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8/17/2019 Topic 1 Introduction_ECV5223 (1)
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Lecturer : Dr Nor Azizi Safiee
ECV 5223
STEEL STRUCTURES
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Marks distribution
Assignments and Quizzes 10 %
Project 10 %Test 1 20 %
Test 2 20 %
Final 40 %
Assignments 10 %
Project 20 %
Test 1 15 %
Test 2 15%
Final exam 40 %10 %
Test 1 20 %
Test 2 20 %
inal 40 %
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References
1. Morris L.J. and Plum, D.R. (1996). Structural Steelwork
Design to BS5950. Longman2. Nethercot, D.A. (1996). Limit States Design of Structural
Steelwork. 2nd Edition. E&FN Spon, London
3. Bresler, B., Lin, T.Y. and Scalzi, J.B.(1968). Design of Steel
Structures. John Wiley and Sons, Inc.4. Ambrose J. and Tripeny, P. (2007). Simplified Design of Steel
Structures. New York: Prentice Hall.
5. Segui, W.T. (2007). Steel design. Cengage Learning.
6. Graham W.O and Brian D.C. (1989). Structural SteelworkConnections. Butterworths & Co.
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Learning Outcomes
Explaining the principles of materials and design of steelstructures
Analyze the structural steel members and systems
Implement the design of steel structure
Synopsis
This course covers limit state design method, connectiondesign, elastic and plastic beam design, portal frame design,
multi storey frame design, and fire engineering.
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Content/Syllabus
Introduction to steel structures
Design of connection
Elastic Design of continuous construction – beam, portalframe, multi storey
Plastic Design of continuous construction – beam, portalframe, multi storey
Fire Engineering Design
Design project steel structure
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Introduction
Structural steelwork can be either a single member or an
assembly of a number of steel sections connected together insuch a way that they perform a specified function.
To fulfill the design requirement, the complete design processand relationships between the behavior and analysis of steel
structures and their structural design have to be considered. Steel sections can be produced by hot rolled and cold rolled
The standard cross section are obtained by the hot rolling ofsteel billets in a rolling mill while for the complex shapes, areproduced by cold formed from steel sheet.
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Typical structural steel sections commonly used as steel members
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Structural Steel Material
Steel material was in the form of wrought iron, produced byheating ore in a blast furnace.
In early nineteenth century, cast iron and wrought iron were used invarious types of bridges.
Steel – an alloy of primarily iron and carbon, with fewer impuritiesand less carbon than cast iron. In 1855, steel began to displacewrought iron and cast iron in construction.
structural steel was widely used in construction of bridge, highrise building, roof truss, electricity transmission tower,warehouse, factory, offshore structure
In the civil engineering field steel is in competition principally withreinforced and prestressed concrete, timber and brickwork.
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Why steel ?
Easy tofabricate
Greatstrength
Highstiffness
Goodductility
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Item Comments
Ease of
installation/construction
No formwork, minimum
cranage
Speed of installation
process
Much of the structure can
be prefabricated away from
the site
Modifications at a later
date
Extensions/strengthening
relatively straightforward
Low self-weight Permits large clear spans
Good dimensional control Prefabrication in the shop
ensures accurate work
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US steel building
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Future steel structures
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J. Mayer’s Metropol Parasol,
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Properties of Steel
Strength – measured in tensile test where a small coupon of
material is pulled in a testing machine until it fractures The results of a tensile test are normally presented in terms
of a stress-strain curve for material (figure).
The relationship between stress and strain is linear elastic up
to the proportional limit and obeys Hooke’s law. As the strain is increased until proportional limit where the
curve tends to depart from linearity, the stress at this pointknown as proportionality limit stress, σ pl
Further straining will result in the stress yielding at a yield stress,σy (material no longer behaves elastically)
The stress then remains constant, eventhough the strain continuesto increase – called yield plateau or plastic range (plastic flow ofmaterial and measurement of the ductility)
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Properties of Steel
Typical steel possess yield plateau of at least 10 or 12 times the
strain at yield before strain hardening begins. The initial slope of this part of the curve is termed the strain
hardening modulus, Est.
A maximum value of stress is reached correspond to theultimate tensile stress, σult.
Thereafter stress appears to decrease (specimen begins to neckdown) until fracture finally occurs and this stress known as fracturestress, σf .
The behavior of most structural steel to be very similar incompression and tension, with the compressive yield stress being5% higher on average than the tensile value.
