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RAKENNUSTEKNIIKKA Olli Ilveskoski 30.08.2006 rev2 10.01.2007 30 INTRODUCTION OF DESIGN PRINCIPLES ESDEP Course http://www.kuleuven.ac.be/bwk/materials/Teaching/master/toc.htm SUUNNITTELUPERUSTEET https://www.virtuaaliamk.fi/opintojaksot/030501/1132142124407/1133882367215/1136369095512/113636 9418395.html.stx - Rakennejärjestelmät - Jäykistystavat - Liitostyypit - Rakennetyypit - Rakennetyyppien liittymät Kuva: TRY ry:n / Risto Liljan Oppimisympäristön Runkosuunnittelun "Suunnitteluperusteet " -osassa tutustutaan rakennejärjestelmiin, jäykistystapoihin, liitostyyppeihin, rakennetyyppeihin ja rakennetyyppien liittymiin, jotka luovat perustan varsinaisen mitoituksen aloittamiselle.

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INTRODUCTION OF DESIGN PRINCIPLES

ESDEP Coursehttp://www.kuleuven.ac.be/bwk/materials/Teaching/master/toc.htm

SUUNNITTELUPERUSTEET https://www.virtuaaliamk.fi/opintojaksot/030501/1132142124407/1133882367215/1136369095512/1136369418395.html.stx

- Rakennejärjestelmät- Jäykistystavat- Liitostyypit- Rakennetyypit- Rakennetyyppien liittymät

Kuva: TRY ry:n / Risto Liljan Oppimisympäristön Runkosuunnittelun "Suunnitteluperusteet " -osassa tutustutaan rakennejärjestelmiin, jäykistystapoihin, liitostyyppeihin, rakennetyyppeihin ja rakennetyyppien liittymiin, jotka luovat perustan varsinaisen mitoituksen aloittamiselle.

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MITOITUS https://www.virtuaaliamk.fi/opintojaksot/030501/1132142124407/1133882367215/1136369180381/1136369611545.html.stx

Kuva: TRY ry:n / Risto Liljan Oppimisympäristön mitoitusperusteet

- Yleistä- Runkojärjestelmät- Palkit- Profiilipellit- Katto- ja seinäorret- Ohutuumapalkit- Ristikot- Pilarit- Jäykistys- Liitokset- Palosuojaus- Liittorakenteet- Tuotemallinnus ja suunnitelmat

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Kuva: Määräykset ja ohjeetTRY ry:n / Risto Liljan Oppimisympäristö

Kuva: RajatilamitoitusTRY ry:n / Risto Liljan Oppimisympäristö

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Kuva: KuormatTRY ry:n / Risto Liljan Oppimisympäristö

Kuva: KuormatTRY ry:n / Risto Liljan Oppimisympäristö

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Kuva: KuormatTRY ry:n / Risto Liljan Oppimisympäristö

Kuva: MateriaaliominaisuudetTRY ry:n / Risto Liljan Oppimisympäristö

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Kuva: PoikkileikkausluokatTRY ry:n / Risto Liljan Oppimisympäristö

Kuva: MitoitustaTRY ry:n / Risto Liljan Oppimisympäristö

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Kuva: MitoitustaTRY ry:n / Risto Liljan Oppimisympäristö

Kuva: MitoitustaTRY ry:n / Risto Liljan Oppimisympäristö

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Kuva: MitoitustaTRY ry:n / Risto Liljan Oppimisympäristö

Kuva: Palkin mitoitustaTRY ry:n / Risto Liljan Oppimisympäristö

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Opiskelija perehtyy teräksen suunnitteluperusteisiin.ks ESDEP – oppimisympäristö: http://www.terasrakenneyhdistys.fisuomenkielinen versio

Abreviations of the ESDEP Course

Course Contents

STEEL CONSTRUCTION: INTRODUCTION TO DESIGN

Lecture 1B.1 : Process of Design

Lecture 1B.2.1 : Design Philosophies

Lecture 1B.2.2 : Limit State Design Philosophy and Partial Safety Factors

Lecture 1B.3 : Background to Loadings

Lecture 1B.4.1 : Historical Development of Iron and Steel in Structures

Lecture 1B.4.2 : Historical Development of Steelwork Design

Lecture 1B.4.3 : Historical Development of Iron and Steel in Buildings

Lecture 1B.4.4 : Historical Development of Iron and Steel in Bridges

Lecture 1B.5.1 : Introduction to the Design of Simple Industrial Buildings

Lecture 1B.5.2 : Introduction to the Design of Special Industrial Buildings

Lecture 1B.6.1 : Introduction to the Design of Steel and Composite Bridges: Part 1

Lecture 1B.6.2 : Introduction to the Design of Steel and Composite Bridges: Part 2

Lecture 1B.7.1 : Introduction to the Design of Multi-Storey Buildings: Part 1

Lecture 1B.7.2 : Introduction to the Design of Multi-Storey Buildings: Part 2

Lecture 1B.8 : Learning from Failures

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Abreviation of the Workgroup Contents

Lecture 1B.1 : Process of Design

Top

1. DESIGN OBJECTIVES

2. HOW DOES THE DESIGNER APPROACH HIS NEW TASK?

3. HOW DOES THE DESIGNER DEVELOP HIS STRUCTURAL SYSTEM?

3.1 Pose an Initial Concept that may well Satisfy the Functions

3.2 Recognise the Main Structural Systems and Contemplate the Necessary Strength and Stiffness

3.3 Assess Loads Accurately and Estimate Sizes of Main Elements

3.4 Full Structural Analysis, using Estimated Element Sizes with Suitable Modelling of Joints, Related to Actual Details

3.5 Communicate Design Intentions through Drawings and Specifications

3.6 Supervise the Execution Operation

3.7 Conduct Regular Maintenance

3.8 Differences of Emphasis in Design Approach Compared to that of a Medium Sized Building

3.8.1 Single houses

3.8.2 Bridges

3.8.3 Offshore oil rigs

4. CONCLUDING SUMMARY

5. REFERENCES

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Previous | Next | Contents

ESDEP WG 1B

STEEL CONSTRUCTION:

INTRODUCTION TO DESIGN

Lecture 1B.1: Process of Design

OBJECTIVE/SCOPE

To introduce the challenge of creative design and to explain approaches by which it may be achieved.

PREREQUISITES

A general knowledge of basic applied mechanics is assumed and prior encouragement should be given to read J E Gordon's three books [1,2,3].

