ODOT LRFD Foundations - Pages LRFD Foundations May 07, 2013 ... Seismic design not mandatory for...

Ohio Department of Transportation

John R. Kasich, Governor Jerry Wray, Director

ODOT LRFD Foundations

May 07, 2013

Alexander Dettloff, P.E. Foundation Engineer

Office of Geotechnical Engineering

2

AASHTO 6th Ed. versus 5th Ed.

Check the Plan Sheets:

AASHTO 6th Edition (2012) versus 5th Edition (2010)

3

Important Changes in AASHTO 6th Ed.

Eccentricity Limits Section 11.6.3.3

Soil: Middle 2/3 B (was middle ½ B)

Rock: Middle 9/10 B (was middle ¾ B)

New Section 11.5.4 Extreme Event Limit State

Changes all other following Section 11.5 numbers

Seismic design not mandatory for Seismic Zones 1

through 3 (all of Ohio)


4

Important Changes in AASHTO 6th Ed.

Section 10.8.3.6.3 - Group Reduction Factors

for Bearing Resistance of Shafts in Sand


5

Coulomb vs. Rankine Earth Pressure

Short-heeled vs. Long-heeled Walls

Incline EH load at δ for Coulomb

Incline EH load at β for Rankine

Coulomb versus Rankine Earth Pressure

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Coulomb Earth Pressure

Coulomb Earth Pressure

7

Broken Back Slope Analysis

Calculate B instead of β for infinite slope

For h, don't use B, use β

Broken Back Slope Analysis

Please note: In this

figure, “B” is the

“notional” or effective

slope angle.

However, in other

publications (by

FHWA), β′ or βeq are

often used.

8

Global (Overall) Stability


9


Resistance Factor 0.75 = Factor of Safety 1.3

Resistance Factor 0.65 = Factor of Safety 1.5

“Contains or Supports a Structural Element”

Includes:

Another wall above the lower wall

Bridge Abutment or Wing Walls

Walls with bridge foundations above or behind

(including deep foundations) where a wall failure

surface could intersect the bridge foundations

Do not cite irrelevant Factor of Safety for the

condition being analyzed


10


Pay attention to stability model limits


11

Pile Foundations Driven to Rock

Cite Factored Load per Pile (Qp) per BDM

202.2.3.2.a

Compare Qp to maximum structural resistance

of pile, RR max = Pr = φc Pn = φc As Fy, where

φc = 0.50 for severe driving conditions

Fy = 50 ksi for H-piles

Assume pile embedment in bedrock to top of

cored bedrock per BDM 202.2.3.2.a

Include embedment in pile cap and round up

Estimated Pile Length to nearest 5 feet, per

BDM 202.2.3.2

Pile Foundations Driven to Rock

12

Driven Friction Pile Foundations

Cite Factored Load per Pile (Qp) and UBV

(Rndr) per BDM 202.2.3.2.b

Use φDYN = 0.70 to calculate UBV

Do not use Maximum UBV per pile

Compare UBV to Ultimate Capacity not to

Driving Resistance with driving strength loss

Provide justification if using a pile larger than

required by the Factored Load per Pile

Include embedment in pile cap and round up

Estimated Pile Length to nearest 5 feet, per

BDM 202.2.3.2


13


Determine minimum pipe pile wall thickness

per Construction and Material Specifications,

Section 507.06.D, UBV / 900 kip/in

Ensure Wave Equation Drivability Analyses

correspond to Static Analyses (typically

GRLWEAP versus DRIVEN)

Soil strata match in depth and strength

UBV capacity reached at same depth

UBV plot the same shape and magnitude

Use an appropriate shaft gain/loss factor per

DRIVEN user manual, Chapter 5, and GRLWEAP

manual Sections 6.1 and 6.2.


14

Driven Pile Foundations

When performing Drivability Analysis, per

BDM Section 202.2.3.2.b & AASHTO 10.7.8,

compare calculated driving stresses to Maximum Permissible Driving Stress, σdr

For Steel Piles, per AASHTO 10.7.8, σdr = 0.9 φda fy

Per AASHTO 6.5.4.2, φda = 1.00

Per ASTM A252, for pipe piles,

Grade 1, fy = 30 ksi (not allowed)

Grade 2, fy = 35 ksi (most common, default)

Grade 3, fy = 45 ksi (must be specified for use)

Driven Pile Foundations

15

Drilled Shaft Foundations

Self weight of drilled shaft is counteracted by

Archimedes’ principle of buoyancy:

γeffective self weight = γconcrete – γsoil

Friction drilled shaft includes side resistance

and end-bearing in soil.

