LRFD Design of Shallow Foundations Nominal Geotechnical Resistances

LRFD Design of Shallow Foundations

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

Nominal Geotechnical Resistances n LRFD Service Limit State n n n Overall Stability Vertical (Settlement) and Horizontal Movements LRFD Strength Limit State n n n 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 cl T WT l N a WT T l WT a N T N tan f cl T

Resistance Factors ASD Factors of Safety Soil/Rock Parameters and Ground Water Conditions Based On: In-situ or Laboratory Tests and Measurements No Site-specific Tests LRFD Slope Supports Abutment or Other Structure? Yes No 1. 5 1. 3 1. 8 1. 5

Stability Wrap-Up n Unfactored loads n n Applied stress must be limited n n n Service Limit State Footings supported in a slope f ≤ 0. 65 (FS ≥ 1. 5) Stress criteria for stability can control footing design

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

Impact on Structures

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

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

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 e. B = M B / P e. L = M L / P

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

Applied Stress Beneath Effective Footing Area

Stress Applied to Soil Strip Footing

Footings on Rock Trapezoidal Distribution

Footings on Rock Triangular Distribution

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

Strength Limit State Bearing Resistance

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

Bearing Resistance Mechanism Ground Surface Df s v = Df B 3 B>Df d’ e = C + s’ tan f Soil Shear Strength c Pp b’ I a b c Pp b’ 1 2 a b 2 3 d

Table 10. 5. 5. 2. 1 -1 Resistance Factors for Geotechnical Resistance of Shallow Foundations at the Strength Limit State METHOD/SOIL/CONDITION Bearing Resistance b Sliding ep RESISTANCE FACTOR 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 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 Soil on soil 0. 90 Passive earth pressure component of sliding resistance 0. 50

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

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

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

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

Bearing Pressure (k. Pa) Width vs. Resistance - ASD Shear Failure controls Settlement controls 800 600 400 0 0. 0 1. 0 2. 0 3. 0 4. 0 Footing width, B (m) Allowable Bearing Capacity, FS = 3. 0 Bearing Pressure for 25 -mm (1 in) settlement 5. 0

Settlement vs. Bearing Resistance

Nominal Bearing Resistance (ksf) Width vs. Resistance - LRFD 35 25 15 5 0 4 8 12 16 Effective Footing width, B’ (m) Strength Limit State Service Limit State 20

Recommended Practice n For LRFD design of footings on soil and rock; n n n 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 n As per Limit State concepts, presentation of design recommendations needs to be modified n

Strength Limit State Resistance Factors METHOD/SOIL/CONDITION Bearing Resistance Sliding ep RESISTANCE FACTOR All methods, soil and rock 0. 45 Plate Load Test 0. 55 Precast concrete placed on sand 0. 90 Cast-in-Place Concrete on sand 0. 80 Clay 0. 85 Soil on soil 0. 90 Passive earth pressure component of sliding resistance 0. 50