RELATED LECTURES

Since this lecture deals with the process of design in general terms almost all other lectures are related to it in some way. Those sections which are most closely associated with it are 1B:Introduction to Design, 14: Structural Systems: Buildings, 15A: Structural Systems: Offshore, 15B: Structural Systems: Bridges, and 15C: Structural Systems: Miscellaneous

SUMMARY

The lecture begins by considering a definition of design and some objectives. It discusses how a designer can approach a new problem in general and how a structural designer can develop a structural system. It concludes by considering differences of emphasis in design approach for different classes of structure.

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2. HOW DOES THE DESIGNER APPROACH HIS NEW TASK?

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3. HOW DOES THE DESIGNER DEVELOP HIS STRUCTURAL SYSTEM?

Prepare a set of initial assumptions for possible materials and the structural 'Frame', 'Planar' or 'Membrane' load-bearing system [7] that might be compatible with the 'volumes of space' as shown in Figure 8. These assumptions will be based on previous knowledge and understanding of actual constructions[8-13] or structural theory, see Figure 9 a, b, as well as the current availability of materials and skills. Initial consultations may be needed with suppliers and fabricators, e.g. for large quantities or special qualities of steel.

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3.8 Differences of Emphasis in Design Approach Compared to that of a Medium Sized Building

3.8.1 Single houses

Most "traditionally" built timber and masonry houses include some standard steel elements, e.g. hot-rolled steel beams to span larger rooms and support walls, hollow section columns for stair flights, cold-rolled lintels over window openings, stainless steel wall ties and straps, also nails, screws and truss-rafter nail plates.

Cold-rolled galvanised or stainless steel sections can be made up into truss-rafters and replace timber in repetitive conditions. Similar sections can be made up as stud walls, but fire protection of the thin-walled sections will require careful attention, especially for multi-storey houses.

A main steel structural frame may be used for houses, but integration of services, thermal control, fire protection in multi-storeys, corrosion and fabrication costs of elegant jointing must be designed appropriately. Various types of profiled or composite panel cladding can be used for the exterior.

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3.8.2 Bridges

The magnitudes of gravity loading are often relatively greater in bridges, and particular load patterns need to be assessed; also trains of moving wheel loads will occur giving marked dynamic effects. Dynamic effects of wind loading are significant in long-span structures. Accessibility of site, constructability of massive foundations, type of deck structure and regular maintenance cost will govern the system adopted. Aesthetics for users and other observers are important; long distance scale should be appropriately slender but psychologically strong; careful attention is needed for fairly close viewing of abutments and deck underside.

3.8.3 Offshore oil rigs

The scale of the whole operation will be very many times that of an onshore building. Gravity loading, wind speeds, wave heights and depth of water are significant design parameters for structure size and stability (here larger elements cause larger wind and wave loads). The scale of the structure also poses special problems for fabrication control, floating out, anchorage at depth by divers and, not least, cost, see Figure 1. Later when the design life is complete, the problems of dismantling should be easy, if considered during the initial design.

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4. CONCLUDING SUMMARY

• This lecture introduces the challenge of creative design and suggests a holistic strategy for designing structural steelwork. It seeks to answer questions about what a designer is trying the achieve and how he can start putting pen to paper. It illustrates how a successful design is iterated, through qualitative ideas to quantitative verification and finally execution.

• Creative and imaginative design of structures is most challenging and fun - now try it and gain confidence for yourself. Do not be afraid of making mistakes. They will only be eliminated by repeating and exploring many other solutions. Make sure the design is right before it is built, using your own personal in-built checking mechanisms.

5. REFERENCES

[1] Gordon, J. E. 'The New Science of Strong Materials', Pelican.

[2] Gordon, J. E. 'Structures', Pelican.

[3] Gordon, J. E. 'The Science of Structures and Materials', Scientific American Library, 1988.

[4] Jones, J. C. 'Design Methods', Wiley 2nd Edition 1981.

A good overview of general design methods and techniques.

[5] Broadbent, G. H. 'Design in Architecture', Wiley, 1973.

Chapters 2, 13, 19 and 20 useful for designing buildings.

[6] De Bono, E. eg: 'Lateral Thinking' or 'Practical Thinking' or 'The Use of Lateral Thinking', Pelican.

[7] LeGood, J. P, 'Principles of Structural Steelwork for Architectural Students', SCI, 1983 (Amended 1990).

A general introduction and reference booklet to buildings for students.

[8] Francis, A. J, 'Introducing Structures, Pergamon, 1980.

A good overview text, especially Chapter 11 on Structural Design.

[9] Lin, T. Y. and Stotesbury, S. D, 'Structural Concepts and Systems for Architects and Engineers', Wiley, 1981.

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Chapters 1-4 give a very simple and thoughtful approach to total overall structural design, especially for tallish buildings.

[10] Schodek, D. L, 'Structures', Prentice Hall, 1980.

Good clear introductory approach to structural understanding of simple concepts, also especially chapter 13 on structural grids and patterns for buildings.

[11] Otto, F, 'Nets in Nature and Technics', Institute of Light Weight Structures, University of Stuttgart, 1975.

Just one of Otto's excellent booklets which observe patterns in nature and make or suggest possible designed forms.

[12] Torroja, E, 'Philosophy of Structures', University of California Press, 1962.

Still a unique source book.

[13] Mainstone, R. J, 'Developments in Structural Form', Allen Lane, 1975.

Excellent scholarly historical work, also chapter 16 on 'Structural Understanding and Design'.

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Lecture 1B.2.2 : Limit State Design Philosophy and Partial Safety Factors

Top

1. INTRODUCTION

2. PRINCIPLES OF LIMIT STATE DESIGN

3. ACTIONS

3.1 Characteristic Values of Actions (Gk, Qk and Ak)

3.2 Design Values of Actions (Gd, Qd and Ad)

4. MATERIAL PROPERTIES

4.1 Characteristic Values of Material Properties

4.2 Design Values of Material Properties

5. GEOMETRICAL DATA

6. PARTIAL SAFETY FACTORS

7. ULTIMATE LIMIT STATE

8. SERVICEABILITY LIMIT STATE

8.1 Deflections

8.2 Dynamic Effects

9. STRUCTURAL DESIGN MODELS

10. CONCLUDING SUMMARY

11. GLOSSARY

12. REFERENCES

13. ADDITIONAL READING

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Previous | Next | Contents

ESDEP WG 1B

STEEL CONSTRUCTION:

INTRODUCTION TO DESIGN

Lecture 1B.2.2: Limit State Design Philosophy and Partial Safety Factors

OBJECTIVE/SCOPE

To explain the philosophy of limit state design in the context of Eurocode 3: Design of Steel Structures. To provide information on partial safety factors for loads and resistance and to consider how the particular values can be justified.

