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DESIGN PHILOSOPHIES

DESIGN PHILOSOPHIES. Design should address: UNDERSTRENGTH OVERLOAD. Allowable Stress Design (ASD) Plastic Design (Collapse Design) Load and Resistance Factor Design (LRFD). Required Strength. Available Strength (Nominal Resistance). Allowable Stress Design (ASD) - AISCS 1923.

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DESIGN PHILOSOPHIES

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  1. DESIGN PHILOSOPHIES Design should address: UNDERSTRENGTH OVERLOAD • Allowable Stress Design (ASD) • Plastic Design (Collapse Design) • Load and Resistance Factor Design (LRFD) Required Strength Available Strength (Nominal Resistance)

  2. Allowable Stress Design (ASD) - AISCS 1923 Philosophy: Maximum stress must not exceed allowable stress Fmax : maximum stresses due to working loads Fall : allowable stresses F.S. : Factor of Safety > 1 (understrength) Flim: Limit of Usefulness (FyFuFcr etc) Can be expressed in terms of Strength

  3. Allowable Stress Design (ASD) Comments • Real Safety Against Failure is Unknown • Conservative • One Load Factor for All Load Types

  4. Plastic Design (PD) - AISCS 1963 Takes Advantage of Ductility and Ultimate Strength Philosophy:Limit of Structural Usefulness is load Pu that causes a plastic mechanism to form Failure Load Reaches failure under factored loads but safe under service loads

  5. Load and Resistance Factor Design (LRFD) - 1986 Makes full use of test information, design experience, engineering judgment Probabilistic Analysis Limit State: Describe a condition at which a structure or part of it ceases to perform the intended function

  6. Load and Resistance Factor Design (LRFD) - 1986 • Typical Strength Limit States • Plastic Strengths • Buckling • Fracture • Fatigue • Overturning • Typical Serviceability Limit States (under normal service loads) • Excessive Deflections • Slipping • Vibrations • Cracking • Deterioration

  7. Load and Resistance Factor Design (LRFD) - 1986 For Each Limit State LRFD Satisfies Rn : nominal resistance Qi : applied loads f : resistance factor < 1 (understrength) g : load factor > 1 (overload)

  8. AISC Manual • AISC Specification • Design Aids • Catalog of most widely available structural shapes • Editions 1-9 ->ASD • Editions 10, 11, 12 -> LRFD Edition 1, 2, 3 • Current Edition 13 incorporates ASD and LRFD

  9. AISC Manual • Part 1 – Dimensions and Properties • Part 2 – General Design Considerations • Part 3 – Design of Flexural members • Part 4 – Design of Compression Members • Part 5 – Design of Tension Members • Part 6 – Design of Members Subject to Combined Loading • Parts 7 to 15 – Connections • Part 16 Specifications and Codes • Part 17 Misc. Data and Mathematical Information

  10. AISC Specifications (PART 16 of Manual) • Main Body • Alphabetically organized to chapters A-M • Major Headings labeled with chapter designation followed by number • Further subdivisions are numerically labeled • Appendices • Appendices 1-7 • Commentary • Background and elaboration on provisions of the Specification. • Organized in the same way as the specification

  11. Design RequirementsSpecifications - Chapter B • B1 General Provisions • B2 Loads and Load Combinations (LRFD or ASD) • Applicable building codes • In the absence use SEI/ASCE 7 • B3 Design Basis • B3.1 Required Strength: Required strength determined by structural analysis based on load combinations • B3.2 Limit State: No applicable strength or serviceability limit state shall be exceeded when structure subjected to all load combinations • B3.3 Design for Strength Using LRFD • B3.4 Design for Strength Using ASD • B3.5 Design for Stability • etc

  12. Design for Strength ASD LRFD

  13. Load Combinations - LRFD • Manual – Part 2 page 2-8 for LRFD

  14. Load Combinations - ASD • Manual – Part 2 page 2-9 for ASD

  15. Resistance Factorsf(Manual pp. 2-9) LRFD f=0.9 for limit states involving yielding f=0.75 for limit states involving rupture ASD W=1.67 for limit states involving yielding W=2.0 for limit states involving rupture

  16. LRFD – Probabilistic Approach Structural Safety: Acceptably small probability of Q (demand) exceeding R (resistance) Load and Resistance Factors assure that probability is negligibly small

  17. LRFD - Design Criteria

  18. LRFD - Design Criteria b: Safety Index or reliability index The larger the safer

  19. LRFD - Design Criteria For Example

  20. EXAMPLES

  21. Example I A floor system has W24x55 sections spaced 8 ft on center supporting a dead load 50 psf and live load 80 psf. The W sections are simply supported. Determine nominal moment capacity 4’ 8’ 4’

  22. Example II

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