SCI-BRE-BCSA Wind Guidance.pdf

ELECTRONIC PUBLICATIONSCI ED001

Recommended Application of

BS 6399-2

© 2002 The Steel Construction Institute ii SCI ED001

This document has been prepared by: The Steel Construction Institute Silwood Park Ascot Berkshire SL5 7QN Tel: 01344 623345 Fax: 01344 622944 Email: [email protected] The author of the document is:

Mr D G Brown BEng CEng MICE

2002 The Steel Construction Institute

Enquiries concerning reproduction of this document should be sent to The Steel Construction Institute, at the address given above.

Although care has been taken to ensure, to the best of our knowledge, that all data and information contained herein are accurate to the extent that they relate to either matters of fact or accepted practice or matters of opinion at the time of publication, The Steel Construction Institute, the authors and the reviewers assume no responsibility for any errors in or misinterpretations of such data and/or information or any loss or damage arising from or related to their use.

Publication Number: SCI ED001

© 2002 The Steel Construction Institute iii SCI ED001

FOREWORD

This guidance note has been prepared to assist designers using BS 6399-2:1997. The guidance sets out a recommended procedure for applying the provisions of the Standard, particularly for those designers using the Standard for the first time. The recommendations are not intended to produce the “best” answer in every case – rather that they will produce good results in most cases. With increasing experience, designers will learn where the recommended approach can be foreshortened without penalty, and where extra calculation effort will reap benefits.

A comprehensive guide to the Standard, Guide to evaluating design wind loads to BS 6399-2 by Dr C Bailey will be published later in 2002[3]. It explains the different methods in the Standard that may be used to calculate wind loads.

This document was drafted by Mr D Brown of the Steel Construction Institute, with significant contribution by others in developing the understanding of how to use the Standard. In particular, the contribution of Dr C Bailey of BRE was considerable. He was responsible for many of the studies clarifying the Standard, and for liaison with the Code Committee.

Valuable comment and advice has also been received from:

Dr D Moore BRE

Mr P J Williams BCSA

Mr C M King SCI

Financial support from the DLTR is gratefully acknowledged.

© 2002 The Steel Construction Institute v SCI ED001

Contents Page No.

FOREWORD iii

SUMMARY vi

1 INTRODUCTION 1 1.1 Scope 1 1.2 Recommendations 1

2 OVERVIEW OF THE RECOMMENDED PROCESS 2 2.1 Calculation stages 2 2.2 The dependence of the dynamic pressure, qs on wind direction 2

3 DYNAMIC CLASSIFICATION 6

4 WIND SPEED 7 4.1 Basic wind speed 7 4.2 Site wind speed 7 4.3 Effective wind speed 8

5 DYNAMIC PRESSURE 12

6 CALCULATION OF LOADS 15 6.1 Overview of the recommended process 15 6.2 Pressure coefficients 15 6.3 Overall loads 20 6.4 Frame deflections 21

7 REFERENCES 22

© 2002 The Steel Construction Institute vi SCI ED001

SUMMARY

This short document gives recommendations on how to apply the provisions of BS 6399-2:1997 Loadings for buildings. Code of practice for wind loads. Its main objective is to help designers minimise the design wind loads, as higher (more conservative) wind loads may lead to unnecessarily conservative steel designs. The publication gives the overview of two principal calculation methods: the directional method and the standard method. Within each method, the calculation process is split into a number of sequential stages:

• dynamic classification of the structure

• calculation of wind speeds

• calculation of the dynamic pressure, qs

• calculation of the loads on the building.

Recommendations covering all of these principal stages are given in the publication. It also includes notes, highlighted in a tinted box, as helpful advice in addition to the preceding text.

© 2002 The Steel Construction Institute 1 SCI ED001

1 INTRODUCTION

1.1 Scope This short guidance document is intended to help designers using BS 6399-2:1997[1]. It gives recommendations on how to apply the provisions of the Standard, with particular application to steel structures. The objective of this publication is to help designers minimise the design wind loads, as higher (more conservative) wind loads may lead to unnecessarily conservative steel designs.

The guidance may not give the minimum wind load in every circumstance. In some cases, designers could achieve the same wind load with less work. In some circumstances further reductions in the wind loads could result from following a more refined approach.

More detailed guidance on the use of the Standard is given in Guide to evaluating design wind loads to BS 6399-2 [3]. It is expected that when published, later in 2002, designers will refer to this comprehensive guide.

Throughout this publication, BS 6399-2 is referred to as the Standard. References to figures and tables in the Standard, are designated by a single number, for example Figure 6. Figures and tables in this publication are enumerated with a two-part number, for example Figure 3.1.

