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AASHTO’s LRFD Specifications for Foundation and Earth Retaining Structure Design (Through 2006 Interims and Beyond) Jerry A. DiMaggio, P.E. Principal Bridge/Geotechnical Engineer FHWA, Washington D. C. Existing Specifications Standard LRFD 17th Edition 3rd Edition “AASHTO and FHWA have agreed that all state DOT’s will use LRFD for design of NEW structures by 2007.” AK 95% AK AASHTO LRFD Survey May 2005 WA 100% WA MT 35% ND MT OR 100% MN 40% MN WI WI WY WY UT CO 90% CO KS 50% KS MO IL 5% IL ININ MO AZ NM NM OK 100% OK WV TN NC VT 5% VT 0- 24 - 10 0- 2-2 NJ DE MD SC 50% SC MS MS TX VA VA NC TN AR AR 5% TX 13% PA WV 80% KY CA PA 100% OH OH KY CA AZ NY IA NE UT 75% NY 50% MI MI IA 5% NE 60% NV ME SD ID NV ME 100% SD 10% ID 100% OR ND LA LA HI Full Implementation 50-90% Partial Implementation 26-50% Partial Implementation 11-25% Partial Implementation 1-10% Partial Implementation q No Implementation AL AL GA GA FL 100% FL PR NH MA RI CT Earthwork and walls: ASD Superstructure: LRFD Substructure: LRFD/ASD Foundations: ASD Reasons for Not Adopting • • • • • • Human nature. No perceived benefits. Unfamiliarity with LRFD methods. Lack of confidence in the computed results. Perceived errors and inconsistencies. A specification that did not reflect current design practices. What is FHWA doing? • • • • • Bridge Design examples. NHI LRFD Training Courses. FHWA Technical Assistance. FHWA/ NCHRP Calibration efforts. AASHTO Section 11 and 10 Revisions. Bridge Design Examples Concrete Steel http://www.fhwa.dot.gov/bridge/lrfd/examples.htm NHI LRFD Training Courses Course 130082A LRFD for Highway Bridge Substructures and Earth Retaining Structures FHWA/ NCHRP Activities • NCHRP Project 12-66, Specifications for Serviceability in the Design of Bridge Foundations • NCHRP Report 507, Load and Resistance Factor Design (LRFD) for Deep Foundations FHWA/ NCHRP Activities • Publication No. FHWA-NHI-05-052, Development of Geotechnical Resistance Factors and Downdrag Load Factors for LRFD Foundation Strength Limit State Design Revisions to Section 10 • • • • Compiled by a Technical Expert Panel Review and input from A Technical Working Group (TWG) Accepted by AASHTO Subcommittee T-15 in June 2005 in Newport, Rhode Island To be published in 2006 Interim http://bridges.transportation.org/?siteid=34&c=downloads •Attachments to Agenda Item 39 Section 3 revisions •Attachments to Agenda item 40 Section 10 revisions Topics Included • • • • • • • • Subsurface investigations Soil and rock properties Shallow foundations Driven piles Drilled shafts Rigid and flexible culverts Abutments Walls (All types) Topics NOT Included • • • • • • Integral abutments Micropiles Augercast piles Soil nails Reinforced slopes All soil and rock earthwork features. Section 10 Contents 10.1 SCOPE NO SIGNIFICANT CHANGE 10.2 DEFINITIONS UPDATED UPDATED, CONSISTANT 10.3 NOTATION 10.4 SOIL AND ROCK PROPERTIES 10.5 LIMIT STATES AND RESISTANCE FACTORS 10.6 SPREAD FOOTINGS 10.7 DRIVEN PILES 10.8 DRILLED SHAFTS REORGANIZED, NEW CONTENT NEW CONTENT PROPERTY INFO Section 10.4 Soil and Rock Properties GEC 5 Sabatini, 2002 Subsurface Investigations Mayne, 2002 Section 10.4 Soil and Rock Properties 10.4.6 SELECTION OF DESIGN PROPERTIES • • • • • Soil Strength Soil Deformation Rock Mass Strength Rock Mass Deformation Erodibility of rock Section 10.5 Limit States and Resistance Factors • • • Resistance factors revised Additional discussion on the basis for resistance factors Additional discussion of extreme event considerations Articles 3.4.1 and 3.11.8 Downdrag • Methods for computing • Load Factors • Use of minimum load factors clarified Piles, -method Piles, -method Drilled Shafts, O’neill and Reese (1999) Maximum 1.4 Minimum 0.25 1.05 1.25 0.30 0.35 Section 10.6 Spread Footings Eccentricity provisions