Eurocode for Structural Loading

information paper

CI/SfB (J4) September 1998

Eurocode 1

IP 13/98

The code for structural loading

Part 1

Basis of design, dead, imposed, re, snow and wind loads JB Menzies and H Gulvanessian

This two-part Information Paper describes the evolution of Eurocode 1, summarises its contents, and gives some background information on its derivation. It describes the assumptions and requirements of Eurocode 0 Basis of design,

to explain the context in which Eurocode 1 is intended to be used. It gives references, where possible, to enable practising engineers to obtain further insight into the basis and use of the Codes requirements.

This rst part covers basis of design, dead, imposed, re, snow and wind loads. Part 2 covers thermal actions, actions during execution, accidental actions, traffic loads on bridges, actions in silos and tanks, and actions induced by cranes and machinery.

The Structural Eurocodes form a unified set of international codes of practice. This set provides The European code of practice Actions on structure— s, the basis for the limit state design of a wide range commonly known as Eurocode 1 — is a comprehensive of building and civil engineering structures that modern code of practice providing information on all actions (loads) that it is normally necessary to consider in includes buildings, bridges, masts, towers, silos, the design of building and civil engineering structures. Its tanks, chimneys and geotechnical structures. preparation, in ten Parts, began in 1985. All Parts have When completed as European standards (EN), been accepted for publication as European prethere will be ten Eurocodes for structural design standards (ENV), ie codes for voluntary experimental (see Table 1), comprising as a whole a portfolio use. A ve-year programme to convert them to European of more than 50 Parts[1,2]. standards (EN) has recently begun. Development of When European standard Eurocode 1 is issued, it Eurocode 1 is now in an advanced stage. will comprise ten EN Parts (see Table 1 The Structural Eurocodes Table 2). The Parts are EN Known as Title Number referred to in this IP by number of Parts their proposed EN numbers. These Parts 1990 Eurocode 0 Basis of design 1 will provide the actions 1991 Eurocode 1 Actions on structures 10 (loads) for use with 1992 Eurocode 2 Design of concrete structures 4 Eurocodes 2 to 9 as 1993 Eurocode 3 Design of steel structures 14 appropriate for design 1994 Eurocode 4 Design of composite steel and concrete structures 4 and verification on the 1995 Eurocode 5 Design of timber structures 3 basis of overall principles 1996 Eurocode 6 Design of masonry structures 5 which are given in 1997 Eurocode 7 Geotechnical design 3 Eurocode 0. 1998 Eurocode 8 Design of structures for earthquake resistance 6 Eurocode 1

1999

Eurocode 9

Design of aluminium structures

3

2 Table 2 The Parts of Eurocode 1: Actions on structures Proposed Title

Current

EN number

ENV number

1991-1-1

Densities, self-weight and imposed loads

1991-1-2

Actions on structures exposed to re

1991-2-2

1991-1-3

Snow loads

1991-2-3

1991-1-4

Wind actions

1991-2-4

1991-1-5

Thermal actions

1991-2-5

1991-1-6

Actions during execution

1991-2-6

1991-1-7

Accidental actions due to impact and explosions

1991-2-7

1991-2

Trafc loads on bridges

1991-3

1991-3

Actions in silos and tanks

1991-4

1991-4

Actions induced by cranes and machinery

1991-5

The evolution of Eurocode 1 Structural design practice varies substantially across Europe. Different design loads, design methods, fabrication and construction techniques have evolved based on local tradition and circumstances. Variations in economic and sociological standards are reflected in local practice and industry together with differences arising from the local climate: this ranges widely, from the continental climate in central Europe to the maritime climates of the north and northwest, to the warmer climates of the south. National codes of practice mirror the local national situation. Some countries have sophisticated national structural design codes; others have no codes for specific types of structure and use national codes from other countries. The preparation of the Structural Eurocodes as a whole is being undertaken against this background with the primary objective of achieving convergence to a consistent structural design practice throughout Europe. They are being prepared initially by project teams familiar with both traditional European experience and with the results of research in the field. They are being subjected to substantial peer review. Inevitably in such a major programme involving so many engineers from 18 participating countries, there are a few areas where the documents overlap or are inconsistent. These shortfalls are being addressed in the current programme to convert the Eurocode prestandards into European standards. Another objective of the Eurocodes is to provide design rules for everyday use and to provide the basis for innovative design. For construction projects as a whole and for the manufacture of specific construction products, the aim is to facilitate innovative design, and to ensure that the Eurocodes do not prevent the development of innovative products.

