British Code BS Part 2 Wind 20loads - Download as PDF File .pdf), Text File .txt) or read online. bs code. Kb. 1 building type factor in accordance with BS Table 1. for ramed building with structural walls arround lifts and stairs only (e.g. office buildings of open. Building data. Type of roof. Flat. Length of building. L = mm. Width of building. W = mm. Height to eaves. H = mm.

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BRITISH STANDARD BS Incorporating Amendment No. 1 and Corrigendum No. 1 Licensed copy: x x, Universiti Sains Malaysia, Version correct . Wind Loading: A Practical Guide to BS Home · Wind Loading: A Practical Guide to downloads Views 8MB Size Report. DOWNLOAD PDF. The subsequent calculations make use of some or all of the following documents: BS BS BS BS BS

Skip to main content. Log In Sign Up. BS Loading for buildings. Part 2 Wind loads. Code of practice for wind loads ICS BS British Constructional Steelwork Association Ltd.

L 1 should be used. Th e reference height H, is the height of the peak of the gable. In this crmumstance, the folfowing apply. The positive pressures in table 28 appl Y where the gable is dhectly exposed to the wind but give conservative values for the whole gable. Fur all other wind angles, pressures on non-vertical wallx should be taken as the same as for vertical walls 3.

The reference height If, is the height above ground of the top of the wall. NUI X. The pressure coeffi ci ents for non-veti i cal wal l s i n tabl e 29 are essenti al l y i denti cal to the pressure cc. Iterns a to e define the zones, using figure 33 aa reference. For all other wings the overall crosswind breadth of the building should be used.

Wind B wing for wind Limkof wedge c direc tion c.

The wall in which the recess should be assessed as if the recess d]d not exist, as shown in figure 34a. Forpeak cladding loads at the mouth of the narrow recess, additional locti zones A at the external edge of the walls of the recess should be defined aaindkatedinfigurc 34a. The relevant pku-shape for calculating b is that of the whole building. The relevant plan-shape for calculating b is that of any upwind wing, or of the whole building, respectively.

The resulting frictional forces should be applied in acconhwe with 3. Zones of pressure coefficient are defined for each section from the upwind comer as given in figure The shape of the roof in figure 35 represents a typical arbitrary roof plan. Thereference height Hris the height above ground of the top of the roof. The crosswind breadth B zn d inwind depth D are defined in figure 2. Figure 36a shows the completed assignment for the arbitrary shape and wind direction used in figure Figure 36b shows the zones for the same shaped roof but a different wind direction.

The examples in figure 36 cover most conditions likely to be encountered. Sharp eaves represent the most onerous loading condition highest suction. Pressure coefficients for other common types of eaves are given in 3. The r efer ence hei ght H, i sthe hei ght above gr ound of the top of the parapet.

External pressure coefficients for each zone are given in table 32 dependent on the mtio of the corner radius r of the eaves to the scaling length b. External pressure coefficients for each zone are given in table 33 dependent on the pitch of the mansard eaves a. Paxt 2: Wher e bvthpmi ti ve and negati ve val.

Interpolar iarE2. I posi ti ve and negati ve val ues are gi ven, both val ues shoul d be consi der ed. In addkion, two further zones, X and Y, around the base of the inset storeys are defined in figure 37, where the scaling parameter b is bawd on the dimensions of the upper, irrsct Storey. The frictional drag coefficient should be assumed to act over alf of zone G of such roofs, with values as given in table 6.

The resulting frictional forces should be applied in accordance with 3. L4 and table 29, affowing for the differences in definition of zones.

A-frame buildirr , are also better interpreted as duopitch roofs, frdfing under the pmtilons of 3. These zones are 3. Because of the fluctuations of wind direction found in practice and in order to give the expected range of asymmetric loading, both patterns should be considered.

I nterpol ati on may be betwem val ues of the same si gn. NUTE r!. Key for m on opit ch r oofs 3. In this case, when the roof is long in the wind direction, i. These zones are defined from the upwind comer of each face. NUI E The pi tch angl e a i s taken as posi ti ve when the r cmf has a central r i dge and negati ve when the r oof has a central tr ough. Because of the fluctuations of wind direction found in practice and in order to give the expected range of Wmmetric loading, both patterns should be considered.

This load caae should be compared with the standard load case defined in figure 40 and the more onerous condition should be used. When theresul t of i nterpol ati ng: OC Zones of external pressure coefficient are defined in figare Therefererrce height Hristhe height above ground of the ridge. External pressure coefficient. The size of each of these zones is given in figure The width of each of these additional zonesin plan isshowrrin figare 41b.

The boundary between each pair ofadditional zones, T-U, V-W and X-Y, is the mid-point of the respective hip or main ridge. In such cases, the governing criterion is the form of the upwind comer for the wind dlrectirm being considered. Owing to the way that parapets around roofs change the positive pressures expected on upwind pitches with large positive pitch angles to suction, neglecting their ef feet is not always conservative.