This publication includes notes, highlighted in a tinted box, as helpful advice in addition to the preceding text.

1.2 Recommendations Advice is contained in this note is taken from three sources:

• The Standard, incorporating Amendment No. 1

• BRE Digest 436[2]

• The draft design guide[3]

All references to the Standard refer to the Standard incorporating Amendment No. 1, issued 27 March 2002. Readers should note that Amendment No. 1 introduces some significant changes to the 1997 Standard.


2 OVERVIEW OF THE RECOMMENDED PROCESS

2.1 Calculation stages BS 6399-2 contains two principal calculation methods: the directional method and the standard method. The directional method is primarily suited to computer calculation. The standard method is appropriate for hand calculation, but may be up to 30% more conservative than the directional method. Clause 3.4 of the Standard allows hybrid methods, which are a combination of the directional and standard methods.

Within each method, it is convenient to split the calculation process into a number of sequential stages:

• dynamic classification of the structure

• calculation of wind speeds

• calculation of the dynamic pressure, qs

• calculation of the loads on the building.

Recommendations covering all of these principal stages are given in the following Sections. Generally, the recommendations follow the stages in the flow chart given in Figure 1 of the Standard. Each stage is introduced and recommendations are made.

2.2 The dependence of the dynamic pressure, qs on wind direction

The calculation of the dynamic pressure is based on an effective wind speed on the structure and that, in turn, is based on a site wind speed. The effective wind speed is directionally dependent, because the factors that modify the basic and site wind speeds (the altitude factor, directional factor, and terrain and building factor) may be directionally dependent.

The simplest and most conservative approach when determining the dynamic pressure is to ignore any variation around the site and simply take the worst case (i.e. the most onerous directional factor, the most onerous terrain category, and the closest distance from the sea), and assume that these three factors are coincident. In some locations, this will be a realistic combination, and hence the simplest approach will produce the correct value for the dynamic pressure. For the majority of sites, the combination of worst-case factors will be unrealistic and lead to a conservative (i.e. higher) value for the dynamic pressure.

The most accurate approach is to calculate the dynamic pressure at 30° intervals, using the appropriate altitude factor, directional factor, and building and terrain factor within each 30° segment. This leads to a unique value of the dynamic pressure for each of the 12 directions considered. The value of dynamic pressure to be used in design is then the highest of those calculated in


any of the 12 directions. The calculation effort in calculating the dynamic pressure in 12 directions is considerable.

The recommended approach is to determine the appropriate factors in each of four 90° segments and to use these to determine the dynamic pressure in each of the four quadrants. This approach reduces the calculation effort involved whilst producing a reduced dynamic pressure compared to the simple approach. In each of the four quadrants, the most onerous altitude factor, the most onerous directional factor and the most onerous terrain and building factor are used to calculate the dynamic pressure. The dynamic pressure to be used in design may be conservatively taken as the largest of the four values calculated.

Whichever approach is adopted, it is critically important for factors around the full 360° be considered. For example, if only four specific directions that are normal to the building faces were considered, as shown in Figure 2.1, it would be quite possible to miss a more onerous altitude factor, directional factor, or terrain and building factor. Clause 2.1.1.2 makes it clear that if the designer chooses wind directions that are normal to the building faces, a range of wind directions within 45° each side of the normal to the building faces must also be considered, as shown in Figure 2.2. This requirement results in the calculation of the dynamic pressure within four 90° quadrants, as described above.

2.2.1 Choice of quadrants Quadrants may be chosen aligned in any direction around the site, and need not be aligned with respect to the faces of the structure. Figure 2.3 shows a situation where the quadrants have been chosen to not align with the axis of the building.

Figure 2.1 Directions normal to building faces

45°

45°45°

45°

45°

45°

45°

45°

Figure 2.2 Range of directions to be considered


It will usually be convenient to choose quadrants based on North, South, East and West as shown in Figure 2.4. It is not essential that the quadrants be aligned with respect to the axes of the building. The primary benefit of using quadrants to determine the dynamic pressure arises from a consideration of the site rather than being related to the building orientation. If the orientation of the building is unknown, the dynamic pressure to be used in design (the maximum of the four values calculated) will be used in each orthogonal direction.

In some circumstances, when the orientation of the building is known, it may be beneficial to align the quadrants with respect to the building axes. The designer may wish to use different dynamic pressures in the orthogonal loadcases, and if so, the quadrants should be arranged as shown in Figure 2.2. Aligning the four quadrants with respect to the building axes may be particularly beneficial if the building is asymmetric or if the building has particular features, such as dominant openings, where a reduction in the wind pressure on the face with the dominant openings may help mitigate the effects of the opening.