1991-2-1

To help meet this objective, a distinction is made in Eurocode texts between Principles and Application rules. Principles comprise: ● general statements and definitions for which there is no alternative; ● requirements and analytical models for which no alternative is permitted unless specifically stated. Application rules are generally recognised rules, which follow the principles and satisfy their requirements. Use of alternative rules different from the application rules given in Eurocodes 0 and 2 to 9 is permitted provided that the alternative rules meet the relevant principles and will achieve at least the same structural reliability. A proposal to develop an international set of codes of practice for structural design was first agreed in 1974 by several technical-scientific organisations based largely in Europe[3]. Following preparatory work by these organisations, the Commission of the European Communities (CEC) together with the European Free Trade Association (EFTA) took the initiative for developing the Structural Eurocodes at the end of the 1970s by establishing a steering committee to oversee the work. In 1989 the responsibility for their development was transferred to the European Committee for Standardisation (CEN). Financial contributions to the costs of the work continued to be provided by the CEC and EFTA. Technical Committee CEN/TC250 was set up to complete the task of preparation and implementation. Since English was the most widely spoken and understood language, a decision was made in 1991 to conduct meetings and to prepare the Eurocodes in English in the first instance. Once each Eurocode Part is approved for publication as a

3 pre-standard (ENV) it is translated into the other two official CEN languages, French and German, and published in the three languages. In some cases, CEN member bodies have translated these pre-standards into their own local language. Most Eurocodes are in the form of European pre-standards (ENV) issued for experimental use and comment. During the development of these ENV Eurocodes, existing national codes have either not been updated or they have been amended, usually to make them more compatible with the emerging ENVs. In some member countries there are no national codes comparable to some of the developing Eurocodes. At present the ENV Eurocodes contain ‘boxed’ values, eg for partial factors and load combination factors, which national standards bodies may modify for experimental use of the pre-standard in their country. A programme to convert the ENV Eurocodes to European standards (EN) has begun recently. Their issue as European standards will lead to the withdrawal from use of the national codes of practice in the different European countries which are members of CEN. There will be a period of perhaps five years when national codes co-exist with the European standard Eurocodes before the national codes are withdrawn. Updating of the EN Eurocodes will follow normal CEN procedures, each document being reviewed and amended to keep abreast of the advances in construction technology on a fiveyear cycle. Until the mid-1980s, Eurocode 1 consisted only of common safety requirements and common principles and rules. These common unified rules were published in 1984[4]. They were not operational but provided a basis for preparing the operational Eurocodes 2 to 8. Eurocode 9: Design of aluminium structures was not added to the Eurocode preparation programme until 1992. The initial work of Eurocode preparation did not therefore include development of rules for actions (loading). It was not until 1984 that the steering committee agreed to a proposal that an enquiry on national codes and standards concerned with actions be undertaken by BRE amongst Member States. The report of the enquiry concluded that the preparation of a

