BS 8104-1992 Code of practice for assessing exposure of walls to wind-driven rain《墙壁抗风雨侵袭评估实用规程》.pdf

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1、BSI BSr8104 O1 1624667 0453843 522 .% BRITISH STANDARD Code of practice for Assessing exposure of walls to wind-driven rain Code de bonne pratique pour lvaluation de lexposition des muis la pluie chasse par le vent Bestimmung der Beanspruchung von Wanden durch Schlagregen 5s 8104 : 1992 BSI BSa8104

2、O1 Lb24669 0453844 469 .- BS il (b) rain penetration through the masonry, for which the local spell index is the most significant factor. It should be emphasized that neither index is precise enough to enable fine distinctions between degrees of exposure to be made. Hence the user should always take

3、 account of local knowledge and experience. Since the publication of DD 93, further work has included: (1) research into the relation between local building environments and instances of rain penetration; (2) research into the effect of local environment and of building size and design features on r

4、ainfall actually falling on walls in buildings. This further work has led to suggestions for a number of modifications to DD 93. Basic meteorological data remain largely unchanged except that the Channel Islands have now been added to the maps (appendix B). ktors to modify rainfall according to loca

5、l conditions are now based on empirically determined data from experimental sites rather than solely on wind speed data as previously. in addition, factors to allow for building shape and size have been introduced, also based on data from expenmental sites. Future work will include: (i) regular revi

6、ews of climatic data to take into account long term climatic changes; (ii) research into the relation between exposure and the properties of individual materials, such as absorptive capacity, and the effects of evaporation and moisture content. This code of practice represents a standard of good pra

7、ctice and therefore takes the form of recommendations. Compliance with a British Standard does not of itself confer immunity from legai obligations. 2 Code of practice O Introduction The quantity of rain falling on a vertical surface, such as a wall, at any point depends on both the intensity of the

8、 rainfall and the wind speed. The Building Research Establishment Report Driving Ruin Index (1976) postulated that the quantity of rain falling on a vertical surface was proportional to the rate falling on a horizontal surface and to the local wind speed, and incorporated maps of an annual wind-driv

9、en rain index, which is the product of the annual average airfield wind speed and annual average rainfall on the horizontal. Appropriate correction factors were given to compute the local value. (One version of this method, which expresses the index as a logarithmic value, is described in BS 5618.)

10、More recently the Meteorological Office has produced improved values, based on the fact that prolonged rainfall is usualiy associated with stronger than average winds. New maps for the airfield annual wind-driven rain index have been prepared by summing the hourly products of wind speed and rainfall

11、 during the 33 year period 1959 to 1991. These data form the basis for the method of prediction of quantities of rainfall on vertical surfaces, which is described in this code of practice. Features of this code of practice are that it allows calculations cf driving rainfall for different orientation

12、s, it allows annual average values to be calculated as well as quantities for the worst likely spell in any 3 year period and it allows corrections to be made for ground terrain, topography, local shelter, and the constructional characteristics of the building concerned. Site investigations have giv

13、en information not only on the appropriate choice of modifying factors but also on the reproducibility and accuracy of rainfall data calculated for a given building. This is reflected in the degree of precision with which factors R, 7: O and Ware given in this code. Any extra precision which would b

14、e achieved by a closer definition of the buildingkite description would not be sufficient to justify the much increased complexity that this would lend to the code. Appendix A gives worked examples using the wind-driven rain maps in appendix B and the topography factors in appendix C. Further detail

15、s of the background to this code are given in appendix D. 1 Scope This British Standard gives recommendations for two methods for assessing exposure of walls in buildings to wind-driven rain, namely the local spell index method and the local annual index method. It should be noted that this code is

16、not appropriate in respect of the weathertightness of components such as windows. However, the performance of such components can be influenced by the design of the surrounding structure and appendix E may be helpful in this respect. NOTE. The titles of the publications referred to in this standard

17、are listed on the inside back cover. Other references will be found throughout the text, indicated by bracketed numbers thus I i, and these are listed in the bibliography in appendix F. 2 Definitions For the purposes of this British Standard, the following definitions apply. 2.1 spell A period, or s

18、equence of periods, of wind-driven rain on a vertical surface of given orientation. A spell is of variable length and can include several periods of wind-driven rain interspersed with periods of up to 96 h without appreciable wind-driven rain (see also appendix D). 2.2 airfield index The quantity of

19、 driving rain that would occur 10 m above ground level in the middle of an airfield, at the geographical location of the proposed wall. 2.3 airfield annual index, DA The average annual airfield index for a given direction, in litres per square metre per year. 2.4 airfield spell index, Ds The airfiel

20、d index for a given direction during the worst spell likely to occur in any 3 year period, in litres per square metre per spell. 2.5 wall annual index, (b) orientation of proposed wall, i.e. the direction the wall faces; (c) description of the general terrain upwind of the proposed wall, including a

21、ny marked changes in ground level within 1 km upwind caused by hills, cliffs, etc.; (d) description of any obstructions close to the proposed wall, e.g. neighbouring buildings or trees, including description of any clear line of sight between and beyond closest obstructions; (e) description of chara

22、cteristics of proposed wall, e.g. gable wall, eaves wall, overhangs present, storey height and breadth of wall. 4 t 5.2 Step by step procedure 5.2.1 Look at the appropriate wind-driven rain map in appendix B. Find the subregion in which the site is located (indicated by the code adjacent to the symb