Pressures on the parapet walls should be determined using the procedure in 2. For the part of the roof below the top of the parapet, external prwssure coefficients should be deterrrrirredin accordance with 3.

For anY Par t of the roof that is above the top os the parapet, i. External pressure coefficients should be determined in accordance wi th 3. For the part of the roof below the top of the parapet, exterrmf pressure coefficients should be determined in accordance with 3. For any part of the roof that is above the top of the parapet, i. External pressure coefficients should be deterrrrined in accordance with 3.

The reduction factorx of table 31 should be used only for the verge zones Q to S with the parapet height h deterrrrined at the upwirrd comer of each respective zone. For the part of the roof below the top of the parapet, external pressure coefficients shorrfd be determined in accordance with 3. For any part of the mof that is above the top of the parapet, i.

External pressure coefficients should be determined in accordance with 3. The reduction factors of table 31 should not be applied to any zone.

I nterpol ati on may be used. The frictional drag coefficient should be assumed to act over zones F and P only of such roofs, with the values as gjven in table 6. The resulting frictional forces should be applied in accodance with 3. L 1 Multipitch roofs are defined as roofs in which each span is made up of pitches of two or more pitch angles, as shown in figure 22 for the standard method. The form in figure 22a is commonly known as a mansard roof. The eaves zones A to D should be excluded when the pitch angle is greater than that of the pitch below, as shown in figure 22b.

Ridge zones on all other downwind faces should be excluded. When the wi nd di recti on i s normal ta the eaves, i. When the wi nd i s normal to the gabl es, i. The frictiorvd drag coefficient should be assumed to act over only zones F and P of such roofs, with values as given in table 6. The resulting frictional forces should be added to the normal pressure forces in accordance with 3. When necessary, interpolation should be used between the orthogonal wind directions to obtain values for the other wind directions.

FOr the si ngl e wal l , use pressure coeffi ci ents for wai l s en i n tabl e L 1 Appl i cabi l i ty Standsrd effective wind speeds should be combmed with directional prcasure coefficients in csaes where the form of the building is well defined but the exposure of the site is not well defined. This results in a load case for each wind direction for which pressure coefficients are given, usually tweive. In the standard method the method for significant topography 2. The standard method for effective wind speeds sssumes that the site is 2 km from the edge of a town, with sites closer to the edge treated as being in country terrain snd sites further into the town treated as being at 2 km, thus, the potential benefits of shelter from the town exposure are not exploited for any locations except those at exactly 2 km from the edge.

Appl i cati on 3. Annex A BS6 3 9 9: I nfor mati on to enabl e desi gmem t. Currently, the network numbers about stations and the main archi ve comprises hourly mean wind speeds and wind directions, together with details of the maximum gust each hour.

Many of these stations have past rvcords spanning several decades, although the computer-held ones generally begin in about , Conventionally, estimation of the extreme wind clinratc in temperate regions has involved the analysis of a series of annual maximum wind speeds, for example using the method proposed by Gumbel [8]. The main dffiadvantage of methods using only annual maxbnum values is that many other useful data within each year are discarded.

This approach. A storm was defined as a period of at lea. Such periods were identified for 50 anemogr-aph stations, evenly dtiributed over the United Kingdom and mostly having standard exposures, using their records during the period to 19S0. At the majority of these stations, the average number of storms each year was about Three types of new extreme wind information were needed: Analyses of extreme wind speeds are performed in terms of their probability of occurrence.

The standard measure of probability is the cumulative distribution function CDF , conventionally given the symbol P used elsewhere in this standard for wind load , and corresponding to the annual risk of not being exceeded. The reciprocal of the annual risk is sometimes referred to as the return period and is best interpreted as the mean interval between recurrences when averaged over a very long period. The definition of return period rapidly becomes invafid for periods less than about 10 years The period between individual recurrences varies considerably fmm this mean vafue, so the concept of return period is not very useful and is open to misinterpretation.

The concept of annual risk is less open to misinterpretation and should be viewed as the risk of exceeding the design wind speed in each yeaf the building is exposed to the wind. The wind speed V associated with a certain annual cumulative risk P of not being exceeded may be found from: Equati on B. Extreme-value theory predicts that the ETl distribution should be a better fit to dynamic pressure than to wind speed.

It was found that the rate of convergence of storm maxima to the FTl model was faster for the g model than the v, model. Corrections were then made to the individual station estimates to ensure that when all the values were plotted on a map, they represented a height of 10 m above ground in open, level termin at mean sea level. Isotachs were then drawn to be a best fit to the wind speeds plotted.

Fkting the dynamic pressure g to the FTl model has been standard practice in most of Europe for many yeas, whereas the practice in the UK had previously been to fit the wind speed V. Whereas at higher risks, for example for frequent service conditions, the q model predicts higher wind speeds than before.

While adoption of the better q model brings the UK into line with European practice, it fdao implies that previous practice at small risks was overconservative, but that service conditions may have been unconservative. Tlese changes are alao reflected in the expression for probabtity factor SP in annex D.

