lecture 1 bec304

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Lecture 1

CHAPTER 1

INTRODUCTION

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UNCERTAINTY IN STRUCTURAL DESIGN

Action (loading)

Strength of materials

Structural behaviour

A designer cannot guarantee that a structurewill be absolutely safe, but only that the risk

of failure will be extremely small. This isachieved by introducing safety factors into thedesign calculations.

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STRUCTURAL RESPONSE

Structural members are subject to severalactions:

Tension

Compression Moment

Shear

Torsion

These forces can act alone or in combination.

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APPLICATIONS

Factory

Stadium

Building

Transmission tower

Warehouse

Power Plant

Discount store

Etc.

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BROAD CATEGORIES OF STEEL

BUILDING CONSTRUCTION

TYPE MAIN USE MAIN CONSIDERATION

Bearing wall Low rise, lightly loaded Design of steelwork

normally straightforwardSteel frame Wide variety of types and

size of building

Simple construction or

continuous construction,

depending on types of

joints used.

Long span Coverage of large column-free areas Special form of beam maybe required

High rise Tall buildings i.e. more than

20 storeys

Wind load

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FLOWCHART FOR STEEL

CONSTRUCTIONOwner/Developer

Architect

Engineer

Main Contractor

Sub-Contractor

(fabricator)

Stockist

Manufacturer

Sub-Contractor

(others)

Quantity Surveyor

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STRUCTURAL IDEALISATION

A suitable structural system must be selected before carrying outanalysis and design.

Factors might influence the choice: The span involverequirement for long spans or large clear floor

areas.

The vertical loadingpresence of heavy load or need cranes? The horizontal loadinghow to resist horizontal (wind) loading? Rigid

joint? Bracing? Shear wall?

The services requiredwater, gas, electricity, necessary pipeworkand ducting.

The ground conditiontypes of foundation i.e. pad, raft, piled etc.

Otherstemperature effect, appearance etc.

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ADVANTAGES

Advantages:

High strength/weight ratio. Thus self-weight is relatively low.Permits heavy loads and large clear spans. Suitable for high rise,long span bridges and structures on the soft soil.

Good ductilitysteel experiences large plastic deformation before

failure occurs, thus provide enough warning to fix or evacuate thestructures.

Isotropic behaviour.

Ease and speed of erectionrelative economy.

Quick to repair.

Repetitive use. Relative ease of fabrication.

Modifications at a later date.

Good dimensional control.

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STRUCTURAL STEEL ELEMENTS

Beam & plate girder, horizontal bracing

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Column, strut & connection

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Bracing & ties

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STEEL PRODUCTION

Can be divided into three stages:

a) Iron productiona chemical process of four raw materialsi.e. iron ore, blast furnace, coke and limestone. The finalproduct is cast iron with high content of carbon, sulphur,phosphorus.

b) Steel productionprocess to reduce carbon, sulphur andphosphorus in cast iron. If required, chromium, nickel andmanganese are added to produce corrosion resistancematerial.

c) Rolling processsteel billets are rolled to producerequired steel sections.

Steel usually contains 98% iron + other chemicals.

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TYPICAL HOT ROLLED STEEL SECTIONS

UNIVERSAL BEAM UNIVERSAL COLUMN CHANNEL

ANGLE HOLLOW

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GEOMETRICAL AXES

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TYPICAL HOT ROLLED CROSS-SECTIONS

1. Open sections

a) Universal beams UKB

primary function is to carry loads transverse to its

longitudinal axis.

Structural member subject to bending and shear.

Usually horizontal and supports floors in buildings.

b) Universal columns UKC

Primary function is to carry loads in compression along its

longitudinal axis.

Generally vertical and support beams.

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c) Anglesequal or unequalused for purlins,

truss members and bracing.

d) Teesproduced by cutting UKB or UKC into twoparts. Normally used truss members, ties and

light beam sections.

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2. Hollow sections - circular, square, rectangle.

Produced from flat steel profile. Very efficient

in compression. Widely used for lattice

girders, building frames, purlins, sheetingrails.

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TYPICAL STRESS-STRAIN CURVE FOR

MILD STEEL

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TERMINOLOGIES USED IN EC3

The main differences in terminologies are:

Actions Loads, imposed displacements, thermal strains

Effects Internal bending moments, axial forces etc.

Resistance Capacity of a structural element to resist bending moment, axial

force, shears etc.

Verification Check

Execution Construction, fabrication, erection etc.

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EN 1990 states that a structure shall have

adequate:

Structural resistance

Serviceability

Durability

Fire resistance robustness

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LIMIT STATE DESIGN

The principles of limit state design are set out

briefly and the relevant design situations are

classified as:

Persistent Condition of normal use

Transient Temporary conditions e.g. during repair

Accidental Exceptional conditions applicable to the structure or

to its exposure e.g. fire, explosion or impact

Seismic Condition is applicable to the structure under

seismic events

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EN 1990: EUROCODE BASIS OF

STRUCTURAL DESIGNEQU Loss of static equilibrium of the structure or any part of it considered as

a rigid body, in which:

-minor variations in the value or the spatial distribution of actions from

a single source are significant.

STR Internal failure of the structure or structural elements, including

footings, piles, basement walls, etc., in which the strength of

construction materials or excessive deformation of the structure

governs.

GEO Failure or excessive deformation of the ground in which the strength of

soil or rock are significant in providing resistance.

FAT Fatigue failure of the structure or structural elements.

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BASIS OF STRUCTURAL DESIGN

Limit State Design

Design for limit state shall be based on the use of structuraland load models for relevant limit state.

It shall be verified that no limit state is exceeded when

relevant design values for actions, material properties, orproduct properties, and geometrical data are used in thesemodels.

The verifications shall be carried out for all relevant designsituations and load cases.

The requirements of clause 3.5(1) should be achieved bythe partial factor method described in Section 6.

As an alternative, a design directly based on probalisticmethods may be used.

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LOAD COMBINATION

Fundamental combinations of actions may be

determined from EN1990 using either:

Equation 6.10

Less favourable of equation 6.10a and 6.10b.

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VARIABLE ACTIONS Qk

In equation 6.10, the full value of the leadingvariable action is applied Q, 1Qk, 1.5 (i.e. 1.5 xcharacteristic imposed load).

The leading variable action is the one that leads

to the most unfavourable effect (i.e. the criticalcombination).

To generate the various load combinations, eachvariable action should be considered in turn as

the leading one, (and consideration should begiven to whether loading is favourable orunfavourable).

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FAVOURABLE AND UNFAVOURABLE

LOADING

Loads may be considered as unfavourable orfavourable in any given combination, dependingon whether they increase or reduce the effects(bending moments, axial forces, etc.) in the

structural members.

For unfavourable dead load: G= 1.35

For favourable dead load: G= 1.00

For unfavourable variable load: Q= 1.5

For favourable variable load: Q= 0

lecture 1 bec304

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