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Building Research Establishment

Snow loads in Scotland Comparison of BS EN 1991-1-3 and BS 6399: Part 3 for small buildings

January 2012

Report prepared by: Professor Haig Gulvanessian CBE Associate BRE Building Research Establishment Orion House Scottish Enterprise Technology Park East Kilbride Glasgow G75 ORD The opinions expressed in this report are those of the author. Report commissioned by: Directorate for the Built Environment Building Standards Division Denholm House Almondvale Business Park Livingston EH54 6GA Tel: 01506 600 400 Fax: 01506 600 401 e-mail: [email protected] web: www.scotland.gov.uk/bsd © Crown Copyright 2011 Applications for reproduction of any part of this publication should be addressed to: BSD, Directorate for the Built Environment, Denholm House, Almondvale Business Park, Livingston, EH54 6GA This report is published electronically to limit the use of paper, but photocopies will be provided on request to Building Standards Division.

Contents Foreword Executive Summary 2 1 Introduction 3 1.1 Client requirements 3 1.2 Present situation and difference and similarities between BS EN 1991- 1-3 Snow loads and BS 6399: Part 3 3 1.3 Approach 4 2 Characteristic Ground Snow Load sk from BS EN 1991-1-3 5 3 Characteristic Ground Snow Loads from BS 6399: Part 3 8 4 Comparison for sk determined from BS EN 1991-1-3 and s0 values from BS 6399: Part 3 12 5 Discussion on effects of drifting 19 5.1 Drifting patterns for snow on duo-pitched roofs in EN 1991-1-3 19 5.2 Study of roof 20 6 Conclusions and Recommendations 22 6.1 Objectives of the Study 22 6.2 To determine whether the replacement of withdrawn British Standards with Eurocodes (BS EN 1991-1-3) has made snow loading more onerous in terms of small buildings as defined in clause 4.3.2 of BS 6399-3:1988 22 6.3 To recommend a simplified method based on BS 6399: Part 3 Clause 4.3.2, which may be included in the Scottish small buildings guide 22

Foreword Structural Eurocodes were introduced in Scotland on 1 October 2010. This research has been commissioned by the Scottish Government, Buildings Standards Division (BSD) as a result of a number of enquiries from Local Authority Verifiers, designers and conservatory manufacturers. The research compares ground snow load calculated to BS EN 1991-1-3 (including the National Annex) against that calculated to the withdrawn British Standards BS 6399-3:1988, mainly considering small buildings as defined in clause 4.3.2 of BS 6399-3. The comparison tables given in this report compare ground snow loads (which takes into account geographical location and site altitude), they do not compare the actual imposed roof loads (which take into account more aspects such as the slope of the roof and drifting). BSD were concerned that structural engineers and manufacturers involved with the design and construction of small buildings would need to carry out complex analysis, including snow load shape coefficients for snow drifting on roofs of small buildings e.g. extensions, conservatories and new build houses. The research also considers whether the simplified approach for small buildings described in clause 4.3.2 of BS 6399-3 could be adopted in the future. BS EN 1991-1-3 does not provide a similar simplified approach when calculating snow drifting on roofs of small buildings The BSD commissioned the Building Research Establishment to carry out the research.

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Executive Summary The objectives of the study were as follows:

• to determine whether the replacement of withdrawn British Standards with Eurocodes (BS EN 1991-1-3) has made snow loading more onerous in terms of small buildings as defined in clause 4.3.2 of BS 6399-3:1988; and

• to recommend a simplified method based on BS 6399: Part 3 Clause 4.3.2. The following steps were taken

• Using BS EN 1991-1-3 determine sk (snow load on the ground) for different zones 3, 4 and 5 and altitudes to 1000m above sea level.

• Using BS 6399-3 determine site values s0 (snow load on the ground) for different for values of sb ranging from 0.5 kN/m2 to 1.0 kN/m2 for zones 3, 4 and 5 and altitudes to 1000m above sea level.

• Compare the results from i) BS EN 1991-1-3 and ii) BS 6399:3. • Consider increases in effects of actions for the asymmetric drifting case after drifting

to the uniform load case before drifting. • Recommend a simplified approach, based on BS 6399: Part 3 Clause 4.3.2.

The following conclusions were drawn:

• For the majority of cases the snow load on the ground is more onerous using BS 6399-3 than using BS EN 1991-1-3. BS EN 1991-1-3 is however more onerous at altitudes greater than 150m for zones 3 and 4; and altitudes greater than 800m for zone 5.