This was done by comparing results from the storms analysis for Northern Ireland with a map, prepared by the Irish Meteorological Service, showing isotach. Annex B BS6 2 9 9: After correction for site exposure, the directional characteristics of extreme winds showed no significant variation with location anywhere in the United Kingdom, with the strongest winds blowing from dmctions south- west to west.

This enabled one set of direction factom to be proposed. The ratios calculated refer to a given risk in each sector. However, due to contributions from other sectom, the overall risk wiIl be greater than the required value. The direction factor Sd has been derived by adjusting sectorial ratios to ensure an evenly distributed overall risk. Given the risk of a value being exceeded by month, the risk in any longer period is the sum of the monthly risks.

The seasonal characteristics of strong winds also show no significant variation across the UK so, again, one set of factors could be proposed. The strongest winds usually occur in mid-winter and the leaat windy period is between June and August. A more recent anaiyais of the full year records for ten of the original 50 sites showed an improved analysis accuracy but the values were not signfilcantly different from the original analysis.

This gives further cotildence that the 1l-year period of the original analysis was representative. Advice can also be obtained from the Meteorological Office at the following addresses. The full analysis of the governing relationships leads to equations which are too complex for codification purposes. A numerical evaluation and curve-fitting exercise carried out for practical prismatic buildings, including portd-fmme structures, showed that simplifications could be made to the algebraic relationships with only marginal loss of accuracy within a range of mildly dynamic structures.

VahI eS of ffb arc given in table 1. Northern Ireland: Values of the building type factor Kb given in table 1, have been derived from data obtained from a large number of completed buildings and other structures. Annex D BS6 3 9 9: A number of vslues of Sp for standard values of Q sre relevant: NI JTE 1.

The annual mcde, cor r espondi ng to the most l i kel y annual maxi mum val ue. The standard desi gm val ue, cor r espondi ng ta a mean r ecur r ence i nterval of 50 year n.

NOTE 4. The desi gn ri sk for bri dges, cor r espondi ng t. NUTE 5. The annual ri sk comqm. Eack-cal cul ated awnni ng the parti al factor l oad for the ul ti mate l i mi t i s YC- 1. NI JTE 6. The desi gn ri sk for nucl ear i nstal l ati ons, cor r espondi ng to a mean r ecur r ence i nterval of 10 COOyew. If values of S, are used they should be taken fmm table D.

RUt 2: By deftig three basic terrain categories wind speeds can be derived for any interrnediate category or to account for the influence of differing upwind categories to that of the site. The three basic categories defined in 1.

This applies to the sea, but also to infmrd lakes which are large enough and close enough to affect the wind speed at the site. Although this standard dots not cover offshore structures, it is necessary to define such a category so that the gradual deceleration of the wind speed from the coast inland can be quantitled and the wind speed for any land-baaed site can be determined.

This covers a wide range of terrain, from the flat open level, or nearly level country with no shelter, such as fens, airt3elds, moorland or farmfand with no hedges or walls, to undulating countryside with obstmctions such as occasional buildings and windbreaks of trees, hedges and walls. Examples are farmlands and country estates and, in reality, all terrain not othe- defined or sea or town. This terrain includes suburban rcgioms in which the general level of ruof tops is about 5 m above und level, encompassing all two storey domestic housing, provided that such buildings are at least as dense as normal suburban developments for at least m upwind of the site.

The aer odynami c mugbn- of foresk3 and mature vmdand i s si mi l ar to town terrai n z. I t i s i nadvkabl e to take advan-age of the shel ter pr ovi dr n by woodl and unl ess i t i s per manent not l i kel y to be cl ear fd. The ar tment of wind speed characteristics as the wind flows from one terrain to another is not inatantr.

At a change frum a smoother to a rougher surface the mean wind speed is gradually slowed down near the ground and the turbulence in the wind increases. This adjustment requires time to work up through the wind prufde and at any site downwind of a change in terrain the wind speed is at some intermediate flow between that for the smooth terrain and that for the fufly developed rough terrain.

The resulting gradual deceleration of the mean speed and increase in turbulence has been accounted for in tables 22 and 23 by defining the site by ita distance downwind from the coast and, in addition if it is in a town, by its distance from the edge of the town. Shelter of a site from a town upwind of the site has not been allowed for, other than if the site is in a town itself.

However, [8] and [15] give information on how to take such effects into account. It is important, if directional effects need to be considered, to take full account of the effects of te- upwind of the site in conjunction with the direction factor.

This becomes even more important if the effects of topography also need to be considered, as the topographic increment Sh can be large. Ehacd on this work the normal practical range of duplacement heights has been found to be 0. When considering low-rise He. For the purposes of these procedures the simplifkd formula was thus considered adequate. However, the value of the gust factor in terms of the gust period t is not of direct application to design.

The problem is rather to determine, for static structures, the appropriate gust speed which will envelop the stmcture or component to produce the maximum loading thereon. Simifar percentage changes would apply for different sizes and heights. The resulting values of size a are then shown as the abscissa on the graph of figure F.

