HISTORY OF STRUCTURAL STEEL DESIGN CODES 1923-2011 REMINISCENSE OF A LIFE IN STEEL RESEARCH TED GALAMBOS MINNEAPOLIS, USA
STEEL DESIGN EVOLUTION IN MY LIFETIME
Cartoon From the New Yorker Magazine
MOTIVATIONS FOR CHANGE • • • • • • • • •
INDUSTRY Composite design Stud shear connectors High strength bolts Welding Welded connections Bolted connections Many steel grades High-Performance steels • etc
• • • • • • • • • • •
ACADEMIA Plastic design Stability criteria Computer methods Direct design Bracing rules Plate girder design Ponding Fatigue Fracture etc
TWO PATHS • INDUSTRY
• THEORY
• RESEARCH
• RESEARCH
• DESIGN STANDARDS
• DESIGN STANDARDS
• PRACTICE
• PRACTICE
EXAMPLE OF INDUSTRY INITIATED PRACTICE • • • • •
COMPOSITE BEAMS FIRST STRUCTURES ~1935 RESEARCH AT ILLINOIS AND LEHIGH 1950s AASHO ROAD TEST BRIDGES 1959-61 BOOK BY VIEST, FOUNTAIN, SINGLETON, 1958 • First in AASHO Standard 1957 • First in AISC Specification 1963 • Continuous research up to this date and beyond
EXAMPLE OF RESEARCH INITIATED PRACTICE • PLASTIC DESIGN AND INELASTIC INSTABILITY • Realization around 100 years ago: there is life beyond the elastic limit. • Column experiments by Engesser (~1890s) • First tests of plastic mechanism (1914, Budapest) • Tests, theory and practice (John Baker, 1930s and 1940s)
PLASTIC DESIGN AT LEHIGH 1950s-1970s • Plastic design leads to a better understanding of behavior under load, and it gives economy • Inelastic stability is much more difficult than elastic stability: research had to proceed on both fronts. • Theory alone was not enough: testing had to be integral part of the development • Research was not enough: results must be directed to usefulness in practice.
PLASTIC DESIGN RESEARCH RESULTS • • • • • • • • • •
Continuous beam and frame design Column formulas Interaction equations for beam-columns Connection design rules (welded and bolted joints) Multi-story design methods Post-buckling strength of plate girders is recognized Many applications in seismic design Applications in bridge design SEISMIC DESIGN Etc.
FRITZ ENGINEERING LABORATORY
PROGRESS!?! • We live in a world of rules, and so, inevitably, design standards must be formulated! • There are many standards, but we will concentrate on only one: • “Specification for structural steel buildings” • By the American Institute of Steel Construction = AISC
LENGTH OF AISC SPECIFICATION
Page Numbers
200
150
100
50
0 1920
1940
1960
1980
Year
2000
2020
THE BEGINNING 1923 ALLOWABLE STRESS DESIGN LENGTH OF AISC SPECIFICATION
Page Numbers
200
150
9 pages, 3 formulas
100
50
0 1920
1940
1960
1980
Year
2000
2020
LIMIT STATES DESIGN, 1963 LENGTH OF AISC SPECIFICATION
Page Numbers
200
150
93 pages
100
50
0 1920
1940
1960
1980
Year
2000
2020
LOAD AND RESISTANCE FACTOR DESIGN, 1986 LENGTH OF AISC SPECIFICATION 200
Page Numbers
91 pages 150
100
50
0 1920
1940
1960
1980
Year
2000
2020
COMBINED SPECIFICATION,2005 LENGTH OF AISC SPECIFICATION 200
Page Numbers
196 pages 150
100
FOR SURE: 2010 WILL BE
50
BIGGER
0 1920
1940
1960
1980
Year
2000
2020
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.” (2005)
AISC 1923! • To obtain a satisfactory structure, the following major requirements must be fulfilled: • (a) The material used must be suitable, of uniform quality, and without defects affecting the strength and service of the structure. • (b) Proper loads and and conditions must be assumed in the design. (c) and (d)…. • (e)The computations and design must be properly made …and the structure and its details shall possess the requisite strength and rigidity.