• A simple tabular method considering uniformly distributed roof loads only is given

and this recommendation can only be for roofs within the scope described in Clause 4.3.2 of BS 6399- 3.

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1 Introduction 1.1 Client requirements BRE have been asked by the Building Standards Division of the Scottish Government as to whether or not the replacement of withdrawn British Standards with Eurocodes has made snow loading more onerous in terms of small buildings as defined in clause 4.3.2 of BS 6399-3:1988. There does not appear to be a similar simplified approach when working out snow drifting for small buildings in BS EN 1991-1-3 or the corresponding National Annex. It was therefore a concern that designers / engineers of small buildings would need to include analysis of snow load shape coefficients for snow drifting on roofs of small buildings e.g. extensions, conservatories and houses. 1.2 Present situation and Difference and similarities between BS EN 1991-1-

3: Snow loads and BS 6399: Part 3 1.2.1 Situation regarding roofs of small buildings As identified by the Client, BS EN 1991-1-3 does not include prescriptive rules for determining snow loads on particular roofs as defined by Clause 4.3.2 of BS 6399: Part 3.

1.2.2 Difference and similarities between BS EN 1991-1-3 and BS 6399: Part 3 a) The Small Buildings clause is not included in BS EN 1991-1-3 b) Imposed loads due to maintenance are given in BS EN 1991-1-1: Densities; self weight and imposed loads on buildings. The scope of BS EN 1991-1-3 is limited to snow loads only c) In accordance with Clause 3.3.2 (1) of BS EN 1991-1-1 roofs of buildings (particularly domestic roofs where the allowed imposed loads cover maintenance only), imposed loads need not be considered to act simultaneously with snow loads and / or wind actions. This is different to BS 6399:3 where they have to be considered together d) Maximum altitude greater in BS EN 1991-1-3 (1,500m) than in BS 6399 Part 3 (500m) e) For snow loads on the ground BS EN 1991-1-3 provides a zoned map compared with isopleths in BS 6399: Part 3 f) The methods of determining snow loads on roofs for both undrifted and drifted snow for pitched roofs and flat roofs close to other construction works are the same in both codes. In both codes, drift cases can be considered as Accidental Actions. However, in BS 6399: Part 3 all loads due to drifting may be assumed to be exceptional and thus treated as accidental loads. In BS EN 1991-1-3, only drifting on the following types of roof may be considered as accidental: • Multi-span roofs; • Roofs abutting and close to taller construction works; • Roofs where drifting occurs at projections, obstructions and parapets.

1.2.3 Differences in Notation and Symbols

a) BS EN 1991-1-3 sk is the characteristic value of snow on the ground at the relevant site determined from the Z, the Zone number given by the BS EN 1991-1-3 snow map and A, the altitude of the site [kN/m2]

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s is the snow load on the roof [kN/m2] b) BS 6399:3 sb is the basic snow load in accordance with snow map in BS 6399: Part 3. [kN/m2] so the site snow load on the ground at the relevant site determined from A, the altitude of the site [kN/m2] sd is the snow load on the roof [kN/m2] 1.3 Approach The approach will be as follows:

• to determine whether or not the replacement of withdrawn British Standards with Eurocodes (BS EN 1991-1-3) has made snow loading more onerous in terms of small buildings as defined in clause 4.3.2 of BS 6399-3:1988; and

• to recommend a simplified method based on BS 6399: Part 3 Clause 4.3.2. The following steps have been taken:

• Using BS EN 1991-1-3 determine sk snow load on the ground for different zones 3, 4 and 5 and altitudes to 1000m above sea level (See Section 2 and Figure 1 of this paper).

• Using BS6399:3 determine site values s0 (snow load on the ground) for different values of sb ranging from 0.5 kN/m2 to 1.0 kN/m2 for zones 3, 4 and 5 and altitudes to 1000m above sea level (See Section 3).

• Compare the results from i) BSEN 1991-1-3 and ii) BS 6399:3 (See Section 4). • Consider increases in effects of actions for the asymmetric drifting case after drifting

to the uniform load case before drifting; • Recommend a simplified approach, based on BS 6399: Part 3 Clause 4.3.2.