Factor gt is given in table 24 for various heights and building sizes. Wind Engineering. Wind Engi neeri ng in thsEighties. CIRfA, Wind Effects on. New York: John Wiley and Sons, NRCC No. National Council of Canada, 19S0. Tlwasawmmi of dssign wind spsed data: Building Research Establishment, Reprinted with amendment Rmt 2: Static structures. Butterworth Scientific, Building Research Establishment, Background, damage SWTS?

Y,WirJ -ddata ad St? J o-urnal of Wind Errginsering and Industrial Aerudynnmics. Note on directional and seaaonal assessment of extreme winds for design. J vurrzd of Wind Engineering and Industrial Aerodynamics , 12, Extreme wind climate of the United Kingdom. J ournal of Wind Engineering and Industrial Aervdy?

The estimation of extreme winds. J ournal of Industrial Aerodynamics. BRE Microcomputer package, Garston: Strong winds i. Mean hourly wirrd speeds. Engineering Sciences Data Item S ESDU International, An smdytical approach to wind velocity gust factors.

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Enquiries about copyright should be made to the Copyright Manager at Chiswick. C2L 5! F;Od 50 ytors. Flag for inappropriate content. Related titles. Jump to Page. Search inside document. Francis Zigi. Kong Chin Tuan. Yannis Alexandru. Lina Ngah. Kevin Domun. Philip Amankwah. Sourav Biswas. Vasanthapragash Nadarajha. Rachelle C. Sauting Lam. Brukadah Williams Onwuchekwa. Anushka Abeysinghe. Gautam Sharma. Mohamed Habola.

Tharmalingam Vijay. More From MdShahbazAhmed. Eurocode 7: Geotechnical Design - Worked examples. Panagiotis Xanthos. Mesfin Derbew. Muhammad Saqib Abrar. Popular in Pressure. Dante Orlando. Plant Safety and Pressure Relieving Operations. Victor Iriondo. Serge Lorenz Glinogo Villasica. Michael Ross. Jun Hao Heng. Jahid Jahidul Islam Khan. It gives better estimates of effective wind speeds in towns and for sites affected by topography.

Combination a is appropriate when the form of the building is well defined, but the site is not; the cases of relocatable buildings or standard mass-produced designs are typical examples. Such hybrid combinations should be applied only in accordance with 3. Standard method 2 Licensed copy: When the building is doubly-symmetric, e. When the building is singly-symmetric, three orthogonal cases are required, e.

When the building is asymmetric, four orthogonal cases are required. When symmetry is used to reduce the number of orthogonal load cases, both opposing wind directions, e.

Values of size effect factor are given in Figure 4, dependent on the site exposure see 1. For external pressures the diagonal dimension a is the largest diagonal of the area over which load sharing takes place, as illustrated in Figure 5. For internal pressures an effective diagonal dimension is defined in 2. Load effects, for example bending moments and shear forces, at any level in a building should be based on the diagonal dimension of the loaded area above the level being considered, as illustrated in Figure 5c.

NOTE 2 As the effect of internal pressure on the front and rear faces is equal and opposite when they are of equal size, internal pressure can be ignored in the calculation of overall horizontal loads on enclosed buildings on level ground. In practice, option b will not produce significantly lower values than a unless the combination of location, exposure and topography of the site is unusual. Its calculation in the standard method depends on whether topography is considered to be significant, as indicated by the simple criteria in Figure 7.

When topography is not considered significant, Sa should be calculated using the procedure in 2. When topography is significant, Sa should be calculated using the procedure in 2.

NOTE In this case the value of Sa, based on the site altitude, compensates for residual topography effects. In reading the value of s from these figures, the location with respect to the crest of the feature is scaled to the lengths of the upwind LU or downwind LD slopes as follows: The basis for the derivation of the values in Figure 9a and Figure 9b and Figure 10a and Figure 10b is given in Annex G.

In this case, a value of s may be derived from Figure 9a and Figure 9b and Figure 10a and Figure 10b, and the smaller value used. NOTE When the direction factor is used with other factors that have a directional variation, values from Table 3 should be interpolated for the specific direction being considered, or the largest tabulated value in the range of wind direction may be selected. For permanent buildings and buildings exposed to the wind for a continuous period of more than 6 months a value of 1.

Equation D. The reference height Hr for each part should be taken as the height to the top of that part.

The diagonal dimension, a, should be taken for the loaded area being considered. NOTE For all sites inside towns except exactly at the upwind edge or at a distance of 2 km from the upwind edge the simplifications of the standard method produce a larger value of Sb than the directional method.

If the loads produced by the standard method are critical to the design, the use of the hybrid combination given in 3. Table 4 — Factor Sb for standard method Site in country or up to 2 km into town Site in town, extending U 2 km upwind from the site Effective height Closest distance to sea upwind Effective height Closest distance to sea upwind He He km km m k 0. NOTE 2 The figures in this table have been derived from reference [5]. NOTE The standard pressure coefficients may be used for buildings and elements of generally similar shape.

