National Application Document Part A Buildings

Introduction

This National Application Document (NAD) has been prepared by Subcommittee B/525/1. It has been developed from:

a) a textual examination of ENV 1991-2-4;

b) a limited parametric calibration against BS 6399: Part 2 and CP 3 : Chapter V : Part 2;

c) trial calculations.

This NAD encompasses wind actions as covered by ENV 1991-2-4. Specific UK requirements relating to" loads resulting from densities, self weight and imposed loads (ENV 1991-2-1), fire actions (ENV' 1991-2-2), snow loads (ENV 1991-2-3), thermal actions (ENV 1991-2-5), loads and deformations imposed during execution (ENV 1991-2-6) and accidental actions (ENV 1991-2-7) are given separately in the appropriate NADs for ENV 1991-2.

During the ENV period of provisional application, it has been assumed that the design codes to be used should be the relevant British Standards appropriate to the structure.

Care should be exercised if the wind action derived from ENV 1991-2-4 is used in conjunction with other design codes. Where other documents are used for design aspects, the values given will not necessarily produce the required level of reliability. Departure from any of the requirements given in the NAD should be agreed with the relevant Authority.

NOTE. The UK NADs for Eurocodes as already published l e.g. ENV 1992. ENV 199:3) do not permit the use of ENV 1991-2-4 in conjunction with those ENVs; but they recommend the use of CP.'i Chapter V with modifications. If tnal use of ENV 1991-2-4 is to be undertaken in conjunction with the already published design Eurocodes. modifications may be necessary.

1 Scope

This NAD provides information to enable ENV 1991-2-4 : 1995 to be used for the design of the following structures to be located in the UK or to be designed for UK loading:

a) building structures up to a height of 200 m. their components and appendages:

b) cantilevered structures;

c) highway and railway bridges up to 200 m span and for cycle track footbridges up to 30 m span.

ENV 1991-2-4 does not at present provide sufficient information for wind actions on lattice towers, guyed masts and chimneys.

NOTE Tins is being developed in ENV 199:3-:i!'

Specific information on wind actions on bridses is contained in Part Bn.

Whilst this NAD can be used for the structural appraisal and assessment of existing construction, for assessing repairs and modifications, or for assessing change of use, it will need to be used in conjunction with other appropriate documentation.

2 References

2.1 Normative references

This National Application Document incorporates, by dated or undated reference, provisions from other publications. These normative references are made at the appropriate places in the text and the cited publications are listed on page lvii. For dated references, only the edition cited applies; any subsequent amendments to or revisions of the cited publication apply to this Part of DD ENV 1991 only when incorporated in the reference by amendment or revision. For undated references, the latest edition of the cited publication applies, together with any amendments.

2.2 Informative references

This National Application Document refers to other publications that provide information or guidance. Editions of these publications current at the time of issue of this NAD are listed on page lvii, but reference should be made to the latest editions.

3 Partial safety factors, combination factors and other values a) The values for partial safety factors on wind actions for buildings y should be those given in table 9.2 of ENV 1991-1: 1994 as modified by the NAD for that Part.

b) The values for combination and reduction factors lF for buildings should be those given in table 9.3 of ENV 1991-1 : 1994 as modified by the NAD for that Part.

c) For different levels of reliability the partial safety factors on wind action. 7, given in table 9.2 of ENV 1991-1 : 1994 would need to be factored. For example, for nuclear installations 7 would need to be multiplied by 1.6.

4 Notation

ENV 1991-24 uses some notation that differs from that adopted in BS 6399 : Part 2 and other British Standards. The ENV notation should be adopted when using this NAD.

In preparation.

5 Additional recommendations

5.1 General

It is recognized that there are several alternative methods of deriving the application rules provided in ENV1991-2-4. It is recommended that during the ENV period, the procedures as set out in informative annexes B and C of ENV 1991-24: 1995 are followed.

5.2 Meteorological information

5.2.1 Section 7. Reference wind a) Clause 7.1

The air density to be used for structures designed in the United Kingdom should be taken as p = 1.225 kg/m2.

5.2.2 Annex A. Meteorological information and national wind maps a) Figure A. 11

The wind map should be replaced by figure 1 for the design of structures in the United Kingdom. For structures to be built in Northern Ireland the data provided in A.9 of ENV 1991-24 should not be used.

NOTE. It shouid be noted that the map wind speeds are identical to those in BS 6399: Part 2 despite the fact that the values are 10 mm averages as opposed to hourly mean values in the British Standard. This is due to the rougher terrain used as the standard category in ENV 1991-2-4 (z0 = 0.05) compared with that in BS 6399 : Part 2 (z0 = 0.03).

5.2.3 Section 8. Wind parameters a) Clause 8

ENV 1991-24 provides four categories of terrain as defined in 8.2 and given in table 8.1. To assist in the selection of the appropriate terrain category photographs/diagrams of typical terrains are provided in annex A of this NAD.

For circumstances where there are changes in terrain category close to the site, the simplified procedure set down in 8.3 (2) and (3) should not be used for structures higher than [15]m. In such circumstances the procedure set down in annex B may be followed.

This necessitates the introduction of a further terrain category 0, which represents open sea and has the parameters z0 = 0.003, k^ - 0.16.

