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Published byNoelle Kenning Modified about 1 year ago

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LRFD Design of Shallow Foundations

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Nominal Geotechnical Resistances ASD Failure Modes ASD Failure Modes Overall Stability Overall Stability Bearing Capacity Bearing Capacity Settlement Settlement Sliding Sliding Overturning Overturning

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

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StabilizeDestabilize Service Limit State Global Stability

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Global Stability Factor of Safety – Method of Slices WTWT WTWT WTWT WTWT N N T T T T l l clcl clcl N tan f

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

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Stability Wrap-Up Unfactored loads Unfactored loads Service Limit State Service Limit State Applied stress must be limited Applied stress must be limited Footings supported in a slope Footings supported in a slope ≤ 0.65 (FS ≥ 1.5) ≤ 0.65 (FS ≥ 1.5) Stress criteria for stability can control footing design Stress criteria for stability can control footing design

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Service Limit State Design – Settlement Cohesive Soils Cohesive Soils Evaluate Using Consolidation Theory Evaluate Using Consolidation Theory Cohesionless Soils Cohesionless Soils Evaluate Using Empirical or Other Conventional Methods Evaluate Using Empirical or Other Conventional Methods Hough Method Hough Method

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Impact on Structures

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Settlement of Granular vs. Cohesive Soils Relative importance of settlement components for different soil types Relative importance of settlement components for different soil types Elastic Elastic Primary Consolidation Primary Consolidation Secondary Settlement (Creep) Secondary Settlement (Creep)

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Settlement of Granular vs. Cohesive Soils Structural effects of settlement components Structural effects of settlement components Include Transient Loads if Drained Loading is Expected and for Computing Initial Elastic Settlement 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 Transient Loads May Be Omitted When Computing Consolidation Settlement of Cohesive Soils

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Hough Method Settlement of Cohesionless Soils

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Stress Below Footing Boussinesq Pressure Isobars

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Nominal Bearing Resistance at Service Limit State RnRn BfBf For a constant value of settlement

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Eccentricity of Footings on Soil e B = M B / P e L = M L / P

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Effective Dimensions for Footings on Soil B′ = B – 2e B B′ = B – 2e B L′ = L – 2e L L′ = L – 2e L

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Applied Stress Beneath Effective Footing Area

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Stress Applied to Soil Strip Footing

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Footings on Rock Trapezoidal Distribution

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Footings on Rock Triangular Distribution

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Use of Eccentricity and Effective Footing Dimensions Service Limit State Service Limit State Nominal Bearing Resistance Limited by Settlement Nominal Bearing Resistance Limited by Settlement Strength Limit State Strength Limit State Nominal Bearing Resistance Limited by Bearing Resistance Nominal Bearing Resistance Limited by Bearing Resistance Prevent Overturning Prevent Overturning All Applicable Limit States All Applicable Limit States

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Strength Limit State Bearing Resistance

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Strength Limit State Design – Bearing Resistance Footings on Soil Footings on Soil Evaluate Using Conventional Bearing Theory Evaluate Using Conventional Bearing Theory Footings on Rock Footings on Rock Evaluate Using CSIR Rock Mass Rating Procedure Evaluate Using CSIR Rock Mass Rating Procedure

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d a d’ = C + s ’ tan f Soil Shear Strength Df B>Df B Ground Surface s v = D f Pp Pp c c b a I b’ bb’ Bearing Resistance Mechanism

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Table Resistance Factors for Geotechnical Resistance of Shallow Foundations at the Strength Limit State METHOD/SOIL/CONDITIONRESISTANCE FACTOR Bearing Resistance bb 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 Test0.55 Sliding Precast concrete placed on sand 0.90 Cast-in-Place Concrete on sand 0.80 Cast-in-Place or precast Concrete on Clay0.85 Soil on soil0.90 ep Passive earth pressure component of sliding resistance 0.50

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Footings on Rock Service Limit State – use published presumptive bearing Service Limit State – use published presumptive bearing Published values are allowable therefore settlement-limited Published values are allowable therefore settlement-limited Procedures for computing settlement are available Procedures for computing settlement are available

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Very little guidance available for bearing resistance of rock 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 Proposed Specification revisions provide for evaluating the cohesion and friction angle of rock using the CSIR Rock Mass Rating System Footings on Rock – Strength Limit State

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CSIR Rock Mass Rating System CSIR Rock Mass Rating developed for tunnel design CSIR Rock Mass Rating developed for tunnel design Includes life safety considerations and therefore, margin of safety Includes life safety considerations and therefore, margin of safety Use of cohesion and friction angle therefore may be conservative Use of cohesion and friction angle therefore may be conservative

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

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Width vs. Resistance - ASD Settlement controls Shear Failure controls Footing width, B (m) Bearing Pressure (kPa) Allowable Bearing Capacity, FS = 3.0 Bearing Pressure for 25-mm (1in) settlement

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Settlement vs. Bearing Resistance

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Width vs. Resistance - LRFD Effective Footing width, B’ (m) Nominal Bearing Resistance (ksf) Strength Limit State Service Limit State

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

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Summary Comparison of ASD and LRFD for Spread Footings Same geotechnical theory used to compute resistances, however Same geotechnical theory used to compute resistances, however As per Limit State concepts, presentation of design recommendations needs to be modified As per Limit State concepts, presentation of design recommendations needs to be modified

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METHOD/SOIL/CONDITION RESISTANCE FACTOR Bearing Resistance All methods, soil and rock 0.45 Plate Load Test0.55 Sliding Precast concrete placed on sand 0.90 Cast-in-Place Concrete on sand 0.80 Clay0.85 Soil on soil0.90 ep Passive earth pressure component of sliding resistance 0.50 Strength Limit State Resistance Factors

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