Where the building or element shape falls outside the scope of the tabulated pressure coefficients in 2. The majority of conventional buildings, such as cuboidal, or composed of cuboidal elements, with different roof forms such as flat, monopitch, duopitch, hipped and mansard, are included.

Where considerable variation of pressure occurs over a surface it has been subdivided into zones and pressure coefficients have been provided for each zone. External pressure coefficients are given in 2. When the procedure of 2. Values outside this range should be obtained from 3. Overall forces may be calculated using the pressure coefficients of Table 5 together with equation 23 of 3. See 2. Where the well or bay extends across more than one pressure zone, the area-average of the pressure coefficients should be taken.

The loaded zones on the face should be divided into vertical strips from the upwind edge of the face with the dimensions shown in Figure 12, in terms of the scaling length b, making no special allowance for the presence of the cutout. The scaling length b is determined from the height H and crosswind breadth B of the windward face.

The loaded zones on the face are divided into vertical strips immediately downwind of the upwind edges of the upper and lower part of the face formed by the cut-out.

The scaling length b1 for the zones of the upper part is determined from the height H1 and crosswind breadth B1 of the upper inset windward face. The scaling length b2 for the zones of the lower part is determined from the height H2 and crosswind breadth B2 of the lower windward face. The reference height for the upper and lower part is the respective height above ground for the top of each part. The pressure coefficients for zones A, B and C may then be obtained from Table 5. For the inset walls, provided that the upwind edge of the wall is inset a distance of at least 0.

However, the reference height Hr is taken as the actual height of the top of the wall above ground. Where the upwind edge of the wall is flush, or inset a distance of less than 0.

The reference height for zone E should be taken as the top of the lower storey The greater negative pressure suction determined for zone E or for the zone A in item a , should be used. The frictional drag coefficient should be assumed to act over all zone C of such walls, with values as given in Table 6. The resulting frictional forces should be added to the normal forces as described in 2.

Table 6 — Frictional drag coefficients Type of surface Frictional drag coefficient Smooth surfaces without corrugations or ribs across 0. These pressure coefficients are also applicable to silos, tanks, stacks and chimneys. It is therefore the region with the highest risk of fatigue damage to cladding fixings.

External pressure coefficients for flat roofs with edge parapets are given in Table 8, dependent upon the ratio of the height h of the parapet, defined in Figure 17a , to the scaling length b. The zones start from the edge of the flat part of the roof as defined in Figure 17b.

The zones start from the edge of the flat part of the roof as defined in Figure 17c. NOTE 3 In zone D, where both positive and negative values are given, both values should be considered. NOTE 4 Values of coefficients for other wind directions are given in 3. NOTE 5 For pitched roofs with curved or mansard eaves, the values in this table may be compared with the appropriate values in Table 9, Table 10 or Table 11 and the least negative values used.

Figure 18 — Key for inset storey 2. However, a further zone around the base of the inset storeys should be included, as shown in Figure 18, where b is the scaling parameter from 2. The pressure coefficient in this zone should be taken as that of the zone in the adjacent wall of the upper storey as determined from 2.

NOTE Hipped roof forms are treated separately in 2. Owing to the asymmetry of this roof form, values are given for three orthogonal load cases: Values are given for two wind directions: For duopitch roofs of greater disparity in pitch angles see reference [6]. The definitions of loaded zones and pitch angles are given in Figure The data in Table 11 may be applied to hipped roofs where main faces and hipped faces have different pitch angles, provided the pitch angle of the upwind face is used for each wind direction, as indicated in Figure Negative pitch angles occur when the roof is a hipped-trough form.

For pressure coefficients for skew-hipped roofs and other hipped roof forms see reference [6]. The key in Figure 22 indicates where edge zones should be omitted.

In these cases two sets of values are given and they should be treated as separate load cases.

NOTE 2 Interpolation for intermediate pitch angles may be used between values with the same sign. Two sets of values are given at these pitch angles and they should be treated as separate load cases. However, reduced values of external pressure coefficients may be derived from Table 9 or Table 10, as appropriate, as follows: For steeper roofs, the effects of parapets should be taken into account by using the procedure given in 3.

Larger overhangs should be treated as open-sided buildings, with internal pressure coefficients determined using the provisions of 2. NOTE 3 Load cases cover all possible wind directions. When using directional effective wind speeds, use: NOTE 2 Interpolation for intermediate pitch angles may be used between values of the same sign. Values of Cp for intermediate blockages may be linearly interpolated between these two extremes, and applied upwind of the position of maximum blockage only.

Where the local coefficient areas overlap the greater of the two given values should be taken. For monopitch canopies the centre of pressure should be taken to act at 0. For duopitch canopies the centre of pressure should be taken to act at the centre of each slope.

