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Summary

Introduction to Steel Design Motivation Eurocodes Introduction to EN 1990 Basis of design Limit state design Combination of actions 1 Motivation Iconic steel structure Very large span Curved shape Cloud Forest Sky Park Flower Dome 2 Motivation Complex connection 4 Motivation Light structure Movable ...

Description

Introduction to Steel Design √ Motivation √ Eurocodes √ Introduction to EN 1990 • Basis of design • Limit state design • Combination of actions

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• • •

Motivation

Iconic steel structure Very large span Curved shape

Cloud Forest

Sky Park

Flower Dome 2

Motivation •

Complex cross-section

The key structural components of the Flower Dome conservatory structure

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•

Motivation

Complex connection

4

Motivation • •

Light structure Movable structure

National stadium

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6

Motivation • •

Fast Fabrication Light Foundation

www.tensilehangars‐steelstructures.com

http://www.steelstructures.co.za/

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Advantages of Steel as a Structural Material Y The many advantages of steel can be summarized as follows: • High Strength • Uniformity • Elasticity • Ductility • Toughness • Weldability

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Disadvantages of Steel as a Structural Material Y Steel also has many disadvantages that make reinforced concrete as a replacement for construction purposes. Y The disadvantages of steel can be summarized as follows: • Fireproofing Cost • Brittle Fracture • Fatigue • Susceptibility to Buckling • For most structures, the use of steel columns is very economical because of their high strength-to-weight ratios. However, as the length and slenderness of a compressive column is increased, its danger of buckling increases.

How to design to achieve balance between safety and economy? 9

Disadvantages of Steel as a Structural Material

Lateral-Torsional buckling of a beam

Local buckling of a column 10

Disadvantages of Steel as a Structural Material

Concrete beam section

Steel beam section

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Shear buckling

Web buckling

Flange buckling

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Structural Elements Y Structural elements are categorized based up on the effect in them. For example • Columns/struts – subjected to compressive axial force only • Beams/girders – subjected to flexural loads, i.e. shear force & bending moment • Beam-column members – subjected to combined axial force & flexural loads

• Purlins – carrying roof sheeting

• Bracing – diagonal struts and ties •

Ties – subjected to tensile axial force only

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Type of Steel Structural Elements Purlins

Columns Beams Bracings 14

Basic Steel Structural Systems Y Trusses

Y Frames • Beams • Girders • Columns

Y Space trusses/frames

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Structural Elements Y In trusses: • All the members are connected using pin/hinge connections • All external forces are applied at the pins/hinges • All truss members are subjected to axial forces

Y In fames: • The beams are subjected to flexural loads only • In braced frames • The columns are subjected to compressive axial forces only • The diagonal members are subjected to tension/compression axial forces only • In moment frames • The vertical members (beam-columns) are subjected to combined axial & flexural loads 16

Structural Connections Truss connection

Simple Shear connection

Moment resisting connection

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Structural Design Y To meet client requirements and statutory requirements Y The design process is an iterative process. Assume dimensions, structural conditions and cross sections Structural Analysis Selection of cross sections to satisfy structural requirements Does the design violate the initial assumptions?

YES

NO Final Design 18

Structural Design Y Optimal structural design shall achieve balance between the following requirements: Strength Serviceability

Optimal design Economy

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Eurocodes EN 1990 - Basis of structural design EN 1991 - Actions on structures EN 1992 – EN 1996 EC2 - Concrete structures EC3 - Steel structures EC4 - Composite structures EC5 - Timber structures EC6 - Masonry structures EC9 – Aluminum structures

EN 1997 - Geotechnical design EN 1998 - Structures for earthquakes 20

Eurocodes vs Structural design

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System of Eurocodes for Steel Structures EN 1090 – Part 1: Delivery conditions for prefabricated steel component EN 1990: Basis of structural design Product standards for steel materials

EN 1090 – Part 2: Execution of steel structures

EN 1991: Action on structures EN 1993: Design rules for steel structures

Part 1: General Rules Part 2: Steel Bridges Part 3: Towers, masts, Chimneys Part 4: Silos, Tanks, Pipelines Part 5: Steel Piling Part 6: Crane supporting structures 22

EN 1993 Part 1: General Rules for Steel Structures

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National Annexes Y Every country using Eurocodes will also have its own National Annexes Y The National Standard incorporating Eurocodes (for e.g. SS EN 1993-1-1) must contain the full, unaltered text of that Eurocode, including all Annexes. Y The National Annex may only include information on those parameters within clauses that have been left open for national choice.

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National Annexes Y Allows for differences in: • Geography, climatic conditions • Level of safety, durability, economy

Y NA contains: • • • •

Nationally determined parameters Country specific data (e.g. wind maps) Decisions on application of informative annexes References to NCCI

It cannot modify the content of the Eurocodes text in any way.

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EN 1990 Basis of Design Y EN1990 states that a structure shall be designed adequately and you are required to demonstrate adequacy in: Bending resistance

• Structural resistance • Serviceability

Compression resistance Buckling resistance Tension resistance

• Durability • Fire resistance • Robustness

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Eurocode Subscripts Extensive use of sub-scripts generally helpful: Subscript

Definition

Example

Ed

Design value of an effect

MEd Design bending moment

Rd

Design resistance

MRd Design resistance for bending

El

Elastic property

Wel

Elastic section modulus

Pl

Plastic property

Wpl

Plastic section modulus

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Actions, Effects and Resistances Y Actions (F): • • •

direct actions - applied loads indirect actions - temperature changes, vibrations both essentially produce same effect

Y Effects of action (E): • •

on structural members and whole structure for example bending moments, shear forces, deflections

Y Resistances (R): •

capacity of a structural element to resist bending moment, axial force, shear, etc.