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2 Characteristic Ground Snow Load sk from BS EN 1991-1-3

Figure 1: Characteristic snow loads (sk)on the ground map for the UK; and formulation to determine snow load on the ground at desired altitude A

Determination of sk sk = (0,15 + [0,1Z + 0,05]) + ((A – 100)/525)) sk = Characteristic ground snow load (kN/m2) Z = Zone number (obtained from map) A = Site altitude (m)

From the information contained in Figure 1 (ref: Figure NA.1 from and Clause NA 2.8 from BSEN 1991-1-3) major cities, town and areas have been prescribed the following zones

City, Town and Areas Zone Number Edinburgh 3 Glasgow 3 Dundee 3 Aberdeen 4 Inverness 4 Scottish Highlands 4 & 5

Table 1: Prescribed zone (BS EN 1991-1-3) for major cities etc in Scotland

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Tables 2i, 2ii and 2iii give values for sk for different altitudes for Zones 3, 4 and 5 2i) Z = 3: Areas (e.g. Glasgow, Edingurgh, Dundee etc) – Zone 3: sk = (0,15 + [0,1Z + 0,05]) + ((A – 100)/525)) Altitude (m) Ground snow load sk (kN/m2) 0 0.33 10 0.34 25 0.36 50 0.40 100 0.50 150 0.60 200 0.69 250 0.79 400 1.07 600 1.45 800 1.83 1000 2.21 2ii) Z = 4) (Areas e.g. Aberdeen, Inverness, Scottish Highlands) – Zone 4 sk = (0,15 + [0,1Z + 0,05]) + ((A – 100)/525)) Altitude (m) Ground snow load sk (kN/m2) 0 0.41 10 0.43 25 0.46 50 0.50 100 0.60 150 0.70 200 0.79 250 0.89 400 1.17 600 1.55 800 1.93 1000 2.31

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2iii) Z = 5) (Areas e.g. Scottish Highlands) – Zone 5 sk = (0,15 + [0,1Z + 0,05]) + ((A – 100)/525)) Altitude (m) Ground snow load sk (kN/m2) 0 0.51 10 0.53 25 0.56 50 0.60 100 0.70 150 0.80 200 0.89 250 0.99 400 1.27 600 1.65 800 2.03 1000 2.41

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3 Characteristic Ground Snow Loads s0 from BS6399- 3

From the information contained in Figure 1 from this paper (ref: Figure NA.1 and Clause NA 2.8 from BS EN 1991-1-3) and BS 6399: Part 3 Figure 1; major cities, town and areas have been prescribed the following zones (from BS EN 1991-1-3) and basic snow load on the ground sb, assumed at 100m above sea level (BS 6399: Part 3)

City, Town and Areas Zone Number: EN 1991-1-3

sb from BS 6399-3

Table Number

Edinburgh 3 0.5 3i Glasgow 3 0.5 3i Dundee 3 0.5 3i Aberdeen 4 0.6 3ii Inverness 4 0.7 3iii Scottish Highlands 4 & 5 0.8 to 1.0 3iv and 3v

Table 2: Prescribed zone (BS EN 1991-1-3) for major cities etc in Scotland and values of sb from BS 6399:3 In BS 6399 Part 3 from Clause 6.2 the site snow load s0 is determined as follows: s0 = sb + salt ((A – 100)/100) for altitudes greater than 100m but less than 500m where salt = 0.1sb + 0.09, sb is the basic snow load on the ground and A is the site altitude. and s0 = sb for altitudes up to 100m above sea level *NOTE 1 - However BS 6399-3 does also state that the equation for altitude greater than 100m may also be used for altitudes up to 100m and points out that in these cases the correction term salt ((A – 100)/100) will automatically be negative. Therefore the results of s0 = sb + salt ((A – 100)/100) is shown in brackets () beside the result of s0 = sb for altitudes up to 100m above sea level in the tables below. ** NOTE 2 - BS 6399-3 states that caution is necessary in applying the snow load calculations for sites at altitudes above 500 m and specialist advice should be obtained in such situations EN 1991-1-3 applies to altitudes of 1500m above sea level. Tables 3i to 3v give values for s0 for different sb values and altitudes for Zones 3, 4 and 5