If the orientation of the building is unknown, or the quadrants are deliberately chosen to misalign with the faces of the building, the maximum of the four values of dynamic pressure must be used in each orthogonal loadcase.

90°

90°

Site

Figure 2.3 Deliberate misalignment between building axis and

quadrants

N

EW

S

North-westquadrant

North-eastquadrant

South-westquadrant

South-eastquadrant

Figure 2.4 Recommended orientation of quadrants


If topography is significant only over a relatively narrow range of directions, it will generally be beneficial to “capture” the significant topography within one quadrant. The same recommendation applies to any other important feature, such as terrain category or distance from the sea. It is generally advantageous if any such features can be “captured” in a single quadrant, rather than impacting on two quadrants. Figure 2.5 shows two alternative orientations, when a site is relatively close to a large stretch of inland water. In this case, the Standard recommends that the “distance from the sea” must be measured from the inland water, leading to a more onerous building and terrain factor. In the first orientation of quadrants, the proximity of the inland water affects two quadrants. In the second orientation, the proximity of the inland water affects only one quadrant.

Inland water affects NE and SE quadrants

Inland water 'capured'in one quadrant

Figure 2.5 Alternative orientation of quadrants


3 DYNAMIC CLASSIFICATION

The Standard allows loads on mildly dynamic buildings to be calculated by amplifying the static loads. The amplification is described within the Standard as dynamic augmentation. The initial part of the design process:

• checks that the building falls within the provisions of the Standard, and,

• determines the dynamic augmentation factor, Cr, for use later in the process when determining the overall loads on the building.

To determine Cr, the building-type factor, Kb, should be determined from Table 1. Having determined the building-type factor, Figure 3 is used to calculate the dynamic augmentation factor, choosing the appropriate curve and using the overall height of the building. If the intersection of curve and building height falls in the shaded zone of Figure 3, the building is too dynamic for the methods described in the Standard and the designer is directed by the Standard to sources of specialist advice.

Notes 1. Table 1 adequately describes orthodox buildings. Building-type factors for

unorthodox structures should be chosen by taking a view on their likely dynamic response.

2. Note that the building height in Figure 3 is shown as a log scale.


4 WIND SPEED

4.1 Basic wind speed The basic wind speed Vb should be taken from Figure 6, for the geographical location of the building. Interpolation between windspeed contours is allowed.

4.2 Site wind speed The site wind speed is calculated from the basic wind speed by applying a number of factors, discussed in the following Sections.

4.2.1 Altitude factor, Sa

The altitude factor depends on:

• the altitude above sea level

• any significant topography.

If topography is not considered to be significant, the altitude factor is simply 1+ (altitude /1000). Altitude is measured in metres.

If topography is considered significant, the altitude factor is taken as the greater of:

• the altitude factor as calculated above ( 1 + altitude / 1000), or

• a value dependent on the altitude at the base of the topographic feature, the effective slope and a topographic location factor. Designers are referred to Clause 2.2.2.2.3 and Figure 8 of the Standard.

Notes 1. The influence of the altitude factor on the value of the dynamic pressure is

very significant. The effect of the altitude factor is squared, meaning that a 100 m difference in height can produce as much as 20 percent change in the dynamic pressure

2. If the ground level for 1 km around the site is flat, or slopes at less than 1 in 20, topography is not significant.

3. Slopes which are steeper than 1 in 20 are considered to be significant topography

4. The significance of topography depends on the wind direction. Topography may be significant when the wind is blowing from certain directions, but is unlikely to be significant in all wind directions

5. If topography is significant, note that the altitude to be used is the altitude at the base of the topographic feature.

4.2.2 Directional factor, Sd

Values of the directional factor are found in Table 3. Interpolation is allowed. Sd should be taken as the maximum value that appears anywhere in the quadrant


under consideration. If quadrants between 0°, 90°, 180°, 270° and 360° are chosen, the values in Table 4.1 should be used.

Table 4.1 Values of Sd, when quadrants orientated from 0°

Quadrant Bearing range Sd

North-east 0° - 90° 0.78

South-east 90° - 180° 0.85 South-west 180° - 270° 1.00

North-west 270° - 360° 0.99

4.2.3 Seasonal factor, Ss

In most circumstances, the seasonal factor has a value of 1.00. If the structure will be exposed to the wind for less than one year (i.e. a temporary structure), a seasonal factor of less than 1.00 may be used. Annex D of the Standard provides values of Sd for periods as short as a single month. Structural designers choosing a seasonal factor of less than 1.00 need to be confident that the structure will only be exposed to wind during the expected period.

It is generally recommended that a seasonal factor of 1.00 be used.