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M,
Typical stress-strain curve for structural mild steel obtained from
a tensile test
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Properties of Steel
Ductility – the ability to undergo large deformations before
fracturing and measured by percentage of elongation. This property enables small regions that are very highly
stressed to yield, thereby relieving this concentration ofstress without undue distress to the structure as a whole.
Adequate ductility is also a prerequisite for the use of theplastic design methods.
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Design Requirements
The design of any structure must be judged by whether it
fulfils the required function safely, can be built with economyand can maintain an acceptable appearance for its specifiedlifetime.
It follows that the design of structural steelwork also will be
assessed by these criteria of safety, economy and appearance. The design of structural steel is based on limit state theory in
accordance with BS5950: Structural Use of Steelwork inBuilding.
The designer selects a number of criteria by which to assessthe proper functioning of the structure and then checkswhether they have been satisfied.
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Limit State Philosophy
Limit states design provides the basic frame within which theperformance of the structure can be assessed against variouslimiting conditions
In formulating procedures nowadays it is customary to do so in away which recognizes the inherent variability of loads, materials,construction practices and approximations made in design
Limit states design philosophy allows a more consistent factor ofsafety against failure and more economical use of materialscompared to the working stress approach
There are two levels of limit state, Ultimate Limit State andServiceability Limit State as considered in BS5950
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Limit State Philosophy
The ultimate limit state may be defined as the point beyond
which the structure would be unsafe and the serviceabilitymay be defined as the point beyond which the structure
becomes unserviceable.
The two limit states summarized in Table below
The load carrying capacity of each member and connectionas determined by the relevant provisions of the code should
be such that the factored loads would not cause any failure.
Structural integrity is another new requirement introducedin the BS 5950
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Ultimate limit state Serviceability limit state
Strength (yielding, rupture, buckling and transformation intoa mechanism)
Excessivedeflection/deformation
Stability against overturning orsway
Excessive vibration
Fracture due to fatigue Repairable damage due to fatigue
Brittle fracture Corrosion and durability
Elastic or plastic instability
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Ultimate Limit States
a) Load factors
The structure being unsafe or on the point of collapsewhen it reaches the limit states of strength or stability
Therefore, necessary to ensure that there is an adequatefactor of safety against failure
Factored load should be applied in the most unfavourablerealistic combination for the part or effect underconsideration
To consider this, the specified loads should be multiplied by
the relevant partial factors, f given in Table 2 BS 5950 Part1.
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Ultimate Limit States
a) Load factors (continue..)
Following load combinations should be checked in the caseof buildings not subject to loads from travelling cranes
Combination loads Design load
1 Dead load and imposedload
1.4Gk +1.6Qk
2 Dead load and windload
1.0Gk +1.4Wk
3 Dead load, imposedload and wind load
1.2Gk +1.2Qk+1.2Wk
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Ultimate Limit States
c) Stability
In accordance with the code static equilibrium, resistance tohorizontal forces and sway should be checked
STABILITY
Resistance tohorizontal
forces
Sway
stiffness
Staticequilibrium
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Ultimate Limit States
c) Stability (continue..)
Static equilibrium The factored loads, considered separately and in combination,
should not cause the structure or any part of it (including thefoundations) to fail by sliding, overturning or uplift at anystage inclusive of erection and demolition
The combination of dead, imposed and wind loads should beto have the most severe effect on the stability limit state underconsideration
Variation in dead load probably during construction or other
temporary condition should take into account Provide sufficient bracing to maintain stability if the members
are incapable of keeping themselves in equilibrium
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Ultimate Limit States
c) Stability (continue..)
Resistance to horizontal forces All structures (including portions between expansion joints)
should have adequate resistance to horizontal forces in orderto provide a practical level of robustness against the effects of
incidental loading. Resistance to horizontal forces should be provided by using
one or more of the systems which include triangulated bracing, moment-resisting joints, cantilever columns, shear
walls and specially designed staircase enclosures such as liftcores
In doing so reversal of load direction should be accommodated
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Ultimate Limit States
c) Stability (continue..)
Sway stiffness All structures (including portions between expansion joints)
should have sufficient sway stiffness so that the vertical loadsacting with lateral displacements of the structure do not result
in excessive secondary forces in the members or connections If there exists “second order” (P-) effects to significant
extent, they should be allowed for in the design of thestructure.
Sway stiffness is provided by sufficient bracing to limit swaydeformations and prevent twisting of the structure on plan.
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Secondary moment created by the “P-” effect
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Ultimate Limit States
c) Stability (continue..)
