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DESIGN PHILOSOPHIES Design should address: UNDERSTRENGTH OVERLOAD Allowable Stress Design (ASD) Plastic Design (Collapse Design) Load and Resistance Factor.

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Presentation on theme: "DESIGN PHILOSOPHIES Design should address: UNDERSTRENGTH OVERLOAD Allowable Stress Design (ASD) Plastic Design (Collapse Design) Load and Resistance Factor."— Presentation transcript:

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 F max : maximum stresses due to working loads F all : allowable stresses F.S.: Factor of Safety > 1 (understrength) F lim : Limit of Usefulness (F y F u F cr etc) Can be expressed in terms of Strength Philosophy:Maximum stress must not exceed allowable stress

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

4 Plastic Design (PD) - AISCS 1963 Takes Advantage of Ductility and Ultimate Strength Philosophy:Limit of Structural Usefulness is load P u 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) 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) 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) R n : nominal resistance Q i : applied loads  : resistance factor < 1 (understrength)  : load factor > 1 (overload) For Each Limit State LRFD Satisfies

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 Requirements Specifications - 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 ASDLRFD

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 Factors  (Manual pp. 2-9)  =0.9 for limit states involving yielding  =0.75 for limit states involving rupture LRFD  =1.67 for limit states involving yielding  =2.0 for limit states involving rupture ASD

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  : 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

23


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