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3i) Scotland Areas (e.g. Glasgow, Edinburgh, Dundee): 0.5kN/m2 at altitudes of 100m s0 = 0.5 + 0.14 ((A – 100)/100) Altitude (m) Ground snow load so (kN/m2) 0 0.50 (0.36)* 10 0.50 (0.37)* 25 0.50 (0.40)* 50 0.50 (0.43)* 100 0.50 150 0.57 200 0.64 250 0.71 400 0.92 600** 1.20 800** 1.48 1000** 1.76 *See NOTE 1, above **See NOTE 2, above 3ii) Scotland Areas (e.g. Aberdeen): 0.6kN/m2 at altitudes of 100m s0 = 0.6 + 0.15 ((A – 100)/100) Altitude (m) Ground snow load so (kN/m2) 0 0.60 (0.45)* 10 0.60 (0.47)* 25 0.60 (0.49)* 50 0.60 (0.53)* 100 0.60 150 0.68 200 0.75 250 0.83 400 1.05 600** 1.35 800** 1.65 1000** 1.95 *See NOTE 1, above **See NOTE 2, above

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3iii) Scotland Areas (e.g. Inverness): 0.7kN/m2 at altitudes of 100m s0 = 0.7 + 0.16 ((A – 100)/100) Altitude (m) Ground snow load so (kN/m2) 0 0.70 (0.54)* 10 0.70 (0.56)* 25 0.70 (0.58)* 50 0.70 (0.62)* 100 0.70 150 0.78 200 0.86 250 0.94 400 1.18 600** 1.50 800** 1.82 1000** 2.14 *See NOTE 1, above **See NOTE 2, above 3iv) Scotland Areas (e.g. Highlands): 0.8kN/m2 at altitudes of 100m

s0 = 0.8 + 0.17 ((A – 100)/100) Altitude (m) Ground snow load so (kN/m2) 0 0.80 (0.63)* 10 0.80 (0.65)* 25 0.80 (0.67)* 50 0.80 (0.72)* 100 0.80 150 0.89 200 0.97 250 1.06 400 1.31 600** 1.65 800** 1.99 1000** 2.33 *See NOTE 1, above **See NOTE 2, above

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3v) Scotland: (e.g. Highlands): 1.0kN/m2 at altitudes of 100m

s0 = 1.0 + 0.19 ((A – 100)/100) Altitude (m) Ground snow load so (kN/m2) 0 1.00 (0.81)* 10 1.00 (0.83)* 25 1.00 (0.86)* 50 1.00 (0.91)* 100 1.00 150 1.01 200 1.19 250 1.29 400 1.57 600** 1.95 800** 2.33 1000** 2.71 *See NOTE 1, above **See NOTE 2, above

4 Comparison of ground snow loads sk determined from EN 1991-1-3 and s0 from BS6399- 3

Key to Tables Higher in BS 6399:3 Higher in BS EN 1991-1-3 Same Value The comparison tables below compare ground snow loads, they do not compare imposed roof loads.

City, Town and Areas Zone Number: EN 1991-1-3

sb from BS 6399:3

For Comparisons see table

Edinburgh 3 0.5 4i Glasgow 3 0.5 4i Dundee 3 0.5 4i Aberdeen 4 0.6 4ii Inverness 4 0.7 4iii Scottish Highlands 5 0.8 4iv Scottish Highlands 5 1.0 4v

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4i) Scotland – Zone 3 (e.g. Glasgow, Dundee, and Edinburgh) in BS EN 1991-1-3, sb = 0.5kN/m2 in BS 6399-3:

Ground snow load Altitude (m)

2i) BS EN 1991-1-3: Map in UK National Annex current UK method sk = (0,15 + [0,1Z + 0,05]) + ((A – 100)/525)) Z = 3 (kN/m2)

3i) BS6399- 3 for sb = 0.50 (e.g. Glasgow, Edinburgh and Dundee) s0 = 0.5 + 0.14 ((A – 100)/100) (kN/m2)

% increase/decrease of BS EN 1991-1-3 loads (2i) compared with BS 6399-3 (3i) (+ = BS EN > BS) (- = BS EN < BS)

0 0.33 0.50 (0.36) -34% (-8%)* 10 0.34 0.50 (0.37) -32% (-8%)* 25 0.36 0.50 (0.37) -28% (-3%)* 50 0.40 0.50 (0.43) -20% (-7%)* 100 0.50 0.50 0 150 0.60 0.57 +5% 200 0.69 0.64 +8% 250 0.79 0.71 +11% 400 1.07 0.92 +16% 600** 1.45 1.20 +21% 800** 1.83 1.48 +24% 1000** 2.21 1.76 +26% * BS 6399-3 states that for altitudes up to100m the formula s0 = sb should be used. However it also states that s0 = sb + salt ((A – 100)/100) may also be used. Therefore the ground snow load using s0 = sb + salt ((A – 100)/100 is shown in brackets () beside the result of s0 = sb for altitudes up to 100m above sea level ** BS 6399-3 states that caution is necessary in applying the snow load calculations for sites at altitudes above 500 m and specialist advice should be obtained in such situations EN 1991-1-3 applies to altitudes of 1500m above sea level.

