AASHTO LRFD Section 11 Abutments, Piers, and Walls

1. AASHTO LRFD Section 11 Abutments, Piers, and Walls

2. AASHTO Section 11 • Design specifications for: • Conventional gravity/semigravity walls • Non-gravity cantilevered walls • Anchored walls • Mechanically Stabilized Earth (MSE) walls • Prefabricated modular walls

3. Common Load Groups for Walls

4. Load Definitions • DC – dead load of structural components and attachments • EV – vertical pressure from dead load of earth fill • EH – horizontal earth pressure load • ES – earth surcharge load • LS – live load surcharge (transient load)

5. Surcharge Loads • Earth surcharge AASHTO Section 3.11.6.1 and 3.11.6.2 • Live load surcharge AASHTO 3.11.6.4

6. Conventional Retaining Walls • Strength Limit States • Sliding • Bearing resistance • Eccentricity • Service Limit States • Vertical settlement • Lateral wall movement • Overall stability

7. External Failure Mechanisms Sliding Failure Overturning Failure Deep-Seated Sliding Failure Bearing Failure

8. Load Factors for Conventional Walls b b 1.50 EHsin(b+d) 1.50 EHsin(b+d) 0.90 DC 1.25 DC 1.50 EH 1.50 EH 1.00 EV 1.35 EV b+d b+d 1.50 EHcos(b+d) 1.50 EHcos(b+d) 1.00 WAV 1.00 WAV 1.00 WAH 1.00 WAH Load Factors for Bearing Resistance Load Factors for Sliding and Eccentricity

9. Conventional Walls - Summary • Use resistance factors for spread footings or deep foundations, as appropriate (Section 10.5) • Eccentricity limited to: • e/B < 0.25 for soil (compare to ASD 0.167) • e/B < 0.375 for rock (compare to ASD 0.25)

10. Non-gravity Cantilevered Walls • Strength Limit States • Bearing resistance of embedded portion of wall • Passive resistance of embedded portion of wall • Flexural resistance of wall/facing elements • Service Limit States • Vertical wall movement • Lateral wall movement • Overall stability

11. Resistance Factors • Code allows increase in Resistance Factors for temporary walls but specific guidance is not provided

12. Pressure Diagrams – Discrete Elements ASD LRFD

13. Non-gravity Cantilevered Walls • Below excavation line, multiply by 3b on passive side of wall and 1b on active side of wall for discrete elements • Look at forces separately below excavation line on passive side and active side (because different load factors)

14. Non-gravity Cantilevered Walls • Factor embedment by 1.2 for continuous wall elements • Do not factor embedment for discrete wall elements (conservatism of 3b assumption)

15. Example • Cantilevered sheet pile wall retaining a 10-ft deep cut in granular soils • Assume 36 ksi yield stress for sheet pile • Compare required embedment depth and structural section for ASD and LRFD • Load Factor of 1.5 used for EH (active)

16. Example Geometry

17. Example Results Since Z is about 1.15 to 1.20 times S, similar section would be acceptable

18. Anchored Walls • Strength Limit States • Bearing resistance of embedded portion of wall • Passive resistance of embedded portion of wall • Flexural resistance of wall/facing elements • Ground anchor pullout • Tensile resistance of anchor tendon • Service Limit States • Same as non-gravity cantilevered wall

19. Apparent Earth Pressure Diagrams • Based on FHWA-sponsored research • Builds upon well-known Terzaghi-Peck envelopes • Appropriate for walls built in competent ground where maximum wall height is critical design case • Same diagram shape for single or multi-leveled anchored walls

20. Recommended AEP for Sands 2/3 H1 2/3 H1 H1 H1 Th1 p p Th1 1/3 H H2 Th2 H Hn Thn 2/3 (H-H1) Hn+1 2/3 Hn+1 R R (a) Walls with one levelof ground anchors (b) Walls with multiplelevels of ground anchors

21. LRFD Check on Tensile Breakage • Guaranteed Ultimate Tensile Strength (GUTS) • Select tendon with:

22. Resistance Factors for Ground Anchors – Tensile Rupture • Resistance factors are applied to maximum proof test load • For high strength steel, apply resistance factor to GUTS

23. Comparison to ASD – Tensile Rupture • ASD • 0.8 GUTS > 1.33 Design Load (DL = EH + LS) • 0.8 GUTS > 1.33 EH + 1.33 LS • LRFD • f GUTS > gp EH + 1.75 LS • 0.8 GUTS > 1.5 EH + 1.75 LS Maximum proof test load must be at least equal to the factored load

24. Anchor Bond Length • Lb = anchor bond length • Tn = factored anchor load • Qa = nominal anchor pullout resistance

25. Nominal Anchor Pullout Resistance • Qa = nominal anchor pullout capacity • d = anchor hole diameter • ta = nominal anchor bond stress • Lb = anchor bond length

26. Preliminary Evaluation Only • Bond stress values in AASHTO should be used for FEASIBILITY evaluation • AASHTO values for cohesionless and cohesive soil and rock

27. Presumptive Nominal Bond Stress in Cohesionless Soils

28. Resistance Factors – Anchor Pullout • Using presumptive values for preliminary design only • Where proof tests conducted to at least 1.0 times the factored anchor load

29. Comparison to ASD – Anchor Pullout 1.1 1.05 1.0 Rock (FS = 3.0, f = 0.50) LRFD/ASD 0.95 Sand (FS = 2.5, f = 0.65) 0.9 0.85 Clay (FS = 2.5, f = 0.70) 0.8 0 5 10 15 20 Dead Load / Live Load

30. Final Anchor Design • Section 11.9.4.2 Anchor Pullout Capacity • “For final design, the contract documents shall require that verification tests or pullout tests on sacrificial anchors in each soil unit be conducted …” • Different than current ASD practice, but intent is not to require, in general, pullout testing

31. Bearing Resistance of Wall Element • Assume all vertical loads carried by portion of wall below excavation level • Code refers designer to section on spread or deep foundations for analysis methods • Resistance factors used are for static capacity evaluation of piles or shafts (i.e.,  = 0.3 to 0.5  FS ~ 3.0 to 4.5) • Resistance factors should be modified to correlate to FS = 2.0 to 2.5 for bearing resistance evaluation

32. MSE Walls • Strength Limit States • Same external stability checks as for conventional gravity walls • Tensile resistance of reinforcement • Pullout resistance of reinforcement • Structural resistance of face elements and face element connection • Service Limits States • Same as for conventional gravity walls

33. MSE Walls – External Stability

34. MSE Walls – Internal Stability • Check pullout and tensile resistance at each reinforcement level and compare to maximum factored load, Tmax

35. Maximum Factored Load • Apply factored load to the reinforcements • sH = factored horizontal soil stress at reinforcement (ksf) • Sv = vertical spacing of reinforcement AASHTO 11.10.6.2.1-2

36. Factored Horizontal Stresses • Factored Horizontal Stress • gP = load factor (=1.35 for EV) • kr = pressure coefficient • sV = pressure due to resultant of gravity forces from soil self weight • DsH = horizontal stress AASHTO 11.10.6.2.1-1

37. Reinforcement Tensile Resistance • Tal = Nominal long-term reinforcement design strength • f = Resistance factor for tensile resistance AASHTO 11.10.6.4.1-1

38. Resistance Factors for Tensile Resistance

39. ASD/LRFD Tensile Breakage Example of Steel Strip Reinforcement

40. Other Developments • LRFD for Soil Nails – NCHRP 24-21 • Draft LRFD Design and Construction Specification for Micropiles

41. The End ?