CTC 422 Design of Steel Structures Introduction

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

Material Properties of Steel Yield stress, F y – 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), F u – the maximum stress a material is capable of developing

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

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

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

Allowable Stress Design - ASD Yield is considered failure Actual stress ≤ allowable stress f ≤ F all = F y / F.S. (or F u / 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

Allowable Stress Design - ASD Equations and notation for ASD changed in 13 th Ed. Steel Design Manual Approach is the same Allowable strength ≥ Applied service load effect R n / Ω ≥ R a Where: R n = Nominal strength Ω = Factor of Safety R a = Required allowable strength based on service loads

Load and Resistance Factor Design - LRFD Design strength ≥ Required strength ΦR n ≥ R u Where: R n = Nominal strength Φ = Strength reduction factor (≤ 1) R u = 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)

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

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 W L 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)

Steel Design Manual 13 th 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