Eurocode for actions on structures was feasible. With the agreement of the steering committee a small task group was established to advise on the steps necessary. The task group was supported by national bodies including BRE, Centre Scientifique et Technique due Batiment (CSTB), Institut für Bautechnic (IfBt), and the Danish Building Research Institute (SBI). An outline for a comprehensive Eurocode for actions was proposed, together with suggestions for the first stages of the work, based on preparatory studies by task group members. The proposal was accepted by the steering committee in 1985. An inherent feature of the proposed Eurocode was that it would be consistent with the existing drafts of Eurocodes 2 to 8 and suitable for structural design based on the limit state concept using the partial safety factor format. Eurocode 1 then became entitled Basis of design and actions on structures embracing the existing Part covering basis of design and the new additional Parts covering actions. Priority was first given in developing the Actions Parts of Eurocode 1 to the most important actions related to the design of building structures. The aim was to have the Parts covering these actions – dead loads, imposed loads, snow loads, wind loads, and actions due to fire – available for use when, or as soon as possible after, publication for experimental use by CEN of Eurocode 2: Part 1 Design of concrete structures (ENV 1992-1-1) and Eurocode 3: Part 1: Steel structures: General rules (ENV 1993-1-1). These two Eurocodes were in the most advanced stage of preparation at the time and the availability of loading requirements was seen as important to facilitate their future development. Subsequently, the scope of the work on actions was extended to include traffic loads on bridges (road and rail) and loads in silos and tanks leading finally to the portfolio of parts indicated in Table 2. For conversion to European standards (EN), a decision was made in 1997 to divide ENV Eurocode 1: Basis of design and actions on structures into two separate documents: EN 1990: Basis of design EN 1991: Actions on structures.

4 Eurocode 0: The basis of the structural Eurocodes Eurocodes as a whole, including Eurocode 1, adopt the general requirements and assumptions for safety and serviceability of structures and methods of design and verification as described in Eurocode 0: Basis of design. It was published in the UK as ENV 1991-1 in 1996. The main requirement is that structures and structural elements are designed, executed and maintained so that, with appropriate degrees of reliability, they will: ● perform adequately under all expected actions; ● withstand all actions and other influences likely to occur during execution and in use and have adequate durability in relation to maintenance costs; ● not be subsequently damaged disproportionately to the original cause in the case of exceptional hazards such as fire, explosion, impact or human error. The use of the term appropriate degrees of reliability allows the reliability to be different for different structures, eg where consequences of failure are high, a higher level of reliability may be chosen. The validity of use of the design principles depends on a number of assumptions; they include: ● Structures are designed by appropriately qualified and experienced personnel and execution is carried out by personnel having the necessary skill and experience. ● The construction materials and products are used as specified in the Eurocodes or in the relevant material or product specifications. Adequate quality assurance measures are applied. ● The structure will be adequately maintained. The basis of design and verification given in Eurocode 0 includes guidelines on: ● limit states; ● classifications concerning actions and environmental influences; ● material properties and geometrical data; ● modelling for structural analysis and resistance; ● design assisted by testing; ● verification by the partial factor method. These guidelines for meeting the requirements have generally been the basis for the preparation of the more detailed principles and rules given in Eurocodes 1 to 9 (Table 1).

Eurocode 0 defines an action (F) as either: ● a direct action, ie force (load) applied to the structure ● an indirect action, ie an imposed or constrained deformation or an imposed acceleration caused, for example, by temperature changes. Actions are described by a model, eg vehicle axle spacing, and their magnitude is commonly represented by a single scalar. The scalar may adopt several representative values, eg dominant or non-dominant action. Several scalars are used when the action is multicomponent. More complex representations are required for fatigue and dynamic actions. Actions are classified by: ● their variation in time, ie: permanent actions (G); variable actions (Q); and accidental actions (A); ● their variation in space, ie: fixed actions (eg self-weight); free actions (eg wind and snow loads); ● their nature and/or structural response, ie: static actions; dynamic actions. The term single action is also used to define an action which is statistically independent in time and space from any other action acting on the structure. The characteristic value of an action is its main representative value. The self-weight of a structure can be represented by a single characteristic value Gk provided the variability of G is small, and can be calculated on the basis of the nominal dimensions and the mean unit mass. If the variability of G is not small and the statistical distribution is known, two values are used; an upper value Gk, sup and a lower value Gk, inf . More information on this subject has been given by Ostlund[5]. A variable action has the following representative value – see Figure 1: the characteristic value Q k the combination value ψ0Q k the frequent value ψ1Q k the quasi-permanent value ψ2Q k The combination value ψ0Qk takes account of the reduced probability of simultaneous occurrence of the most unfavourable values of several independent variable actions. It is used for the verification of ultimate limit states and irreversible serviceability limit states. The frequent value ψ1Q k is used for verification of

