Keirl, Steve. 2019. Ihde’s Praxis Philosophies and Design ...
DESIGN PHILOSOPHIES Design should address: UNDERSTRENGTH OVERLOAD Allowable Stress Design (ASD)...
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Transcript of DESIGN PHILOSOPHIES Design should address: UNDERSTRENGTH OVERLOAD Allowable Stress Design (ASD)...
DESIGN PHILOSOPHIES
Design should address: UNDERSTRENGTHOVERLOAD
• Allowable Stress Design (ASD)• Plastic Design (Collapse Design)• Load and Resistance Factor Design (LRFD)
RequiredStrength
Available Strength(Nominal Resistance)
Allowable Stress Design (ASD) - AISCS 1923
Fmax : maximum stresses due to working loadsFall : allowable stressesF.S. : Factor of Safety > 1 (understrength)Flim : Limit of Usefulness (Fy Fu Fcr etc)
Can be expressed in terms of Strength
..lim
max SF
FFF all
Philosophy: Maximum stress must not exceed allowable stress
Allowable Stress Design (ASD)
• Real Safety Against Failure is Unknown• Conservative• One Load Factor for All Load Types
Comments
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
uw PPFL .).(Failure Load
Reaches failure under factored loads but safe under service loads
Load and Resistance Factor Design (LRFD) - 1986
Makes full use of test information, design experience, engineering judgmentProbabilistic Analysis
Limit State: Describe a condition at which a structure or part of it ceases to perform the intended function
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
Load and Resistance Factor Design (LRFD) - 1986
nii RQ Rn : nominal resistanceQi : applied loads : resistance factor < 1 (understrength) : load factor > 1 (overload)
For Each Limit State LRFD Satisfies
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
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
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
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
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
LRFD – Probabilistic Approach
Structural Safety: Acceptably small probability of Q (demand) exceeding R (resistance)
Load and Resistance Factors assure that probability is negligibly small
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’