Evolution of desing codes for steel structures

EVOLUTION OF DESIGN CODES FOR STEEL STRUCTURES Ted Galambos Emeritus Professor of Structural Engineering University of Minnesota, USA Department of Civil Engineering

The speaker

Galambos

My credentials • Member on Committee on Specification for AISC about 40 years • Former member of AISI (CF steel) and Aluminum Ass. Spec. committee.

• Former Member of CSA S16.1 committee • Accused of being father of LRFD in US • Translated DIN 4114 into English in 1956

Purposes of the presentation • Observations about the status of Structural Engineering in the year 2009 • Reminiscence of a 50 year career in steel design standards developlent • Contemplations on the future of steel design standards

GENERAL OBSERVATIONS • We are experiencing the positive gradient in the history of our profession • The computer has liberated us from tedious work of drafting and calculation • but • The computer permits us to be intellectually lazy and thoughtless

WE CAN DESIGN ANYTHING! • Complicated structures can be analyzed and designed

WE CAN DESIGN ANYTHING! • Creativity can make structures act like a bird (Milwaukee Art Museum) • CALATRAVA CAN DO IT!

CHALLENGES FOR STRUCTURAL ENGINEERS • Rehabilitation for new use • Evaluate and repair damaged structures • Deconstruction of large structures • Design for catastrophes: earthquake, windstorm, ice storm, water surge, fire, blast, etc.

• Life-cycle design: build, renovate, demolish

CHALLENGES FOR STRUCTURAL ENGINEERS • • • •

Design for rapid construction “Green” structures “Sustainability” Structures with control mechanisms: active, passive • Monitoring behavior of structures • Creative use of new materials • Coastal structural engineering

How to develop standards to meet these challenges?

• Limit states on strength need to be defined for design engineers • Building codes need a basis for safe design • Building officials need guidance for checking proposed structures • Economists need means for optimizing costs • Fabricators need guidance for details of connections and for erection

EVOLUTION OF SPECIFICATIONS • Experience and judgment of individual builders • “In-house” standards • Codes of professional and industrial associations • Formalized legal codes

HISTORY OF AISC SPEC. • 1st edition 1923 experience of past successful design, Allowable Stress Design (ASD) • Major revisions significant inputs from research • 1963: Limit States Design (LSD) disguised as ASD • 1986: LSD (Load and Resistance Factor Design, LRFD) • 2005: LSD served up as LRFD or ASD

CURRENT STANDARDS MAINTENANCE PROCESS • Example: American Institute of Steel Construction • Committee on specification originates changes and maintains content • Strict rules on consensus • Strict distribution of membership: • 1/ 3 producers, 1/ 3 users, 1/ 3 researchers

CURRENT STANDARDS MAINTENANCE PROCESS • • • •

Committee on specification prepares draft Public review of draft RESEARCH Resolution of negatives Submission to and approval by American National Standards Association (ANSI) • Adoption by “model codes” (IBC) • Adoption as legal building code by governments

Year of Design Code adoption Criteria pages

Commentary Committee Researcher pages Members Members

1923

ASD

11

0

5

0

1936

“

19

0

*

*

1949

“

30

0

*

*

1963

LSD/ASD 44

46

26

5

1969

“

103

44

36

6

1978

“

93

68

43

9

1989

“

83

68

43

14

1986

LSD

91

66

42

14

1993

“

110

92

46

13

1999

“

124

113

46

14

2005

“

196

231

40

12

AISC Specification

2005 1923

AISC Handbook2005

THE ENGINEER

TOO MANY RULES ! … --- …

LEGAL BUILDING CODES MODEL BUILDING CODES (IBC) INDUSTRY DESIGN STANDARDS APPROVED BY ANSI (AISC, ACI, AA, AISI etc.)

