Ohio Department of Transportation John R. Kasich, Governor Jerry Wray, Director ODOT LRFD Foundations Alexander Dettloff, P.E. Foundation Engineer Office of Geotechnical Engineering May 07, 2013
AASHTO 6 th Ed. versus 5 th Ed. Check the Plan Sheets: AASHTO 6 th Edition (2012) versus 5 th Edition (2010) 2
Important Changes in AASHTO 6 th Ed. Eccentricity Limits Section 11.6.3.3 Soil: Middle 2 / 3 B (was middle ½ B) Rock: Middle 9 / 10 B (was middle ¾ B) New Section 11.5.4 Extreme Event Limit State Changes all other following Section 11.5 numbers Seismic design not mandatory for Seismic Zones 1 through 3 (all of Ohio) AASHTO 6 th Edition (2012) versus 5 th Edition (2010) 3
Important Changes in AASHTO 6 th Ed. Section 10.8.3.6.3 - Group Reduction Factors for Bearing Resistance of Shafts in Sand AASHTO 6 th Edition (2012) versus 5 th Edition (2010) 4
Coulomb vs. Rankine Earth Pressure Short-heeled vs. Long-heeled Walls Incline EH load at δ for Coulomb Incline EH load at β for Rankine Coulomb versus Rankine Earth Pressure 5
Coulomb Earth Pressure Coulomb Earth Pressure 6
Broken Back Slope Analysis Calculate B instead of β for infinite slope For h, don't use B, use β Please note: In this figure, B is the notional or effective slope angle. However, in other publications (by FHWA), β or β eq are often used. Broken Back Slope Analysis 7
Global (Overall) Stability Global (Overall) Stability 8
Global (Overall) Stability Resistance Factor 0.75 = Factor of Safety 1.3 Resistance Factor 0.65 = Factor of Safety 1.5 Contains or Supports a Structural Element Includes: Another wall above the lower wall Bridge Abutment or Wing Walls Walls with bridge foundations above or behind (including deep foundations) where a wall failure surface could intersect the bridge foundations Do not cite irrelevant Factor of Safety for the condition being analyzed Global (Overall) Stability 9
Global (Overall) Stability Pay attention to stability model limits Global (Overall) Stability 10
Pile Foundations Driven to Rock Cite Factored Load per Pile (Q p ) per BDM 202.2.3.2.a Compare Q p to maximum structural resistance of pile, R R max = P r = φ c P n = φ c A s F y, where φ c = 0.50 for severe driving conditions F y = 50 ksi for H-piles Assume pile embedment in bedrock to top of cored bedrock per BDM 202.2.3.2.a Include embedment in pile cap and round up Estimated Pile Length to nearest 5 feet, per BDM 202.2.3.2 Pile Foundations Driven to Rock 11
Driven Friction Pile Foundations Cite Factored Load per Pile (Q p ) and UBV (R ndr ) per BDM 202.2.3.2.b Use φ DYN = 0.70 to calculate UBV Do not use Maximum UBV per pile Compare UBV to Ultimate Capacity not to Driving Resistance with driving strength loss Provide justification if using a pile larger than required by the Factored Load per Pile Include embedment in pile cap and round up Estimated Pile Length to nearest 5 feet, per BDM 202.2.3.2 Driven Friction Pile Foundations 12
Driven Friction Pile Foundations Determine minimum pipe pile wall thickness per Construction and Material Specifications, Section 507.06.D, UBV / 900 kip/in Ensure Wave Equation Drivability Analyses correspond to Static Analyses (typically GRLWEAP versus DRIVEN) Soil strata match in depth and strength UBV capacity reached at same depth UBV plot the same shape and magnitude Use an appropriate shaft gain/loss factor per DRIVEN user manual, Chapter 5, and GRLWEAP manual Sections 6.1 and 6.2. Driven Friction Pile Foundations 13
Driven Pile Foundations When performing Drivability Analysis, per BDM Section 202.2.3.2.b & AASHTO 10.7.8, compare calculated driving stresses to Maximum Permissible Driving Stress, σ dr For Steel Piles, per AASHTO 10.7.8, σ dr = 0.9 φ da f y Per AASHTO 6.5.4.2, φ da = 1.00 Per ASTM A252, for pipe piles, Grade 1, f y = 30 ksi (not allowed) Grade 2, f y = 35 ksi (most common, default) Grade 3, f y = 45 ksi (must be specified for use) Driven Pile Foundations 14
Drilled Shaft Foundations Self weight of drilled shaft is counteracted by Archimedes principle of buoyancy: γ effective self weight = γ concrete γ soil Friction drilled shaft includes side resistance and end-bearing in soil. Rock-socketed drilled shaft includes side resistance and end-bearing in rock, but neglects side resistance in soil, due to greater amount of displacement required to engage side resistance in soil. Drilled Shaft Foundations 15
