1. LRFD Design of Shallow 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 State Global Stability
Stabilize
Destabilize

5. Global Stability Factor of Safety – Method of SlicesWT WT
N tan f
N tan f cl l cl l T N T WT N a WT a T T

6. Resistance Factors LRFD ASD Factors of Safety
Soil/Rock Parameters and Ground Water Conditions Based On:
Slope Supports Abutment or Other Structure?
Yes No
In-situ or Laboratory Tests and Measurements
1.5
1.3
No Site-specific Tests
1.8
LRFD

7. Stability Wrap-Up Unfactored loads Applied stress must be limited
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 Method Settlement of Cohesionless Soils

13. Stress Below Footing Boussinesq 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 Soil Strip Footing

19. Footings on Rock Trapezoidal Distribution

20. Footings on Rock Triangular 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 State Bearing 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 Df B 3 b’ 1 b 3
B>Df 2 2 d’ d a
e = C + s’ tan f
Soil Shear Strength Pp c b a I b’

25. METHOD/SOIL/CONDITION
Table Resistance Factors for Geotechnical Resistance of Shallow Foundations at the Strength Limit State
METHOD/SOIL/CONDITION
RESISTANCE FACTOR
Bearing Resistance b
Theoretical method (Munfakh, et al. (2001), in clay
0.50
Theoretical method (Munfakh, et al. (2001), in sand, using CPT
Theoretical method (Munfakh, et al. (2001), in sand, using SPT
0.45
Semi-empirical methods (Meyerhof), all soils
Footings on rock
Plate Load Test
0.55
Sliding 
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
ep
Passive earth pressure component of sliding resistance

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
Shear Failure controls
Settlement controls
800
Bearing Pressure (kPa)
600
400
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 - LRFD35 25
Nominal Bearing
Resistance (ksf) 15 5
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. METHOD/SOIL/CONDITION
Strength Limit State Resistance Factors
METHOD/SOIL/CONDITION
RESISTANCE FACTOR
Bearing
Resistance 
All methods, soil and rock
0.45
Plate Load Test
0.55
Sliding t
Precast concrete placed on sand
0.90
Cast-in-Place Concrete on sand
0.80
Clay
0.85
Soil on soil
ep
Passive earth pressure component of sliding resistance
0.50