Rock-socketed drilled shaft includes side

resistance and end-bearing in rock, but

neglects side resistance in soil, due to greater

amount of displacement required to engage

side resistance in soil.


16


Axial End-Bearing Resistance in Bedrock

AASHTO 10.8.3.5.4c-1; qp = 2.5 qu, Ncr* = 2.5.

Minimum rock socket = 1.5 B, where B = shaft

diameter (see also FHWA GEC 10, Equation 13-

20, and the associated narrative).

FHWA GEC 10, Equation 13-21 is a refinement of

the above equation for rock where spacing (sv) and

aperture (td) of discontinuities (horizontal joints) is

well known. Ncr* = 0.4 to 5.1 Requires sv ≥ 1.0 feet

and td ≤ 0.25 inch. Do not use otherwise; default to

above AASHTO 10.8.3.5.4c-1.

AASHTO 10.8.3.5.4c-1 and GEC 10, Equation 13-

20 are for “massive rock.”


17

Drilled Shaft Foundations Per GEC 10, “Massive rock can be defined, for

purposes of bearing capacity analysis, as rock

mass for which the effects of discontinuities are

insignificant. Practically, if joint spacing is more

than four to five times the shaft diameter, or if

jointing is horizontal but the joints are tight (no

compressible or gouge-filled seams) the rock can

be treated as massive.”

Consider AASHTO Equation 10.8.3.5.4c-2 or

GEC 10 Equation 13-24 if:

Rock is highly fractured and adversely jointed

Rock contains solution cavities or voids

Open seams contain compressible material (clay)


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Please Note:

AASTO Eqn. 10.8.3.5.4c-2 uses RMR, Rock Mass

Rating system.

GEC 10 Equation 13-24 uses GSI, Geologic

Strength Index.

The next edition of AASHTO will go to GSI


19


For Lateral Resistance using LPILE, when

foundation is Embedded in Rock, always use

LPILE Weak Rock Model, per FHWA GEC 10

12.3.3.4.4. Use of qu > 1000 psi results in a

non-critical error, which should be ignored.

LPILE Strong Rock Model is calibrated to one

certain vuggy limestone formation in southern

Florida, which crushes under load: resistance

drops to zero if deflection equals 0.24% of the

shaft diameter. This does not reflect the p-y

behavior of Ohio rocks.


20

Wall RF Calibrated to

old Factor of Safety:

For example:

γEH =1.50 / φτ = 1.00

: FoS = 1.5

Wall Resistance Factors: AASHTO Section 10 vs. 11

Resistance Factors for Walls

21

CIP Gravity and Semi-Gravity Walls

For Sliding on Granular soils, use AASHTO Equation 10.6.3.4-2, Rτ = V tan δ.

Please note that “δ” in this equation does not equal

δ angle of interface friction in 3.11.5.3.

For Sliding on Cohesive soils, use AASHTO

Figure 10.6.3.4-1.

Note: bedding on “6.0 in. of compacted granular

material” is always assumed, and the comparison

between Su and 0.5σv is always performed.

This limits sliding resistance in stiffer clays to ½ the

contact pressure (which would be less than Su),

and limits sliding resistance in softer clays to Su,

which would be less than ½ the contact pressure.

Cast-in-Place Gravity and Semi-Gravity Walls

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23


For AASHTO Figure 10.6.3.4-1, read this as

“For footings that rest on clay, where footings

are supported on at least 6.0 in. of compacted

granular material, the sliding resistance may

be taken as the lesser of: The cohesion of the

clay, or one-half the normal stress on the

interface between the footing and soil, as

shown in Figure 10.6.3.4-1 for retaining

walls.”


24

Mechanically Stabilized Earth Walls

For Sliding on Granular soils, using AASHTO

Equation 10.6.3.4-2, compare internal friction

angle, ϕr, to external friction angle, ϕf.

Since an MSE wall is composed of soil, a check of

internal sliding along the base should also be

made, and compared to external sliding along the base, i.e. δ = ϕf or ϕr. Whichever generates the

least resistance should be used for foundation

sliding resistance, per AASHTO 11.10.5.3.

Mechanically Stabilized Earth (MSE) Walls

25

Mechanically Stabilized Earth Walls

For Sliding on Cohesive soils:

Do not use the trapezoidal or triangular pressure

distribution shown in AASHTO Figure 10.6.3.4-1.