Additionally, duopitch canopies should be able to support forces with one slope at the maximum or minimum and the other slope unloaded. Advice is given in reference [6]. The resulting frictional drag coefficient should be assumed to act over zone D on flat roofs see Figure 16 for all wind directions; and over zone D for monopitch or duopitch roofs see Figure 19 and Figure 20 and zone J for hipped roofs in Figure 21 only when the wind is parallel to the ridge.

Values of frictional drag coefficient should be obtained from Table 6 and the resulting frictional forces combined with the normal pressure forces as described in 2. Values of frictional drag coefficient should be obtained from Table 6 and the resulting frictional forces combined with the normal forces as described in 2.

NOTE If there are fascias at the eaves or verges see 2. This will result in net wind loads on internal walls. A method for calculating the internal pressures in multi-room buildings is given in reference [6]. For external walls, provided there are no dominant openings, the internal pressure coefficient Cpi should be taken as either —0.

The maximum net pressure coefficient Cp across internal walls should be taken as 0. The relevant diagonal dimension a for the internal pressure may be taken as: The relevant diagonal dimension a depends on the size of the dominant opening relative to the internal volume and may be taken as the greater of: Table 17 — Internal pressure coefficients Cpi for buildings with dominant openings Ratio of dominant opening area to sum of remaining Cpi openings and distributed porosities 2 0.

The relevant diagonal dimension a for use with these coefficients is the diagonal dimension of the open face.

This load case should be allowed for by using a net pressure coefficient of 2. More details are given in reference [6]. For the single wall, use pressure coefficients for walls given in Table 5. For sharp-edged shapes the pressure coefficients remain approximately constant over the whole range of wind speeds likely to be encountered. However, for circular sections the pressure coefficients vary with wind speed and diameter. For circular elements whose diameter is greater than about mm the values in this section are conservative.

These net pressure coefficients should be taken to act on the projected area normal to the wind. Vertical or inclined sections may be taken as being divided into parts of length at least twice the crosswind breadth, L U 2B, and the reference height Hr should be taken as the height above ground of the top of each part. In the case of sections cantilevered from the ground or another plane surface, such as a roof, the length L should be taken as twice the protruding length.

The length L between free ends should be taken as the length of each element, i. When the lattice is dense or shielded, as with multiple lattices frames, the degree of conservatism can be large.

A simplified method of calculating the wind loads on unclad building frames which accounts properly for the shielding effects is given in reference [7], based on the full method given in reference [6]. The load on non-solid walls should be obtained using the net area of the walls. Moderate porosity in this region, i. For porous walls and fences with solidity less than 0. Values of shelter factor to reduce the net pressure coefficient are plotted in Figure Shelter remains significant up to spacings of 20 wall heights.

At very close spacings the net pressure coefficient on the downwind sheltered wall can be zero or can reverse in sign. A minimum limit to the shelter factor of 0. If the gap is less than half the height of the board, then it should be treated as a free-standing wall in accordance with 2. Directional method 3 Licensed copy: This is illustrated in Figure 29 for the case of a rectangular-plan building.

However, since peak loads on each face of buildings do not act simultaneously, the resulting summation would be conservative. NOTE 1 The factor 0. For buildings with flat roofs, or where the contribution to the horizontal loads from the roof is insignificant, the overall load in the wind direction P may be taken, without significant loss of accuracy, as: Face loads are then resolved vectorially to give the overall load in the wind direction.

Walls aligned exactly parallel to the wind give no resolved component in the wind direction. In the case of rectangular buildings, the procedure gives the exact result of the orthogonal cases in 2. For polygonal buildings this may be conservatively based on the concept of the smallest enclosing rectangle. When determining overall forces on the building, the contribution of frictional forces should be taken to act in the wind direction and added to the normal pressure load given by P.

The expression for the directional internal surface pressure pi given in 3. NOTE In the directional method, topographic effects are determined separately from altitude effects. When topography is not to be considered, the altitude factor Sa should be determined from: When topography is to be considered, the altitude factor Sa should be determined from: The effective wind speed should be calculated at the effective height He, determined from the reference height Hr in accordance with 1.

Reference heights Hr are defined with the pressure coefficient data for each form of building.

For buildings whose height H is greater than the crosswind breadth B in the wind direction being considered, some reduction in overall loads may be obtained by dividing the building into a number of parts in accordance with 2. It also modifies the hourly mean site wind speed to an effective gust wind speed. The terrain and building factor Sb should be determined from 3. Load effects, e. NOTE 2 The figures in this table have been derived from reference [8].

The dimension of the building which determines the value of the gust peak factor is the length of the diagonal a of the loaded area over which load sharing takes place see Figure 5. Separate values should be used depending upon whether wind loads are being calculated for the whole building, portions of the building or individual components.

Table 23 — Adjustment factors Tc and Tt for sites in town terrain Effective height Factor Upwind distance from edge of town to site He km m 0. NOTE 2 For sites in towns less than 0. NOTE 3 The figures in this table have been derived from reference [8]. NOTE The derivation of the gust peak factor is described in Annex F, which also includes mathematical equations to derive gt.