2010 1923
PERFORMANCE-BASED DESIGN ?
STABILITY DESIGN CHANGES THROUGH THE YEARS: WHY • Riveted plate girders Welded thinwebbed girders • Carbon steel High strength steels • Slide-rule Computer • Simple frames with heavy cladding Complicated architecture with light cladding • Heavy members Thin-walled structures
INCREASE OF YIELD POINT 1928 : Fy = 30ksi ; Fa ≤ 15ksi ; Fb ≤ 18ksi 1936 : Fy = 33ksi ; Fa ≤ 17 ksi ; Fb ≤ 20ksi 1963 : Fy = Many; Fa ≤ 0.6Fy ; Fb ≤ 0.66Fy
1 ksi = 6.895 MPa
STRENGTH LIMIT STATES INSTABILITY LIMITS
FRACTURE
BUCKLING
YIELDING
STABILITY LIMITS IN AISC • • • • • •
ELEMENT STABILITY: LOCAL BUCKLING Width- thickness limits Compact Non-compact Slender
GENERAL AISC LOCAL BUCKLING RULES FOR AXIAL COMPRESSION F cr
Fy 1.0
0.65
0
0.7
1.0
1.24
b Fy t E
12 ( 1 − ν 2 )
π2
×
1 k
FLANGE LOCAL BUCKLING Fcr Fy
k = 0.708 bf 1.415 − 0.74 2t f
1.0
Fy E 0.69
0.65 COMPACT NO
N-
b f 2t f
T C PA M CO
0.56
1.03
=
(
12 1 −ν 2
)
b Fy f 2t f
k = 0.763 SLENDER
0
Fy E
2
π 2 Ek
bf
Fy
2t f
E
2
LOCAL BUCKLING OF BEAMS Mn Mp
CLASS 1 & 2
1.0
CLASS 3
Mp
0.891
0.625
AISC S16.1 CLASS 4
My
0.7My
0.5 0.439 0.587
0 0
0.38
0.5
1.0
bf
Fy
2t f
E
STABILITY LIMITS IN AISC • MEMBER STABILITY • Flexural buckling • Lateral-Torsional Buckling
COLUMN CURVES IN AISCS, 1923-1989, Fy=33ksi
Fa/Fy
0.4
0.2
0
0
50
100
L/r 1928 1949 1963
150
200
LATERAL-TORSIONAL BUCKLING IN AISCS, W21X50, Fy=33ksi
Fb/Fy
0.6
0.4
0.2
0
0
50
100
Lb/ry 1923 1949 1963
150
200
STABILITY CONCERNS IN AISC • FRAME STABILITY • Effective length
STABILITY CONCERNS IN AISC • Interaction equations • Beam-Column Design
DESIGN OF PLANAR SWAY RIGID FRAMES • 1923-1936 ALLOWABLE STRESS DESIGN • Analysis: “The computations must be… properly made…” • Elastic first-order analysis, “exact” or “approximate” • Ketchum,”Structural Emgineers’ Handbook”, 1924
f axial + f bending ≤ Fallowable
DESIGN OF PLANAR SWAY RIGID FRAMES • 1936 – 1963, ASD • Interaction equation first introduced in 1936
fa fb + ≤ 1.0 Fa Fb
DESIGN OF PLANAR SWAY RIGID FRAMES
• 1963 LIMIT STATES DESIGN (disguised as ASD!) • Effective length concept introduced and required, K>=1.0!!!!!! • New Interaction Equations, recognizing second-order effects!!!!! • Moment gradient effect • The professors finally got their message across!!!!
