Transcript AASHTO Section 10 Revisions - Virginia Department of

LRFD Design of
Shallow Foundations
Nominal Geotechnical
Resistances
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ASD Failure Modes
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Overall Stability
Bearing Capacity
Settlement
Sliding
Overturning
Nominal Geotechnical
Resistances
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LRFD Service Limit State
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Overall Stability
Vertical (Settlement) and Horizontal
Movements
LRFD Strength Limit State
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Bearing Resistance
Sliding
Eccentricity Limits (Overturning)
Service Limit State
Global Stability
Stabilize
Destabilize
Global Stability Factor of Safety
– Method of Slices
+
WT
N tan f
cl
T
WT
l
N
a WT
T
l
WT a
N
T
N tan f
cl
T
Resistance Factors
ASD Factors of Safety
Soil/Rock Parameters and
Ground Water Conditions
Based On:
In-situ or Laboratory Tests and
Measurements
No Site-specific Tests
LRFD
Slope Supports
Abutment or
Other
Structure?
Yes
No
1.5
1.3
1.8
1.5
Stability Wrap-Up
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Unfactored loads
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Applied stress must be limited
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Service Limit State
Footings supported in a slope
f ≤ 0.65 (FS ≥ 1.5)
Stress criteria for stability can control
footing design
Service Limit State Design –
Settlement
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Cohesive Soils
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Evaluate Using Consolidation Theory
Cohesionless Soils
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Evaluate Using Empirical or Other Conventional
Methods
Hough Method
Impact on Structures
Settlement of Granular vs.
Cohesive Soils
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Relative importance of settlement
components for different soil types
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Elastic
Primary Consolidation
Secondary Settlement (Creep)
Settlement of Granular vs.
Cohesive Soils
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Structural effects of settlement
components
Include Transient Loads if Drained
Loading is Expected and for Computing
Initial Elastic Settlement
Transient Loads May Be Omitted When
Computing Consolidation Settlement of
Cohesive Soils
Hough Method
Settlement of Cohesionless Soils
Stress
Below
Footing
Boussinesq
Pressure
Isobars
Nominal Bearing Resistance at
Service Limit State
For a constant value
of settlement
Rn
Bf
Eccentricity of Footings on Soil
L
B
P
P
ML
eB = MB / P
eL = M L / P
MB
e
eL
B
B’
L’
Effective Dimensions for
Footings on Soil
B′ = B – 2eB
 L′ = L – 2eL
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L
B
P
ML
MB
e
eL
B
B’
L’
Applied Stress Beneath Effective
Footing Area
L
B
P
ML
MB
e
eL
B
B’
q
L’
Stress Applied to Soil
Strip Footing
Footings on Rock
Trapezoidal Distribution
Footings on Rock
Triangular Distribution
Use of Eccentricity and Effective
Footing Dimensions
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Service Limit State
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Strength Limit State
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Nominal Bearing Resistance Limited by
Settlement
Nominal Bearing Resistance Limited by Bearing
Resistance
Prevent Overturning
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All Applicable Limit States
Strength Limit State
Bearing Resistance
Strength Limit State Design –
Bearing Resistance
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Footings on Soil
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Evaluate Using Conventional Bearing Theory
Footings on Rock
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Evaluate Using CSIR Rock Mass Rating Procedure
Bearing Resistance Mechanism
Ground
Surface
Df
sv =  Df
B
3
B>Df
d’
e = C + s’ tan f
Soil Shear Strength
c
Pp
b’ I
a
b
c
Pp
b’
2
1
a
b
2
3
d
Table 10.5.5.2.1-1 Resistance Factors for Geotechnical Resistance of Shallow
Foundations at the Strength Limit State
METHOD/SOIL/CONDITION
Bearing
Resistance
b

Sliding
ep
RESISTANCE FACTOR
Theoretical method (Munfakh, et al. (2001), in clay
0.50
Theoretical method (Munfakh, et al. (2001), in sand,
using CPT
0.50
Theoretical method (Munfakh, et al. (2001), in sand,
using SPT
0.45
Semi-empirical methods (Meyerhof), all soils
0.45
Footings on rock
0.45
Plate Load Test
0.55
Precast concrete placed on sand
0.90
Cast-in-Place Concrete on sand
0.80
Cast-in-Place or precast Concrete on Clay
0.85
Soil on soil
0.90
Passive earth pressure component of sliding
resistance
0.50
Footings on Rock
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Service Limit State – use published
presumptive bearing
Published values are allowable
therefore settlement-limited
Procedures for computing settlement
are available
Footings on Rock –
Strength Limit State
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Very little guidance available for
bearing resistance of rock
Proposed Specification revisions
provide for evaluating the cohesion and
friction angle of rock using the CSIR
Rock Mass Rating System
CSIR Rock Mass Rating System
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CSIR Rock Mass Rating developed for
tunnel design
Includes life safety considerations and
therefore, margin of safety
Use of cohesion and friction angle
therefore may be conservative
LRFD vs. ASD
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All modes are expressly checked at a
limit state in LRFD
Eccentricity limits replace the
overturning Factor of Safety
Bearing Pressure (kPa)
Width vs. Resistance - ASD
Shear Failure
controls
Settlement
controls
800
600
400
0
0.0
1.0
2.0
3.0
4.0
Footing width, B (m)
Allowable Bearing Capacity, FS = 3.0
Bearing Pressure for 25-mm (1in) settlement
5.0
Settlement vs. Bearing
Resistance
12
10
N=30
qa, ksf
8
N=25
6
N=20
N=15
4
N=10
2
N=5
0
0
2
4
6
8
B, ft
10
12 14
Nominal Bearing
Resistance (ksf)
Width vs. Resistance - LRFD
35
25
15
5
0
4
8
12
16
Effective Footing width, B’ (m)
Strength Limit State
Service Limit State
20
Recommended Practice
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For LRFD design of footings on soil
and rock;
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Size footings at the Service Limit State
Check footing at all other applicable Limit States
Settlement typically controls!
Summary Comparison of ASD
and LRFD for Spread Footings
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Same geotechnical theory used to
compute resistances, however
As per Limit State concepts,
presentation of design
recommendations needs to be modified
Strength Limit State Resistance Factors
METHOD/SOIL/CONDITION
Bearing
Resistance
Sliding
f
f
fep
RESISTANCE
FACTOR
All methods, soil and rock
0.45
Plate Load Test
0.55
Precast concrete placed
on sand
0.90
Cast-in-Place Concrete on
sand
0.80
Clay
0.85
Soil on soil
0.90
Passive earth pressure
component of sliding
resistance
0.50