4.2.4 Probability factor, Sp

The probability factor has a value of 1.00 or less. Structural designers should only use a probability factor of less than 1.00 if they wish to amend the standard design risk. Using a probability factor of 1.0 represents a once-in-50-year risk, equivalent to an annual risk of 0.02. Annex D of the Standard provides a number of values of the probability factor for various annual risks.

If a dominant opening is assumed closed, the accidental case of the door or window being open must be considered (see Section 6.2.6). For this check, a new dynamic pressure is calculated, taking Sp as 0.8.

In all other circumstances it is recommended that a probability factor of 1.00 is used.

4.2.5 Expression for site wind speed, Vs

The site wind speed, Vs, should be calculated from:

Vs = Vb × Sa × Sd × Ss × Sp

Note that four values of the site wind speed will be calculated, one for each quadrant.

4.3 Effective wind speed The effective wind speed is derived from the site wind speed, depending on the terrain and the building height.

4.3.1 Terrain category In order to determine the terrain and building factor, the terrain category, the effective height and the distance from the sea must all be calculated.


Three categories of terrain are defined in the Standard:

• Sea (which includes large stretches of inland water)

• Town

• Country

Town is defined as built-up areas in which the average level of rooftops is at least 5 m above ground level. The town category includes two-storey domestic housing, as long as the plan area of the upwind buildings is at least 8% of the total area in the segment being considered.

To be in the town category, the town must extend at least 100 m upwind of the site.

Country terrain is any terrain that is neither sea nor town terrain.

Annex E of the Standard gives more details of the terrain categories.

Note that the terrain category may be different in each of the four quadrants.

4.3.2 Effective height If a site falls in the town terrain category, a reduced effective height may be calculated, in accordance with Clause 1.7.3. This is beneficial, since the dynamic pressure increases with the effective height of the structure. In order to calculate a reduced effective height, the height of the upwind buildings and the spacing between the upwind shelter and the site must be known.

If the height of the upwind shelter is not known, the note to Clause 1.7.3.3 allows the height to be estimated from the average number of storeys, assuming each storey height is 3 m. BRE Digest 436 [2] recommends that the average height be calculated over a distance of 100 m upwind from the site.

If the spacing between the site and the upwind buildings is not known, BRE Digest 436 recommends that a spacing of 20 m be assumed in suburban and urban areas.

Reduced effective heights can only be calculated for sites in the town terrain category. There is no reduction for sites in the country terrain category.

Note that the calculation of effective height may result in a different effective height in each quadrant, since this depends on the average height of the upwind buildings and the spacing between the site and the upwind buildings.

Notes 1. The Standard assumes irreversible urbanization. This means that the

degree of shelter is assumed not to decrease over time

2. BRE Digest 436 confirms that the design wind speed should not be adversely affected by the demolition of any individual neighbouring building.


4.3.3 Distance from the sea In order to determine the terrain and building factor, the distance from the sea must be calculated. The effective wind speed reduces as distance from the coast increases. Table 4.2 gives recommended accuracy when measuring distance from the sea.

Note that the distance from the sea will undoubtedly differ in each of the four quadrants.

Table 4.2 Recommended accuracy for measuring distance from the sea

Distance from the sea Recommended accuracy

Between 100 and 300 m Nearest 100 m

Between 300 m and 1 km Nearest 200 m Between 1 km and 3 km Nearest 500 m

Between 3 km and 10 km Nearest km

Between 10 km and 30 km Nearest 2 km Between 30 km and 100 km Nearest 10 km

Note BRE Digest 436 offers advice on calculating the distance from the sea when the site is adjacent to an estuary or inland stretches of water. Conservatively, the distance from the sea may be taken as the distance from any water, although this will often be unduly onerous.

4.3.4 Terrain and building factor, Sb

Two alternatives for calculating the terrain and building factor are provided in this section. The first, and simplest, involves the use of Table 4. The second, involving additional calculations, follows the process described in the directional method. The second approach will generally result in a smaller value of Sb, particularly near the edge of a town, and is recommended in order to produce the most advantageous (i.e. smallest) factor.

Use of Table 4

Table 4 is used to determine the terrain and building factor, having determined the terrain category and calculated a reduced effective height (if the site is in the town terrain category), and knowing the distance from the sea. The terrain and building factor is likely to differ in each of the four quadrants considered.

Table 4 is divided into two halves. The right-hand side covers sites that are at least 2 km into a town. The left-hand side of Table 4 covers sites in the country, and sites up to 2 km into towns.

Notes 1. If a reduced effective height has been calculated for a site in the town

terrain category, and yet the site is less than 2 km into the town, (thus falling into the left-hand side of Table 4), the terrain and building factor should still be calculated on the basis of the reduced effective height.

