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					  LRFD Design of
Shallow Foundations
Nominal Geotechnical
Resistances
     ASD Failure Modes
         Overall Stability
         Bearing Capacity
         Settlement
         Sliding
         Overturning
Nominal Geotechnical
Resistances
     LRFD Service Limit State
         Overall Stability
         Vertical (Settlement) and Horizontal
          Movements
     LRFD Strength Limit State
         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   WT
  cl                                N tan f
                 l                   cl
   T                       l
            N                          T
             a WT               N
                         WT a
            T
                                T
Resistance Factors
             ASD Factors of Safety
                                    Slope Supports
    Soil/Rock Parameters and         Abutment or
    Ground Water Conditions              Other
           Based On:                  Structure?
                                     Yes       No
  In-situ or Laboratory Tests and
                                     1.5     1.3
           Measurements
       No Site-specific Tests        1.8     1.5


            LRFD
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
Service Limit State Design –
Settlement
   Cohesive Soils
       Evaluate Using Consolidation Theory
   Cohesionless Soils
       Evaluate Using Empirical or Other Conventional
        Methods
       Hough Method
Impact on Structures
Settlement of Granular vs.
Cohesive Soils
   Relative importance of settlement
    components for different soil types
       Elastic
       Primary Consolidation
       Secondary Settlement (Creep)
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
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
                ML                P
                                                    MB

 eB = MB / P
 eL = M L / P            e
                         B                 eL
                             B’
                                      L’
Effective Dimensions for
Footings on Soil
 B′ = B – 2eB
 L′ = L – 2eL
                      L
                                                 B

                 ML                P
                                                     MB



                          e
                          B                 eL
                              B’
                                       L’
Applied Stress Beneath Effective
Footing Area

                       L
                                                  B

                  ML                P
                                                      MB



                           e
                           B                 eL
                               B’
                                        L’
              q
Stress Applied to Soil
Strip Footing
Footings on Rock
Trapezoidal Distribution
Footings on Rock
Triangular Distribution
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
Strength Limit State
Bearing Resistance
Strength Limit State Design –
Bearing Resistance
   Footings on Soil
       Evaluate Using Conventional Bearing Theory
   Footings on Rock
       Evaluate Using CSIR Rock Mass Rating Procedure
  Bearing Resistance Mechanism

                       Ground
                       Surface           sv = g Df

 Df      B
                                                3    b’   1   b   3
B>Df
                                                     2        2
                                           d’             a           d
        e = C + s’ tan f
       Soil Shear Strength



                                  b’ I     b
                             c                  c
                                     a
                             Pp                Pp
Table 10.5.5.2.1-1 Resistance Factors for Geotechnical Resistance of Shallow
Foundations at the Strength Limit State




                        METHOD/SOIL/CONDITION                                 RESISTANCE FACTOR
                       Theoretical method (Munfakh, et al. (2001), in clay          0.50
                       Theoretical method (Munfakh, et al. (2001), in sand,
                                                                                    0.50
                       using CPT
                       Theoretical method (Munfakh, et al. (2001), in sand,
Bearing                                                                             0.45
                b     using SPT
Resistance
                       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
Sliding
                       Soil on soil                                                 0.90
                       Passive earth pressure component of sliding
                 ep                                                                0.50
                       resistance
Footings on Rock
   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

   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
   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
   All modes are expressly checked at a
    limit state in LRFD
   Eccentricity limits replace the
    overturning Factor of Safety
           Width vs. Resistance - ASD
                           Shear Failure                 Settlement
                               controls                  controls
Bearing Pressure (kPa)




                         800

                         600

                         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
Settlement vs. Bearing
Resistance
                12


                10

                                                  N=30
                8
                                                  N=25
      qa, ksf



                6                                 N=20

                                                  N=15
                4
                                                  N=10
                2
                                                  N=5
                0
                 0   2   4   6   8   10   12 14

                             B, ft
    Width vs. Resistance - LRFD

                   35
Resistance (ksf)
Nominal Bearing




                   25


                   15


                   5

                        0     4         8         12       16   20
                            Effective Footing width, B’ (m)
                                    Strength Limit State
                                    Service Limit State
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!
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
Strength Limit State Resistance Factors
                                                RESISTANCE
          METHOD/SOIL/CONDITION                    FACTOR
Bearing      f     All methods, soil and rock      0.45
Resistance

                   Plate Load Test                 0.55

Sliding      f    Precast concrete placed
                                                   0.90
                      on sand
                   Cast-in-Place Concrete on
                                                   0.80
                     sand
                   Clay                            0.85
                   Soil on soil                    0.90
             fep   Passive earth pressure
                     component of sliding          0.50
                     resistance

				
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