RELATED LECTURES

Lecture 1B.1: Process of Design

Lecture 1B.3: Background to Loadings

Lecture 1B.8: Learning from Failures

Lecture 2.4: Steel Grades and Qualities

Lecture 2.5: Selection of Steel Quality

SUMMARY

The need for structural idealisations is explained in the context of developing quantitative analysis and design procedures. Alternative ways of introducing safety margins are discussed and the role of design regulations is introduced. The philosophy of limit state design is explained and appropriate values for partial safety factors for loads and strength are discussed. A glossary of terms is included.

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1. INTRODUCTION

The fundamental objectives of structural design are to provide a structure which is safe and serviceable to use, economical to build and maintain, and which satisfactorily performs its intended function. All design rules, whatever the philosophy, aim to assist the designer to fulfil these basic requirements. Early design was highly empirical. It was initially based largely upon previous experience, and inevitably involved a considerable number of failures. Physical testing approaches were subsequently developed as a means of proving innovative designs. The first approaches to design based upon calculation methods used elastic theory. They have been used almost exclusively as the basis for quantitative structural design until quite recently. Limit state design is now superseding the previous elastic permissible stress approaches and forms the basis for Eurocode 3 [1] which is concerned with the design of steel structures. In the following sections the principles of limit state design are explained and their implementation within design codes, in particular Eurocode 3, is described.

2. PRINCIPLES OF LIMIT STATE DESIGN

The procedures of limit state design encourage the engineer to examine conditions which may be considered as failure - referred to as limit states. These conditions are classified into ultimate and serviceability limit states. Within each of these classifications, various aspects of the behaviour of the steel structure may need to be checked.

Ultimate limit states concern safety, such as load-carrying resistance and equilibrium, when the structure reaches the point where it is substantially unsafe for its intended purpose. The designer checks to ensure that the maximum resistance of a structure (or element of a structure) is adequate to sustain the maximum actions (loads or deformations) that will be imposed upon it with a reasonable margin of safety. For steelwork design the aspects which must be checked are notably resistance (including yielding, buckling, and transformation into a mechanism) and stability against overturning (Figure 1). In some cases it will also be necessary to consider other possible failure modes such as fracture due to material fatigue and brittle fracture.

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Serviceability limit states concern those states at which the structure, although standing, starts to behave in an unsatisfactory fashion due to, say, excessive deformations or vibration (Figure 2). Thus the designer would check to ensure that the structure will fulfil its function satisfactorily when subject to its service, or working, loads.

These aspects of behaviour may need to be checked under different conditions. Eurocode 3 for instance defines three design situations, corresponding to normal use of the structure, transient situations, for example during construction or repair, and accidental situations. Different actions, i.e. various load combinations and other effects such as temperature or settlement, may also need to be considered (Figure 3).

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Despite the apparently large number of cases which should be considered, in many cases it will be sufficient to design on the basis of resistance and stability and then to check that the deflection limit will not be exceeded. Other limit states will clearly not apply or may be shown not to govern the design by means of quite simple calculation.

At its most basic level limit state design simply provides a framework within which explicit and separate consideration is given to a number of distinct performance requirements. It need not necessarily imply the automatic use of statistical and probabilistic concepts, partial safety factors, etc., nor of plastic design, ultimate load design, etc. Rather it is a formal procedure which recognises the inherent variability of loads, materials, construction practices, approximations made in design, etc., and attempts to take these into account in such a way that the probability of the structure becoming unfit for use is suitably small. The concept of variability is important because the steelwork designer must accept that, in performing his design calculations, he is using quantities which are not absolutely fixed or deterministic. Examples include values for loadings and the yield stress of steel which, although much less variable than the properties of some other structural materials, is known to exhibit a certain scatter (Figure 4). Account must be taken of these variations in order to ensure that the effects of loading do not exceed the resistance of the structure to collapse. This approach is represented schematically in Figure 5 which shows hypothetical frequency distribution curves for the effect of loads on a structural element and its strength or resistance. Where the two curves overlap, shown by the shaded area, the effect of the loads is greater than the resistance of the element, and the element will fail.

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Proper consideration of each of the limits eliminates the inconsistencies of attempting to control deflection by limiting stresses or of avoiding yield at working load by modifying the design basis (formula, mathematical model, etc.) for an ultimate resistance determination.

The procedure of limit state design can therefore be summarised as follows:

• define relevant limit states at which the structural behaviour is to be checked.

• for each limit state determine appropriate actions to be considered.• using appropriate structural models for design, and taking account of the

inevitable variability of parameters such as material properties and geometrical data, verify that none of the relevant limit states is exceeded.

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3. ACTIONS

An action on a structure may be a force or an imposed deformation, such as that due to temperature or settlement. Actions are referred to as direct and indirect actions respectively in Eurocode 3.

Actions may be permanent, e.g. self-weight of the structure and permanent fixtures and finishes, variable, e.g. imposed, wind and snow loads, or accidental, e.g. explosions and impact (Figure 6). For earthquake actions, see Lectures 17 and Eurocode 8 [2]. Eurocode 1 [3] represents these by the symbols G, Q and A respectively, together with a subscript - k or d to denote characteristic or design load values respectively. An action may also be classified as fixed or free depending upon whether or not it acts in a fixed position relative to the structure.

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3.1 Characteristic Values of Actions (Gk, Qk and Ak)

The actual loadings applied to a structure can seldom be defined with precision; liquid retaining structures may provide exceptions. To design a structure for the maximum combination of loads which could conceivably be applied would in many instances be unreasonable. A more realistic approach is to design the structure for 'characteristic loads', i.e. those which are deemed to have just acceptable probability of not being exceeded during the lifetime of the structure. The term 'characteristic load' normally refers to a load of such magnitude that statistically only a small probability, referred to as the fractile, exists of it being exceeded.

Imposed loadings are open to considerable variability and idealisation, typically being related to the type of occupancy and represented as a uniform load intensity (Figure 7). Dead loads are less variable although there is evidence that variations arising in execution and errors can be substantial, particularly in the case of in-situ concrete and finishes such as tarmac surfacing on road bridges.

Loadings due to snow, wind, etc. are highly variable. Considerable statistical data on their incidence have been collated. Consequently it is possible to predict with some degree of certainty the risk that these environmental loads will exceed a specified severity for a particular location.

3.2 Design Values of Actions (Gd, Qd and Ad)

The design value of an action is its characteristic value multiplied by an appropriate partial safety factor. The actual values of the partial factors to be used depend upon the design situation (normal, transient or accidental), the limit state and the particular combination of actions being considered. Corresponding values for the design effects of actions, such as internal forces and moments, stresses and deflections, are determined from the design values of the actions, geometrical data and material properties.