2. Interpolation or logarithmic interpolation of Table 4 is allowed. Ordinary linear interpolation of Table 4 is recommended.


Determination of Sb from the directional method

In order to determine the terrain and building factor using the directional method:

• Equation 28 (Clause 3.2.3.2.2) should be used for sites in country terrain

• Equation 29 (Clause 3.2.3.2.3) should be used for sites in town terrain

Equation 28 uses values from Table 22. Equation 29 uses values from Tables 22 and 23. Interpolation within both tables is recommended. When measuring distance into town, the recommended accuracy of measurement is shown in Table 4.3.

Table 4.3 Recommended accuracy of measurement from edge of town

Distance from the edge of town Recommended accuracy

Between 100 m and 300 m Nearest 100 m

Between 300 m and 1 km Nearest 200 m Between 1 km and 3 km Nearest 500 m

Between 3 km and 10 km Nearest km Between 10 km and 30 km Nearest 10 km

Notes 1. Note that Table 22 is headed “Upwind distance from sea to site” and

Table 23 is headed “Upwind distance from edge of town to site”. Although the tables look similar, the potential for confusion is clear.

2. In Equations 28 and 29, the term gt is used. The value of gt should always be taken as 3.44 when following the recommendations in this guide. The Standard and Hybrid methods both use the size effect factor, Ca, when determining loads on surfaces, as described in Section 6.2.5. In these circumstances, Clause 3.2.3.3.3 specifies that gt should be taken as 3.44.

3. In Equations 28 and 29, the term Sh is used. This factor allows for significant topography. When following the recommendations in this guide, significant topography has already been accounted for when calculating the altitude factor, Sa (Section 4.2.1). Since the effect of any significant topography has already been included in Sa, the factor Sh should be set to zero.

4.3.5 Expression for effective wind speed, Ve

The effective wind speed is given by:

Ve = Vs × Sb

Since both the site wind speed and the terrain and building factor vary in each quadrant, the effective wind speed is likely to be different in each quadrant.


5 DYNAMIC PRESSURE

The dynamic pressure is given by:

qs = 0.613 × Ve2

It is highly likely that the dynamic pressure will differ in each quadrant. If the quadrants have not been orientated with respect to the building faces, the highest of these four values should be used in design. Such a situation is illustrated in Figure 5.1, where the quadrants have not been chosen to relate to the building axis. A value of dynamic pressure will be calculated for each quadrant, here designated qs1, qs2, qs3, and qs4.

As the designer will typically wish to consider two orthogonal loadcases (wind at 0° and 90°), the maximum of these four values (here designated qmax) will be applied in both orthogonal loadcases, as shown in Figure 5.2.

If the quadrants have been aligned with respect to the building axes as shown in Figure 5.3, the designer has the opportunity simply to apply the maximum value in both orthogonal loadcases, as Figure 5.2, or to associate the different dynamic pressures with their respective loadcases. If the four values of dynamic pressure in Figure 5.3 are designated qs5, qs6, qs7, and qs8, then these may be applied in the four orthogonal directions shown in Figure 5.4.

s1qQuadrant 1

q

q

qQuadrant 4

s4

Quadrant 3

s3

s2

Quadrant 2

Figure 5.1 Deliberate misalignment of quadrants and building faces

q

q

max

max Figure 5.2 Dynamic pressure for orthogonal loadcases


For most structures that are doubly-symmetric and rectangular on plan, designers typically consider only two loadcases, with wind directions along the structure and across the structure. If quadrants have been aligned with the building axes (Figure 5.3), the two orthogonal loadcases and appropriate values of dynamic pressure are shown in Figure 5.5.

qs5

qs7

q

Quadrant 5

Quadrant 7

qQuadrant 6

s6

Quadrant 8

s8

Figure 5.3 Quadrants chosen to align with building axes

q

qq

q

s5

s8 s6

s7 Figure 5.4 Four orthogonal loadcases with differing values of dynamic

pressure

qs6

Larger of or qs8

q qLarger of

ors7 s5 Figure 5.5 Values of dynamic pressure for two orthogonal loadcases


In many cases, the difference between the values given by the approach illustrated in Figure 5.2 (building orientation unknown or ignored) and those given by Figure 5.5 will be small. If the wind loads are a critical consideration in the design, or the structure has some form of asymmetry or has dominant openings, the approach illustrated in Figure 5.4 is recommended.


6 CALCULATION OF LOADS

6.1 Overview of the recommended process Pressure coefficients are determined from the appropriate table for the building size and configuration. Both external and internal pressure coefficients should be determined.