23、ol o). Where the site lies on a boundary between two subregions, the calculation should be made for both subregions and the highest figure taken. Find the geographical increment, i, for the site of the proposed wall. These increments are shown between contour lines; if the site lies on a contour lin

24、e, the larger increment should be used. Increments may be positive or negative. Values of i are shown on the maps in steps of 1 up to 6 and in steps of 2 above 6. 5.2.2 Examine the rose for the particular subregion and method (local annual index or local spell index). Each rose gives 12 values corre

25、sponding to different orientations. Select the rose value, r, nearest to the orientation of the proposed wall. In case of doubt, take the highest of the rose values for the two possible wall orientations. 5.2.3 Add the rose value to the geographical increment to obtain the map value WL. NCJE. Some w

26、orked examples are given in appendix A. )nA = rA + i, or mS = YS + i NOTE. In crossing a subregion boundary either r or i or both may change. (See 5.2.1.) 5.2.4 Obtain the airfield index, DA or Ds, corresponding to the map value from table 1 or 2. NUE The difference in magnitude between airfield ann

27、ual and spell indices for a given map value reflects the different parameters being assessed (see clause 3). 5.2.5 Assess the character of the general terrain upwind of the proposed wall, using the descriptions given in table 3, to obtain a terrain roughness factor R. In case of doubt as to which of

28、 the two adjacent terrain roughness categories is appropriate then select the one giving the highest value of R 5.2.6 The wind-driven rain indices given in appendix B take account of the general level of the site above sea level. They do not allow for local topographical features such as hills, clif

29、fs or escarpments which can affect the wind speed in their vicinity. Hence, consider whether the ground facing the proposed wall slopes away with an average slope of more than 1 in 20 within 1 km of the wall. If it does, use appendix C to calculate topography factor 7: If the slope is less than 1 in

30、 20, use table 4 to assess whether other local circumstances have an influence on the speed of wind approaching the wall. 5.2.7 Assess the exposure of the proposed wall in terms of the nearest obstructions. This can vary markedly between buildings depending upon their proximity to other buildings, t

31、rees, roads, open spaces such as parks and playing fields, land which slopes downwards from the front of the exposed wall and the edge of a built-up area. Likely changes in the degree of exposure, particularly any arising from any impending development, e. g. the felling of trees, should be taken in

32、to account. Obtain a value for the obstruction factor O. This relies on the line of sight from the proposed wall. If the horizontal view is obstructed, for example by walls or other buildings of similar dimensions to the proposed wail, then the obstruction factors shown in table 5 should be used for

33、 that direction (see figure 3). If the view is unobstructed then the obstruction factor is 1.0. NCEE I. If there are other buildings nearby but there is III unobstructed horizontal line of sight, for example over their roofs, then the obstruction factor is 1 .O (see figui-e :I). N(KE 2. Some biiildi

34、ngs within estates have a clear line of 4ght down open roads. These roads can funnel wintl-driven min even for wind directions not precisely down the road The wall should be conservatively estimated to have an obstruction factor of I .O. NOTE 3. For long walls it may be necessary to make, spai-ate N

35、UTE 4 In practical circumstances it is often necrsary to consider obstructions and lines of sight in aggregate For example, for most suburban sites of two storey housing on flat ground with typical spacing and disposition of block, a value of 0.5 can be assumed for houses within the site, provitlrtl

36、 thrrt, are no roads leading away from the proposed wall to opeii country. Other more detailed examples are given in appendix A. 5.2.8 The amount of rain incident on a wall depends on the type of wall (e.g. gable or eaves), on its height, and on other features such as overhangs or string courses. In

37、 addition, the distribution of rainfall intensity varies significantly from one part of a wall to another (for background see appendices D and E). Obtain the wall factor W for the required position on the wall from table 6. 5.2.9 Calculate the wall annual index, ;a - - 30 * 26 30 27 21 29- -24 3; 3

38、29- 23 y 10 / 26. 2 29- -24 - 19 / 27 10 - I ;9 23 I )9 26 / I 16 -I9 19 26 22 28- -2L 0 28 2o 27 - Il 1; ;9 - 2s. 22 11- -2L 28- -24 29 , 27 lb- -22 26, 25 26 27 27 I 7 Soeil rose and annual rdse vaiues for map 1 lh- -11 15- -Il 15 10 I 18 18 13, 16- 18 / 18 5 6 I /6 :5 15 l6 I L 15? , 8? 19, 019 1

39、3 - 24- -20 25?/ 22 25 25 2L 15 16 5 4 ?6, I )? 0, /5 12- -0 ?11 19, ?18 23- -20 . 14? 22 24? / 25 24 15 ;5 :4 16 l7 18 ?/ 18 ?18 21- -19 24? 21 5 14? / 7? 11- 15 2: :3 -B 12 17 I :5- L- ?6 -8 1) ?11 I 15 - 5- I /5 ?5 10 -1 I 15 l3 - I _ .- BSI BS*8104 01 Lb24bb 0453872 323m Appendix B Bsa104 : 1992 rl O1 E b- o O u i) + O m O P O Q O ln O rt O rl O N 2 O O Y CI. m O Co O c- ii 4 O LT C 4 c rr . 302 mo - 3 n -3 O n o N 5 O t- O m O P- O Q O Ln O 4 O m O Cu * O O N IA Ln O a) O P I 1% I IF O O 4 O m Map 3 28