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LRFD Update for Materials/Geotechnical At GRAC Meeting John Schuler, PE Program Manager

LRFD Update for Materials/Geotechnical At GRAC Meeting John Schuler, PE Program Manager Virginia DOT Materials Division October 31, 2011. Purpose of Presentation Provide common ground between Materials & Bridge, give Materials & Geotechs background on LRFD initiative.

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LRFD Update for Materials/Geotechnical At GRAC Meeting John Schuler, PE Program Manager

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  1. LRFD Update for Materials/Geotechnical • At GRAC Meeting • John Schuler, PE • Program Manager • Virginia DOT Materials Division • October 31, 2011

  2. Purpose of Presentation • Provide common ground between Materials & Bridge, give Materials & Geotechs background on LRFD initiative

  3. LRFD – poor choice of words? • Concrete – 1950s • Steel – 1960s • Transportation (Geotech) – 1990s

  4. Why LRFD? • Steel vs Pre-Stressed Industries? • Purpose – Uniform Safety (not economy)

  5. Data obtained from instrumentation • Main bridge members mostly • Supporting members/substructures hardly • Geotech – not a thought (later calibration Tony Allen WSDOT)

  6. Main Players • Modjeski & Masters • D’Appolonia – Geotech • Baker – later Geotech • Prof. Nowak – Michigan – statistics

  7. FHWA – • LRFD by October 2007 for bridges • LRFD by October 2010 for walls, culverts, etc. • Eventually left up to each state FHWA • Phased in for various items

  8. Importance now? • Required • Standard Specs no longer being updated as of about 2000 • LRFD Spec is excellent reference source – especially geotechnical

  9. Biggest problem states had in going LRFD – finding software! • This was impact to structural side, not geotech nearly as much.

  10. Every VDOT Bridge Engineer who was at VDOT in Spring 2007 received following geotechnical guidance training from CO S & B Division.

  11. LRFD Code Highlights Pertaining to Geotechnical Design

  12. LRFD Code Highlights • AASHTO LRFD Bridge Design Specifications • Section 3 for Loads and load factors • Section 10 for Foundations • Section 11 for Abutments, Piers, Walls • Section 12 for Buried Structures

  13. LRFD Code Highlights • In general, LRFD made to match ASD for geotechnical design • C2.6.4.4.2, criticality of scour and economy of scour protection • C3.4.1,expect sliding to control often for spread footings, as horizontal soil force is always maximized

  14. LRFD Code Highlights • 3.11, Earth Pressures (anchored wall pressure distribution change) • Table 10.5.5.2.2-1, better exploration or field testing can increase resistance factor 10%-20% for shallow foundations • RMR for bearing capacity preferred

  15. LRFD Code Highlights • Tables 10.5.5.2.3-1 for driven pile resistance factors • Need to do minimum of 3-4 PDAs on a job • Can increase resistance factor 40% over PDA use if do static load test(s) ($$$)

  16. Geotechnical Parameters • Geotechnical Parameters – Introduction and Guidance on Choosing Them • 4 steps

  17. Geotechnical Parameters • Step 1: Determine soil type • 2 broad classifications of soil • Granular (Gravel, Sand, Silt) • Cohesive (Clay) • The types are determined by sieve test • Boring logs in bridge plans will show soil type

  18. Geotechnical Parameters • Step 2: Determine soil weight • Standard correlations typically used to estimate unit weights • Typically, assume saturated unit weight is 10-20 pcf more than moist unit weight

  19. Geotechnical Parameters • Step 3: Determine soil strength • Look at boring logs for substructure • If soil is granular (gravel, sand, silt) it will have a friction angle • If soil is cohesive (clay, maybe clayey silt) it will have an undrained shear strength • Clayey (sand, silt) may have both cohesion and friction angle

  20. Geotechnical Parameters • Step 3 (cont’d): Determine soil strength • Determine either friction angle or shear strength from SPT corrected blow count N160, CPT data, lab test data • SPT is most common by far • In given column of boring logs, SPT blow counts are a set of 3 numbers – sum the last 2 of 3 to obtain N

  21. Geotechnical Parameters

  22. Geotechnical Parameters • Step 3 (cont’d): Determine soil strength • If soil is granular, correct blow count per correction sheet: • Po is effective vertical soil pressure at depth of N value • N1 = CN*N (AASHTO 10.4.6.2.4-1) • Effective means use buoyant weight of soil (unit weight – 62.4 pcf)