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4. MATERIAL PROPERTIES

Variability of loading is only one aspect of uncertainty relating to structural behaviour. Another important one is the variability of the structural material which is reflected in variations in strength of the components of the structure. Again, the variability is formally accounted for by applying appropriate partial safety factors to characteristic values. For structural steel, the most important property in this context is the yield strength.

4.1 Characteristic Values of Material Properties

The characteristic yield strength is normally defined as that value below which only a small proportion of all values would be expected to fall. Theoretically this can only be calculated from reliable statistical data. In the case of steel, for practical reasons a nominal value, corresponding typically to the specified minimum yield strength, is generally used as the characteristic value for structural design purposes. This is the case in Eurocode 3 which tabulates nominal values of yield strength for different grades of steel.

4.2 Design Values of Material Properties

The design value for the strength of steel is defined as the characteristic value divided by the appropriate partial safety factor. Other material properties, notably modulus of elasticity, shear modulus, Poisson's ratio, coefficient of linear thermal expansion and density, are much less variable than strength and their design values are typically quoted as deterministic.

In addition to the quantified values used directly in structural design, certain other material properties are normally specified to ensure the validity of the design procedures included within codified rules. For instance Eurocode 3 stipulates minimum requirements for the ratio of ultimate to yield strength, elongation at failure and ultimate strain if plastic analysis is to be used [1].

5. GEOMETRICAL DATA

Geometrical data are generally represented by their nominal values. They are the values to be used for design purposes. The variability, for instance in cross-section dimensions, is accounted for in partial safety factors applied elsewhere. Other imperfections such as lack of verticality, lack of straightness, lack of fit and unavoidable minor eccentricities present in practical connections should be allowed for. They may influence the global structural analysis, the analysis of the bracing system, or the design of individual structural elements and are generally accounted for in the design rules themselves.

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6. PARTIAL SAFETY FACTORS

Instead of the traditional single factor of safety used in permissible stress design, limit state design provides for a number of partial safety factors to relate the characteristic values of loads and strength to design values. ISO Standard 2394 [4] suggests the use of seven partial safety factors but these are often combined to simplify design procedures. This is the case in the Eurocodes [1,3] which include factors for actions and resistance. Further details are given in the Appendix.

In principle, the magnitude of a partial safety factor should be related to the degree of uncertainty or variability of a particular quantity (action or material property) determined statistically. In practice, whilst this appears to be the case, the actual values of the partial safety factors used incorporate significant elements of the global safety factor and do not represent a rigorous probabilistic treatment of the uncertainties [5-8].

In essence the characteristic actions (Fk) are multiplied by the partial safety factors on loads (γF) to obtain the design loads (Fd), that is:

Fd = γf Fk

The effects of the application of the design loads to the structure, i.e. bending moment, shear force, etc. are termed the 'design effects' Ed.The design resistance Rd is obtained by dividing the characteristic strengths Rk by the partial safety factors on material γM, modified as appropriate to take account of other considerations such as buckling. For a satisfactory design the design resistance should be greater than the 'design effect'.

7. ULTIMATE LIMIT STATE

The following conditions may need to be verified under appropriate design actions:

a. Ed,dst ≤ Ed,stb

where Ed,dst and Ed,stb are the design effects of destabilising and stabilising actions respectively. This is the ultimate limit state of static equilibrium.

b. Ed ≤ Rd

where Ed and Rd are the internal action and resistance respectively. In this context it may be necessary to check several aspects of an element's resistance. These aspects might include the resistance of the cross-section (as a check on local buckling and yielding), and resistance to various forms of buckling (such as overall buckling in compression, lateral-torsional buckling and shear buckling of webs), as well as a check that the structure does not transform into a mechanism.

c. no part of the structure becomes unstable due to second order effects.

d. the limit state of rupture is not induced by fatigue.

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8. SERVICEABILITY LIMIT STATE

The serviceability limit state is generally concerned with ensuring that deflections are not excessive under normal conditions of use. In some cases it may also be necessary to ensure that the structure is not subject to excessive vibrations. Cases where this is particularly important include structures exposed to significant dynamic forces or those accommodating sensitive equipment. Both deflection and vibration are associated with the stiffness rather than strength of the structure.

8.1 Deflections

At the serviceability limit state, the calculated deflection of a member or of a structure is seldom meaningful in itself since the design assumptions are rarely realised because, for example:

• the actual load may be quite unlike the assumed design load.• beams are seldom "simply supported" or "fixed" and in reality a beam is

usually in some intermediate condition.• composite action may occur.

The calculated deflection is, however, valuable as an index of the stiffness of a member or structure, i.e. to assess whether adequate provision is made in relation to the limit state of deflection or local damage. For this purpose, sophisticated analytical methods are seldom justified. Whatever methods are adopted to assess the resistance and stability of a member or structure, calculations of deflection should relate to the structure of the elastic state. Thus, when analysis to check compliance with the strength limit is based on rigid-elastic or elastic-plastic concepts, the structural behaviour in the elastic phase must also be considered.

Calculated deflections should be compared with specified maximum values, which will depend upon circumstances. Eurocode 3 [1] for instance tabulates limiting vertical deflections for beams in six categories as follows:

• roofs generally.• roofs frequently carrying personnel other than for maintenance.• floors generally.• floors and roofs supporting plaster or other brittle finish or non-flexible

partitions.• floors supporting columns (unless the deflection has been included in the

global analysis for the ultimate limit state).• situations in which the deflection can impair the appearance of the

building.

In determining the deflection it may be necessary to consider the effects of precamber, permanent loads and variable loads separately. The design should also consider the implications of the deflection values calculated. For roofs, for instance,

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regardless of the limits specified in design rules, there is a clear need to maintain a minimum slope for run-off. More stringent limits may need therefore to be considered for nearly flat roof structures.

8.2 Dynamic Effects

The dynamic effects to be considered at the serviceability limit state are vibration caused by machinery and self-induced vibrations, e.g. vortex shedding. Resonance can be avoided by ensuring that the natural frequencies of the structure (or any part of it) are sufficiently different from those of the excitation source. The oscillation and vibration of structures on which the public can walk should be limited to avoid significant discomfort to the users. This situation can be checked by performing a dynamic analysis and limiting the lowest natural frequency of the floor. Eurocode 3 recommends a lower limit of 3 cycles per second for floors over which people walk regularly, with a more severe limit of 5 cycles per second for floors used for dancing or jumping, such as gymnasia or dance halls [1]. An alternative method is to ensure adequate stiffness by limiting deflections to appropriate values.

