Wind Loading Traditional Dwellings

8/8/2019 Wind Loading Traditional Dwellings

1/14

WIND LOADING ON TRADITIONAL DWELLINGS: AMENDMENT OF SIMPLIFIED

DESIGN GUIDANCE

P A Blackmore

Building Research Establishment

1999

EXECUTIVE SUMMARY

The British Standard code of practice for wind loading, CP3: Chapter V: Part 2, is soon to be

withdrawn and replaced by BS6399: Part 2. The methodology used in this new code is quite

different in many respects from that used in CP3. The simplified design approach for wind

loading currently contained in the Scottish Office Small Buildings Guide, which is

currently based on CP3, therefore needs to be modified to reflect the new methodology ofBS6399.

A simplified design approach for wind loads on traditional buildings has been previously

developed for the Department of Environment, Transport and the Regions by BRE under the

guidance of an industry Steering Group. This previous method was developed specifically for

inclusion in Approved Document A of the England and Wales Building Regulations. This

report extends the methodology to the wind climate in Scotland.

This proposed simplified design method has a simple five-step approach and includes factors

that take account of altitude and topography. There are some quite large differences in

allowable building heights between the current and proposed methods, but in general, these

differences are largely beneficial. In many town and country areas of Scotland, the allowable

building height has increased, although at high altitudes or in the northernmost Islands the

allowable building heights are reduced. The proposed method includes a more accurate

treatment of topography, which allows significant increases in building height for all sites on

hilly or moderately hilly terrain.


2/14

Building Research Establishment Centre for Structural Performance

CONTENTS

1 Introduction....................................................................................................................1

2 Background to Previous Work.......................................................................................1

3 Principle Differences Between CP3: ChV: Part 2 & BS6399: Part 2...........................2

4 Description of the Proposed Simplified Method ...........................................................2

5 Implications of the Proposed Changes for Traditional Building Design.......................8

6 Conclusions..................................................................................................................11

7 References....................................................................................................................11

Annex A List of Steering committee members

Annex B Sensitivity analysis of factors affecting design wind load in BS6399

Annex C Assumptions made in the development of the simplified method


3/14


1 INTRODUCTIONThe latest revision to the British Standard Code of Practice for wind loads, BS6399: Part 2

[1] was published in August 1997. The methodology used in this new Code is different from

that used in its predecessor, CP3: Chapter V: Part 2 [2]. Some of the major changes include:

!Basic wind speed - now given as hourly mean values rather than gust values

! Terrain categories - reduced to two from four! Site altitude - now included as a separate factor! Distance of site from coast - now included

Results from calibrations between these two Codes show that in general that the two methods

give similar results, although significant differences in design wind loads can arise in some

circumstances, particularly for sites at high altitudes.

The Scottish Office Small Building Guide and the equivalent guidance for England & Wales and

Northern Ireland, together with a large number of British Standards, including BS 8103: Parts 2

& 3 "Structural design of low rise housing" and BS 5268 Part 3 "Code of practice for trussedrafter roofs", all currently include design guidance based on the former wind Code. These

authoritative documents therefore need to be amended to take account of the provisions in the

latest wind Code.

2 BACKGROUND TO PREVIOUS WORK

A previous study [3], funded by DETR, was undertaken by BRE to develop simplified design

guidance for wind loading based on BS 6399: Part 2, for inclusion in Approved Document A,

BS8103: Parts 2 & 3 and BS5268, to replace current guidance based on CP3: ChV: Part 2. This

work was carried out under the guidance of an industry steering group, membership of which is

given in Annex A. As these foregoing authoritative documents referred to primarily concern the

t ti f t diti l d lli th bj ti f thi t d t d d d


4/14


The steering group considered a number of different simplified methods, which are described in

detail in [3], before eventually settling on a simple five step approach which is described in

Section 4.

3 PRINCIPLE DIFFERENCES BETWEEN CP3: CHV: PART 2 & BS6399: PART 2The principle reasons for the revision of CP3: Chapter V: Part 2 are:

! To take account of the considerable advances in knowledge and experience of windengineering gained in the last 30 years.

! To move from a 3 second gust windspeed to an hourly mean value in order to allow moreaccurate treatment of wind flow over topography and to provide a starting point forfatigue and dynamic response calculations.

! To move towards harmonisation of methodologies with the Eurocode the ISO wind codeand other National Codes.

A full summary of the differences between CP3: ChV: Part 2 and BS6399: Part 2 is given inAnnex B

The factors in BS6399: Part 2 which most affect the design wind loads are the topography factor

(similar to the S1 factor in CP3: ChV: Part 2) and the altitude factor. Topography can increase

wind loads by 21% for every 100m increase in altitude. The effects from shelter, terrain, distance

from the sea or edge of town and size of loaded element, are smaller, although still significant.

Annex B contains a detailed sensitivity analysis of the parameters that affect the design wind

load.