Surface loads are calculated from the product of the dynamic pressure and the net pressure coefficient. If the form of construction is such that load sharing takes place over a significant area, the surface loads may be reduced by a size effect factor, which is related to the size of the area over which load sharing takes place.

Overall loads may be calculated using overall force coefficients (only specified in the draft amendment), or are to be calculated from the loads on the windward and leeward faces. When calculating overall loads based on the loads on the windward and leeward faces, a factor of 0.85 is applied to overall loads (excluding drag) to account for non-simultaneous action between faces. Unless a preliminary design is required, for example when sizing vertical bracing, it is recommended that the overall loads be calculated from the combination of loads on the windward and leeward faces, in accordance with Clause 2.1.3.6 of the Standard.

6.2 Pressure coefficients Within this guide, tables from the Amended Standard are quoted. Designers should note that Amendment No 1 introduces some significant changes to the 1997 Standard.

6.2.1 Overall load coefficients The Standard provides a table of net pressure coefficients that may be used to determine overall loads, instead of summing the effects on the windward and leeward faces. Table 5a from the Standard is reproduced as Table 6.1 below.

Table 6.1 Net pressure coefficients for overall load (Table 5a from the Standard)

D/H B/D

# 1 $ 4

# 0.5 1.2 1.0

1 1.2 0.8

2 1.2 0.8 $ 4 1.1 0.8

Where:

B is the crosswind breadth of the building

D is the inwind depth of the building

H is the building height.


6.2.2 External pressure coefficients, Cpe

Pressure coefficients for walls

Table 5 from the Standard is reproduced below as Table 6.2.

Table 6.2 External pressure coefficients Cpe for vertical walls (Table 5 from the Standard)

Span ratio of building Exposure case Vertical wall face D/H # 1 D/H $ 4

Vertical wall face Isolated Funnelling

Windward (front) 0.85 0.6 Side Zone A –1.3 –1.6

Zone B –0.8 –0.9

Leeward (rear) –0.5 –0.5 Zone C –0.5 –0.9

Notes 1. Table 5 can be used for walls within 15° of vertical. Guidance for inclined

walls can be found in the directional method

2. Interpolation should be used to determine coefficients for span ratios between 1 and 4

3. Note that H is the height of the wall, which is not necessarily the height of the building.

4. The length of zones A, B, and C depends on the scaling length, b. The scaling length, b, is the lesser of the crosswind breadth and twice the height of the building. Depending on the geometry of the building, it is quite possible that zone C does not exist.

Funnelling

If the wind can blow down the gap between buildings, funnelling (which increases the pressure coefficients on the facing elevations) may occur. Funnelling may occur when the gap between the two buildings is between b/4 and b, where b is the scaling length.

The Standard specifies that if the two buildings are sheltered by upwind buildings such that the effective height before the lower of the two buildings is no greater than 0.4Hr, funnelling may be disregarded. The effective height depends on the structure height, Hr, the height of the upwind shelter, Ho, and the spacing between the structure and the upwind shelter, Xo. Table 6.3 identifies when funnelling should be checked.

Table 6.3 Requirement to check funnelling

Spacing Requirement for funnelling check

If X < 2H0 No funnelling if H0 ≥ 1.5Hr

If X ≥ 6 H0 Funnelling may occur – check gap

If 2H0 < X < 6H0 No funnelling if H0 – 0.1X ≥ 0.3 Hr

It is recommended that it be made clear to designers whether funnelling is to be considered by providing information about the gap between buildings.


Designers should always state whether funnelling has been considered and this information should be recorded in the as-built documentation and the Health and Safety file.

Pressure coefficients for flat roofs

External pressure coefficients for flat roofs (with roof slopes between +5° and -5°) are given in Table 8 of the Standard.

Pressure coefficients for monopitch and duopitch roofs

External pressure coefficients for monopitch and duopitch roofs are given in Tables and 9 and 10 respectively. Table 11 covers hipped roofs. Part of Table 10, for duopitch roofs, is reproduced as Table 6.4.

