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

Design for Strength

n

a

RR nu RR

ASD LRFD

nii RQ ..

limmax SF

FFF all

Load Combinations - LRFD

• Manual – Part 2 page 2-8 for LRFD

Load Combinations - ASD

• Manual – Part 2 page 2-9 for ASD

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

LRFD - Design Criteria

0ln ,1 0

Q

R

Q

RQR

mean)deviation/ standard(,

ln22

VVV

QR

QR

mm

LRFD - Design Criteria

: Safety Index or reliability index

The larger the safer

LRFD - Design Criteria

RV

n

m eR

R 55.0

For Example

EXAMPLES

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’

Example II