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LRFD

LRFD. Load & Resistance Factor Design. IMPLEMENTATION. AASHTO REQUIRES ALL STATES TO BE FULLY COMPLIANT BY 2007. Reality – As of 2011 All States Are at 90 % Compliance. WHY LRFD ?. Lighter Structures Better Service (Performance). WHAT IS LRFD?. A PROBABILISTIC APPROACH TO DESIGN.

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LRFD

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  1. LRFD Load& Resistance Factor Design

  2. IMPLEMENTATION AASHTO REQUIRES ALL STATES TO BE FULLY COMPLIANT BY 2007 Reality – As of 2011 All States Are at 90 % Compliance

  3. WHY LRFD ? • Lighter Structures • Better Service (Performance)

  4. WHAT IS LRFD? A PROBABILISTIC APPROACH TO DESIGN ALL ELEMENTS OF A STRUCTURE SHARE THE SAME RISK RESULTS– LESS OVERDESIGN AND BETTER SERVICE

  5. LRFD METHODOLOGY STRENGTH – Will it bend, break, or fail under various load combinations? SERVICE – Will it perform within expected limits?

  6. LRFD Differences from ASD • Same • Determining Resistance • Determining Deflection • Different • Comparison of load and resistance • Specific separation of resistance and deflection

  7. LRFD STRENGTH Σηiδi ≤ФRη Factored Load Factored Resistance

  8. LRFD STRENGTH Load Cases Basic Wind and Others Extreme Event Load factors depend on case type and probability of variation: e.g. δ LL = 1.75, δ DL = 1.5

  9. LRFD SERVICE DISPLACEMENT LIMITATIONS SLIDING STABILITY PILE DRIVING DAMAGE ANALYSIS IS BY TYPICAL METHODS δ = 1.0 UNLESS OTHERWISE REQUIRED NORMAL LOAD CASE EXTREME EVENT CASE e.g. SCOUR

  10. Wall Service Failures • Base Sliding • Low Weight, Weak Subgrade • Overturning • Low Weight • Excessive Settlement • Compressible Bearing Soils • Base Too Narrow • Global Stability • Low Strength bearing soils, narrow base, not enough embedment, high gwt • Bearing Capacity • Same as Global Stability

  11. Wall Service Failures • Anchor Pullout • Anchor Too Short, Overest. Capacity, Poor Grouting, Not Enough Testing • Tendon Failure • Failure to assess load • Bulging • Failure Plane within Backfill • Internal Sliding • Reinforced Zone too Short • Facing Connection Failure • Design Error

  12. Leading Causes of Non-Structural Failure • Post & Panel Walls • Piles Too Short • Piles Widely Spaced • Sx Too Low • Global Stability • Other Walls • Excessive Settlement • Bearing Capacity / Global Stability • Anchor Capacity

  13. Modular Block Walls?

  14. DESIGN PROCESS • Obtain Adequate Geotechnical Info • Use Parameters within Normal Distribution • Follow AASHTO Guidelines • Perform a Reality Check • Don’t Be Cheap

  15. Load and Resistance Factors Excel Design Examples

  16. Deep Foundation Design Process • Decide deep foundation type • Select resistance factor • Compute resistances • Layout foundation group and analyze at the strength limit state • Check the service limit state

  17. Strength Limit State Checks Driven Piles Drilled Shafts • Structural resistance • Axial geotechnical resistance • Driven resistance • Structural resistance • Axial geotechnical resistance

  18. Service Limit State Checks Driven Piles Drilled Shafts • Global Stability • Vertical Displacement • Horizontal Displacement • Global Stability • Vertical Displacement • Horizontal Displacement

  19. Selection of Resistance factors C • Strength limit state • Structural Resistance • Geotechnical Resistance • Driven Resistance (piles only) • Service limit state • Resistance factor = 1.0 (except global stability)

  20. Determining Geotechnical Resistance of Piles • Field methods • Static load test • Dynamic load test (PDA) • Static analysis methods • Driving Formulae

  21. Geotechnical Resistance Factors for Piles • Site Variability Defined in NCHRP Report 507 • Range of Values of Resistance Factors Depends on Number of Static Load Tests AASHTO Table 10.5.5.2.2-2

  22. Geotechnical Resistance Factors for Piles • Test 1% to 50% of Production Piles, Depending on Site Variability and Number of Piles Driven • Site Variability Defined in NCHRP Report 507 AASHTO Table 10.5.5.2.2-1 & 3

  23. Geotechnical Resistance Factors for Piles PDA 0.65 Load Test 0.75 Load Test and PDA 0.8 AASHTO Table 10.5.5.2.2-1

  24. Geotechnical Resistance Factors Pile Static Analysis Methods AASHTO Table 10.5.5.2.2-1

  25. Comparison to ASD Service Load = 2794 kips

  26. Driven Performance Limit

  27. LRFD STRENGTH Σηiδi ≤ФRη Factored Load Factored Resistance THANK YOU

  28. What Do You See in Common?

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