LRFD Design of Shallow Foundations

1
LRFD Design ofShallow Foundations
2
Nominal Geotechnical Resistances

• ASD Failure Modes
• Overall Stability
• Bearing Capacity
• Settlement
• Sliding
• Overturning

3
Nominal Geotechnical Resistances

• LRFD Service Limit State
• Overall Stability
• Vertical (Settlement) and Horizontal Movements
• LRFD Strength Limit State
• Bearing Resistance
• Sliding
• Eccentricity Limits (Overturning)

4
Service Limit StateGlobal Stability
Stabilize
Destabilize
5
Global Stability Factor of Safety Method of
Slices
WT
WT
N tan f
N tan f
cl
l
cl
l
T
N
T
WT
N
a
WT
a
T
T
6
Resistance Factors
ASD Factors of Safety ASD Factors of Safety ASD Factors of Safety
Soil/Rock Parameters and Ground Water Conditions Based On Slope Supports Abutment or Other Structure? Slope Supports Abutment or Other Structure?
Soil/Rock Parameters and Ground Water Conditions Based On Yes No
In-situ or Laboratory Tests and Measurements 1.5 1.3
No Site-specific Tests 1.8 1.5
LRFD
7
Stability Wrap-Up

• Unfactored loads
• Service Limit State
• Applied stress must be limited
• Footings supported in a slope
• f 0.65 (FS 1.5)
• Stress criteria for stability can control footing
  design

8
Service Limit State Design Settlement

• Cohesive Soils
• Evaluate Using Consolidation Theory
• Cohesionless Soils
• Evaluate Using Empirical or Other Conventional
  Methods
• Hough Method

9
Impact on Structures
10
Settlement of Granular vs. Cohesive Soils

• Relative importance of settlement components for
  different soil types
• Elastic
• Primary Consolidation
• Secondary Settlement (Creep)

11
Settlement of Granular vs. Cohesive Soils

• 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

12
Hough MethodSettlement of Cohesionless Soils
13
Stress Below FootingBoussinesq Pressure Isobars
14
Nominal Bearing Resistance at Service Limit State
For a constant value of settlement
Rn
Bf
15
Eccentricity of Footings on Soil
eB MB / P eL ML / P
16
Effective Dimensions for Footings on Soil

• B' B 2eB
• L' L 2eL

17
Applied Stress Beneath Effective Footing Area
18
Stress Applied to SoilStrip Footing
19
Footings on RockTrapezoidal Distribution
20
Footings on RockTriangular Distribution
21
Use of Eccentricity and Effective Footing
Dimensions

• Service Limit State
• Nominal Bearing Resistance Limited by Settlement
• Strength Limit State
• Nominal Bearing Resistance Limited by Bearing
  Resistance
• Prevent Overturning
• All Applicable Limit States

22
Strength Limit StateBearing Resistance
23
Strength Limit State Design Bearing Resistance

• Footings on Soil
• Evaluate Using Conventional Bearing Theory
• Footings on Rock
• Evaluate Using CSIR Rock Mass Rating Procedure

24
Bearing Resistance Mechanism
Ground Surface
sv ? Df
B
Df
b
b
1
3
3
BgtDf
2
2
d
d
a
e C s tan f
Soil Shear Strength
25
Table 10.5.5.2.1-1 Resistance Factors for
Geotechnical Resistance of Shallow Foundations
at the Strength Limit State
METHOD/SOIL/CONDITION METHOD/SOIL/CONDITION METHOD/SOIL/CONDITION RESISTANCE FACTOR
Bearing Resistance ?b Theoretical method (Munfakh, et al. (2001), in clay 0.50
Bearing Resistance ?b Theoretical method (Munfakh, et al. (2001), in sand, using CPT 0.50
Bearing Resistance ?b Theoretical method (Munfakh, et al. (2001), in sand, using SPT 0.45
Bearing Resistance ?b Semi-empirical methods (Meyerhof), all soils 0.45
Bearing Resistance ?b Footings on rock 0.45
Bearing Resistance ?b Plate Load Test 0.55
Sliding ?? Precast concrete placed on sand 0.90
Sliding ?? Cast-in-Place Concrete on sand 0.80
Sliding ?? Cast-in-Place or precast Concrete on Clay 0.85
Sliding ?? Soil on soil 0.90
Sliding ?ep Passive earth pressure component of sliding resistance 0.50
26
Footings on Rock

• Service Limit State use published presumptive
  bearing
• Published values are allowable therefore
  settlement-limited
• Procedures for computing settlement are available

27
Footings on Rock Strength Limit State

• 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

28
CSIR Rock Mass Rating System

• 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

29
LRFD vs. ASD

• All modes are expressly checked at a limit state
  in LRFD
• Eccentricity limits replace the overturning
  Factor of Safety

30
Width vs. Resistance - ASD
Settlementcontrols
Shear Failurecontrols
800
600
Bearing Pressure (kPa)
400
0
0.0 1.0 2.0 3.0 4.0 5.0
Footing width, B (m)
Allowable Bearing Capacity, FS 3.0 Bearing
Pressure for 25-mm (1in) settlement
31
Settlement vs. Bearing Resistance
32
Width vs. Resistance - LRFD
35
25
Nominal Bearing Resistance (ksf)
15
5
0 4 8 12 16 20
Effective Footing width, B (m)
Strength Limit State Service Limit State
33
Recommended Practice

• For LRFD design of footings on soil and rock
• Size footings at the Service Limit State
• Check footing at all other applicable Limit
  States
• Settlement typically controls!

34
Summary Comparison of ASD and LRFD for Spread
Footings

• Same geotechnical theory used to compute
  resistances, however
• As per Limit State concepts, presentation of
  design recommendations needs to be modified

35
Strength Limit State Resistance Factors
METHOD/SOIL/CONDITION METHOD/SOIL/CONDITION METHOD/SOIL/CONDITION RESISTANCE FACTOR
Bearing Resistance ? All methods, soil and rock 0.45
Bearing Resistance ? Plate Load Test 0.55
Sliding ?t Precast concrete placed on sand 0.90
Sliding ?t Cast-in-Place Concrete on sand 0.80
Sliding ?t Clay 0.85
Sliding ?t Soil on soil 0.90
Sliding ?ep Passive earth pressure component of sliding resistance 0.50