MATERIAL CODES (ASTM)

LOAD STANDARDS (ASCE 7)

MANUFACTURING STANDARDS (BOLTS, WELDING)

WHY WE SHOULD COMPARE DESIGN CODES? • Globalization of design and construction • Safety of designs when differing codes interact • To show that despite differing appearance there is a common background

What is in common in modern steel design standards? • Common theory and common research • Example: steel column design

THEORY

RESEARCH

APPLICATION

2000

BATTERMAN JOHNSTON TALL, BEER, SCHULTZ BJORHOVDE

NAGOYA DATA BANK EUROPEAN TESTS LEHIGH TESTS

1950

CHWALLA, JEZEK COLUMN THEORY WESTERGAARD, OSGOOD RESIDUAL STRESS

1900

von KARMAN INELASTIC ANALYSIS

RANKINE (1861)

ROS, BRUNNER COLUMN TESTS

INITIALLY OUT-OF STRAIGHT COLUMNS

MODERN DESIGN SPECIFICATIONS EUROPEAN COLUMNCURVES SSRC COLUMN CURVES DIN 4114

COLUMN FORMULAS  Fcr = 0.658 Fy if λ ≤ 1.5    AISC   0.877 Fy if λ >1.5   Fcr = 2 λ   λ2

 CSA  Fcr = Fy 1 + λ 

(

−1 2n n

)

 Fy  RCDF  2n 2n 1 + − 0.

1 5 λ 

(

 n = 1.34  

)

1/ n

  n=1.4  

COMPARISON OF COLUMN FORMULAS 1.0

φ Fcr / Fy

0.8 0.6 0.4 0.2 0.0 0.0

0.5

1.0

1.5

2.0

(L / r)(1/π)(Fy / E)1/2

2.5

3.0

What is in common?

• Common theory and common research • Limit states: common basis for design • Load and resistance factor design (LRFD), Limit States Design (LFD), Design with Partial Safety Factors

BASIC CODE FORMAT

k

φi Rni ≥ ∑ γ j Q j j =1

FOR i LIMIT STATES FOR j LOAD COMBINATIONS

•Load combination rule

γ D QD + γ m Qm +

k

∑

j = 1; j ≠ m

γ jQ j

DEAD LOAD MAXIMUM POINT-IN-TIME LOAD ARBITRARY POINT-IN-TIME LOADS

What is in common?

• Common theory and common research • Limit states: common basis for deign • Load and resistance factor design (LRFD), Limit States Design (LFD), Design with Partial Safety Factors • Factors are based on probabilistic methods: Level 2 methods used for developing Level 1 equations

LOAD EFFECT

UNSAFE

R=Q

R<Q R-Q < 0 SAFE

R≥Q R-Q ≥ 0 RESISTANCE

LOAD EFFECT Q PROBABILITY DENSITY

0.004

RESISTANCE R

0.003

0.002

0.001

0

0

500

1000

1500

VALUE OF Q or R

2000

β σ R +σQ PROBABILITY DENSITY

2

2

0.0030

mean

0.0025 0.0020

Probability 0f 0.0015 failure 0.0010 0.0005 0.0000 -400

0

400

R-Q

800

1200

1600

PROBABILITY DENSITY

β=2.5

0.0030

β=3.9

0.0025 0.0020

β=4.4

0.0015 0.0010 0.0005 0.0000 -400

0

400

R-Q

800

1200

1600

What is in common?

• Common theory and common research • Limit states: common basis for deign • Load and resistance factor design (LRFD), Limit States Design (LFD), Design with Partial Safety Factors • Factors are based on probabilistic methods: Level 2 methods used for developing Level 1 equations • Calibration to time-tested methods

SELECT BRIDGES (WSD, LFD)

RELIABILITY ANALYSIS: β

RELIABILITY ANALYSIS FOR φ FOR ELEMENT & MATERIAL

TARGET RELIABILITY βT

DESIGN EQUATION FORMAT

φ Rn ≥ ∑ γ i Qi

OPTIMIZE γi FOR ALL TYPES OF LOADS AND COMBINATIONS FOR ALL TYPES OF BRIDGES

FRAME DESIGN METHODS COMPARED • Common features: • Elastic 2nd-order analysis is required • Limit states: first plastic hinge to form, member or element buckling • Direct 2nd-oder analysis is preferred • First-order analysis with force amplification factors is permitted