9. STRUCTURAL DESIGN MODELS

No structural theory, whether elastic or plastic, can predict the load-carrying resistance of a structure in all circumstances and for all types of construction. The design of individual members and connections entails the use of an appropriate structural theory to check the mode of failure; sometimes alternative types of failure may need to be checked and these may require different types of analysis. For example, bending failure by general yielding can only occur when the plastic moment is attained; however bending failure is only possible if failure does not occur at a lower load level by either local or overall buckling.

Serviceability limit states are concerned with the performance of the structure under service loading conditions. The behaviour should therefore be checked on the basis of an elastic analysis, regardless of the model used for the ultimate limit state design.

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10. CONCLUDING SUMMARY

• Limit state design procedures require formal examination of differentconditions which might lead to collapse or inadequate performance.

• The effect of various actions is compared with the corresponding resistance of the structure under defined failure criteria (limit states).

• The most important failure critera are the ultimate limit state (collapse) and the serviceability limit state of deflection.

• In checking each limit state, appropriate design models must be used to provide an accurate model of the corresponding structural behaviour.

• Separate partial safety factors are introduced for loading and material. These factors are variable quantities and the precise values to be used in design reflect the degree of variability in the action or resistance to be factored.

• Different combinations of action may also require different values of safety factor.

• This flexible approach helps provide a more consistent level of safety compared with other design approaches.

11. GLOSSARY

A limit state is a condition beyond which the structure no longer satisfies the design performance requirements.

The ultimate limit state is a state associated with collapse and denotes inability to sustain increased load.

The serviceability limit state is a state beyond which specified service requirements are no longer met. It denotes loss of utility and/or a requirement for remedial action.

Characteristic loads (Gk, Qk, Ak) are those loads which have an acceptably small probability of not being exceeded during the lifetime of the structure.

The characteristic strength (fy) of a material is the specified strength below which not more than a small percentage (typically 5%) of the results of tests may be expected to fall.

Partial safety factors (γ G, γ Q, γ M) are the factors applied to the characteristic loads, strengths, and properties of materials to take account of the probability of the loads being exceeded and the assessed design strength not being reached.

The design (or factored) load (Gd, Qd, Ad) is the characteristic load multiplied by the relevant partial safety factor.

The design strength is the characteristic strength divided by the appropriate partial safety factor for the material.

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Lecture 1B.3 : Background to Loadings

Top

1. INTRODUCTION

2. PERMANENT ACTIONS

2.1 Dead Loads

3. VARIABLE ACTIONS

3.1 Imposed Loads

3.2 Permitted Reductions in Imposed Load

3.3 Superimposed Bridge Loads

3.4 Crane Loads

3.5 Environmental Loads

3.6 Wind Loads

3.7 Snow Loads

3.8 Wave Loading

3.9 Temperature Effects

3.10 Retained Material

3.11 Seismic Loads

3.12 Accidental Loads

4. CONCLUDING SUMMARY

5. REFERENCES

6. ADDITIONAL READING

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Previous | Next | Contents

ESDEP WG 1B

STEEL CONSTRUCTION:

INTRODUCTION TO DESIGN

Lecture 1B.3: Background to Loadings

OBJECTIVE/SCOPE:

To provide an introduction to the sources of loads on structures and how loads can be quantified for the purpose of structural design.

RELATED LECTURES:

Lecture 1B.2.1: Design Philosophies

SUMMARY:

Various types of loads (dead, imposed and environmental) and their classification as permanent, transient or accidental within Eurocode 1: Basis of Design and Actions on Structures, is considered. Calculations for dead loads on the basis of material densities and component sizes are explained. Means of estimating imposed loads based upon usage and the implications of change of use are discussed. Loads due to snow, temperature and seismic effects are considered briefly. The statistical treatment of wind and wave loads, and their dependence upon wind speed and wave height respectively, are described. The importance of load characteristics, other than simply their magnitude, is considered. These characteristics include fatigue, dynamic and aerodynamic effects. Simplified treatments for dynamic loads are described.

1. INTRODUCTION

Structures are subject directly to loads from various sources. These loads are referred to as direct actions and include gravity and environmental effects, such as wind and snow. In addition deformations may be imposed on a structure, for instance due to settlement or thermal expansion. These 'loads' are indirect actions. In applying any quantitative approach to structural analysis, the magnitudes of the actions need to be identified. Furthermore, if the structure is to perform satisfactorily throughout its design life, the nature of the loads should be understood and appropriate measures taken to avoid problems of, for instance, fatigue or vibration.

The magnitude of loads cannot be determined precisely. In some cases, for instance in considering loads due to the self-weight of the structure, it might be thought that values can be calculated fairly accurately. In other cases, such as wind loads, it is only possible to estimate likely levels of load. The estimate can be based on observation of previous conditions and applying a probabilistic approach to predict

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maximum effects which might occur within the design life of the structure. (In fact, the extensive wind records which are now available mean that wind loads can often be predicted with greater accuracy than self-weight). Loads associated with the useof the structure can only be estimated based on the nature of usage. Insufficient data is available in most cases for a fully statistical approach and nominal values are therefore assigned. In addition, problems of change of use and fashion can occur.

In analysing structures it is rare to consider all loadings acting simultaneously. This approach may be because the most severe condition for parts of the structure occurs when some other combination of load is considered. Alternatively it may be that the possibility of such a condition actually occurring is extremely small. However, the risk of coexistence of apparently unrelated loads may be greater than is first imagined. Correlations can be produced from unexpected sources or from coincidences which, although physically unconnected, are temporarily connected. For example, floor and wind loads would normally be considered as unrelated. However, in hurricane areas residents on the coast might be expected to move their ground floor contents to upper floors if a hurricane warning, with associated tidal surge, were given. This circumstance could very easily produce extreme floor loads in combination with extreme wind loads. This case may be a very special one but there are others. The risk of fire may not be considered correlated with high wind loads, yet in many parts of the world high winds are more likely in winter, which is also the period of greatest fire risk.

For these reasons it is convenient to consider loads under various categories. The categories can then be ascribed different safety factors and applied in various combinations as required. Traditionally, loadings have been classified as dead, superimposed and environmental loads. These classes include a wide range of gravity effects, seismic action, pressures due to retained material or liquids, temperature induced movement, and, for marine structures, water movement. The Eurocodes on actions and steelwork design [1, 2] classify loads and other actions as permanent, variable and accidental. These classes of action will be considered in more detail in the following Sections.

