., sliding, bearing, e) is controlling a design.

(Not an AASHTO Term)

Lesson 8 – Learning OutcomesParticipants will be able to:• Discuss the basic principles of

LRFD• Identify and discuss strength,

service, and extreme event limit states

• Discuss the application of various load and resistance factors as well as load combinations

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Any Questions?