Per AASHTO C11.10.5.4, “Due to the flexibility of

MSE walls, a triangular pressure distribution at the

wall base cannot develop, even if the wall base is

founded on rock, as the reinforced soil mass has

limited ability to transmit moment. Therefore, an

equivalent uniform base pressure distribution is

appropriate for MSE walls founded on either soil or

rock.”

One-half the average vertical stress σ′v over the

entire base width B should be used.

Mechanically Stabilized Earth (MSE) Walls

26

MSE or Spread Footing Founded Walls

For Sliding and Eccentricity of Load, use Limit

State “Strength I-a” per AASHTO Figure

C11.5.6-2.

CIP Gravity, Semi-Gravity, and MSE Walls

27


For Bearing Resistance, use Limit State

“Strength I-b” per AASHTO Figure C11.5.6-1.


28


Apply LS load directly for Bearing, only apply

LS load indirectly (as a component of earth

pressure) for Sliding and Eccentricity.


29


Because of differing Load Factors and

application of loads, “Strength I-a” bearing

pressure is always less than “Strength I-b”

bearing pressure.

Bearing is always more critical in the

“Strength I-b” limit state.

Sliding and Eccentricity are always more

critical in the “Strength I-a” limit state.

Do not perform “Strength I-a” bearing

analyses, and do not perform “Strength I-b”

sliding or limiting eccentricity analyses.


30


For bearing analyses on soil, always use

effective base width B′ = B – 2e, and uniform

bearing pressure per AASHTO Sections

10.6.1.3 and 11.6.3.2.

For this case, calculate eccentricity (e) with

“Strength I-b” limit state loading.

For bearing analyses on rock, use a linearly

distributed pressure over the entire base

width B.

For MSE Walls on rock, use the average

pressure over the entire base width B.


31

Cantilever Non-Gravity Walls

For Minimum Embedment, use maximum of:

1. Minimum Embedment depth to resist Factored

(Strength I) Axial Loads; however, this is typically

neglected for non-anchored walls.

2. Minimum Embedment depth to meet Service-

ability deflection limit at head of wall (under

Service I limit state loading), typically:

1% of exposed height, if adjacent and above roadway

2 inches, if adjacent and below existing roadway

1% of length above bedrock otherwise

3. Minimum Embedment Depth for moment

equilibrium about toe (point F), per AASHTO

Figure 3.11.5.6-1 through Figure 3.11.5.6-3


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Cantilever Non-Gravity Walls For AASHTO Figure 3.11.5.6-1 through Figure

3.11.5.6-3,

b = width of vertical member

S = spacing of vertical members

Use b where S > 3b

Use S where S ≤ 3b

Use Strength I factored load and factored resistance

γEH = 1.50 per AASHTO Table 3.4.1-2

γLS = 1.75 per AASHTO Table 3.4.1-2

φp = 0.75 per AASHTO Table 11.5.7-1

See AASHTO C11.8.4.1 for an example of how to use

the load and resistance factors with Figure 3.11.5.6-3.

Similar principles can be used with Figure 3.11.5.6-1 and

Figure 3.11.5.6-2.


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Cantilever Non-Gravity Wall

For Structural Design, calculate Maximum

Moment and Shear in vertical members, both

within exposed height (cantilever section) and

in vertical member as a whole.

Consider exposed height as unbraced member

Unbraced Length (Lb per AASHTO 6.10.8.2.3) =

Exposed Height

Consider embedded length as continuously braced

member


35

Anchored Walls

Use Apparent Earth Pressure (AEP) model

per AASHTO Section Section 3.11.5.7

Use Load Factor γEH = 1.35 for AEP per

AASHTO Table 3.4.1-2

For Minimum Embedment, use maximum of:

1. Minimum Embedment depth to resist Axial

Loads, including anchor loads and self weight

2. Minimum Embedment depth per FHWA GEC 4,

Section 5.5 for passive lateral resistance.


equilibrium about toe

Anchored Walls

36

Anchored Walls


equilibrium about toe

Anchored Walls

37

Anchored Walls

3. Determine Minimum Embedment Depth for

moment equilibrium about toe, where:

b = width of vertical member

S = spacing of vertical members

Pp = 3/2 φp kp γ′ b D2, where S > 3b

Pp = ½ φp kp γ′ S D2, where S ≤ 3b

φp = 0.75 per AASHTO Table 11.5.7-1

ΣMF = MR + DO R + Do/3 Pp = 0

Dmin = 1.2 DO

These are Strength I Factored Loads

Soil Nail Walls are not Anchored Walls:

design soil nail walls per FHWA GEC 7

Anchored Walls

38

Thank You

Questions?

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