Steel Structures - Trent Global · 2 Introduction Structures may be classified on the basis of...
Transcript of Steel Structures - Trent Global · 2 Introduction Structures may be classified on the basis of...
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Introduction
Structures may be classified on the basis ofmaterials used for construction, as follows:
• Steel structures
• Aluminium structures
• Concrete structures
• Composite structures
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Steel
• Steel, as a building
material has been
extensively used in
various structures in
Singapore.
• To facilitate safe and
economical design,
standards and codes
are prepared.
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Advantages of Steel
Steel has many advantages as a structuralmaterial.
• Steel members have high strength per unitweight.
• Being light, steel members can be convenientlyhandled and transported.
• Properly maintained steel structures have a longlife.
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Advantages of Steel (Contd)
• The properties of steel mostly do notchange with time.
• Steel, being a ductile material, does notfail suddenly, but gives visible evidence ofimpending failure by large deflections.
• Additions and alterations can be madeeasily to steel structures.
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Advantages of Steel (Contd)
• They can be erected at a faster rate.
• Steel has the highest scrap value amongst all
building materials.
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Disadvantages of Steel
Steel has the following disadvantages as a
structural material.
• Steel structures when placed in exposed
conditions are subjected to corrosion. Therefore,
they require frequent painting.
• Steel structures need fire proof treatment, which
increases cost.
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Uses of Structural Steel
• Use of steelwork in building construction is
gaining popularity in view of:
- speed of construction
- less labour intensive
- cleaner work site
- economy in terms of construction cost
- allow design flexibility due to excellent strength
and predictable behaviour under loading
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Projects in Singapore
• Singapore National Library
• The Esplanade
• Capital Tower
• Gardens by the Bay
• Singapore Sports Hub
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Mild Steel
Composition:
• Carbon 0.23 – 0.25%
• Sulphur 0.055%
• Phosphorus 0.055%
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Properties of Mild Steel
• Its structure is fibrous with dark bluish colour.
• It is malleable and ductile
• It is difficult to harden and temper.
• Its specific gravity is 7.8.
• It can easily be welded, riveted and forged.
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Properties of Mild Steel (Contd)
• It is more tough and elastic than cast iron and
wrought iron.
• It is equally strong in compression, tension and
in shear.
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Uses of Mild Steel
• As rolled structural sections like I-section, T-section, channel section, angle iron, plates,round and square rods in construction works.
• Mild steel round bars are extensively used asreinforcements in Reinforced CementConcrete (RCC).
• Mild steel tubes are finding much use instructures.
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Uses of Mild Steel (Contd)
• Plain and corrugated sheets of mild steel are
being used as roof coverings.
• Mild steel is also used in the manufacture of
various tools and equipments, machine parts,
towers and industrial buildings.
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High Tensile Steel
Composition
• Carbon 0.8%
• Manganese 0.6%
• Silicon 0.2%
• Sulphur 0.05%
• Phosphorus 0.05%
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Uses of High Tensile Steel
• It is essentially a low carbon steel and the
percentage of carbon is kept less than 0.15%. It
is also known as high strength steel.
• High tensile steel is normally used in pre-
stressed concrete
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Types of Steel
Steel is given different names based on itsalloyed composition:
• Carbon Steel
• Alloy Steel
• Structural Steel
• Sheet Steel
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Mechanical Properties of Steel
The most important mechanical properties of
steel are its
• strength,
• elasticity and
• plasticity,
• characterised by stresses and elongations as
well as its tendency to brittle failure.
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Behaviour of Steel under Tension
• Stress-strain diagram for mild steel and high
strength steel is shown below:
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Stress-Strain Diagram
The load-deformation or conventional stress-strain diagram is very important, as it is thebasis for accepting the steel for most of thestructural requirements.