Table 6.4 External pressure coefficients Cpe for duopitch roofs of buildings (extracted from Table 10 of the Standard)

Zone for 22 = 00 Zone for 22 = 900 Pitch angle A B C E F G A B C D

–1.8 –1.2 –0.6 –0.9 –0.3 –0.4 –2 –1.1 –0.6 –0.5 5°

0 0 0 –0.9 –0.3 –0.4

–1.1 –0.8 –0.4 –1.3 –0.9 –0.5 –1.6 –1.5 –0.6 –0.4 15°

0.2 0.2 0.2 –1.3 –0.9 –0.5

–0.5 –0.5 –0.2 –0.9 –0.5 –0.5 –1.2 –1.1 –0.6 –0.5 30°

0.8 0.5 0.4 –0.9 –0.5 –0.5

Designers familiar with the 1997 Standard should note that the changes to Tables 9, 10 and 11 are associated with a revision to Clause 2.1.3.7, which covers asymmetric loads. Monopitch, duopitch, and hipped roofs are specifically covered by Tables 9, 10 and 11, and therefore no other allowance need be made for asymmetric loads. Asymmetric loads on structures not specifically covered must be allowed for by reducing the design wind load by 40% on those parts of the structure where the effect of the load is beneficial. Note that two loadcases are specified for wind at 0° (across the building), as can be seen in Table 6.4 above. For example, at a roof slope of 5°, the first loadcase has external pressure coefficients of –1.8, –1.2, –0.6 on zones A, B and C respectively. The second loadcase has no external load at all on zones A, B and C.

Notes 1. Coefficients should be interpolated between the roof slopes given.

2. See Section 6.2.3 for recommendations on simplifying the range of pressure coefficients for orthodox roofs.

Internal pressure coefficients, Cpi

Internal pressure coefficients are given in Table 16. BRE Digest 436 advises that Cpi = +0.2 is now less likely to be a critical design case. In most cases, design of portal frames should proceed with a single internal pressure coefficient of Cpi = -0.3. This assumes that there are no dominant openings (or that these are closed during a storm), and reasonably equal permeability on all faces of the building[2]. Permanent non-dominant openings in the walls, such as ventilators,


will not affect the internal pressure coefficient, provided that they are distributed approximately equally around the perimeter of the building.

6.2.3 Simplified net pressure coefficients for portal frame roofs For portal frame buildings, where the internal pressure coefficient is taken as -0.3, simplified net pressure coefficients may be used for design. Full details are given in References 3 and 4. If a more refined approach is necessary, the coefficients in the Standard and draft amendment should be used.

Partial details from Reference 3 are given in Table 6.5. The zones referenced in Table 6.5 are shown in Figure 6.1. Table 6.5 may be used for the design of purlins and for the design of frames as described below, provided that:

• bw/10 ≤ (half the span of the purlin), and

• Cpi is taken as –0.3

Purlin design

Purlins may be designed using the coefficients in Table 6.5. it is expected that purlins will be designed using the coefficients for zone X, and checked for zone Y, reducing the spacing of the purlins if necessary.

Frame design

When designing a portal frame, the coefficients in zone Y of Table 6.5 may be ignored, since the zone X coefficients are conservative.

Table 6.5 Simplified net coefficients for duopitch roofs

Zone for 22 = 0° Pitch angle X Y

–1.14 –1.64 5°

0 0

–0.94 –1.14 15°

0.50 0.50

–0.74 –0.94 30°

0.80 1.10

6.2.4 Surface pressures Surface pressures are calculated from the product of the dynamic pressure multiplied by a pressure coefficient, multiplied by a size effect factor. The size effect factor (see Section 6.2.5) is smaller or equal to 1.0. As the size effect

L

L

b /2L b /2L

b /10

b /10

x

x

y y

y y

Figure 6.1 Key for Table 6.5


factor for external and internal pressures will be considerably different, it is recommended that the external and the internal surface pressures be calculated, before combining the two pressures to give a net pressure. It is not conservative to take the size effect factor as 1.0 for both external and internal pressures, as the most onerous net pressure will result from a smaller “relieving” pressure, resulting from a size effect factor less than 1.0. The size effect factor could be as small as 0.52.

6.2.5 Size effect factor, Ca

The size effect factor is taken from Figure 4 of the Standard, and depends on a diagonal dimension, a.

External pressures

Dimension a for external pressures is taken as the diagonal over which load sharing takes place. Some knowledge of structural response is therefore necessary when determining the wind loads. Typical examples of dimension a for external pressures are:

• For a gable post: the diagonal of the loaded height and post spacing.

• For roof bracing: the diagonal of the entire loaded area carried by the bracing.

• For a purlin: the diagonal of the spacing and span. For continuous systems, dimension a may be based on the purlin spacing and two spans.

• For an individual portal frame: the diagonal based on frame spacing and height to apex.

The last bullet above implies that load sharing does not take place between main frames. In most cases, load sharing will take place, due to the shear stiffness of the roof cladding, as long as this is positively fixed to the purlins. However, standing-seam roof systems, and others without positive fixing, have insufficient shear stiffness to justify load sharing over more than one bay.