Pδ δ AND P∆ ∆ IN UNBRACED FRAMES, Pδ δ and P∆ ∆ CAN OCCUR

2H

HL

P∆ ∆ Pδ δ

L H P

M diagram

LOAD

SECOND-ORDER ELASTIC ANALYSIS

1.2Dn+1.6Ln

FIRST ORDER ANALYSIS

SECOND-ORDER ANALYSIS

Mrequired MOMENT

INTERACTION EQUATIONS Pu 0.85 M u Mu + ≤ 1.0; ≤ 1.0 φ Pn φ Mn φ M n Canada/RCDF/SA

Pu Mu + ≤ 1.0 φ Pn φ M n

Eurocode & Australia

Pu 8 M u Pu + ≤ 1.0; if ≥ 0.2 φ Pn 9 φ M n φ Pn Pu Mu Pu + ≤ 1.0; if ≤ 0.2 2φ Pn φ M n φ Pn

United States

INTERACTION EQUATIONS COMPARED 1.0 CANADA / SA / RCDF Eurocode / Australia USA - AISC

Pu / φ Pn

0.8 0.6 0.4 0.2 0.0 0.0

0.2

0.4

0.6

Mu / φ Mn

0.8

1.0

AUSTRALIA AS4100 (1999?)

• Second-order analysis required for actual design (factored) loads.

• Column strength Pn is determined with effective length factor K>1.0 for sway frames • Alternate choice: “Advanced analysis”

CANADA / RCDF (?)/SA • Second-order analysis is required with design (factored) loads plus a “notional” lateral load at each story level of 0.005ΣPgravity.

• Column strength Pn is determined with effective length factor K=1.0 for sway frames

Eurocode 3 CHOICE #1 • Second-order analysis is required with design (factored) loads plus a “notional” lateral load at each story level of 0.005ΣPgravity.

• Column strength Pn is determined with effective length factor K=1.0 for sway frames

Eurocode 3 (2004) CHOICE #1 • Can also make analysis of “imperfect” structure with 0.005 x height as “initial out-of-plumb”.

• Reductions permitted for allowances due to height of structure and number of columns in a story.

Eurocode 3 (2004) CHOICE #2 â&#x20AC;˘ Second-order analysis required for actual design (factored) loads. â&#x20AC;˘ Column strength Pn is determined with effective length factor K>1.0 for sway frames

American (AISC/05) Choice #1 • Second-order analysis required for actual design (factored) loads.

• Column strength Pn is determined with effective length factor K>1.0 for sway frames • Minimum lateral load of 0.002ΣPgravity.

• Under some conditions may use K=1.0

American (AISC/05) Choice #2 • “Direct 2nd-order analysis with notional lateral load of 0.002ΣPgravity. required for all factored load conditions.

• Use reduced stiffness of 0.8EI and 0.8EA

• Column strength Pn is determined with effective length factor K=1.0 for sway frames

CRITIQUE OF MODERN STRUCTURAL STEEL DESIGN CODES • Too many features to “serve” special products: composite design, different bolt tightening methods, prefab metal buildings etc.

• Too many codes: CF steel, Stainless Steel, Aluminum, composite design, steel joists • Too many choices for methods of frame design

CRITIQUE • Emphasis on strength limit states, even though most designs are controlled by serviceability • Code committees are dominated by “high-end” design professionals and professors • Many legal constraints, including the language: “…shall be permitted…” for “…may…”

CRITIQUE • Disincentive for sponsoring research to improve criteria satisfactory to the sponsors • Research is often crisis driven • Codes are driven by seismic considerations • Everybody is happy with LSD format, so there is delay in implementing probability concepts • Tendency to diverge by industry, nation: TRADITION!

PERFORMANCE BASED DESIGN PBD • A new Buzzword?