Values of Sh should be derived for each wind direction considered and used in conjunction with the corresponding direction factor Sd. When the topography is defined as not significant by the simple criteria in Figure 7, the terrain may be taken as level and the topographic increment Sh may be taken as zero.

It should be noted that Sh will vary with height above ground level, from maximum near to the ground reducing to zero at higher levels, and with position from the crest, from maximum near the crest reducing to zero distant from the crest.

In situations of multiple hills or ridges, this procedure is appropriate when applied to the single hill or ridge on which the site is situated. Before any reduction in wind speeds is considered specialist advice should be sought.

NOTE Values of Sh may be derived from model-scale or full-scale measurements or from numerical simulations. In this case, a value of s may be derived from both Figure 9 and Figure 10 and the smaller value used. In many cases, it will not be clear whether topography or altitude dominates. As each is assessed differently by the directional method, it is necessary to calculate the effective wind speed Ve twice, as follows, and to take the larger value of Ve obtained: This procedure is recommended to determine the limit of topographic influence downwind of a cliff or escarpment.

Values of s from Figure 9 should be used as upper bound values. Zones A and B should be defined, measuring their width from the upwind edge of the wall. If zones A and B do not occupy the whole of the wall, zone D should be defined from the downwind edge of the wall. If zone D does not occupy the remainder of the face, zone C is then defined as the remainder of the face between zones B and D.

The reference height Hr is the height above ground of the top of the wall, including any parapet, or the top of the part if the building has been divided into parts in accordance with 2. The crosswind breadth B and inwind depth D are defined in Figure 2.

In this circumstance, the following apply. The positive pressures in Table 26 apply where the wall is directly exposed to the wind but give conservative values for the whole wall. In such cases there may be any number of faces greater than or equal to 3. The wind direction, principal dimensions and scaling length remain as defined in 3. NOTE Instead of calculating the crosswind breadth B and inwind depth D for the complex building plan, these dimensions may be determined from the smallest rectangle or circle which encloses the plan shape of the building.

NOTE 2 When the result of interpolating between positive and negative values is in the range —0. The reference height Hr is the height of the peak of the gable. The positive pressures in Table 28 apply where the gable is directly exposed to the wind but give conservative values for the whole gable.

For all other wind angles, pressures on non-vertical walls should be taken as the same as for vertical walls. The reference height Hr is the height above ground of the top of the wall. NOTE The pressure coefficients for non-vertical walls in Table 29 are essentially identical to the pressure coefficients for steep pitched roofs in 3.

However, steep-pitched surfaces which meet along the top edge to form a ridge, e. Items a to e define the zones, using Figure 33 as reference. For all other wings the overall crosswind breadth of the building should be used. NOTE Instead of calculating the crosswind breadth B and inwind depth D for the complex building plan, these dimensions may be determined from the smallest rectangle or circle which encloses the plan shape of the upwind wing or of the whole building, respectively.

NOTE 3 When the result of interpolating between positive and negative values is in the range —0. The wall in which the recess should be assessed as if the recess did not exist, as shown in Figure 34a. The pressure coefficient corresponding to the position of the recess should be applied to all the walls inside the recess.

For peak cladding loads at the mouth of the narrow recess, additional local zones A at the external edge of the walls of the recess should be defined as indicated in Figure 34a.

The relevant plan-shape for calculating b is that of the whole building. The procedure in 3. The relevant plan-shape for calculating b is that of any upwind wing, or of the whole building, respectively. The frictional drag coefficient should be assumed to act over all zone C and D of such walls, with values as given in Table 6. The resulting frictional forces should be applied in accordance with 3. This general method also accounts for the variations in high local suction around the periphery of the roof caused by various common forms of eaves detail.

Figure 35 — Key to general method for flat roofs 3. Zones of pressure coefficient are defined for each section from the upwind corner as given in Figure The shape of the roof in Figure 35 represents a typical arbitrary roof plan.

The reference height Hr is the height above ground of the top of the roof. NOTE Instead of calculating the crosswind breadth B and inwind depth D for the complex building plan at every wind angle, these dimensions may be determined from the smallest rectangle or circle which encloses the plan shape of the building.

Figure 36a shows the completed assignment for the arbitrary shape and wind direction used in Figure Figure 36b shows the zones for the same shaped roof but a different wind direction. The examples in Figure 36 cover most conditions likely to be encountered.

Sharp eaves represent the most onerous loading condition highest suction. Pressure coefficients for other common types of eaves are given in 3. The external pressure coefficients given in Table 30 for zones A to D for flat roofs with sharp eaves should be multiplied by the appropriate reduction factors given in Table 31, dependent on the height of the parapet h, as defined for the standard method in Figure 17, and the eaves height H or crosswind width B.

External pressure coefficients for each zone are given in Table 32 dependent on the ratio of the corner radius r of the eaves to the scaling length b. NOTE 2 Where both positive and negative values are given both values should be considered. NOTE 3 Where both positive and negative values are given, both values should be considered.