variable load Q

5

Qk

characteristic value

ψ0Q k

combination value

ψ1Q k

frequent value

ψ2Q k

quasi-permanent value

Time Figure 1 Representative values

ultimate limit states involving accidental actions and reversible limit states. The quasi-permanent value ψ2Q k is also used for ultimate limit state verification involving accidental actions and for reversible serviceability limit states. The Eurocode values given for ψ0, ψ1, ψ2 for buildings are shown in Table 3 together with those adopted in the UK application documents. The ENV Eurocodes are not totally consistent in the values given for combination factors: some slightly different values appear in Eurocodes 2 to 5 primarily because they referred to the current BSI loading codes in 1992. To determine actions for use in design the following steps are required: ● for each relevant design situation identify critical load cases; ● for each critical load case determine design values for the effects of actions in combination using the partial factors for actions; Table 3 ψ factors for buildings — from DD ENV 1991-1 Action ψ0 boxed

UK*

EC0

● combine the design actions using the rules given; ● verify that the effects of design actions do not exceed the design resistance for ultimate limit states or the performance criteria for serviceability limit states. The partial factors for actions in the ENV Eurocodes for ultimate limit states are not totally consistent yet. Tables 4 and 5 show comparisons. There are also some differences between the ENV Eurocodes in partial factors and combinations of actions used for serviceability limit states. The theoretical background to Eurocode 0 is discussed by Vrouwenvelder[6] and a background document was published in 1996[7]. A designer’s handbook is available[8].

ψ1 boxed

ψ2 UK*

EC0

boxed

UK*

EC0

Imposed loads in buildings domestic, residential

0.7

0.5

0.5

0.4

0.3

0.2

ofces

0.7

0.7

0.5

0.6

0.3

0.3

0.7

0.7

0.6

0.6

congregation areas

0.7

shopping

0.7

0.7

0.6

storage

1.0

roof (including snow)

0.6

0.7

0.2

0.2

0.0

0.0

vehicle weight < = 30 kN

0.7

0.7

0.7

0.7

0.6

0.6

vehicle weight < = 160 kN

0.7

0.7

0.5

0.7

0.3

0.6

roofs

0.0

0.9

0.3

0.8

Trafc loads in buildings

Wind loads on buildings

0.6

Temperature (non-re) in buildings

0.6

0.0 0.7

0.5

0.0 0.2

0.5

0.0

0.0

0.0

Crane loads vertical

0.7

0.6

0.3

horizontal

0.7

0.6

0.3

0.9 (vertical + horizontal)

0.7

0.6

0.3

* Values determined for use of Eurocodes 2 and 3 before Eurocode 0 was published

6 Table 4 Partial factors for actions in ENV Eurocodes (ultimate limit states) Action description EC0 EC2 EC3 EC4 Permanent Variable

1.00

1.00

1.00

1.00

1.00

1.00

1.00

1.35

1.35

1.35

1.35

1.35

1.35

1.35

0.00

0.00

0.00

0.00

0.00

1.50

1.50

1.50

1.50

1.50

1.50

1.50

1.50

1.50

1.50

1.35

2nd and more favourable

0.90

1.00

1.10

1.10

0.90

0.90

unfavourable

1.10

1.34

1.35

1.35

1.10

1.10

1.00

1.00

both

Vectorial

EC7

favourable favourable

Prestressing

EC6

unfavourable unfavourable Single permanent

EC5

favourable

0.9/1.0

0.9

unfavourable

1.2/1.0

1.20

favourable

Earth and water pressure

0.80

0.80

1.35

1.35

1.50

0.70 1.35

Notes Vectorial refers to cases where the components of a vectorial effect vary independently. EC5 introduced a set of reduced partial coefcient for specied small structures which is as follows: Permanent Variable

favourable

1.00

unfavourable

1.20

favourable

0.00

unfavourable

1.35

EC2 states that snow drift loads should be treated as a variable action with the characteristic value taken at 0.7. EC3, EC4 and EC5 state that local snow drifting on roofs should be treated as an accidental action.