In limit state design, characteristic values of actions are used as the basis of all calculations. They are values which statistically have only a small probability of being exceeded during the life of the structure. To provide a margin of safety, particularly against collapse, partial safety factors are applied to these characteristic values to obtain design quantities. In principle, different partial safety factors can be applied depending on the degree of uncertainty or variability of a particular type of action. In practice, whilst this appears to be the case, the actual values of partial safety factors used incorporate significant elements of the global safety factor and do not represent a rigorous probabilistic treatment of the uncertainties of the actions.

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2. PERMANENT ACTIONS

Permanent actions, as the name implies, are always present and must be considered in all cases. They comprise what are traditionally referred to as dead loads, but may also include permanent imposed loads due, for instance, to machinery or stored material.

2.1 Dead Loads

Dead loads are gravity loads due to the self weight of the structure and any fixtures or finishes attached to it (Figure 1). Their magnitudes can be estimated with reasonable confidence based on prescribed dimensions and a knowledge of material density. Even so, variations due to constructional tolerances and natural variations in materials, will exist. Furthermore, fixtures, fittings and finishes may be replaced or modified during the life of the structure. This possibility has been recognised in calculating loads on bridge decks, for which a separate load category of 'superimposed dead load' is included to allow for surfacing which is likely to be replaced a number of times during the life of the bridge. For this situation there is consequently a much greater potential for variability than for other dead loads.

A similar condition exists within certain types of building with respect to partitions (Figure 2). Where the position of walls is predetermined their weight can simply be included as a dead load. For more speculative development, internal partitions will be the responsibility of the client and their layout is likely to change many times during the life of the building. An allowance, as an equivalent uniformly distributed load, is therefore normally made.

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Schedules of densities for common building materials are listed in Eurocode 1 [1] and manufacturers of proprietary products, such as cladding, blockwork, raised floors, etc. provide information on weights. Together with specified dimensions, these data enable dead loads to be calculated. Where dead loads are not strictly evenly distributed over a plan area, such as timber floor joists located at discrete intervals, they are often represented as an equivalent uniformly distributed load for convenience in design calculations. As long as the equivalent magnitude is determined in a rational manner, any differences between this simplified approach and a more rigorous analysis taking account of the actual location of the joists will be negligible.

To determine dead loads, consider, for example, the case of a floor consisting of a 150mm thick reinforced concrete slab with 50mm lightweight screed and a 15mm plaster soffit. Details are shown in Figure 3 together with densities for each material. The total dead load per square metre of floor plan can be calculated as follows:

lightweight screed 15 x 0,05 = 0,75 kN/m2

rc slab 24 x 0,15 = 3,60

plaster 12 x 0,015 = 0,18

total dead load = 4,53 kN/m2

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In addition an allowance would normally be made for any services or fittings (electric lighting, pipework, etc.) fitted to the underside of the slab or located within the screed or under a raised floor (Figure 4). This case is another where an equivalent uniformly distributed load is used to represent load sources distributed in an uneven manner. A value between 0,1 and 0,3 kN/m2 is normally adequate to cover such installations.

The weight of walls can be treated in a similar manner to floors by considering the various component parts and summing the weights per square metre on elevation. For example, consider a cavity wall consisting of a tile-hung brick outer leaf (100mm thick) and a plastered blockwork inner leaf (150mm thick) as shown in cross-section in Figure 5.

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The total dead load is determined as follows:

tiles 0,6 kN/m2

brickwork 2,1

blockwork 1,4

plaster 0,2

total dead load of wall 4,3 kN/m2

By multiplying this value by the height of the wall, the load intensity as a line load on the supporting structure can be determined.

Loads due to internal lightweight stud or blockwork partitions cannot normally be treated in such a rigorous manner since their location is often not known at the design stage and in any case may change during the life of the building. Instead an allowance is made within the assessment of imposed loads which is described under variable actions.

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3. VARIABLE ACTIONS

Variable actions comprise loads which are not always acting but may exist at various times during the normal use of the structure. They include loads due to the occupation of a building and traffic on bridges (imposed loads), snow and wind loads (environmental loads), and temperature effects (Figure 6). They do not include accidental conditions such as fire, explosion or impact.

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3.1 Imposed Loads

Imposed loads - sometimes referred to as "superimposed", "super" or "live" loads -are those loads due directly to the use of the structure. For buildings, they are concerned with the occupancy by people, furniture, equipment, etc. For bridges they are due to traffic, whether pedestrian or vehicular.

Clearly these conditions will be almost constantly changing and are rather more difficult to quantify than dead loads. For buildings, the approach has therefore been to relate imposed load levels to occupancy, and to base them on observation and sensible deduction. Eurocode 1: Basis of Design and Actions on Structures [1] distinguishes between four classes of loaded floor area as follows:

• areas of dwellings, offices, etc.• garage and traffic areas.• areas for storage, production machinery and filing.• areas serving as escape routes.

The first class is further subdivided into four categories according to their specific use. They are residential (including hospital wards, hotel bedrooms etc.), public premises (such as offices, hotels, hospitals, schools, leisure centres etc.), public premises susceptible to overcrowding (including assembly halls, conference rooms, theatres, shopping areas and exhibition rooms), and public premises susceptible to overcrowding and accumulation of goods (including areas in warehouses and department stores).

The characteristic values of the imposed loads for these different categories are given in Table 1. Thus domestic residences attract a lower imposed load than office accommodation; areas of public assembly, where large numbers of people could gather at any one time, are prescribed a high superimposed load. Storage areas must be particularly carefully considered and Eurocode 1 includes details of densities for a range of stored materials. Some of these, such as steel strip, will generate high loads, but even apparently innocuous conditions, such as filing stores, can experience very high loading levels. Escape routes must be designed for relatively high imposed loads.

Although such loads are used in limit state design in a semi-probabilistic way and are referred to as characteristic values (implying a statistical basis for their derivation) little data is available. A proper statistical analysis is not therefore possible and values specified are nominal quantities. One study which was conducted into office accommodation in the UK [4] revealed a wide variation in actual load levels for similar building occupancies. In all cases the load levels measured were considerably less than the characteristic values specified for the structural design. However, this observation must be viewed with some caution since design must allow for extreme conditions, misuse and panic situations.

Note that, although imposed loading will rarely be evenly distributed, a uniform distribution of load intensity is normally assumed (Figure 7).

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3.2 Permitted Reductions in Imposed Load

The nominal values of imposed load associated with different classifications of building occupancy and use represent extreme conditions. In many cases the probability of such conditions existing simultaneously throughout a building is remote. In recognition of this remote possibility some reductions in imposed load intensity may be permitted. Reduction applies particularly to columns in multi-storey buildings where it increases with the number of floors supported by a particular length of column. Typical reductions range from 10% to 30% and apply to imposed loads only. No reductions are permitted in dead load or for certain types of imposed load - notably in the case of storage areas, crane loads, and loads explicitly allowed for such as those due to machinery or due to people in public premises susceptible to overcrowding.