4.0 DESCRIPTION OF THE PROPOSED SIMPLIFIED METHOD

This method has been implemented in the form of a flow chart based on a simple five step


5/14


The flow chart and accompanying figures (Figures 1 and 2) and tables (Tables 1, 2 and 3) for the

proposed simplified method are shown below, and summarised in Figure 3.

Calculate

value of factor S

from:

Read Map

wind speed Vfrom Figure 1

Find the topographic zone for

the site from Figure 2 and

obtain factor T from Table 1

Obtain value of factorA

from Table 2

Step 1: Read windspeed V from Figure 1

Step 2: Use Figure 2 to determine which topographic zone the site

is situated in and use Table 1 to determine the value of the

Factor T for the appropriate topographic zone and average

ground slope around the site.

Step 3: Use Table 2 to determine the value of Factor A.

Step 4: Calculate the value of Factor S from

S = V x T x A


6/14



7/14


zone 2 zone 1 zone 2

Lu Ld

zone 2 zone 1 zone 2 zone 3

Lu

0.4Lu0.4Lu 0.4Ld 0.4Ld

0.25Lu0.25Lu 0.4Lu 0.4Lu 1.2Lu

Hills and Ridges

Cliffs and Escarpments

Figure 2 Topographic zones for Factor T


8/14


Table 1 Factor T

Factor TTopographic category and average slope of whole

hillside, ridge, cliff or escarpment Zone 1 Zone 2 Zone 3

Category 1: Nominally flat terrain, average slope < 1/20

Category 2: Moderately steep terrain, average slope 1/5

1.0

1.24

1.36

1.0

1.13

1.20

1.0

1.10

1.15

Note: Outside of these zones factor T = 1.0

Table 2 Factor A

Site Altitude

(m)

Factor A

0

50

100

150

200

300

400

1.00

1.05

1.10

1.15

1.20

1.30

1.40

Table 3 Maximum allowable building height (m)Country Sites Town Sites*

Distance to the coast Distance to the coastFactor

S50km 50km

23

24

2526

27

28

29

30

15

15

118

6

4.5

3.5

3

15

15

14.510.5

8.5

6.5

5

4

15

15

1513

10

8

6

5

15

15

1515

15

13.5

11

9

15

15

1515

15

15

13

11

15

15

1515

15

15

14.5

12.5


9/14


Table 1 Factor T

Factor TTopographic category and average

slope of whole hillside, ridge, cliff

or escarpmentZone 1 Zone 2 Zone 3

Category 1: Nominally flat terrain,

average slope < 1/20

Category 2: Moderately steep

terrain, average slope < 1/5

Category 3: Steep terrain,

average slope > 1/5

1.0

1.24

1.36

1.0

1.13

1.20

1.0

1.10

1.15

Note: Outside of these zones factor T = 1.0

Figure 1 Map of basic windspeed

Table 2 Factor A

Site Altitude (m) Factor A

0

50

100150

200

300

400

1.00

1.05

1.101.15

1.20

1.30

1.40

Table 3 Maximum allowable building height (m)

Country Sites Town Sites*

Distance to the coast Distance to the coast

Factor

S

50km 50km

23

24

15

15

15

15

15

15

15

15

15

15

15

15

Figure 2 Topographic zones for Factor T

Calculateva f Sl ue of a ctor

from:

V x T x AS =

Read Map

wind speed VfromFigure 1

Find the topographic zone forthe s ite from igure nd

obtain factor T om able

F 2 a

fr T 1

Obtain value of factor A

from Table 2

zone 2 zone 1 zone 2

Lu Ld

zone 2 zone 1 zone 2 zone 3

Lu

0.4Lu0.4Lu 0.4Ld 0.4Ld

0.25Lu0.25Lu 0.4Lu 0.4Lu 1.2Lu

Hills and Ridges

Cliffs and Escarpments


10/14


Figure 3 Summary of the Proposed Simplified Method


11/14


In BS6399: Part 2, for cases where topography is significant the calculation of altitude factor is

based on the altitude at the base of the topographic feature. For simplicity in this proposed

method, the calculation of altitude factor is based on the site altitude even when topography is

significant. This can lead to an overestimation of the wind speed for sites on large hills and

ridges, with the consequence that the maximum allowable building height could be lower than

necessary. Therefore, it is suggested that the following note be added, either in the text or

attached to Table 2: > Note: For sites in topographic categories 2 and 3, a more accurate

assessment of Factor A can be obtained by using the altitude at the base of the topographic

feature instead of the altitudeat the site=. This approach is fully compatible with BS6399: Part

2, and would allow the user to take advantage of this more accurate approach if required.

For some hilly sites the user might not have prior information on the dimensions of the hill and

so will not be able to determine which zone his site lies in. It is suggested that the following note

be added, either in the text or attached to Table 1 to overcome this difficulty; >Note: If the exact

location of the site is not known then assume Zone 1=.

