2 LRFD Update for Materials/Geotechnical At GRAC MeetingJohn Schuler, PEProgram ManagerVirginia DOT Materials DivisionOctober 31, 2011
3 Purpose of Presentation Provide common ground between Materials & Bridge, give Materials & Geotechs background on LRFD initiative
4 LRFD – poor choice of words LRFD – poor choice of words? Concrete – 1950s Steel – 1960s Transportation (Geotech) – 1990s
5 Why LRFD. Steel vs Pre-Stressed Industries Why LRFD? Steel vs Pre-Stressed Industries? Purpose – Uniform Safety (not economy)
6 Data obtained from instrumentation Main bridge members mostly Supporting members/substructures hardly Geotech – not a thought (later calibration Tony Allen WSDOT)
7 Main Players Modjeski & Masters D’Appolonia – Geotech Baker – later Geotech Prof. Nowak – Michigan – statistics
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
9 Importance now? Required Standard Specs no longer being updated as of about 2000 LRFD Spec is excellent reference source – especially geotechnical
10 Biggest problem states had in going LRFD – finding software Biggest problem states had in going LRFD – finding software! This was impact to structural side, not geotech nearly as much.
11 Every VDOT Bridge Engineer who was at VDOT in Spring 2007 received following geotechnical guidance training from CO S & B Division.
12 LRFD Code Highlights Pertaining to Geotechnical Design
13 LRFD Code Highlights AASHTO LRFD Bridge Design Specifications Section 3 for Loads and load factorsSection 10 for FoundationsSection 11 for Abutments, Piers, WallsSection 12 for Buried Structures
14 LRFD Code HighlightsIn general, LRFD made to match ASD for geotechnical designC , criticality of scour and economy of scour protectionC3.4.1,expect sliding to control often for spread footings, as horizontal soil force is always maximizedScour is the leading cause of bridge failure in America. Don’t skimp on scour protection to save money.
15 LRFD Code Highlights3.11, Earth Pressures (anchored wall pressure distribution change)Table , better exploration or field testing can increase resistance factor 10%-20% for shallow foundationsRMR for bearing capacity preferredRMR is Rock Mass Rating System – LRFD code prefers this method to compute bearing capacity on rock, but it is not required.
16 LRFD Code HighlightsTables for driven pile resistance factorsNeed to do minimum of 3-4 PDAs on a jobCan increase resistance factor 40% over PDA use if do static load test(s) ($$$)Be aware, you may need to increase your PDA (Pile Dynamic Test) test quantities over what you previously used.
17 Geotechnical Parameters Geotechnical Parameters – Introduction and Guidance on Choosing Them4 stepsThese slides will not be discussed in depth here in class. For students’ future use if so desired.
18 Geotechnical Parameters Step 1: Determine soil type2 broad classifications of soilGranular (Gravel, Sand, Silt)Cohesive (Clay)The types are determined by sieve testBoring logs in bridge plans will show soil type
20 Geotechnical Parameters Step 2: Determine soil weightStandard correlations typically used to estimate unit weightsTypically, assume saturated unit weight is pcf more than moist unit weight
21 These simple index charts are used by geotechnical engineers much more often than not.
22 Geotechnical Parameters Step 3: Determine soil strengthLook at boring logs for substructureIf soil is granular (gravel, sand, silt) it will have a friction angleIf soil is cohesive (clay, maybe clayey silt) it will have an undrained shear strengthClayey (sand, silt) may have both cohesion and friction angle
24 Geotechnical Parameters Step 3 (cont’d): Determine soil strengthDetermine either friction angle or shear strength from SPT corrected blow count N160, CPT data, lab test dataSPT is most common by farIn given column of boring logs, SPT blow counts are a set of 3 numbers – sum the last 2 of 3 to obtain N
26 Geotechnical Parameters Step 3 (cont’d): Determine soil strengthIf soil is granular, correct blow count per correction sheet:Po is effective vertical soil pressure at depth of N valueN1 = CN*N (AASHTO )Effective means use buoyant weight of soil (unit weight – 62.4 pcf)Only granular soils are corrected for overburden pressure, as for granular soils their strength depends on confinement.
