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CTC 422Design of Steel Structures
Introduction
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Steel as a Building Material
• Advantages• High strength / weight ratio• Properties are homogeneous and predictable• High ductility
• Able to undergo large deformations before failure• Speed and ease of fabrication and erection of steel
structures• Erection of steel structures not as affected by weather• Contributes to sustainable design
• Over 90% of steel used in structural shapes is recycled from scrap
• Disadvantages• Susceptible to corrosion• Adversely affected by high temperatures
• Often requires fire-proofing
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Material Properties of Steel
• Yield stress, Fy – value of stress at which there is a significant increase in strain with little or no increase in stress
• Proportional Limit – value of stress at which stress-strain curve becomes non-linear
• Elastic Limit – greatest value of stress at which no permanent deformation occurs upon unloading
• Elastic Range – from origin to proportional limit• Ultimate strength (tensile strength), Fu – the
maximum stress a material is capable of developing
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Modulus of Elasticity
• Modulus of Elasticity• The ratio of the normal stress on an element to the
corresponding strain• Modulus of elasticity = normal stress / normal strain,
or E = σ ε• Can be determined by the slope of the straight line
portion of the stress-strain curve• Modulus of Elasticity of steel, E = 29,000 ksi
• When stress is below the elastic limit, there is a straight-line relationship between stress and strain
• Hooke’s Law applies: σ = E ε
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Ductility
• Ductility – the ability to undergo significant deformation prior to failure (rupture)• Ductile material - >5% elongation before rupture
• Gradual failure
• Brittle material - <5% elongation before rupture• Sudden failure
• Steel is a ductile material• Elongation at failure approximately 15 – 20%• This makes steel a desirable construction
material
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Objectives of Structural Design
• Structure is adequate to support loads which will be applied during its life• Strength provided ≥ strength required
• Structure will meet serviceability requirements• Deflection• Vibration
• Structure will meet functional requirements
• Structure will meet economic requirements
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Allowable Stress Design - ASD
• Yield is considered failure• Actual stress ≤ allowable stress
• f ≤ Fall = Fy / F.S. (or Fu / F.S in some cases)
• Loads used to calculate actual stress are service loads• Service loads – actual maximum loads
expected during the life of the structure
• Factor of Safety, F.S. is > 1 and is dependent on type of stress
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Allowable Stress Design - ASD
• Equations and notation for ASD changed in 13th Ed. Steel Design Manual• Approach is the same
• Allowable strength ≥ Applied service load effect• Rn / Ω ≥ Ra
• Where:• Rn = Nominal strength• Ω = Factor of Safety• Ra = Required allowable strength based on service
loads
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Load and Resistance Factor Design - LRFD
• Design strength ≥ Required strength• ΦRn ≥ Ru
• Where:• Rn = Nominal strength• Φ = Strength reduction factor (≤ 1)• Ru = Required strength based on factored loads
• Loads used to calculate required strength are factored loads• Factored loads – service loads multiplied by
the appropriate load factors, (usually > 1)
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Loads
• Examples of loads to be considered in design• Dead Load, D• Floor Live Load, L
• This is live load due to occupancy• Roof Live Load, L r
• Snow Load, S• Rainwater or Ice Load, R• Wind Load, W• Earthquake Load, E• Other
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Loads & Load Combinations
• Actual loads and combinations of loads to be used in design are determined by the applicable building code
• Examples of ASD Load Combinations – Service loads• D• D + L• D + W, or D – W• D + 0.75W + 0.75L
• Examples of ASD Load Combinations – Factored loads• 1.4D• 1.2D + 1.6L• 1.2D + 1.6W + 0.5L
• In both types of design, service loads are used to calculate deflections (serviceability)
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Steel Design Manual
• 13th edition – Combined ASD and LRFD Design• Member properties and dimensions• Specifications (Steel design Code) and
Commentary• Design Guides, Tables and Charts
• Beams• Columns• Tension Members• Members Subject to Combined Loading• Connections• More
• Design examples on CD