  23. Geotechnical Parameters

  24. Geotechnical Parameters • Further SPT N corrections: • N60 = (ER/60%)*N(AASHTO 10.4.6.2.4-2) • N160 = CN*N60 (AASHTO 10.4.6.2.4-3) • ER = 60% for drop hammer • ER = 80% for automatic hammer • Unusual to correct for other items

  25. Geotechnical Parameters • Step 3 (cont’d): Determine soil strength • Determine friction angle for granular soils or shear strength for clays from testing (preferred) or standard correlations

  26. Geotechnical Parameters

  27. Geotechnical Parameters • Step 4: Determine soil settlement parameters • Elastic modulus values of soil obtained by testing or correlations • Tables in AASHTO • Poisson’s Ratio • Can use 0.3 for all non-saturated soils • Use 0.5 for all saturated soils

  28. Geotechnical Parameters • Rock • Type of rock is shown on boring logs • RQD is shown on boring logs • Groundwater table shown on boring logs • Need spacing and condition of joints • Need point load or UC tests of rock • Friction between concrete and rock is based on rock friction angle – obtain from tables – typically between 35 and 45

  29. Geotechnical Parameters • Rock (cont’d) • Obtain elastic modulus from AASHTO LRFD (Table C10.4.6.5-1) • Obtain Poisson’s Ratio from AASHTO LRFD (Table C10.4.6.5-2) • 0.2 is a good approximation

  30. Geotechnical Parameters • Exploration • Follow Materials Division MOI Chapter III for number and depth of borings (same as AASHTO, except 20 ft under piles/shafts) • Reckon depth of borings based on applied stresses and pile lengths • Always sample at least 10-ft below EPTE and always core at least 10-ft of rock • Good heuristic – bore 100-ft minimum

  31. Geotechnical Parameters • Exploration (cont’d) • Use drill rig to get SPT N values. Sample frequently within 2B of footing bottom • Use split spoon to get disturbed soil samples for sieve analysis, Atterberg limits, corrosivity tests • Get GROUNDWATER ELEVATIONS! • Affects bearing, settlement, constructability, downdrag, corrosivity, earth pressures

  32. Example - Plan No. 285-84 • Pile capacities in ABLRFD • Generally, you will specify a strength axial capacity and a service axial capacity for a pile • Service axial capacity will essentially be matched to ASD capacity • The specified capacity is generally linked to the structural capacity of the pile – ensure geotechnical capacity is available

  33. Example - Plan No. 285-84 • Steel H-Piles • End-bearing • Service Axial Capacity = 0.25*Fy*Area • Corresponds to 9 ksi – same as ASD • Advantage of 50 ksi steel can be counted on during driving, not for long-term static capacity • Strength Axial Capacity = 0.60*Fy*Area • Article 6.5.4.2 – 0.60 is good driving conditions; 0.50 is severe conditions • Corresponds to 21.6 ksi in good conditions

  34. Example - Plan No. 285-84 • Steel H-Piles • Friction • Service Axial Capacity = Ultimate Geotechnical Capacity / 3 • Matches ASD • Strength Axial Capacity = Ultimate Geotechnical Capacity / 2

  35. Example - Plan No. 285-84 • P/S Concrete Piles • End-bearing • Service Axial Capacity – match to ASD value of about 1.44 ksi (0.33f’c – 0.27fpe, Article 4.5.7.3 of ASD code); • HOWEVER, VDOT practice is limit to ~0.80 ksi • Strength Axial Capacity – use 0.70*f’c*Area (Article 5.5.4.2.1 of LRFD code, simple compression bearing)

  36. Example - Plan No. 285-84 • P/S Concrete Piles • Friction • Service Axial Capacity = Ultimate Geotechnical Capacity / 3 • Matches ASD • Again, LIMIT to bearing stress of ~0.80 ksi • Strength Axial Capacity = Ultimate Geotechnical Capacity / 2

  37. VDOT MSE Wall Analysis Spreadsheet • Overview • Plan No. 285-18 Example • (Univ. Blvd. over 1-66, Prince William County) • John Schuler, PE • Senior Geotechnical Engineer • Virginia DOT Structure & Bridge Division • Spring 2007

  38. Example - Plan No. 285-18 – MSE Wingwalls • VDOT MSE Wall Spreadsheet • Analyze & Iteratively Design MSE walls • Objectives: • Accurate • User-friendly • Transparent

  39. Example - Plan No. 285-18 – MSE Wingwalls • Use the VDOT MSE Wall Spreadsheet • External Stability (Bearing, Sliding, Eccentricity) • Internal Stability for Steel Strips and Steel Grids • Pullout, Tensile Strength, Connection Rupture

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