1. 2 LRFD Update for Materials/Geotechnical At GRAC Meeting John Schuler, PE Program Manager...

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LRFD Update for Materials/Geotechnical

At GRAC Meeting

John Schuler, PEProgram ManagerVirginia DOT Materials DivisionOctober 31, 2011

Purpose of PresentationProvide common ground between

Materials & Bridge, give Materials & Geotechs background on LRFD initiative

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LRFD – poor choice of words?

Concrete – 1950s

Steel – 1960s

Transportation (Geotech) – 1990s

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Why LRFD?

Steel vs Pre-Stressed Industries?

Purpose – Uniform Safety (not economy)

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Data obtained from instrumentation

Main bridge members mostly

Supporting members/substructures hardly

Geotech – not a thought (later calibration Tony Allen WSDOT)

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Main Players

Modjeski & Masters

D’Appolonia – Geotech

Baker – later Geotech

Prof. Nowak – Michigan – statistics

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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

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Importance now?

Required

Standard Specs no longer being updated as of about 2000

LRFD Spec is excellent reference source – especially geotechnical

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Biggest problem states had in going LRFD – finding software!

This was impact to structural side, not geotech nearly as much.

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

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LRFD Code Highlights Pertaining to Geotechnical Design

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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

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• 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

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• 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

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• 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) ($$$)

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Geotechnical Parameters

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

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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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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

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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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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

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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)

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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

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Step 3 (cont’d): Determine soil strength• Determine friction angle for granular soils or

shear strength for clays from testing (preferred) or standard correlations

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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

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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

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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

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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

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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

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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

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Steel H-PilesEnd-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

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Steel H-PilesFriction• Service Axial Capacity = Ultimate Geotechnical

Capacity / 3– Matches ASD

• Strength Axial Capacity = Ultimate Geotechnical Capacity / 2

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P/S Concrete PilesEnd-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)

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P/S Concrete PilesFriction• 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

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VDOT MSE Wall Analysis SpreadsheetOverview

Plan No. 285-18 Example(Univ. Blvd. over 1-66, Prince William County)

John Schuler, PESenior Geotechnical EngineerVirginia DOT Structure & Bridge DivisionSpring 2007

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Example - Plan No. 285-18 – MSE Wingwalls

VDOT MSE Wall Spreadsheet

Analyze & Iteratively Design MSE walls

Objectives:• Accurate• User-friendly• Transparent

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• 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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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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Strips with given input data work for pullout

Strips don’t work for tensile strength or connection strength as input (just an example – use actual manufacturer data)

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QUESTIONS?

¿PREGUNTAS?

FRAGEN?

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VDOT Anchored Wall Analysis SpreadsheetOverviewExample

John Schuler, PESenior Geotechnical EngineerVirginia DOT Structure & Bridge DivisionSpring 2007

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Example – Anchored Wall

VDOT Anchored Wall Spreadsheet

Analyze & Iteratively Design MSE walls

Objectives:• Accurate• User-friendly• Transparent

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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

Materials developing plastic, metal, and concrete pipe LRFD design capability

Plastic already on TeamSite

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http://www.virginiadot.org/business/bridge-LRFD.asp

VDOT Materials Geotechnical TeamSite

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1. 2 LRFD Update for Materials/Geotechnical At GRAC Meeting John Schuler, PE Program Manager...

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