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Types of Actions (EN 1991) Y Permanent actions (G): are those that essentially do not vary with time such as self-weight of structure, fixed equipment Y Variable actions (Q): leading and non-leading actions, and those that can vary with time such as imposed loads, wind loads and snow loads Y Accidental actions (A): are usually of short duration, but high magnitude such as explosions, impacts

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Design Approach in Eurocodes Y Outlined in EN 1990 Basis of Structural Design Y Based on limit state design Y Principal limit states •

Ultimate limit state, concerned with ‘collapse’ D

•

yielding; buckling; overturning

Serviceability limit state, concerned with ‘function’ D

Deflection; vibration

Y Other limit states • • •

Fire resistance Durability Robustness

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Limit State Design Y Limit states: • states beyond which the structure can no longer meets its original design intention Y Ultimate limit states: • states associate with ‘collapse or other similar forms of structural failure, for e.g. strength, fatigue Y Serviceability limit states: • states correspond to ‘function’, beyond which specified service requirements cannot be met, for e.g. deflection, vibration 31

Limit State Design Principles Y Define relevant limit states Y Determine appropriate combined actions {F}, e.g. • applied loads • temperature changes Y Determine design effects {E} • bending moments • deflections Y Determine design resistance {R} Y Ensure no limit state is exceeded {R > E}

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Effects and Resistances Ed ≤ Effect of Actions: Self-Load Wind Snow Variable loads Temperature Fire ....

Rd Resistance: Structure Structural Elements Materials, EModulus etc. cross sections, Area, Moment of Inertia

S 33

Structural Safety

Ed

Rd

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Characteristic and Design Values Y Characteristic values of actions Representative value of action above which not more than a small percentage of the action may exceed during the design working life Y Design values • Design values used to check limit state condition •

Design value of actions The characteristic value of action multiplied by the relevant partial factor for action

•

Design value of strength The characteristic value of strength divided by the relevant partial factor for material

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Partial Factors Y Partial safety factors γ are applied to characteristic values for both actions material to account for variability Y The value of γ depends on: • The limit state under consideration • The variable to which it is applied • The context – e.g. is an action beneficial in relation to the considered effect Y γF for actions (loading) Y γM for resistance (material and modeling uncertainties)

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Partial Factors Action

γF

Permanent Action (G) Unfavourable conditions Favourable conditions Ultimate limit state Variable Action (Q)

γG = 1.35 γG = 1.00

Unfavourable conditions Favourable conditions

γQ = 1.50 γQ = 0.0

Serviceability limit state

γM

Partial factor

Permanent Action (G)

γG = 0.0

Variable Action (Q)

γQ = 1.00

γM γM0

EC 3 value (SG NA value)

application

1.00 (1.00)

Cross-sections

γM1

1.00 (1.00)

Member buckling

γM2

1.25 (1.10)

fracture

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Ultimate Limit State Y Fundamental combination of actions may be determined from EN 1990 using Equation 6.10: 1.5 × combination factor × other variable actions



G, j

G k , j   Q,1Qk,1    Q,i 0,i Qk ,i

j 1

(6.10)

i1

1.5 × leading combination factor variable actions Y Load factors 1.35 and 1.5 are applied when actions are ‘unfavorable’

1.35 × permanent actions

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Combination of Actions Y Identify leading variable action Qk,1 • The leading variable action is the one that leads to the most unfavourable effect (i.e. the critical combination) • To generate the various load combinations, each variable action should be considered in turn as the leading one. Y Other variable actions reduced by a combination factor ψ Y Accounts for probability of simultaneous occurrence of multiple variable loads • Imposed load ψ = 0.7 • Wind load ψ = 0.6 (SS NA = 0.5)

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Combination of Actions Y the load combinations about EC 3 Gk = Dead load (permanent action); Qk = imposed load (variable action); Wk = wind load (variable action) Combination Dead load & imposed load

1.35Gk + 1.5Qk

Dead load & wind load

1.35Gk + 1.5Wk

Dead load, imposed and wind load

1.35Gk + 1.50Wk + 1.05Qk or 1.35Gk + 1.50Qk + 0.75Wk

Leading variable action

Design Value

*1.05 = 0.7 × 1.50 0.75 = 0.5 × 1.50

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References 1. Lam, D. Ang, T.C. and Chiew, S.P., “Structural Steel Work: Design to Limit State Theory”, 4th Edition, CRC Press, Taylor & Francis Group, UK, 2014. 2. Gardner, L. and Nethercot, D.A., “Designers’ Guide to Eurocode 3: Design of Steel Structures General Rules and Rules for Buildings”, Thomas Telford, London, UK, 2005 3. Wald, F., Tan, K. H. and Chiew, S.P., “Design of Steel Structures with Worked Examples to EN 1993-1-1 and EN 1993-1-8”, Research Publishing, Singapore 2011. 4. Chiew, S.P, CV 3012 PPT slides, 2014.

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Thank You!

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