CIE 428 Module A
Instructor: Andrew Whittaker
MODULE A: INTRODUCTION This module of CIE 428 covers the following subjects Specifications for design of steel structures Structural steel Grades of steel Steel shapes Properties of structural steel Concepts in structural steel design Basis of load and resistance factors READING:
Chapters 1 and 2 of Segui Chapters 1 and 2 of Bruneau et al. AISC LRFD Manual of Steel Construction, 3rd Ed.
SPECIFICATIONS There are two key specifications for the design of steel structures 1. American Institute of Steel Construction (AISC) Design of steel buildings and connections www.aisc.org 8/21/2002 8:39 AM
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CIE 428 Module A
Instructor: Andrew Whittaker
2. American Association of State Highway and Transportation Officials (AASHTO) Design of steel/reinforced concrete/timber bridges www.aashto.org Other specifications are available from American Iron and Steel Institute (AISI) Cold-formed steel structures www.steel.org American Railway Engineering Association Steel railway bridges STRUCTURAL STEEL History of engineered construction using metals Iron Chief component of steel Wrought iron first used for tools around 4000 BC Produced by heating ore in a charcoal fire Cast and wrought iron used in the late 18C and early 19C in bridges 8/21/2002 8:39 AM
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CIE 428 Module A
Instructor: Andrew Whittaker
Steel An alloy of primarily iron and carbon Fewer impurities and less carbon than cast iron Began to replace iron in construction in the mid 1800s First steel railroad bridge in 1874 First steel framed building in 1884 GRADES OF STEEL Numerous grades of steel are available in the marketplace. The choice is dependent on Application Yield strength Composition See the summaries on the following sheets from the textbook of Gaylord et al. ASTM A36, A53, A242, A572, A709 Tensile properties
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CIE 428 Module A
Instructor: Andrew Whittaker
STEEL SHAPES Hot-rolled shapes are produced from molten steel in a furnace that is poured into a continuous casting where the steel solidifies but does not cool completely. The partially cooled steel is then passed through rollers to achieve the desired shape. Common structural steel shapes are shown below (from Segui).
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CIE 428 Module A
Instructor: Andrew Whittaker
A sample designation of a steel shape is
W18x50
Bar, plate and HSS shapes are shown below. Hollow steel sections (HSS) are fabricated by either bending plate material into the desired shape and seam welding or hot-working to produce a seamless shape
PROPERTIES OF STRUCTURAL STEEL Stress-strain relationship
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CIE 428 Module A
Instructor: Andrew Whittaker
The stress-strain relationship is the best-known characterization of steel. See the figure below from Segui.
Stress is denoted as f or σ , and is calculated as f = Engineering stress Strain is denoted ε and is calculated as ε =
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∆L L
P A
CIE 428 Module A
Instructor: Andrew Whittaker
Engineering strain
The figure above shows 4 ranges of response Elastic Plastic (yield plateau) Strain hardening Necking and failure (strain softening) Many steels are ductile. Ductility is a measure of post-yield elongation, where elongation is calculated as e=
( L f − L0 ) L0
where Lf = L0 =
An idealized stress-strain relationship is shown in the figure on the following page from Segui. The important descriptors are Fy : the yield point Fu : the ultimate tensile strength 8/21/2002 8:39 AM
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CIE 428 Module A
Instructor: Andrew Whittaker
E: Modulus of elasticity or Young’s modulus (29,000 ksi)
For high-strength steels, the stress-strain relationships are often similar to that shown below (from Segui).
Note from the above figure that Elastic range No well-defined yield point
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CIE 428 Module A
Instructor: Andrew Whittaker
Ultimate tensile strength Because steel design makes use of yield strength and a tensile strength, a definition of yield strength is needed for these steels 0.2% offset (residual strain) method used Chemical composition The chemical composition of a steel determines its mechanical properties of Strength Ductility Hardness (resistance to plastic deformation) Closely related to ultimate strength Toughness The principal components of steel, an alloy, are iron (large %) and carbon (smaller %). Carbon contributes to strength but not ductility. Other components include Manganese (Mn), Silicon (Si), Chromium (Cr), Molybdenum (Mo), Vanadium (V), Nickel (Ni), and Copper (Cu). The concept of carbon equivalent (CE) was introduced to convert into equivalent carbon content the effect of other elements known to increase the hardness of steel. The AWS definition of CE is: CE=C+
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(Mn+Si) (Cr+Mo+V) (Ni+Cu) + + 6 5 15
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CIE 428 Module A
Instructor: Andrew Whittaker
where C is the % carbon, etc. If strength increases, hardness increases, decreases, and weldability decreases.
ductility
If CE is high, say 0.4 to 0.5, then the potential for cracking in the HAZs of welded connections is increased. Limits on CE not found in ASTM standards but other limits are used to control maximum % of elements, etc. Structural steels are often grouped by broad composition, namely, Plain carbon steels Mostly iron and carbon, less than 1% C Low-alloy steels Iron, carbon and other components (