The total curve can be divided into threedistinct regions:
• Elastic Region
• Region of Pure Plasticity
• Region of Plasticity with Work Hardening
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Design Objectives for Steelwork
Structural Steel work can be
• single member
• an assembly of a number of sections
connected together & perform a specified
function
Three Main Design Criteria:
- Safety
- Economy
- Appearance
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Factors Influencing Design of Steel
• The spans involved
• The vertical loading
• The horizontal loading
• The services required
• The ground conditions
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Design Methods
1. Elastic (allowable stress) Design
The process involves:
• Determine the stress of members subject to
service load on the structure & thereafter.
• Design the member so as not to exceed the
allowable strength of material used.
• In view of possible inaccuracies in the loading
and material behaviour, the material factor
used is normally high.
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Design Methods (Contd)
2. Limit State (load factor) Design
The process involves:
• Ultimate limit state which assess the strength
of material, stability against overturning, sway
and fatigue.
• Serviceability limits states which assess the
deflection, durability, corrosion & vibration.
• Determine the stress of member subject to the
ultimate load.
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Limit State Approach
Two limit states considered in the design ofstructural steel work:
1. Ultimate Limit State
- strength
- stability against overturning
- fracture due to fatigue
- brittle fracture
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Limit State Approach (Contd)
2. Serviceability Limit State
- deflection
- vibration
- durability
- corrosion
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Load Factors
• For the limit state, the service or actual loads are
multiplied by load factors and the products are
referred to as ultimate loads.
• For the serviceability limit state, load factor is
unity (1).
• Partial safety factor for dead load = 1.4
• Partial safety factor for live load = 1.6
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Categories of Steel Building
Construction
The majority of steel building fits within one of
the following categories:
Type Main Use
Bearing Wall Low rise, lightly loaded
Steel Frame Wide variety of types and sizes of building
Long Span Coverage of long column-free areas
High Rise Tall buildings i.e. more than 20 storeys
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Types of Load
Dead Load (DL)
- Point load (kN)
- Uniformly distributed load, UDL (kN/m)
Refers to self-weight of steel member and other
permanent parts of the building.
Dead Load of a structure can be computed
based on the density of the material.
For eg: Density of Steel is 7850 kg/m3
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Types of Load (Contd)
Imposed Load (Live Load)
- Point load (kN)
- Uniformly distributed imposed load UDL (kN/m)
Refers to temporary load. eg: people, furniture
Imposed load varies with functional usage of space / room
BS 6399 Part 1 – Code of Practice for DL & LL
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Types of Load (Contd)
Wind Load (WL)
BS 6399 Part 2
Code of Practice for Wind Load
• Wind speed is selected based on the location of
building, topography, surrounding buildings,
height above the ground level, component size
and period of exposure.
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Load Combination
In design of structural steel building, the
following principal combination of loads should
be taken into account:
• Load Combination 1: 1.4 Dead Load + 1.6 Imposed Load
• Load Combination 2: 1.4 Dead Load + 1.4 Wind Load
• Load Combination 3: 1.2 Dead Load + 1.2 Imposed Load + 1.2 WL
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Design Procedure
Design problems start with:
• Selection of type and layout of structure
• Estimate the loading
• Conduct analysis to determine themaximum moments, forces and shear
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Design Procedure (Contd)
• Select the material and proportioning of
members and connection
• Check the performance of the structure
under service load such as deflection,
vibration, corrosion, etc.
• Production of detailed drawing
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Fabrication & Erection
• Simple connection
• Quality welding should be done at the factory
• Bolting is preferred at the site
• Avoid expensive weld – fillet weld is cheaper
than butt weld
• Design joints based on calculated forces rather
than section capacity
• Consider installation sequence in design
• Design for simple construction
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Material Economy
• Use material only when necessary
• High strength & lightweight steel
• Cost per ton for SHS is 30% more than H or I-section
• Be reasonable in design
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Buildability
• Consider an integrated system i.e. mechanical
services, superstructure and foundation
• Use high strength lightweight design to reduce
load on foundation
• Use fast track construction for early return of
investment
• Use complete design to enhance strength and
stiffness as well as for fire protection