Drilled Shaft Foundations Axial End-Bearing Resistance in Bedrock AASHTO 10.8.3.5.4c-1; q p = 2.5 q u, N cr * = 2.5. Minimum rock socket = 1.5 B, where B = shaft diameter (see also FHWA GEC 10, Equation 13-20, and the associated narrative). FHWA GEC 10, Equation 13-21 is a refinement of the above equation for rock where spacing (s v ) and aperture (t d ) of discontinuities (horizontal joints) is well known. N cr * = 0.4 to 5.1 Requires s v 1.0 feet and t d 0.25 inch. Do not use otherwise; default to above AASHTO 10.8.3.5.4c-1. AASHTO 10.8.3.5.4c-1 and GEC 10, Equation 13-20 are for massive rock. Drilled Shaft Foundations 16
Drilled Shaft Foundations Per GEC 10, Massive rock can be defined, for purposes of bearing capacity analysis, as rock mass for which the effects of discontinuities are insignificant. Practically, if joint spacing is more than four to five times the shaft diameter, or if jointing is horizontal but the joints are tight (no compressible or gouge-filled seams) the rock can be treated as massive. Consider AASHTO Equation 10.8.3.5.4c-2 or GEC 10 Equation 13-24 if: Rock is highly fractured and adversely jointed Rock contains solution cavities or voids Open seams contain compressible material (clay) Drilled Shaft Foundations 17
Drilled Shaft Foundations Please Note: AASTO Eqn. 10.8.3.5.4c-2 uses RMR, Rock Mass Rating system. GEC 10 Equation 13-24 uses GSI, Geologic Strength Index. The next edition of AASHTO will go to GSI Drilled Shaft Foundations 18
Drilled Shaft Foundations For Lateral Resistance using LPILE, when foundation is Embedded in Rock, always use LPILE Weak Rock Model, per FHWA GEC 10 12.3.3.4.4. Use of q u > 1000 psi results in a non-critical error, which should be ignored. LPILE Strong Rock Model is calibrated to one certain vuggy limestone formation in southern Florida, which crushes under load: resistance drops to zero if deflection equals 0.24% of the shaft diameter. This does not reflect the p-y behavior of Ohio rocks. Drilled Shaft Foundations 19
Resistance Factors for Walls Wall RF Calibrated to old Factor of Safety: For example: γ EH =1.50 / φ τ = 1.00 : FoS = 1.5 Wall Resistance Factors: AASHTO Section 10 vs. 11 20
CIP Gravity and Semi-Gravity Walls For Sliding on Granular soils, use AASHTO Equation 10.6.3.4-2, R τ = V tan δ. Please note that δ in this equation does not equal δ angle of interface friction in 3.11.5.3. For Sliding on Cohesive soils, use AASHTO Figure 10.6.3.4-1. Note: bedding on 6.0 in. of compacted granular material is always assumed, and the comparison between S u and 0.5σ v is always performed. This limits sliding resistance in stiffer clays to ½ the contact pressure (which would be less than S u ), and limits sliding resistance in softer clays to S u, which would be less than ½ the contact pressure. Cast-in-Place Gravity and Semi-Gravity Walls 21
CIP Gravity and Semi-Gravity Walls Cast-in-Place Gravity and Semi-Gravity Walls 22
CIP Gravity and Semi-Gravity Walls For AASHTO Figure 10.6.3.4-1, read this as For footings that rest on clay, where footings are supported on at least 6.0 in. of compacted granular material, the sliding resistance may be taken as the lesser of: The cohesion of the clay, or one-half the normal stress on the interface between the footing and soil, as shown in Figure 10.6.3.4-1 for retaining walls. Cast-in-Place Gravity and Semi-Gravity Walls 23
Mechanically Stabilized Earth Walls For Sliding on Granular soils, using AASHTO Equation 10.6.3.4-2, compare internal friction angle, ϕ r, to external friction angle, ϕ f. Since an MSE wall is composed of soil, a check of internal sliding along the base should also be made, and compared to external sliding along the base, i.e. δ = ϕ f or ϕ r. Whichever generates the least resistance should be used for foundation sliding resistance, per AASHTO 11.10.5.3. Mechanically Stabilized Earth (MSE) Walls 24
Mechanically Stabilized Earth Walls For Sliding on Cohesive soils: Do not use the trapezoidal or triangular pressure distribution shown in AASHTO Figure 10.6.3.4-1. Per AASHTO C11.10.5.4, Due to the flexibility of MSE walls, a triangular pressure distribution at the wall base cannot develop, even if the wall base is founded on rock, as the reinforced soil mass has limited ability to transmit moment. Therefore, an equivalent uniform base pressure distribution is appropriate for MSE walls founded on either soil or rock. One-half the average vertical stress σ v over the entire base width B should be used. Mechanically Stabilized Earth (MSE) Walls 25