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4ii) Scotland – Zone 4 (e.g. Aberdeen) in BS EN 1991-1-3, sb = 0.6kN/m2 in BS 6399-3:


2ii) BS EN 1991-1-3: Map in UK National Annex current UK method sk = (0,15 + [0,1Z + 0,05]) + ((A – 100)/525)) Z = 4 (kN/m2)

3ii) BS6399: Part 3 for sb = 0.60 (e.g. Aberdeen) s0 = 0.6 + 0.15 ((A – 100)/100) (kN/m2)

% increase/decrease of BS EN 1991-1-3 loads (2ii) compared with BS 6399-3 (3ii) (+ = BS EN > BS) (- = BS EN < BS)

0 0.41 0.60 (0.45)* -32% (-9%) 10 0.43 0.60 (0.47)* -28% (-9%) 25 0.46 0.60 (0.49)* -23% (-6%) 50 0.50 0.60 (0.53)* -17% (-6%) 100 0.60 0.60 0 150 0.70 0.68 +3% 200 0.79 0.75 +5% 250 0.89 0.83 +7% 400 1.17 1.05 +11% 600** 1.55 1.35 +15% 800** 1.93 1.65 +17% 1000** 2.31 1.95 +18% * BS 6399-3 states that for altitudes up to100m the formula s0 = sb should be used. However it also states that s0 = sb + salt ((A – 100)/100) may also be used. Therefore the ground snow load using s0 = sb + salt ((A – 100)/100 is shown in brackets () beside the result of s0 = sb for altitudes up to 100m above sea level ** BS 6399-3 states that caution is necessary in applying the snow load calculations for sites at altitudes above 500 m and specialist advice should be obtained in such situations EN 1991-1-3 applies to altitudes of 1500m above sea level.

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4iii) Scotland – Zone 4 (e.g. Inverness) in BS EN 1991-1-3, sb = 0.7kN/m2 in BS 6399-3:


2ii) BS EN 1991-1-3: Map in UK National Annex current UK method sk = (0,15 + [0,1Z + 0,05]) + ((A – 100)/525)) Z = 4 (kN/m2)

3iii) BS6399: Part 3 for sb = 0.70 (e.g. Inverness) s0 = 0.7 + 0.16 ((A – 100)/100) (kN/m2)

% increase/decrease of BS EN 1991-1-3 loads (2ii) compared with BS 6399-3 (3iii) (+ = BS EN > BS) (- = BS EN < BS)

0 0.41 0.70 (0.54)* -41% (-24%) 10 0.43 0.70 (0.56)* -39% (-23%) 25 0.46 0.70 (0.58)* -34% (-21%) 50 0.50 0.70 (0.62)* -29% (-19%) 100 0.60 0.70 -14% 150 0.70 0.78 -10% 200 0.79 0.86 -8% 250 0.89 0.94 -5% 400 1.17 1.18 -1% 600** 1.55 1.50 +3% 800** 1.93 1.82 +6% 1000** 2.31 2.14 +8% * BS 6399-3 states that for altitudes up to100m the formula s0 = sb should be used. However it also states that s0 = sb + salt ((A – 100)/100) may also be uses. Therefore the ground snow load using s0 = sb + salt ((A – 100)/100 is shown in brackets () beside the result of s0 = sb for altitudes up to 100m above sea level ** BS 6399-3 states that caution is necessary in applying the snow load calculations for sites at altitudes above 500 m and specialist advice should be obtained in such situations EN 1991-1-3 applies to altitudes of 1500m above sea level.

