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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STRUCTURAL STEEL ELEMENTS
Column, strut & connection
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STRUCTURAL STEEL ELEMENTS
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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TYPICAL HOT ROLLED CROSS-SECTIONS
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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TYPICAL HOT ROLLED CROSS-SECTIONS
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
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