31 Geotechnical Parameters Step 4: Determine soil settlement parametersElastic modulus values of soil obtained by testing or correlationsTables in AASHTOPoisson’s RatioCan use 0.3 for all non-saturated soilsUse 0.5 for all saturated soils
32 Geotechnical Parameters RockType of rock is shown on boring logsRQD is shown on boring logsGroundwater table shown on boring logsNeed spacing and condition of jointsNeed point load or UC tests of rockFriction between concrete and rock is based on rock friction angle – obtain from tables – typically between 35 and 45
33 Geotechnical Parameters Rock (cont’d)Obtain elastic modulus from AASHTO LRFD (Table C )Obtain Poisson’s Ratio from AASHTO LRFD (Table C )0.2 is a good approximation
35 Geotechnical Parameters ExplorationFollow 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 lengthsAlways sample at least 10-ft below EPTE and always core at least 10-ft of rockGood heuristic – bore 100-ft minimum
36 Geotechnical Parameters Exploration (cont’d)Use drill rig to get SPT N values. Sample frequently within 2B of footing bottomUse split spoon to get disturbed soil samples for sieve analysis, Atterberg limits, corrosivity testsGet GROUNDWATER ELEVATIONS!Affects bearing, settlement, constructability, downdrag, corrosivity, earth pressures
37 Pile capacities in ABLRFD Example - Plan NoPile capacities in ABLRFDGenerally, you will specify a strength axial capacity and a service axial capacity for a pileService axial capacity will essentially be matched to ASD capacityThe specified capacity is generally linked to the structural capacity of the pile – ensure geotechnical capacity is available
38 Example - Plan No. 285-84 Steel H-Piles End-bearing Service Axial Capacity = 0.25*Fy*AreaCorresponds to 9 ksi – same as ASDAdvantage of 50 ksi steel can be counted on during driving, not for long-term static capacityStrength Axial Capacity = 0.60*Fy*AreaArticle – 0.60 is good driving conditions; 0.50 is severe conditionsCorresponds to 21.6 ksi in good conditionsUse of 36 ksi yield strength has a long history
39 Example - Plan No. 285-84 Steel H-Piles Friction Service Axial Capacity = Ultimate Geotechnical Capacity / 3Matches ASDStrength Axial Capacity = Ultimate Geotechnical Capacity / 2
40 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 of ASD code);HOWEVER, VDOT practice is limit to ~0.80 ksiStrength Axial Capacity – use 0.70*f’c*Area (Article of LRFD code, simple compression bearing)The 0.80 ksi limit corresponds to about 60 tons on a 12” square p/s pile, and about 230 tons on a 24” square p/s pile – traditional upper limits at VDOT.
41 Example - Plan No. 285-84 P/S Concrete Piles Friction Service Axial Capacity = Ultimate Geotechnical Capacity / 3Matches ASDAgain, LIMIT to bearing stress of ~0.80 ksiStrength Axial Capacity = Ultimate Geotechnical Capacity / 2
42 VDOT MSE Wall Analysis Spreadsheet Overview Plan No Example(Univ. Blvd. over 1-66, Prince William County)John Schuler, PESenior Geotechnical EngineerVirginia DOT Structure & Bridge DivisionSpring 2007
43 Example - Plan No. 285-18 – MSE Wingwalls VDOT MSE Wall SpreadsheetAnalyze & Iteratively Design MSE wallsObjectives:AccurateUser-friendlyTransparent
44 Example - Plan No. 285-18 – MSE Wingwalls Use the VDOT MSE Wall SpreadsheetExternal Stability (Bearing, Sliding, Eccentricity)Internal Stability for Steel Strips and Steel GridsPullout, Tensile Strength, Connection Rupture
50 Yellow cells are input (may show up as green on screen) Yellow cells are input (may show up as green on screen). AASHTO says reinforcement length has to be at least 70% of wall height and all reinforcement layers must be of equal length.
57 Again, yellow cells are input. Input as many rows as you call for Again, yellow cells are input. Input as many rows as you call for. Calculations are checked at each row in this table for rupture strengths of strips and of connections to panels.
59 Example - Plan No. 285-18 – MSE Wingwalls Strips with given input data work for pulloutStrips don’t work for tensile strength or connection strength as input (just an example – use actual manufacturer data)
60 Example - Plan No. 285-18 – MSE Wingwalls QUESTIONS?¿PREGUNTAS?FRAGEN?
63 Example – Anchored Wall Use the VDOT Anchored Wall SpreadsheetChecks wall/soldier pile bendingDesigns anchor lengthChecks anchor strengthDesigns soldier pile embedment – against rotation and vertical loadCurrently cannot be used for a cantilever sheeting wallDesign and construction details are a little more detailed than for MSE walls.
65 Main input cells – again in yellow. AASHTO references listed.
66 Earth pressure diagrams from AASHTO/FHWA Geotechnical Engineering Circular (GEC) 4. The pressure diagrams vary slightly for varying rows of anchors, hence the spreadsheet is broken up into worksheets for 1, 2, and up to 3 anchors.
70 This table checks that the discrete vertical elements (typically a soldier pile embedded in a 3-ft diameter concrete caisson, about 8’ on centers) are embedded deep enough to provide rotational restraint against the active earth pressure behind the wall.
71 This input cell that allows user to vary how much vertical anchor component load transfers to the embedded portion of soldier pile below excavation level is important. There is no guidance in AASHTO on this, other than to assume a very, very conservative 100% is transferred.
72 This table checks the embedment of the soldier pile for bearing resistance against vertical loads.
73 Materials developing plastic, metal, and concrete pipe LRFD design capability Plastic already on TeamSite