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4iv) Scotland – Zone 5 (e.g. Scottish Highlands) in BS EN 1991-1-3, sb = 0.8kN/m2 in BS 6399-3:


2iii) BS EN 1991-1-3: Map in UK National Annex current UK method sk = (0,15 + [0,1Z + 0,05]) + ((A – 100)/525)) Z = 5 (kN/m2)

3iv) BS6399: Part 3 for sb = 0.80 (e.g. Scottish Highlands) s0 = 0.8 + 0.17 ((A – 100)/100) (kN/m2)

% increase/decrease of BS EN 1991-1-3 loads (2iii) compared with BS 6399-3 (3iv) (+ = BS EN > BS) (- = BS EN < BS)

0 0.51 0.80 (0.63)* -36% (-19%) 10 0.53 0.80 (0.65)* -34% (-18%) 25 0.56 0.80 (0.67)* -30% (-16%) 50 0.60 0.80 (0.72)* -25% (-17%) 100 0.70 0.80 -13% 150 0.80 0.89 -10% 200 0.89 0.97 -8% 250 0.99 1.06 -7% 400 1.27 1.31 -3% 600** 1.65 1.65 0 800** 2.03 1.99 2% 1000** 2.41 2.33 3% * BS 6399-3 states that for altitudes up to100m the formula s0 = sb should be used. However it also states that s0 = sb + salt ((A – 100)/100) may also be used. Therefore the ground snow load using s0 = sb + salt ((A – 100)/100 is shown in brackets () beside the result of s0 = sb for altitudes up to 100m above sea level ** BS 6399-3 states that caution is necessary in applying the snow load calculations for sites at altitudes above 500 m and specialist advice should be obtained in such situations EN 1991-1-3 applies to altitudes of 1500m above sea level.

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4v) Scotland – Zone 5 (e.g. Scottish Highlands) in BS EN 1991-1-3, sb = 1.0kN/m2 in BS 6399-3:


2iii) BS EN 1991-1-3: Map in UK National Annex current UK method sk = (0,15 + [0,1Z + 0,05]) + ((A – 100)/525)) Z = 5 (kN/m2)

3v) BS6399: Part 3 for sb = 1.0 (e.g. Scottish Highlands) s0 = 1.0 + 0.17 ((A – 100)/100) (kN/m2)

% increase/decrease of BS EN 1991-1-3 loads (2iii) compared with BS 6399-3 (3v) (+ = BS EN > BS) (- = BS EN < BS)

0 0.51 1.00 (0.81) -49% (-37%) 10 0.53 1.00 (0.83) -47% (-36%) 25 0.56 1.00 (0.86) -44% (-35%) 50 0.60 1.00 (0.91) -40% (-34%) 100 0.70 1.00 -30% 150 0.80 1.01 -21% 200 0.89 1.19 -25% 250 0.99 1.29 -23% 400 1.27 1.57 -19% 600** 1.65 1.95 -15% 800** 2.03 2.33 -13% 1000** 2.41 2.71 -11% * BS 6399-3 states that for altitudes up to 100m the formula s0 = sb should be used. However it also states that s0 = sb + salt ((A – 100)/100) may also be used. Therefore the ground snow load using s0 = sb + salt ((A – 100)/100 is shown in brackets () beside the result of s0 = sb for altitudes up to 100m above sea level ** BS 6399-3 states that caution is necessary in applying the snow load calculations for sites at altitudes above 500 m and specialist advice should be obtained in such situations EN 1991-1-3 applies to altitudes of 1500m above sea level.

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4vi) Summary of comparisons in ground snow load

% increase/decrease of BS EN 1991-1-3 loads compared with BS 6399-3

(+ % = BS EN greater than BS)

(-% = BS EN less than BS)

Altitude (m)

Zone 3, sb 0.5kN/m2 (e.g. Edinburgh, Dundee)

Zone 4, sb 0.6kN/m2 (e.g. Aberdeen)

Zone 4, sb 0.7kN/m2 (e.g. Inverness)

Zone 5, sb 0.8kN/m2 (e.g. Scottish Highlands)

Zone 5, sb 1.0kN/m2 (e.g. Scottish Highlands)

0 -34% (-8%)* -32% (-9%) -41% (-24%) -36% (-19%) -49% (-37%) 10 -32% (-8%)* -28% (-9%) -39% (-23%) -34% (-18%) -47% (-36%) 25 -28% (-3%)* -23% (-6%) -34% (-21%) -30% (-16%) -44% (-35%) 50 -20% (-7%)* -17% (-6%) -29% (-19%) -25% (-17%) -40% (-34%) 100 0 0 -14% -13% -30% 150 +5% +3% -10% -10% -21% 200 +8% +5% -8% -8% -25% 250 +11% +7% -5% -7% -23% 400 +16% +11% -1% -3% -19% 600** +21% +15% +3% 0 -15% 800** +24% +17% +6% 2% -13% 1000** +26% +18% +8% 3% -11% ** BS 6399-3 states that caution is necessary in applying the snow load calculations for sites at altitudes above 500 m and specialist advice should be obtained in such situations EN 1991-1-3 applies to altitudes of 1500m above sea level.

