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Design Specifications and Load Factors for Walls

This guide covers design specifications for various types of walls, including conventional and non-gravity walls, as well as load definitions and factors to ensure structural stability and integrity. It includes details on strength and service limit states, external failure mechanisms, and resistance factors for different wall types. The text provides comprehensive information on bearing resistance, passive resistance, flexural resistance, and overall stability for different wall designs.

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Design Specifications and Load Factors for Walls

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  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 ?

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