5.0 IMPLICATIONS OF THE PROPOSED CHANGES FOR TRADITIONAL

BUILDING DESIGN

Eight sites in Scotland were chosen for a comparison between the proposed simplified method

and the current simplified method in the Scottish Office Small Building Guide. These sites were

chosen to give a geographical spread and range of altitudes and distance from the coast.

Site details, including the basic hourly mean wind speed from the proposed method and the basic

gust wind speed from the current Scottish Office Small Building Guide are shown below for the

eight sites selected.

Site Altitude

(m)

Distance to coast

(km)

V (m/s)

(current method)

V (m/s)

(proposed method)Edinburgh

Glasgow

Inverness

Aberdeen

Oban

60

30

20

20

80

1

20

1

0

0 5

50

51

51

49

51 7

23.5

24

24

24

25 3


12/14


Each site has been assumed to be in both Country terrain and Town terrain on flat level ground

and on moderate and steeply sloping ground; for hilly terrain the site has also been assumed to be

at the hill crest and at a point some distance upwind from the crest. This gave a total of about 160

notional site locations for which comparisons between the current Small Buildings Guide method

and this proposed simplified method were made. The maximum allowable building heights using

the current and proposed methods are shown in Table 4, (heights are rounded down to the next

0.1m)

A comparison such as this is somewhat subjective because the current Small Buildings Guide

method is based on four terrain categories whereas the proposed method is based on two

categories. The Country terrain category in BS6399 includes terrain >... from the flat open level,

or nearly level country with no shelter, such as fens, airfields, moorland or farmland with no

hedges or walls, to undulating countryside with obstructions such as occasional buildings and

windbreaks of trees, hedges and walls=. This category therefore encompasses categories 1 and 2

in the Small Buildings Guide. Similarly, the Town category in BS6399 encompasses categories 3and 4 in the Small Buildings Guide. For the purposes of this exercise, Country terrain in the

proposed method has been compared with category 1 in the current method and Town with

category 3.

The results in Table 4 show that there are some quite marked differences between the two

methods. In flat terrain, i.e. the no topography case, the proposed method results in significantly

greater allowable building heights at four sites (Edinburgh, Glasgow, Inverness and Aberdeen),

where the allowable heights have increased from about 6m to 8m to 12m to 15m. At Fort

William there is little change, at Lerwick and Oban there is a small reduction of about 1m, whilst

at Kingussie there is a 50% reduction - about 3.5m, due to the altitude of this site. The largest

and most striking differences occur for the cases where topography is significant. The Small

Buildings Guide uses a very conservative treatment of topography, which assumes that the site is

always on the crest of a very steep hill. This method does not allow any buildings to be

constructed on steeply sloping sites at any of the eight selected locations. Whereas the proposed

method allows, for example, in Glasgow, buildings of up to 7.3m in height on the crest of a steephill. Where the building is assumed to be in Town terrain and away from the crest (as the

majority of buildings are) or on a moderate slope then the proposed method allows building

heights of up to 15m in Glasgow, over 13m in Edinburgh, Inverness and Aberdeen, over 8m in

Oban and Fort William and over 5m in Lerwick and Kingussie. Compare this with the current

method which does not allow any building in these situations


13/14

Building Research Establishm

ent Centre for Structural Performance

6.0 CONCLUSIONS

This proposed simplified design method uses a simple five-step approach, which includes

separate factors to take account of altitude and topography. There are some quite large

differences in allowable building heights, but in general, these differences are largely beneficial.

The main conclusions that can be drawn from this study are:

For flat, level sites in open country or town terrain the proposed simplified method willgenerally give greater maximum allowable building heights that the current Small

Buildings Guide method, the main exception appears to be sites in the northernmost

islands where the allowable heights are slightly lower. For sites at altitudes greater than

about 100m, the maximum allowable building heights could be significantly lower than

those currently allowed by the Small Buildings Guide.

The current simplified method in the Small Buildings Guide is overly conservative forsites on hilly terrain and does not allow any buildings on hilly sites at any of the eight

selected locations. The proposed method treats topography in a more realistic and

accurate manner that allows buildings in all of these previously excluded areas.

The proposed simplified methodology is more complex than that currently included inthe Small Buildings Guide due to the inclusion of altitude and topographic factors.

However, the steering committee feel that this new method is still well within the

capabilities of the users of the Small Buildings Guide.

7.0 REFERENCES

1. BS6399: Part 2: British Standard Code of Practice for Wind Loads, BSI, 1997.2. CP3: Chapter V: Part 2: Code of Practice for Wind Loads, BSI, 1972


14/14


12

Table 4 Comparison of Maximum Allowable Building Heights in Metres Using the Current and Proposed Simplified Methods

No Topography With Topography

Current Proposed Current method1 Proposed Steep slope >1/5 Proposed Moderate slope steeply sloping sites=

Documents

Wind Loading Traditional Dwellings