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5 Discussion on effects of drifting 5.1 Drifting patterns for snow on duo-pitched roofs in BS EN 1991-1-3

Figure 2 shows the snow shape coefficients that need to be used for analysing duo-pitch roofs in accordance with BS EN 1991-1-3 (including the UK National Annex)

A

B

C

Case (iii) µ1(α 1)

Case (ii)

Case (i) µ1(α1) µ1(α 2)

µ1(α 2)

α1 α2

Figure 2: Snow shape coefficients for pitch roofs – Maritime climate (μ values given in Table below)

Drifted snow load shape coefficient for a duo-pitched roof in the UK to be used with Figure 2

Angle of pitch of roof (α i, I = 1,2)

0º ≤ αi ≤ 15º

15º < αi ≤ 30º

30º < αi < 60º

α i ≥ 60º

Snow load shape coefficient μ 1

0,8 0,8 + 0,4(α -15)/15

1,2(60 – α )/30

0,0

In BS 6399 Part 3 for the pitched roofs defined in Clause 4.3.2 Small Buildings, Cases (ii) and (iii) shown in Figure 2 need not be applied and instead the code specifies a udl load that is 1.25 times the calculated value based on site snow load s0. To adopt a similar system for BS EN 1991-1-3 a study was made to determine the increase in maximum bending moment for the asymmetric cases after drifting (Cases (ii) and (iii) compare with the undrifted case (i) see Figure 2. Once the increase is determined, the undrifted case can be considered as a udl, but by specifying a higher value of sk to

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take account of the increase in bending moment when considering the asymmetric cases (ii) and (iii) Figure 2.

5.2 Study of roof

The investigation was made on the symmetrical duo-pitch roof illustrated in Figure 3.

h α

L

C

B

A

Figure 3: Roof considered for study The following parameters were considered Case Pitch α Support

Conditions A & C

L (m) H (m)

1 15o Pinned 3m 0.8m 2 15o Encastre 3m 0.8m 3 30o Pinned 3m 1.732m 4 30o Encastre 3m 1.732m 5 45o Pinned 3m 3m 6 45o Encastre 3m 3m For each of the cases 1 to 6, two loading situations were considered i) A uniform udl on spans AB and BC taking into account the self weight of the roof

and the snow load (undrifted case). Case (i) Figure 2. ii) A uniform udl on span AB including self weight of the roof and the snow load;

and a uniform udl on BC including self weight only (drifted case). Cases (ii) and (iii) Figure 2.

Table 4 below shows the results that were obtained with the increase in bending moment in the asymmetric case compared to the uniform case highlighted.

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Case1 Loading

Situation Moment at A Maximum

Moment span AB

Moment at B

1 - P 15o i) uniform 0 -1.05 1.82 1 - P 15o ii) drifted 0 -1.26 1.32 1 - P 15o Increase of

ii) over i) 0% 20%

(increase in moment)

- 27% (decrease in moment)

2 – E 15o i) uniform 1.3 -0.62 1.18 2 – E 15o ii) drifted 1.45 -0.71 0.86 2 – E 15o Increase of

ii) over i) 11% (increase in moment)

14.5% (increase in moment)

-32% (decrease in moment)

3 – P 30o i) uniform 0 -1.16 2.07 3 – P 30o ii) drifted 0 -1.39 1.50 3 – P 30o Increase of

ii) over i) 0% 19.8%


- 27.5% (decrease in moment)


ii) over i) 12.8% (increase in moment)

14% (increase in moment)


5 – P 45o i) uniform 0 -1.41 2.54 5 – P 45o ii) drifted 0 -1.71 1.84 5 – P 45o Increase of

ii) over i) 0% 21%




ii) over i) 13.5% (increase in moment)

13.6% (increase in moment)

- 27.2% (decrease in moment)