Table 5 Partial factors for actions (accidental situations) Action description EC0 EC2 EC3 Accidental action

EC5

EC6

1.00

1.05

favourable

1.00

1.00

0.90

0.90

1.00

1.00

unfavourable

1.00

1.00

1.05

1.05

1.00

1.00

1.00

1.00

Variable

1.0

EC4

1.00

Notes EC3 and EC4 state that the accidental action partial factor is relevant only where the accidental load is not specied directly. On conversion to EN the intention is to place all partial factors and combination values for actions in Eurocode 0.

EC7

7 Eurocode 1: Part 1.1: Densities, self-weight and imposed loads Scope and eld of application

This Part, which was published as an ENV in 1996, covers: ● the assessment of actions for use in structural design due to the density of construction materials and stored material; ● the self-weight of structural elements and whole structures and some fixed nonstructural items; ● imposed loads on floors and roofs of buildings, but excluding snow, which is covered by Part 1.3. Densities and self-weight

In developing the sections on densities and selfweight, consideration was given to the contents of the National Codes of the CEN member countries and the International Standard ISO 9194. Differences in the scope and specifications were found in the national codes and the guidance was at times contradictory. Additionally little statistical basis exists in general for the load values given in these codes. These differences imposed constraints on the development work. It was not possible to describe the load values as either mean or characteristic values since both of these terms imply some understanding of the underlying statistical distribution of the load values. The loads are therefore described as representative values. For materials where the bulk weight density has significant variability according to its source, a range of values is provided. Methods are provided for assessing the selfweight of construction elements in buildings, for example floors, walls, partitions, roofs, cladding and finishes. For bridges, determination of the self-weight of non-structural elements is also defined. Imposed loads on buildings

To determine imposed loads in buildings, loaded floor and roof areas are categorised into four classes according to their use: ● areas in dwellings, offices etc; ● garages and vehicle traffic areas; ● areas for storage and industrial activities; ● roofs.

Numerical values are given for floors and roof loads in buildings, including parking and vehicle traffic areas. For areas for storage and industrial activities only, guidance for the determination of numerical values is given. Different account is taken of the loading areas for several storeys (when a reduction factor is given) from the loading area within one storey The basis for the determination of the characteristic loads is given elsewhere[9]. For example, for the determination of the characteristic loads in dwellings, offices, schools, hospitals etc the loads are caused by: Furniture and movable objects (eg light movable partitions and loads from commodities and contents of containers). These loads may be subjected to considerable instantaneous changes in magnitude at certain times owing, for example, to changes of tenant or change of use. The load varies very slowly and the magnitude of variations is very small generally in the periods between the major instantaneous changes. Normal use by persons These loads are often periodical and present only during a relatively small part of the time, eg for school rooms only about one quarter of the day. Additionally, the proportion of the load caused by people can be very different, being very high in corridors and lower in residential buildings. The loads from persons may also cause dynamic effects. Extraordinary use Examples include exceptional concentrations of people (these can also give rise to dynamic effects) or of furniture, or the moving or stacking of commodities which may occur during reorganisation etc. These special situations usually occur during a short period of time. They occur sufficiently often during the lifetime of a building to make it necessary to take them into account. Assumptions have also been given[9] to enable characteristic values to be determined statistically. However, the statistical database was found to be poor: and it was very poor for short-term loads. Therefore, the values given in Part 1.1 were derived from comparisons of the values in national codes including values for all classes of imposed loads.

8 Eurocode: Part 1.2: Actions on structures exposed to re Scope and eld of application

This Part was available as a first draft in 1990[10] and it was published as an ENV in 1996. It covers the assessment of actions to be used in the structural design of buildings and civil engineering works where they are required to give adequate performance in fire exposure. It is intended for use with the parts on structural fire design in Eurocodes 2 to 6 and 9[11]. Actions on structures due to fire exposure are classified in the Eurocodes as accidental actions. For fire design, it follows that fire actions are the dominant action. Design situations

The combined occurrence of a fire in a building and an extremely high level of mechanical loads is assumed to be very small. The combinations of mechanical actions which need to be considered have been described elsewhere[11]. Simultaneous occurrence with other independent accidental actions need not be considered. However, Part 1.2 does require consideration of risks of fire in the wake of other accidental actions. Post-fire situations after the structure has cooled down do not need to be considered. For buildings, Part 1.2 requires fire compartments to be designed to prevent fire spread. Only one fully developed fire within one compartment is considered at a time. Determining actions arising in re