3.3 Superimposed Bridge Loads

In practice a highway bridge is loaded in a very complex way by vehicles of varying sizes and groupings. In order to simplify the design process this real loading is typically simulated by two basic imposed loads - a uniformly distributed load and a knife edge load - representing an extreme condition of normal usage (Figure 8). The design is then checked for a further load arrangement representing the passage of an abnormal load. The magnitudes of all these loads are generally related to the road classification, the highway authority's requirements and the loaded length of the bridge.

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For vehicular traffic within buildings, lightweight conditions (less than 16 tonnes) can be dealt with in categories such as cars, light vehicles and medium vehicles. For heavier traffic, highway loading must be considered.

Railway bridge design must take account of static loading and forces associated with the movement of vehicles. As for highway bridges, two models of loading are specified for consideration as separate load cases. They represent ordinary traffic on mainline railways and, where appropriate, abnormal heavy loads. They are expressed as static loads due to stationary vehicles and are factored to allow for dynamic effects associated with train speeds up to 300km/h. Eurocode 1 also gives guidance on the distribution of loads and their effects and specifies horizontal forces due to vehicle motion. Centrifugal forces associated with the movement around curves, lateral forces due to oscillation of vehicles (nosing) and longitudinal forces due to traction and braking are included.

Other aspects of bridge loading which need to be considered include accidental loads and the possibility of premature failure due to fatigue under traffic loading.

3.4 Crane Loads

For buildings fitted with travelling overhead cranes, the loads due to the crane itself and the lifted load are considered separately. The self weight of the crane installation is generally readily available from the manufacturer, and the load lifted corresponds to the maximum lifting capacity of the crane. When a load is lifted from rest, there is an associated acceleration in the vertical direction. In the same way that gravity loads are equal to mass multiplied by the acceleration due to gravity, so the lifting movement causes an additional force. If the load is lifted very gently - that is with little acceleration - this force will be very small, but a sudden snatch, i.e. a rapid rate of acceleration, would result in a significant force. This force is of course in addition to the normal force due to gravity, and is generally allowed for by factoring the normal static crane loads.

Movements of the crane, both along the length and across the width of the building, are also associated with accelerations and retardations, this time in the horizontal plane. The associated horizontal forces must be taken into account in the design of the supporting structure. The magnitude of the forces will depend, as before, on the rates of acceleration. The normal procedure is to calculate the magnitudes on the basis of a proportion of the vertical wheel load.

The approach yields an equivalent static force which can be used in designing the structure for strength. However, the nature of crane loads must also be recognised. The possibility of premature failure due to fatigue under the cyclic loading conditions should be considered.

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3.5 Environmental Loads

Environmental loads are clearly variable actions. For bridges and buildings the most important environmental loads are those due to snow and wind. For marine structures, particularly offshore installations such as oil platforms, loads due to water movements are often dominant. The action of waves generally represents the most severe condition. In certain geographical locations, the effects of earthquakes must be included in the structural analysis. All of these loads from environmental sources are beyond the control of man. It has therefore been recognised that a statistical approach must be adopted in order to quantify corresponding design loads.

The approach is based on the 'return period' which is a length of time to which recorded environmental data, such as wind speeds, snowfall or wave heights, is related. If records are only available over a relatively short period, data for the 'return period' may be predicted. The most severe condition on average over the return period then represents the design value. For a return period of 100 years, for example, it is referred to as the 1 in 100 year wind speed or wave height, etc. The return period normally corresponds to the design life of the structure. Clearly there is a degree of uncertainty about the process of predicting the most severe conditions likely to be encountered. Further simplifications are implicit in translating measured environmental data such as wind speeds or wave heights into loads.

3.6 Wind Loads

Wind forces fluctuate with time but for many structures the dynamic effect is small and the wind load can be treated using normal static methods. Such structures are defined as 'rigid' and Eurocode 1 [1] provides guidance on this classification. For slender structures the dynamic effect may be significant. Such structures are classified as 'flexible' structures and their dynamic behaviour must be taken into account.

The most important parameter in quantifying wind loads is the wind speed. The basis for design is the maximum wind speed (gust) predicted for the design life of the structure. Factors which influence its magnitude are:

• geographical location; wind speeds are statistically greater in certain regions than others. For many areas considerable statistical data is now available and basic wind speeds are published usually in the form of isopleths (Figure 9) which are lines of equal basic wind speed superimposed on a map. The basic wind speed is referred to in Eurocode 1 [1] as the reference wind speed and corresponds to the mean velocity at 10m above flat open country averaged over a period of 10 minutes with a return period of 50 years.

• physical location; winds gust to higher speeds in exposed locations such as coasts than in more sheltered places such as city centres (Figure 10), because of varying surface roughness which reduces the wind speed at

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ground level. This variation is taken into account by a roughness coefficient which is related to the roughness of the terrain and the height above ground level.

• topography; the particular features of a site in relation to hills or escarpments are taken into account by a topography coefficient.

• building dimensions; height is important in particular because wind speeds increase with height above ground level (Figure 11).

• the mean wind velocity is determined by the reference wind velocity factored to account for the building height, ground roughness and topography. The wind pressure is proportional to the square of the mean wind speed. In addition the following parameters are important:

• structural shape; it is important to recognise that wind loads are not simply a frontal pressure applied to the facade of a structure but are the result of a complex pressure distribution on all faces due to the movement of air around the whole structure. The distribution is further complicated by adjacent structures and natural obstructions/variations such as hills, valleys, woodland which may all influence the pattern of air movement and associated pressure distribution.

• roof pitch; this parameter is really a special aspect of structural shape. It is worth noting that roofs with a very shallow pitch may be subject to uplift or suction, whilst steeper roofs - say greater than about 20° - are likely to be subject to a downwards pressure (Figure 12).

• wind direction; pressure distributions will change for different wind directions (Figure 13).

• gust response factor; this factor is used to take into account the reduction of the spatial average of the wind pressure with increasing area due to the non-coincidence of maximum local pressures acting on the external surface of the structure. Thus small parts of a building, such as cladding units and their fixings, must be designed for higher wind pressures than the whole structure. The gust response factor is related to an equivalent height, which corresponds approximately to the centroid of the net wind force on a structure.

