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LRFD Design of Shallow Foundations

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

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LRFD Design of Shallow Foundations

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  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 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. b’ b I c c a Pp Pp Bearing Resistance Mechanism Ground Surface sv =  Df B Df b’ b 1 3 3 B>Df 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

  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 Failurecontrols Settlementcontrols 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

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