Sway stiffness “P-” effects will be insignificant in a low to medium rise
structure where reasonably proportioned bracing is provided,however, does imply that this should be checked even in a
structure of simple construction In the case of symmetrical frame, with symmetrical vertical
loads, the sway effects should be taken as comprising theforces and moments in the frame due to horizontal loads.
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Ultimate Limit States
d) Fatigue
Fatigue need to be considered for a structure or structuralelement that subjected to significant and numerousfluctuations of stress
Stress changes due to normal fluctuations in wind loading isnot a critical factor and hence need not be considered.
However, situations may arise in building structures that mayrequire fatigue checks.
Crane supporting structures, platforms supporting plant ormachinery which cause vibration and slender members with
wind induced oscillation – fatigue check becomes essential BS5950 not fully cover workmanship for cases where fatigue is
critical, refer to specialist literature
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Ultimate Limit States
e) Brittle fracture
Brittle fracture should be avoided by using a steel quality withadequate notch toughness taking into account the effects ofminimum service temperature, thickness of the material, steelgrade, loading speed and stress level.
Brittle fracture is prevented in BS5950-Part 1 by limiting thethickness of steel in particular situations. Design strength ischosen based on thickness.
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Serviceability Limit States
a) Deflection
The deflections of a building or part under serviceability loadsshould not impair the strength or efficiency of the structure orits components, nor cause damage to the finishing.
A check on deflection is an essential part of design and is oftencritical for beams and slender structures.
When checking for deflections the most adverse realisticcombination and arrangement of serviceability loads should beassumed and the structure may be assumed to behaveelastically.
Deflections are usually calculated under unfactored imposedload only. This assumes that dead load deflections will be “builtup” during fabrication and erection or that only imposed loaddeflections will be of significance to the occupants.
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Deflection on beams due to unfactored
imposed load
CantileversBeams carrying plaster or other brittle finishAll other beams
Length/180Span/360
Span/200
Horizontal deflection of columns other than
portal frames
Top of columns in single storey buildingsIn each storey of a building with more than one storey
Height/300Height of storey/300
Gantry Girders
VerticalHorizontal
Span/600Span/500
Suggested deflection limits for typical cases
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Serviceability Limit States
b) Vibration and Oscillation
Vibration and oscillation of building structures should belimited to avoid discomfort to users and damage to contents
No guidance is given in BS 5950 on how to check thiscondition and it is recommended to refer on specialist
literature. Normally used “Design guide on the vibration of floors” –
Publication P076 on the Steel Construction as guidelines
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Serviceability Limit States
c) Durability
In order to ensure the durability of the structure underconditions relevant both to its intended use and to its intendedlife, the following factors should be taken into account indesign
i. The environment of the structure and the degree of
exposureii. The shape of the members and the structural detailing
iii. The protective measures, if any
iv. Whether inspection and maintenance are possible
The most important factor that requires attention in durabilityissue is corrosion. Corrosion of steel will be worse in thepresence of environmental factors such as chlorides andsulphites
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Code of Practice
Particularly BS 5950 is used in designing steelwork in
building. Clauses in BS 5950 covers – sway stability, avoidance of
disproportionate collapse, resistance to brittle fracture, local buckling, lateral torsional buckling, shear resistance,
stiffeners, members subject to combined axial force and bending moment, joints, connections and testing.
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Code of Practice
BS 5950 consists of the following parts:
Part 1 : Code of practice for design f rolled and welded sections Part 2 : specification for materials; fabrication and erection, rolled and
welded sections
Part 3 : Design in composite construction
Part 4 : Code of practice for the design of composite slabs with profiled steelsheeting
Part 5 : Code of practice for the design of cold-formed thin gauge sections
Part 6 : Code of practice for design of light gauge profiled steel sheeting
Part 7 : Specification for materials fabrication and erection of cold-formed
sections and sheeting Part 8 : code of practice for fire resistant design
Part 9 : Code of practice for stressed skin design
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Scope of BS 5950 – Part 1
BS 5950-1 gives recommendations for the design of
structural steelwork using hot rolled sections, flats plates, hotfinished structural hollow sections
The use of this code is primarily intended for building andallied structures not specifically covered by other standards
The recommendations in the code assume that the standardsof material and construction are as specified in BS 5950-2.
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Design MethodsThree basic design methods are recognised in limit state design
philosophy. (Clause 2.1.2 BS 5950)
Designmethods
Continuousdesign
Semi-continuous
design
Simpledesign