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Tabulated procedures enable the above parameters to be accounted for firstly in calculating the design wind speed, and secondly in translating that wind speed into a system of forces on the structure. These equivalent static forces can then be used in the analysis and resistance design of the structure, as a whole. However, certain additional features of wind should also be taken into account:

• local pressures, particularly at corners and around obstructions in an otherwise 'smooth' surface, can be significantly higher than the general level (Figure 14). High local pressures particularly affect cladding and fixing details, but can also be a consideration for structural elements in these areas.

• structures sensitive to wind should be given a more sophisticated treatment. It might involve wind tunnel testing and include the influence of surrounding buildings. Structures which might need to be treated in this way include high-rise buildings, long or slender bridges, masts and towers.

• aerodynamic instability may be a consideration for certain types of structure or component, for example chimneys and masts. Vortex shedding can normally be avoided by the use of strakes (Figure 15). Galloping may be a problem in cables.

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3.7 Snow Loads

Loads due to snow have traditionally been treated by specifying a single load intensity, with possible reductions for steep roof slopes. This approach takes no account of such aspects as the increased snowfall at higher altitudes or of locally higher loads due to drifting. Cases of complete or partial collapse due to snow load are not unknown [5]. A more rational approach is to use a snow map giving basic snow load intensities for a specified altitude and return period similar to the treatment for basic wind speeds (Figure 16). Corrections for different altitudes or design life can then be applied as shown in Table 2. At present the European snow map is provisional and further work is under way to acquire more data.

Allowance for different roof configurations can be dealt with by means of a shape coefficient. It provides for conditions such as accumulations of snow behind parapets, in valleys and at abrupt changes of roof height (Figure 17). In addition to snow falling in calm conditions, it may be necessary to consider the effects of wind. Wind may cause a redistribution of snow, and in some cases its partial removal from roofs. Any changes in snow distribution on roofs due to excessive heat loss through part of the roof or snow clearing operations should be accounted for if such loading patterns are critical. Eurocode 1 [1] does not cover additional wind loads due to the presence of snow or the accretion of ice, nor loads in areas where snow is present throughout the year.

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3.8 Wave Loading

For offshore structures in deep and hostile waters, wave loads can be particularly severe. The loads arise due to movement of water associated with wave action. These movements can be described mathematically to relate forces to physical wave characteristics such as height and wavelength.

The treatment is therefore similar to wind loads in that these physical characteristics are predicted and corresponding forces on the particular structural arrangement then calculated. These calculation procedures are, however, very complicated and must realistically be performed on a computer.

3.9 Temperature Effects

Exposed structures such as bridges may be subject to significant temperature variation which must be taken into account in the design. If it is not provided for in terms of allowing for expansion, significant forces may develop and must be included in the design calculations. In addition, differential temperatures, e.g. between the concrete deck and steel girders of a composite bridge, can induce a stress distribution which must be considered by the designer.

3.10 Retained Material

Structures for retaining and containing material (granular or liquid) will be subject to a lateral pressure. For liquids it is simply the hydrostatic pressure. For granular material a similar approach can be adopted, but with a reduction in pressure depending on the ability of the material to maintain a stable slope - this is the Rankine approach. Ponding of water on flat roofs should be avoided by ensuring adequate falls (1:60 or more) to gutters.

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3.11 Seismic Loads

In some parts of the world earthquakes are a very important design consideration. Seismic actions on structures are due to strong ground motion. They are a function of the ground motion itself and of the dynamic characteristics of the structure.

Strong ground motion can be measured by one of its parameters, the maximum ground acceleration being the parameter most usually adopted for engineering purposes. These parameters are expressed on a probabilistic basis, i.e. they are associated with a certain probability of occurrence or to a return period, in conjunction with the life period of the structure [3].

3.12 Accidental Loads

Accidental actions may occur as a result of accidental situations. The situations include fire, impact or explosion. It is very difficult to quantify these effects. In many cases it may be preferable to avoid the problem, for instance by providing crash barriers to avoid collision from vehicles or roof vents to dissipate pressures from explosions.

Where structures such as crash barriers for vehicles and crowds must be designed for 'impact' the loading is treated as an equivalent static load.

4. CONCLUDING SUMMARY

• There are many sources of structural loads, notably dead loads, those due to the use of the structure and environmental effects such as wind, earthquake, snow and temperature. The loads must be quantified for the purpose of structural design. Dead loads can be calculated. Imposed loads can only be related to type of use through observation on other similar structures. Environmental loads are based on a statistical treatment of recorded data.

• Calculated or prescribed values of loads are factored to provide an adequate margin of safety. The nature, as well as the magnitude, of the loads must be recognised, particularly in terms of dynamic and fatigue behaviour.

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5. REFERENCES

[1] Eurocode 1: Basis of Design and Actions on Structures, CEN (in preparation).

[2] Eurocode 3: Design of Steel Structures: ENV 1993-1-1: Part 1.1, General principles and rules for buildings, CEN, 1992.

[3] Eurocode 8: Structures in Seismic Regions - Design, CEN (in preparation).

[4] Floor Loadings in Office Buildings - the Results of a Survey, BRE Current Paper 3/71, Building Research Establishment, Watford, 1971.

[5] Design Practice and Snow Loading - Lessons from a Roof Collapse, The Structural Engineer, Vol 64A, No 3, 1986.

6. ADDITIONAL READING

1. Monograph on Planning and Design of Tall Buildings, Volume CL, Tall Building Criteria and Loading, American Society of Civil Engineers, 1980.

2. Civil Engineer's Handbook, Butterworths, London, 1974.3. Bridge Aerodynamics Conference, Institute of Civil Engineers, Thomas

Telford, London, 1981.4. On Methods of Load Calculation, CIB Report No 9, Rotterdam, 1967.5. BRE The Designer's Guide to Wind Loading of Building Structures

Part 1 Butterworths, 1985

Part 2, Butterworths, 1990.

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Loaded Areas α

[kN/m2]

Category A

Category B

Category C

Category D

- general

- stairs

- balconies

- general

- stairs, balconies

- with fixed seats

- other

- general

2,0

3,0

4,0

3,0

4,0

4,0

5,0

5,0

Table 1 Imposed loads on floors in buildings

Snow load so [kN/m2]

Altitude [m]

Zone 0 200 400 600

1 0,40 0,49 0,70 0,95

2 0,80 0,98 1,40 1,89

3 1,20 1,47 2,09 2,84

4 1,60 1,97 2,79 3,78

5 2,00 2,46 2,49 4,73

Table 2 Snow loads for zones given in Figure 16

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so = 0,412z

where:

A is the altitude of the site above mean sea level [m]

z is a constant, depending on the snow load zone.