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1. 2 LRFD Update for Materials/Geotechnical At GRAC Meeting John Schuler, PE Program Manager Virginia DOT Materials Division October 31, 2011.

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Presentation on theme: "1. 2 LRFD Update for Materials/Geotechnical At GRAC Meeting John Schuler, PE Program Manager Virginia DOT Materials Division October 31, 2011."— Presentation transcript:

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2 2 LRFD Update for Materials/Geotechnical At GRAC Meeting John Schuler, PE Program Manager Virginia DOT Materials Division October 31, 2011

3 Purpose of Presentation Provide common ground between Materials & Bridge, give Materials & Geotechs background on LRFD initiative 3

4 LRFD – poor choice of words? Concrete – 1950s Steel – 1960s Transportation (Geotech) – 1990s 4

5 Why LRFD? Steel vs Pre-Stressed Industries? Purpose – Uniform Safety (not economy) 5

6 Data obtained from instrumentation Main bridge members mostly Supporting members/substructures hardly Geotech – not a thought (later calibration Tony Allen WSDOT) 6

7 Main Players Modjeski & Masters DAppolonia – Geotech Baker – later Geotech Prof. Nowak – Michigan – statistics 7

8 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

9 Importance now? Required Standard Specs no longer being updated as of about 2000 LRFD Spec is excellent reference source – especially geotechnical 9

10 Biggest problem states had in going LRFD – finding software! This was impact to structural side, not geotech nearly as much. 10

11 Every VDOT Bridge Engineer who was at VDOT in Spring 2007 received following geotechnical guidance training from CO S & B Division. 11

12 12 LRFD Code Highlights Pertaining to Geotechnical Design

13 13 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

14 14 LRFD Code Highlights In general, LRFD made to match ASD for geotechnical design C , 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

15 15 LRFD Code Highlights 3.11, Earth Pressures (anchored wall pressure distribution change) Table , better exploration or field testing can increase resistance factor 10%-20% for shallow foundations RMR for bearing capacity preferred

16 16 LRFD Code Highlights Tables 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) ($$$)

17 17 Geotechnical Parameters Geotechnical Parameters – Introduction and Guidance on Choosing Them 4 steps

18 18 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

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

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22 22 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

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24 24 Geotechnical Parameters Step 3 (contd): Determine soil strength Determine either friction angle or shear strength from SPT corrected blow count N1 60, 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

25 25 Geotechnical Parameters

26 26 Geotechnical Parameters Step 3 (contd): Determine soil strength If soil is granular, correct blow count per correction sheet: P o is effective vertical soil pressure at depth of N value N1 = C N *N (AASHTO ) Effective means use buoyant weight of soil (unit weight – 62.4 pcf)

27 27 Geotechnical Parameters

28 28 Geotechnical Parameters Further SPT N corrections: N 60 = (ER/60%)*N (AASHTO ) N1 60 = C N *N 60 (AASHTO ) ER = 60% for drop hammer ER = 80% for automatic hammer Unusual to correct for other items

29 29 Geotechnical Parameters Step 3 (contd): Determine soil strength Determine friction angle for granular soils or shear strength for clays from testing (preferred) or standard correlations

30 30 Geotechnical Parameters

31 31 Geotechnical Parameters Step 4: Determine soil settlement parameters Elastic modulus values of soil obtained by testing or correlations Tables in AASHTO Poissons Ratio Can use 0.3 for all non-saturated soils Use 0.5 for all saturated soils

32 32 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

33 33 Geotechnical Parameters Rock (contd) Obtain elastic modulus from AASHTO LRFD (Table C ) Obtain Poissons Ratio from AASHTO LRFD (Table C ) 0.2 is a good approximation

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35 35 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

36 36 Geotechnical Parameters Exploration (contd) 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

37 37 Example - Plan No 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

38 38 Example - Plan No 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 – 0.60 is good driving conditions; 0.50 is severe conditions –Corresponds to 21.6 ksi in good conditions

39 39 Example - Plan No Steel H-Piles Friction Service Axial Capacity = Ultimate Geotechnical Capacity / 3 –Matches ASD Strength Axial Capacity = Ultimate Geotechnical Capacity / 2

40 40 Example - Plan No P/S Concrete Piles End-bearing Service Axial Capacity – match to ASD value of about 1.44 ksi (0.33fc – 0.27fpe, Article of ASD code); HOWEVER, VDOT practice is limit to ~0.80 ksi Strength Axial Capacity – use 0.70*fc*Area (Article of LRFD code, simple compression bearing)

41 41 Example - Plan No 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

42 42 VDOT MSE Wall Analysis Spreadsheet Overview Plan No Example (Univ. Blvd. over 1-66, Prince William County) John Schuler, PE Senior Geotechnical Engineer Virginia DOT Structure & Bridge Division Spring 2007

43 43 Example - Plan No – MSE Wingwalls VDOT MSE Wall Spreadsheet Analyze & Iteratively Design MSE walls Objectives: Accurate User-friendly Transparent

44 44 Example - Plan No – 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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54 54 Example - Plan No – MSE Wingwalls The 22-ft strip length works. This is 70% of wall height and is the minimum allowed by AASHTO. Now look at internal stability

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59 59 Example - Plan No – MSE Wingwalls Strips with given input data work for pullout Strips dont work for tensile strength or connection strength as input (just an example – use actual manufacturer data)

60 60 Example - Plan No – MSE Wingwalls QUESTIONS? ¿ PREGUNTAS? FRAGEN?

61 61 VDOT Anchored Wall Analysis Spreadsheet Overview Example John Schuler, PE Senior Geotechnical Engineer Virginia DOT Structure & Bridge Division Spring 2007

62 62 Example – Anchored Wall VDOT Anchored Wall Spreadsheet Analyze & Iteratively Design MSE walls Objectives: Accurate User-friendly Transparent

63 63 Example – Anchored Wall Use the VDOT Anchored Wall Spreadsheet Checks wall/soldier pile bending Designs anchor length Checks anchor strength Designs soldier pile embedment – against rotation and vertical load Currently cannot be used for a cantilever sheeting wall

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73 Materials developing plastic, metal, and concrete pipe LRFD design capability Plastic already on TeamSite 73

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79 ss/bridge-LRFD.asp VDOT Materials Geotechnical TeamSite 79


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