In addition, two further zones, X and Y, around the base of the inset storeys are defined in Figure 37, where the scaling parameter b is based on the dimensions of the upper, inset storey c In zones X and Y the pressures shall be taken as the pressure appropriate to the wall zones A to D on each adjacent inset storey wall from 3. The frictional drag coefficient should be assumed to act over all of zone G of such roofs, with values as given in Table 6.

Steep-pitched surfaces attached to the top of vertical walls are better interpreted as pitched roofs, falling under the provisions of 3. Steep-pitched surfaces springing directly from the ground which meet along the top edge to form a ridge, e. A-frame buildings, are also better interpreted as duopitch roofs, falling under the provisions of 3. These zones are defined from the upwind corner.

Because of the fluctuations of wind direction found in practice and in order to give the expected range of asymmetric loading, both patterns should be considered. In this case, when the roof is long in the wind direction, i.

These zones are defined from the upwind corner of each face. This load case should be compared with the standard load case defined in Figure 40 and the more onerous condition should be used. Zones of external pressure coefficient are defined in Figure The reference height Hr is the height above ground of the ridge.

External pressure coefficients for zones O and P on the downwind faces are given in Table The size of each of these zones is given in Figure The width of each of these additional zones in plan is shown in Figure 41b.

The boundary between each pair of additional zones, T—U, V—W and X—Y, is the mid-point of the respective hip or main ridge. In such cases, the governing criterion is the form of the upwind corner for the wind direction being considered. Owing to the way that parapets around roofs change the positive pressures expected on upwind pitches with large positive pitch angles to suction, neglecting their effect is not always conservative.

Pressures on the parapet walls should be determined using the procedure in 2. For the part of the roof below the top of the parapet, external pressure coefficients should be determined in accordance with 3. For any part of the roof that is above the top of the parapet, i. External pressure coefficients should be determined in accordance with 3. The reduction factors of Table 31 should be used for upwind eaves and verge zones A to D and H to J, with the parapet height h determined at the upwind corner of each respective zone.

The reduction factors of Table 31 should be used only for the verge zones Q to S with the parapet height h determined at the upwind corner of each respective zone.

The reduction factors of Table 31 should not be applied to any zone. The frictional drag coefficient should be assumed to act over zones F and P only of such roofs, with the values as given in Table 6. The form in Figure 22a is commonly known as a mansard roof. NOTE Flat roofs with mansard eaves are dealt with in 3. NOTE The letters designating the zones in Figure 22 which correspond to the standard method should be ignored.

The eaves zones A to D should be excluded when the pitch angle is greater than that of the pitch below, as shown in Figure 22b. Ridge zones on all other downwind faces should be excluded. However, these pressures are reduced in value for the downwind spans.

NOTE When the wind direction is normal to the eaves, i. When the wind is normal to the gables, i. The frictional drag coefficient should be assumed to act over only zones F and P of such roofs, with values as given in Table 6. The resulting frictional forces should be added to the normal pressure forces in accordance with 3. When necessary, interpolation should be used between the orthogonal wind directions to obtain values for the other wind directions.

NOTE If more accurate values are required, internal pressures in enclosed buildings or buildings with dominant openings may also be determined from the distribution of external pressures by calculating the balance of internal flows see reference [6]. The positive value applies when the wind is blowing into the short open face. For the single wall, use pressure coefficients for walls given in Table NOTE Typical examples are relocatable or portable buildings, or mass-produced designs.

This results in a load case for each wind direction for which pressure coefficients are given, usually twelve.

In the standard method the method for significant topography 2. The standard method for effective wind speeds assumes that the site is 2 km from the edge of a town, with sites closer to the edge treated as being in country terrain and sites further into the town treated as being at 2 km, thus, the potential benefits of shelter from the town exposure are not exploited for any locations except those at exactly 2 km from the edge.

Option a is the simplest to implement when topography is not significant; option b ensures that estimates will correspond exactly with the building axes; option c ensures that the most onerous topographic effects are included. These values may be taken to be equivalent to the standard effective wind speeds and used in the standard method.

NOTE Information to enable designers to make a considered judgement of the facilities offered when commissioning wind tunnel tests is available in reference [9]. Annex B informative Derivation of extreme wind information B. Currently, the network numbers about stations and the main archive comprises hourly mean wind speeds and wind directions, together with details of the maximum gust each hour.

Many of these stations have past records spanning several decades, although the computer-held ones generally begin in about Conventionally, estimation of the extreme wind climate in temperate regions has involved the analysis of a series of annual maximum wind speeds, for example using the method proposed by Gumbel [8]. The main disadvantage of methods using only annual maximum values is that many other useful data within each year are discarded. For the preparation of the basic wind speed map given in Figure 6, a superior technique involving the maximum wind speed during every period of windy weather or storm was used.

This approach greatly increased the amount of data available for analysis and enabled the directional and seasonal characteristics of the UK wind climate to be examined.