The basis for determining the thermal and mechanical actions arising in fire situations is given. The main text refers principally to nominal fires using the temperature/time relationships for standard fire conditions and assumed radiation and convection heat transfer characteristics. Nominal fires are assumed to be identical whatever the size or design of the building. Those used in Part 1.2 are mainly the standard fire (ISO 834), the hydrocarbon fire reaching a constant 1100°C after 30 minutes, and the external fire reaching a constant 680°C after 30 minutes. These nominal fires are used to verify that the level of fire resistance of structural elements meets national or other requirements which are expressed in terms of one of these nominal fires. Informative Annexes provide, for the first time in an international standard, models for more realistic calculation of thermal actions. They use so-called parametric temperature-time curves (Annex B) or the equivalent time of

fire exposure approach (Annex E). Parametric fire is a general term which covers fire evolution more in line with real fires and takes into account the main parameters which influence the growth of fires. Parametric temperature-time curves therefore vary mainly with building size, type of construction, fire load, and size of openings. At present the Annexes do not provide all the data needed to allow a performance-based structural fire design. Investigations are continuing with a view to making this approach fully operational when Part 1.2 is converted to a European standard (EN)[12]. The equivalent time of fire exposure approach allows use of realistic fire exposure depending on design fire load density and ventilation for the design of members by tabulated data or simplified rules[11]. The equivalent time is the time in the standard fire exposure (eg ISO 834) for a structural member to reach the maximum temperature obtained when exposed to the realistic fire. The relationships between equivalent time of exposure and parametric temperature-time curves are the subject of continuing studies[13,14]. In the United Kingdom, the thermal and mechanical actions determined using Part 1.2 are intended to be inputs to the verification of the structural design using Eurocodes 2 to 6 and 9 for performance requirements which are defined by Building Regulations[15]. Essentially, the objective is to limit risk to life from fire by meeting the following performance requirements of the structure: ● to maintain loadbearing function during the relevant fire exposure; ● to meet deformation criteria where the separating or protecting function of the construction may be impaired by structural deformation in the fire; ● to maintain separating function, ie no integrity or insulation failure, during the relevant fire exposure where fire compartmentation is required. Annex D of Part 1.2 gives guidance on the determination of fire load densities. The design value is either based on a national fire load classification or a survey of fire loads combined with partial factors to take account of fire consequences, fire frequency and active fire safety measures[11]. Investigations continue to examine methods used in countries worldwide for taking active measures into account with a view to improving this aspect of Part 1.2 on its conversion to a European standard[16].

9 Eurocode 1 : Part 1.3 : Snow loads Scope and eld of application

Published as an ENV in 1996, Part 1.3 provides guidance for the calculation of: ● snow loads on roofs which occur in calm or windy conditions; ● loads imposed by snow sliding down a pitched roof onto snowguards and other obstructions; ● loads due to snow overhanging the cantilevered edge of a roof; ● snow loads on bridges. Part 1.3 closely follows ISO 4355: 1981 and was prepared taking into account the content of existing national codes. It applies to: ● new buildings and structures; ● significant alterations to existing buildings and structures. It does not generally apply to sites at altitudes above 1500 m. Format for taking account of climatic variation

Both the initial deposition and any subsequent movements of snow on a roof are affected by the presence of wind. However, there is little data on the combined action of wind and snow to allow a direct statistical treatment. In design, the lack of data is normally overcome by considering one or more critical design situations. These situations are usually snow deposited when no wind is blowing and snow deposited when the wind speed is sufficient to cause drifting, but without quantifying the precise wind speed. Owing to the climatic variability across Europe, Part 1.3 provides different rules for single snow events (eg in the UK) and multiple snow events. Single snow events occur in regions where the snow fall is considered to be associated with weather systems of about three to four days duration and where there is a reasonable expectation that the snow deposited on roofs will thaw between the arrival of one weather system and the next. This situation requires the separate consideration of either uniform snow load or a drift load as the two are not expected to occur together. Multiple snow events occur where snow is more persistent, where snow falling in calm conditions is followed by further snow carried by another weather system driven by wind, and where there are several repetitions of these

events before significant thawing. In these situations the accumulations are combined into a single load case. Part 1.3 does not actually call these concepts single and multiple events; instead, it provides rules for such eventualities. It is left to the National Competent Authority to specify which should be used for a particular region. Classication of snow loads

