AISC Steel Design Philosophies of Design

ASD – Allowable Stress Design
LFD – Load Factor Design
LRFD – Load and Resistance Factor Design

ASD: Allowable Stress Design

ASD does not recognize different variabilities of different load types.

LFD: Load Factor Design

In LFD, load and resistance are not considered simultaneously.

LRFD: Load & Resistance Factor Design

The LRFD philosophy provides a more uniform, systematic, and rational approach to the selection of load factors and resistance factors than LFD.

Variability of Loads and Resistances:

Suppose that we measure the weight of 100 students…

Now suppose that we measure the strength of 100 ropes…

Reliability Index:


Resistance Factor:

AISC Steel Construction Manual, 13th Edition

  1. Dimensions and Properties
  2. General Design Considerations
  3. Design of Flexural Members
  4. Design of Compression Members
  5. Design of Tension Members
  6. Design of Members Subject to Combined Loading
  7. Design Consideration for Bolts
  8. Design Considerations for Welds
  9. Design of Connecting Elements
  10. Design of Simple Shear Connections
  11. Design of Flexible Moment Connections
  12. Design of Fully Restrained Moment Connections
  13. Design of Bracing Connections and Truss Connections
  14. Design of Bearing Plates, Base Plates, and Column Splices
  15. Design of Hanger Connections, Brackets Plates, etc.
  16. Specifications and Codes
  17. Miscellaneous Data and Mathematical Information Index

Load Combinations:


  1. 1.4D
  2. 1.2D + 1.6L + 0.5(Lr or S or R)
  3. 1.2D + 1.6(Lr or S or R) + (0.5L or 0.8W)
  4. 1.2D + 1.6W + 0.5L + 0.5(Lr or S or R)
  5. 1.2D ± 1.0E + 0.5L + 0.2S
  6. 0.9D ± (1.6W or 1.0E)

ASD (Consistent with ASCE 7)

  1. D
  2. D + L
  3. D + (Lr or S or R)
  4. D + 0.75L + 0.75(Lr or S or R)
  5. D ± (W or E)
  6. D ± 0.75(W or E) + 0.75L + 0.75(Lr or S or R)
  7. 0.6D ± (W or 0.7E)

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