CTC 422Design 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, 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

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 ≤ 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

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

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)

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 + 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)

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