• Has been around a long time • All modern steel design standards have either implicit or explicit clauses permitting PBD. • AISC/05 has explicit criteria for fire resistant design • Seismic design recommendations by FEMA contain PBD

AISC DEFINITION OF PERFORMANCE-BASED DESIGN • “An engineering approach to structural design that is based on agreed-upon performance goals and objectives, engineering analysis and quantitative assessment of alternatives against those design goals and objectives using accepted engineering tools, methodologies and performance criteria.”

AISC DEFINITION OF PRESCRIPTIVE DESIGN

• “A design method that documents compliance with general criteria established in a building code.”

AISC PERFORMANCE OBJECTIVE • “Structural components, members and building frame systems shall be designed so as to maintain their load-bearing function during the design-basis fire and to satisfy other performance requirements specified for the building occupancy.”

PERFORMANCE-BASED DESIGN IN EARTHQUAKE ENGINEERING • EXAMPLE: • “Recommended Seismic Design Criteria For New Steel MomentFrame Buildings.”

• FEMA 350, July 2000

Building performance level

Frequent 50% in 50yr

Ground motion levels

Operational

MCE 2% in 50yr

M

Immediate occupancy

Life safe

Near collapse

POST_EARTHQUAKE STRUCTURAL PERFORMANCE LEVEL DEFINITIONS

• Collapse Prevention: • Structure is on the verge of partial or total collapse. Must carry gravity load demands.

• Immediate Occupancy: • Only limited structural damage has occurred.

FEMA-350 PERFORMANCE DEFINITION • Example: • A design shall provide a 95% level of confidence that the structure will provide Collapse Prevention or better performance for earthquake hazards with a 2% probability of exceedance in 50 years.

• Methods are given for a “simple” or “detailed” evaluation of the level of confidence.

BRAVE NEW WORLD?

• Code committees define the performance requirements and all computations reside in “software”?

CODE AUTHORITY DEFINES GENERAL REQUIREMENTS OWNER, ARCHITECT, ENGINEER FABRICATOR, BUILDER DEFINE PROJECT-SPECIFIC REQUIREMENTS

HAVE CRITERIA BEEN MET ?

PERFORMANCE REQUIREMENTS FOR STEEL STRUCTURES • SERVICEABILITY: drift, deflection, slip, vibration • Economics, utility, user confidence and comfort TOLERANCE LIMITS

?

>

PERFORMANCE

WHAT IS ACCEPTABLE RISK?

CAN DO NOW

STRENGTH REQUIREMENTS • Safety is pre-eminent concern • Limit states: now defined in design standards • Can they be lumped into a small number of performance criteria?

• Overall system buckling, Complete collapse mechanism, Local mechanism, Etc.

SERVICEABILITY FOR ARBITRARY- POINT- IN -TIME DEMANDS THE USUAL

HOW ABOUT SOMETHING IN-BETWEEN?

STRENGTH MAXIMUM LIFETIME DEMANDS THE WORST

INTERMEDIATE STAGE • Most modern codes are really in this stage • Limit states: first hinge formation, member instability, etc.

• Tolerable local damage to provide safety and economic repair

WHAT TO DO NEXT? • Formulate an acceptable framework for PBD • Engineering methods • Systems behavior • Probability • Economics • Research results, old and new • Experience and good judgment

CHALLENGES FOR DESIGN STANDARDS • How to deal with Performance-Based Design?

• How can building authorities validate designs without formulas (FEM)?

• How to develop codes for repair, rehabilitation, re-use, new types of structures, new materials?

• The answer: Continue to keep a healthy research infrastructure.

RESEARCH OPPORTUNITIES • Laboratories are better than ever • Field testing to monitor and to assess strength • Testing from another site via communications network • Provide each structural engineer to engage sometimes in research as part of professional experience

Multi-axial structural testing laboratory at University of Minnesota

THANK YOU VERY MUCH FOR YOUR KIND ATTENTION. QUESTIONS OR DISCUSSION??