P∆ ∆ and Pδδ RL
P∆ ∆
2H
Pδ δ R P
1963 Interaction Equations fa fa fb ≤ 0.15 : + ≤ 1.0 Fa Fa Fb
Pδ δ
fa fa Cm fb > 0.15 : + ≤ 1.0 Fa Fa fa 1 − ′ Fb Fe P∆ ∆ KL
1969-1989 CORRECTION TO 1963 INTERACTION EQUATIONS fa fa fb ≤ 0.15 : + ≤ 1.0 Fa Fa Fb fa fa Cm fb > 0.15 : + ≤ 1.0 Fa Fa fa 1 − ′ Fb Fe fa fb + ≤ 1.0 0.6Fy Fb
0.6Fy Fa
0
fa f + b ≤ 1.0 0.6Fy Fb fa Cm fb + ≤ 1.0 Fa fa 1 − ′ Fb Fe
Fb
TIME FOR A CHANGE • 1960 Researchers developed the groundwork for a probability-based code system of designing structures (in Mexico: Emilio Rosenblueth) • 1969 work started on implementing these ideas into a practical steel code for AISC. • First Load and Resistance Factor Design (LRFD) Specification adopted by AISC in 1986.
1988 LOAD AND RESISTANCE FACTOR DESIGN: LRFD • Second-order effects shall be considered in design. • Effective length factor shall be determined by structural analysis • New column formulas • New interaction equations • Analysis by elastic second-order program • Or analysis by elastic first-order program, modified by B1 and B2 scheme.
ADDITIONS TO LRFD • 1993: Effect of “leaner columns” needs to be considered in calculating KL • 1993: Stiffness reduction due to column instability is permitted • 1999: New chapter added on stability bracing
φPy φPn
Pr 8 M r + = 1.0 φ Pn 9 M n Pr Mr + = 1.0 2φ Pn M n
MEMBER STRENGTH
φMn φMp
B1 and B2 explained
Pδ
P∆
M r = B1 M nosway + B2 M sway Pr = Pnosway + B2 Psway
B1 for Pδδ of individual column
Cm B1 = ≥ 1.0 Pro 1− Pe 1 Pe 1 =
π EI 2
2
L
; Pro = Pnt + Plt
B2 for P∆ ∆ of each story level
B2 =
1 P ∑ 1− ∑P
nt
e2
ΣPnt = Total vertical load supported by the story, including gravity column loads For beams use larger of B2 for upper and lower story
∑P
e2
=∑
π EI 2
( K 2 L)
2
OR
∑P
e2
HL ∑ = 0.85
∆H
“COMBINED” SPECIFICATION, 2005, 2010 • Major additions to the LRFD method of frame design DIRECT DESIGN • Second-order analysis is required • Two methods of second-order analysis are permitted • Three methods of achieving a satisfactory design are presented. • The 2010 code is an elaboration of the 2005 code, with improvements.
METHODS OF ANALYSIS • Second-Order Elastic Analysis, P∆ and Pδ • First-Order Elastic Analysis, with B1 and B2 modifiers.
REQUIRED STRENGTH DETERMINATION • • • •
Direct Analysis Method of Design, DAM Effective Length Method, ELM First-Order Analysis Method, FOAM These are poor names because the use of the word analysis applies to both the computation of the internal forces and the determination of the actions needed to make the analysis.
DIRECT METHOD • Always permitted • Must include initial imperfections and stiffness reductions • Initial imperfection initial out-ofplumb • a) Direct modeling of out-of plumb • b) Notional lateral load, N=0.002 ΣP • Stiffness adjustments; use 0.8EI and 0.8AE in analysis. • BENEFIT: Effective length k=1.0
ALTERNATIVE METHODS • EFFECTIVE LENGTH METHOD • K>1.0; no reduction of EI and AE in the analysis • Restrictions: • Use notional loads, N=0.002 ΣP • Must determine effective length factors • Vertical columns
EFFECTIVE LENGTH METHOD ∆2nd order ∆ 1st order
≤ 1.5 max
K=1.0 is permitted if
∆2nd order ∆ 1st order
≤ 1.1 max
BENEFITS?
FIRST-ORDER ANALYSIS METHOD • K=1 • Permitted if:
P ≤ 0.5 Py ∆ N = 2.1 Σ P ≥ 0.0042 Σ P h ∆2nd order ≤ 1.5 ∆ 1st order max
THANK YOU!