If load sharing over more than one bay is assumed in the design calculations, and hence a larger diagonal dimension is calculated, it is recommended that this feature of the design, and the need for adequate shear stiffness from any replacement cladding system is documented in the Health and Safety file.

Internal pressures

In accordance with Clause 2.6.1.1, the relevant diagonal dimension a for internal pressures may be taken as:

3 storeyofvolumeinternal10 ×=a

Note Note that in Figure 4, the diagonal dimension is a log scale. An approximate value for the diagonal dimension will be satisfactory.

6.2.6 Dominant openings If buildings have dominant openings (see Clause 2.6.2 of the Standard), both the diagonal dimension and the internal pressure coefficients change significantly. Clause 2.6.1.3 states that if a dominant opening is considered to be closed at


ULS, the condition with the door open should be considered as a serviceability limit state.

The intent of the clause is that an alternative ULS loadcase should be considered, representing the accidental opening of a door during a severe storm. This is made clear by BRE Digest 436 [2], which states that when considering this alternative loadcase, the dynamic pressure should be recalculated using a probability factor, Sp, of 0.8. In addition, since this is an accidental loadcase, all load factors used in load combinations should be taken as 1.0.

6.2.7 Frictional drag Frictional drag should be calculated in accordance with Clause 2.1.3.8. Frictional drag is only applied to the most “downwind” zones of walls and roofs, i.e. zone C of walls and zone D of roofs. Frictional drag coefficients for different types of surface are given in Table 6.

6.2.8 Division by parts Division by parts, familiar to designers using CP3:ChV:Part 2 is not allowed unless the building is taller than the cross-wind breadth. Designers are referred to Figure 11 of the Standard. Note that even for tall buildings, division by parts can only be used for positive pressures.

6.3 Overall loads 6.3.1 Calculation of overall loads Overall loads are calculated in accordance with Clause 2.1.3.6 as the sum of the loads on individual surfaces. When calculating overall loads (and only at this stage) the loads are amplified by (1+ Cr), where Cr is the dynamic augmentation factor (see Section 3).

At this stage (and only at this stage) allowance should be made for the non-simultaneous action between faces. To account for this effect, the Standard specifies that a factor of 0.85 be applied when calculating the overall loads.

The non-simultaneous action can be thought of as allowing for the time delay between gusts acting on the faces of the structure. Where there is no time delay, the 0.85 factor should not be applied. Thus in Figure 6.2, there is a time delay between gusts acting on the upwind and downwind faces of the building. In this situation, the factor 0.85 is applied to all loads, at ULS and SLS.

When the wind direction is along the building, as in Figure 6.3, the time delay is only applicable to the pressures on the windward gable, the suction at a section along the building and the suction on the leeward gable. The specified

Figure 6.2 Pressures due to wind across a building


suctions on an individual frame can all occur simultaneously, and the 0.85 factor should not be applied to the loads on an individual frame in these circumstances.

Note Neither the dynamic augmentation nor the 0.85 factor is applied to frictional drag.

6.4 Frame deflections Table 5a of the draft amendment presents overall pressure coefficients (see Section 6.2.2). These produce smaller overall loads than calculating the loads on the windward and leeward faces individually. It is not clear how the implied reductions could be applied to loads on frames when determining forces and moments around the frame, but the reductions can be applied, if required, to the horizontal deflections arising from the wind loads. The reductions in horizontal deflection are given in Table 6.6.

Table 6.6 Reduction factors which may be applied to horizontal deflections due to wind

D/H B/D

≤ 1.0 ≥ 4.0

≤ 0.5 0.89 0.9

1.0 0.89 0.73 2.0 0.89 0.73

≥ 4.0 0.81 0.73

Notes 1. The reduction factors in Table 6.6 can only be applied to the deflections

due to wind[3].

2. The reduction factors are additional to the 0.85 factor which may have been applied in accordance with Section 6.3.1.

Figure 6.3 Pressures due to wind along building


7 REFERENCES

1. BRITISH STANDARDS INSTITUTION BS6399 Loadings for buildings. BS6399-2:1997 Code of practice for wind loads (Incorporating Amendment No. 1, issued 27 March 2002)

2. Building Research Establisment BRE Digest 436, Part 1. Wind loading on buildings – Brief guidance for using BS 6399 BRE, 1999

3. BAILEY, C. G. Guide to evaluating design wind loads to BS 6399-2 The Steel Construction Institute, (to be published)

4. BAILEY, C.G Simplified wind net pressure coefficients for the design of portal frames The Structural Engineer, Volume 80, No. 4 The Institution of Structural Engineers, 2002

SCI-BRE-BCSA Wind Guidance.pdf

Documents

Transcript of SCI-BRE-BCSA Wind Guidance.pdf