clarified B′ = B – 2eB L′ = L – 2eL Q = P/(B’ L’) Applies to geotechnical design for settlement and bearing resistance L B P ML MB e eL B B’ q L’ Section 10.6 Spread Footings Hough method Elastic Settlement of cohesionless soils σ'vo Δσ v 1 ΔH H c log C' σ'vo Section 10.6 Spread Footings NOMINAL RESISTANCE COHESION UNIT WEIGHT DEPTH WIDTH qn = c Ncm + Df Nqm Cwq + 0.5 B Nm Cw Nc sc ic Nq sq dq iq Nsi Water table correction Shear through overburden Inclination Factors Bearing Capacity Factors Shape Correction Factors correction factor Settlement correction factors removed Section 10.7 Driven Piles Settlement of pile groups 4 new diagrams From: Hannigan (2005) Section 10.7 Driven Piles Qt Ht Mt The P-y method specified for horizontal deflection P y Section 10.7 Driven Piles S P P Pm * P y D P-multiplier (Pm) Spacing (S) Row 1 Row 2 3D 0.7 0.5 5D 1.0 0.85 Row 3 0.35 0.7 Section 10.7 Driven Piles Field determination of nominal resistance Static load test Dynamic load test Section 10.7 Driven Piles Static analysis methods • Nordlund – Thurman method added Section 10.7 Driven Piles Static analysis methods • • • Primary use is for pile length estimation for contract drawings Secondary use for estimation of downdrag, uplift resistance and scour effects Should rarely be used as sole means of determining pile resistance Section 10.7 Driven Piles Comp Str ksi Tens Str ksi 30 Requirements for driveability analysis have been added and clarified 20 10 Ult Cap kips Stroke 800 ft 16.0 600 12.0 400 8.0 200 4.0 0 160 320 480 Blows/ft Section 10.7 Driven Piles 10.7.3.2 PILE LENGTH ESTIMATES FOR CONTRACT DOCUMENTS 10.7.6 Determination of minimum pile penetration Section 10.8 Drilled shafts Refers to driven piles section where possible • Downdrag • Group settlement • Horizontal displacement (single and group) • Lateral squeeze • Water table and buoyancy • Scour • Group resistance (cohesive soil only) • Uplift (group and load test sections) • Buckling • Extreme event limit state Section 10.8 Drilled shafts • • Static analysis methods for soil and rock have been updated Consideration of both base and side resistance in rock is now included O’Neill and Reese (1999) Section 10.8 Drilled shafts A+B A+D QS Resistance Total Resistance B+C A Side Resistance D Tip Resistance QP Displacement B C Conclusion Future Enhancements Overall stability • Weight is both a load and a resistance • Service limit state (should be strength limit state) + WT N tan f cl T N T WT l WT l WT N T N tan f cl T Future Enhancements Inclination Factors • Ignored by many practicing engineers • Based on small scale tests and theory • Effect of embedment (Df) • Resistance factors are for vertical load Q Df Future Enhancements Nominal bearing resistance of rock • Very little guidance available • CSIR Rock Mass Rating System proposed • CSIR developed for tunnel design • Includes life safety considerations and therefore, margin of safety • May be conservative Future Enhancements V HH Pile head fixity • Connection details • Effects of axial loads Future Enhancements Dx Dz Serviceability limits NCHRP 12-66 Due April 2006 What Should I Know and Do? • • Become familiar with BOTH the AASHTO standard specifications and LRFD specs. Develop an understanding of your agency’s current design practice What Should I Know and Do? • • Develop and compare results for SEVERAL example problems with LRFD and YOUR standard design practice Translate your current practice to an LRFD format What Should I Know and Do? • Communicate your findings to AASHTO’s SubCommitteee members AASHTO Section 11 • Design specifications for: • • • • • Conventional gravity/semigravity walls Non-gravity cantilevered walls Anchored walls Mechanically Stabilized Earth (MSE) walls Prefabricated modular walls LRFD Specifications for Foundation/ Earth Retaining Structure Design Questions?