5.3 Obstruction heights

Advantage can be taken of the shelter effect of buildings upwind in towns, i.e. in categories m or IV by using the procedures set down in annex C of this NAD.

5.4 Limitations on use of ENV 1991-2-4

As the majority of UK is within 100 km of a coast, the wind characteristics at most sites will be influenced by the sea and so for buildings for which wind loading is a governing condition, the wind parameters need to be defined more precisely than is necessary on mainland Europe. For such buildings the wind structure defined in ENV 1991-24 is not adequate for the unique conditions in the UK.

Appropriate procedures, incorporating the recent advances in wind engineering, are set out in BS 6399 : Part 2 : 1995 and a relatively simple procedure for incorporating these provisions is outlined in annex E of this NAD.

If structures are designed using ENV 1991-24, it is recommended that an additional check is undertaken using the procedure given in annex E of this NAD. If this results in more onerous loading the designer should be satisfied that there is adequate safety, by either using the higher loads or using appropriate measures in conjunction with the ENV loads.

5.5 Pressure coefficients

5.5.1 Section 10. Aerodynamic coefficients a) Clause 10.2.1 (2)

The coefficients are upper bound values as they have to account for any higher coefficients which may occur when the wind blows other than normal or parallel to a face. They have been simplified, for codification purposes, to be applicable over relatively large surface areas; this inevitably leads to some conservatism.

The pressure coefficients given in tables 10.2.1 to 10.2.5 for 1 m2 areas are upper bound values. For normal cladding elements the loading can be based on the 10 m2 values.

Values given for monopitch and duopitch roofs in tables 10.2.3 and 10.2.4 are, for 90" wind directions, particularly onerous and for such cases the directional coefficients should be used (see 5.5.2).

The internal pressure coefficients only apply to enclosed buildings with openings in only one wall. For situations with more than one wall with a dominant opening, reference should be made to clause 2.6 of BS 6399 : Part 2.

d) Clause 10.2.10

This clause should not be followed for tiling and slating. In such circumstances the methodology given in BS 5534 : Part 1 should be used.

e) Clause 10.14

The limitations of '/b < or > 70 are unclear. They are intended to indicate the ranges within which the slenderness reduction factor ¥ x can be used (see figure 10.14.1). Therefore, for items 2, 3 and 4 of table 10.14.1, if '/b > 70, «P x should be taken as 1.0. For item 5 of table 10.14.1 if >/b < 70, ¥ x should be taken as 1.0.

5.5.2 Direction pressure coefficients

To provide more accurate values of pressure coefficients, and values appropriate to the specific wind direction being considered, tables of directional coefficients are given in annex D covering all wind directions, generally in 15° segments.

5.6 Wind actions on bridges

Specific rules for wind actions on bridges will be published as Part B of this NAD.

5.7 Wind actions on lattice towers, masts and chimneys

For lattice towers, guyed masts and chimneys the procedures set out in ENV 1991-2-4 are inadequate. Specific rules are being developed for such structures in ENV 1993-32) and it is recommended that for lattice tower and guyed masts design, the relevant clauses in ENV 1993-3 which are being based on the principles of ENV 1991-2-4 are followed when they become available together with specific rules for the United Kingdom NAD.

-1 la preparation

6399 Wind Map

Kilometres 0 40 80 120 160

0 20 40 60 80 100 Statute miles

Figure 1. United Kingdom — Wind map (reference velocity)

Kilometres 0 40 80 120 160

0 20 40 60 80 100 Statute miles

Figure 1. United Kingdom — Wind map (reference velocity)

Annex A (normative)

Photographs/diagrams of terrain categories

Eurocode Terrain Roughness Curve
Terrain Category
Figure A.2 Terrain category I z0 = 0.01
6399 Terrain Category
Figure A.3 Terrain category II ~, = 0.05
Terrain CatagoryTerrain Category
Figure A.4 Terrain category HI 20 = 0.3

Annex B (normative)

Treatment of transition between terrain categories

B.l Transition coefficient

To account for sites which are located close to the transition between two (or more) terrain categories, a transition coefficient cx Or) is introduced appropriate to a site x km from the edge of a category change. The wind characteristics do not change abruptly at a ciiange of roughness. Rather, the wind velocity adjusts gradually to the new roughness in a transition zone which grows upwards from the ground, as shown by figure B. la).

Within the transition zone the wind speed is given by multiplying the value for the new roughness by the transition factor, cx(.r). Downwind of the transition zone the wind speed has adjusted to the new roughness and this is taken to occur after 10 km from the change in roughness.

After a change to a rougher terrain, the assumption that the wind speed decreases abruptly would not lie safe, so adoption of cx(x) is required in a region extending for 10 km from the increase of roughness, and corresponding values of i ,,r) are given below.

After a change to a smoother terrain, the assumption that the wind speed increases abruptly is safe ;md values of r^.ri for this case are not to be used.

The general equation for the transition factor for an increase in roughness by one category is given by:

where x is the distance in km from the site to the location of the increase in roughness (upwind) (x < 10 km).

Values of cx(x) based on equation B.l are given in table B.l, from which values may be interpolated.

Table B.l. Values of transition factor cx(x)

.r[km]

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