Snow loads are classified as variable free actions. Part 1.3 allows snow loads to be treated as accidental actions in some cases. In particular, for local drifts occurring in climatic regions where single snow events generally occur and local drifting of snow on roofs is considered to form an exceptional load because of the rarity of the occurrence. Method of assessment of snow load on the roof

The snow load on the roof is derived by multiplying the characteristic value of the snow load on the ground by a snow load shape coefficient. In addition, Part 1.3 makes provision for adjustment of the roof snow load using an exposure coefficient factor to allow for abnormal exposure to the elements and a thermal coefficient factor for heat loss through the roof. Characteristic value of snow load on the ground

The snow load on the ground is that assumed to occur in perfectly calm conditions. It is usually determined from records of snow load or snow depth measured in well sheltered areas (ISO 4355 recommends in a deciduous forest). The characteristic value for the snow load on the ground is defined as the value with an annual probability of 0.02 of being exceeded. The variations of the snow load with geographic location is generally given in map form, with separate information for each CEN member country. Snow load shape coefcients

Several different snow load coefficients need to be considered in design. They relate to different climatic conditions before, during and after snow fall. Part 1.3 provides shape coefficients for monopitched, duo-pitched, multi-pitched and cylindrical roofs and coefficients for drifting at abrupt changes in roof height and at obstructions on roofs.

10 Three primary loading situations are identified and are accounted for in the coefficients provided: ● A uniformly distributed layer of snow over the complete roof, likely to occur when snow falls with little wind :balanced load part. ● Either an initially unbalanced distribution by local drifting at obstructions or a redistribution of snow which affects the load distribution on the whole roof, eg snow transported from the windward slope of a pitched roof to the leeward slope: unbalanced load part due to drifting. ● A redistribution of snow from an upper part of the building: unbalanced load part due to sliding. The coefficients provided for the multiple snow event are based on ISO 4355:1981 and the single snow event on BS6399: Part 3. Conversion to EN and future developments

A background document has been prepared on Part 1.3[17]. On conversion, Part 1.3 is expected to be modified to reflect the latest results of research and consideration of the new version of ISO 4355. To provide a fully harmonised code, a reappraisal of ground snow loads is currently in progress with a view to the production of a co-ordinated European snow map.

Eurocode 1: Part 1.4 : Wind actions Scope and eld of application

This Part was published as an ENV in 1997. It enables the assessment of wind loads for the structural design of buildings up to a height of 200 m, chimneys and other cantilevered structures, highway and railway bridges up to a span of 200 m and cycle/foot bridges up to a span of 30 m. It does not cover wind actions on lattice towers, cable stayed and suspension bridges, guyed masts or offshore structures. Basis and procedures

Part 1.4 was based initially on an ISO TC98 document and was developed using inputs from the latest wind engineering practice introduced into national standards in European countries[18]. It gives principles and rules for calculating static and dynamic response[19]. Two procedures are included: a simple approach for those structures whose structural properties do not make them susceptible to dynamic excitation and a detailed procedure for those structures which are likely to be dynamically responsive. Criteria for determining the appropriate procedure are included. Since the data on wind velocities in the different member countries are on different bases, Part 1.4 includes only an indicative European wind map and uses velocities defined in each country for determining the reference wind. Pressure coefficients are based on research undertaken principally at BRE and are upper bound values used for wind directions orthogonal to the building. Informative annexes give a detailed procedure for in-line response of dynamically responsive structures and rules for vortex excitation and other aeroeclastic effects. The technical complexity of wind actions on structures led to a concerted effort to provide a consistent framework for the National Application Documents (NAD)[20]. The United Kingdom’s NAD relating to buildings has been published (DD ENV 1991-2-4: 1997); publication of the companion NAD relating to bridges is expected in 1998.

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