Topic 1 Introduction_ECV5223 (1)

8/17/2019 Topic 1 Introduction_ECV5223 (1)

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Lecturer : Dr Nor Azizi Safiee

[email protected]

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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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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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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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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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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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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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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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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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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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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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

Documents

Topic 1 Introduction_ECV5223 (1)