MSE or Spread Footing Founded Walls For Sliding and Eccentricity of Load, use Limit State Strength I-a per AASHTO Figure C11.5.6-2. CIP Gravity, Semi-Gravity, and MSE Walls 26
MSE or Spread Footing Founded Walls For Bearing Resistance, use Limit State Strength I-b per AASHTO Figure C11.5.6-1. CIP Gravity, Semi-Gravity, and MSE Walls 27
MSE or Spread Footing Founded Walls Apply LS load directly for Bearing, only apply LS load indirectly (as a component of earth pressure) for Sliding and Eccentricity. CIP Gravity, Semi-Gravity, and MSE Walls 28
MSE or Spread Footing Founded Walls Because of differing Load Factors and application of loads, Strength I-a bearing pressure is always less than Strength I-b bearing pressure. Bearing is always more critical in the Strength I-b limit state. Sliding and Eccentricity are always more critical in the Strength I-a limit state. Do not perform Strength I-a bearing analyses, and do not perform Strength I-b sliding or limiting eccentricity analyses. CIP Gravity, Semi-Gravity, and MSE Walls 29
MSE or Spread Footing Founded Walls For bearing analyses on soil, always use effective base width B = B 2e, and uniform bearing pressure per AASHTO Sections 10.6.1.3 and 11.6.3.2. For this case, calculate eccentricity (e) with Strength I-b limit state loading. For bearing analyses on rock, use a linearly distributed pressure over the entire base width B. For MSE Walls on rock, use the average pressure over the entire base width B. CIP Gravity, Semi-Gravity, and MSE Walls 30
Cantilever Non-Gravity Walls For Minimum Embedment, use maximum of: 1. Minimum Embedment depth to resist Factored (Strength I) Axial Loads; however, this is typically neglected for non-anchored walls. 2. Minimum Embedment depth to meet Serviceability deflection limit at head of wall (under Service I limit state loading), typically: 1% of exposed height, if adjacent and above roadway 2 inches, if adjacent and below existing roadway 1% of length above bedrock otherwise 3. Minimum Embedment Depth for moment equilibrium about toe (point F), per AASHTO Figure 3.11.5.6-1 through Figure 3.11.5.6-3 Cantilever Non-Gravity Walls 31
Cantilever Non-Gravity Walls Cantilever Non-Gravity Walls 32
Cantilever Non-Gravity Walls For AASHTO Figure 3.11.5.6-1 through Figure 3.11.5.6-3, b = width of vertical member S = spacing of vertical members Use b where S > 3b Use S where S 3b Use Strength I factored load and factored resistance γ EH = 1.50 per AASHTO Table 3.4.1-2 γ LS = 1.75 per AASHTO Table 3.4.1-2 φ p = 0.75 per AASHTO Table 11.5.7-1 See AASHTO C11.8.4.1 for an example of how to use the load and resistance factors with Figure 3.11.5.6-3. Similar principles can be used with Figure 3.11.5.6-1 and Figure 3.11.5.6-2. Cantilever Non-Gravity Walls 33
Cantilever Non-Gravity Wall For Structural Design, calculate Maximum Moment and Shear in vertical members, both within exposed height (cantilever section) and in vertical member as a whole. Consider exposed height as unbraced member Unbraced Length (L b per AASHTO 6.10.8.2.3) = Exposed Height Consider embedded length as continuously braced member Cantilever Non-Gravity Walls 34
Anchored Walls Use Apparent Earth Pressure (AEP) model per AASHTO Section Section 3.11.5.7 Use Load Factor γ EH = 1.35 for AEP per AASHTO Table 3.4.1-2 For Minimum Embedment, use maximum of: 1. Minimum Embedment depth to resist Axial Loads, including anchor loads and self weight 2. Minimum Embedment depth per FHWA GEC 4, Section 5.5 for passive lateral resistance. 3. Minimum Embedment Depth for moment equilibrium about toe Anchored Walls 35
Anchored Walls 3. Minimum Embedment Depth for moment equilibrium about toe Anchored Walls 36
Anchored Walls 3. Determine Minimum Embedment Depth for moment equilibrium about toe, where: b = width of vertical member S = spacing of vertical members P p = 3 / 2 φ p k p γ b D 2, where S > 3b P p = ½ φ p k p γ S D 2, where S 3b φ p = 0.75 per AASHTO Table 11.5.7-1 ΣM F = M R + D O R + Do / 3 P p = 0 D min = 1.2 D O These are Strength I Factored Loads Soil Nail Walls are not Anchored Walls: design soil nail walls per FHWA GEC 7 Anchored Walls 37
Thank You Questions? ODOT LRFD Foundations 38