1. Design should address: UNDERSTRENGTH OVERLOAD
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 (Fy Fu Fcr 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 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
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 Factors f (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