ODOT LRFD Foundations

Transcription

1 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

2 AASHTO 6 th Ed. versus 5 th Ed. Check the Plan Sheets: AASHTO 6 th Edition (2012) versus 5 th Edition (2010) 2

3 Important Changes in AASHTO 6 th Ed. Eccentricity Limits Section Soil: Middle 2 / 3 B (was middle ½ B) Rock: Middle 9 / 10 B (was middle ¾ B) New Section 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

4 Important Changes in AASHTO 6 th Ed. Section Group Reduction Factors for Bearing Resistance of Shafts in Sand AASHTO 6 th Edition (2012) versus 5 th Edition (2010) 4

5 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

6 Coulomb Earth Pressure Coulomb Earth Pressure 6

7 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

8 Global (Overall) Stability Global (Overall) Stability 8

9 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above or behind (including deep foundations) where a wall failure surface could intersect the bridge foundations Do not cite irrelevant Factor of Safety for the condition being analyzed Global (Overall) Stability 9

10 Global (Overall) Stability Pay attention to stability model limits Global (Overall) Stability 10

11 Pile Foundations Driven to Rock Cite Factored Load per Pile (Q p ) per BDM 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 a Include embedment in pile cap and round up Estimated Pile Length to nearest 5 feet, per BDM Pile Foundations Driven to Rock 11

12 Driven Friction Pile Foundations Cite Factored Load per Pile (Q p ) and UBV (R ndr ) per BDM 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 Driven Friction Pile Foundations 12

13 Driven Friction Pile Foundations Determine minimum pipe pile wall thickness per Construction and Material Specifications, Section 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

14 Driven Pile Foundations When performing Drivability Analysis, per BDM Section b & AASHTO , compare calculated driving stresses to Maximum Permissible Driving Stress, σ dr For Steel Piles, per AASHTO , σ dr = 0.9 φ da f y Per AASHTO , 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

15 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

16 Drilled Shaft Foundations Axial End-Bearing Resistance in Bedrock AASHTO c-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 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 c-1. AASHTO c-1 and GEC 10, Equation are for massive rock. Drilled Shaft Foundations 16

17 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 c-2 or GEC 10 Equation if: Rock is highly fractured and adversely jointed Rock contains solution cavities or voids Open seams contain compressible material (clay) Drilled Shaft Foundations 17

18 Drilled Shaft Foundations Please Note: AASTO Eqn c-2 uses RMR, Rock Mass Rating system. GEC 10 Equation uses GSI, Geologic Strength Index. The next edition of AASHTO will go to GSI Drilled Shaft Foundations 18

19 Drilled Shaft Foundations For Lateral Resistance using LPILE, when foundation is Embedded in Rock, always use LPILE Weak Rock Model, per FHWA GEC 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

20 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

21 CIP Gravity and Semi-Gravity Walls For Sliding on Granular soils, use AASHTO Equation , R τ = V tan δ. Please note that δ in this equation does not equal δ angle of interface friction in For Sliding on Cohesive soils, use AASHTO Figure 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

22 CIP Gravity and Semi-Gravity Walls Cast-in-Place Gravity and Semi-Gravity Walls 22

23 CIP Gravity and Semi-Gravity Walls For AASHTO Figure , 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 for retaining walls. Cast-in-Place Gravity and Semi-Gravity Walls 23

24 Mechanically Stabilized Earth Walls For Sliding on Granular soils, using AASHTO Equation , 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 Mechanically Stabilized Earth (MSE) Walls 24

25 Mechanically Stabilized Earth Walls For Sliding on Cohesive soils: Do not use the trapezoidal or triangular pressure distribution shown in AASHTO Figure Per AASHTO C , 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

26 MSE or Spread Footing Founded Walls For Sliding and Eccentricity of Load, use Limit State Strength I-a per AASHTO Figure C CIP Gravity, Semi-Gravity, and MSE Walls 26

27 MSE or Spread Footing Founded Walls For Bearing Resistance, use Limit State Strength I-b per AASHTO Figure C CIP Gravity, Semi-Gravity, and MSE Walls 27

28 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

29 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

30 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 and 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

31 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 through Figure Cantilever Non-Gravity Walls 31

32 Cantilever Non-Gravity Walls Cantilever Non-Gravity Walls 32

33 Cantilever Non-Gravity Walls For AASHTO Figure through Figure , 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 γ LS = 1.75 per AASHTO Table φ p = 0.75 per AASHTO Table See AASHTO C for an example of how to use the load and resistance factors with Figure Similar principles can be used with Figure and Figure Cantilever Non-Gravity Walls 33

34 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 ) = Exposed Height Consider embedded length as continuously braced member Cantilever Non-Gravity Walls 34

35 Anchored Walls Use Apparent Earth Pressure (AEP) model per AASHTO Section Section Use Load Factor γ EH = 1.35 for AEP per AASHTO Table 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

36 Anchored Walls 3. Minimum Embedment Depth for moment equilibrium about toe Anchored Walls 36

37 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 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

38 Thank You Questions? ODOT LRFD Foundations 38