1 P is Pitched and E is Encastre. 15o etc signify the pitch of the roof Table 5: Change in bending moment (highlighted) in the asymmetric (i.e. drifted) case compared to the uniform case

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6 Conclusions and Recommendations

6.1 Objectives of the Study The objectives of the study were as follows: • To determine whether the replacement of withdrawn British Standards with

Eurocodes (BS EN 1991-1-3) has made snow loading more onerous in terms of small buildings as defined in clause 4.3.2 of BS 6399-3:1988; and

• To recommend a simplified method based on BS 6399: Part 3 Clause 4.3.2, Note - The comparison tables given in this report compare ground snow loads, they do not compare design loads. 6.2 To determine whether the replacement of withdrawn British Standards

with Eurocodes (BS EN 1991-1-3) has made snow loading more onerous in terms of small buildings as defined in clause 4.3.2 of BS 6399-3:1988

By examining the summary of comparison of ground snow loads Table 4vi) it can be seen that for the majority of cases the snow load on the ground results are more onerous using BS6399:3 than using BS EN 1991-1-3. BS EN 1991-1-3 is more onerous at altitudes greater than 150m for zone 3 and 4; and altitudes greater than 800m for zone 5. However BS 6399-3 states that caution is necessary in applying the snow load calculations for sites at altitudes above 500 m and specialist advice should be obtained in such situations EN 1991-1-3 applies to altitudes of 1500m above sea level, therefore there is doubt on the results from BS 6399: Part 3 above 500m a.m.s.l..

6.3 To recommend a simplified method based on BS 6399: Part 3 Clause 4.3.2

Provided the recommendation given below is strictly applied to the roofs within the scope described in Clause 4.3.2 of BS 6399: Part 31 and considering that BS EN 1991-1-3 generally gives lower snow loads on the ground than BS 6399: 3 (tables 3i to 3v). The % increase of critical bending moments due to the asymmetric case (i) and (ii) in figure 2 (i.e. drifted) compared to the uniform case (i) in Figure 2 (i.e. before drifting) is 13% to 21% (reference Table 5).

The BS 6399:3 definition of a small building is based on the sizes and past satisfactory performance of domestic houses. Provided the restrictions of Clause 4.3.2 are strictly adhered to, the recommendation below may be considered for use for small domestic buildings and small non-domestic buildings.

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Note 1 This recommendation applies to any buildings where no access is provided to the roof (other than necessary for cleaning and maintenance) which has

It is recommended that for roofs that are within the scope of Clause 4.3.2 of BS 6399: Part 3 that a uniformly distributed load is used for snow loads which is the greater of

i) 1.25 times snow load for the ground sk as determined from BS EN 1991-1-3; or

ii) The values given in the Table below

Imposed Roof Loads (kN/m2) Zones Altitudes ≤ 100m Altitudes > 100m

and ≤ 250m Altitudes > 250m

1 to 4 0.75 1.50 See BSEN 1991-1-3

5 1.00 1.50 See BSEN 1991-1-3

For roofs whose slope (pitch) α is between 30o and 60o the values given in i) and ii) above may be reduced by [(60 – α)/30]. For roofs where α is greater than 60o the minimum udl may be taken as zero.

• A roof area no larger than 200m2 in plan; or • A width no greater that 10m and a pitched roof with no parapets

[N.B. The intention of the above bullet points are as follows

• 1st bullet point is to cover for example a large detached house; and • 2nd bullet point is to cover for example a row of terrace houses.}

Provided that there are no other buildings within 1.5m of its perimeter, and provided the roof configuration also meets one of the following conditions

• The roof has no abrupt changes of height greater than 1m, at which drifting could occur

• The area of the lower part of the roof, on which a drift could form, is not greater that 35m2

For the purpose of this recommendation the roof area is defined as the total covered area, in plan of the entire building structure. Also chimneys and dormers whose vertical elevation area, against which a drift may form, is less that 1m2 can be ignored as an abrupt change of height [N.B. The intention of the above bullet points are as follows

• 1st bullet point ensures that significant local drifting will not be a problem; and • 2nd bullet point again ensures that significant local drifting will not be a problem by

restricting lower parts of roofs to 35m2. This will accommodate a double garage or a small single storey extension attached to a house. Therefore extensions of greater 35m2 may need to be designed to BS EN 1991-1-3 and the UK National Annex.]

snow loads in scotland comparison of bs en 1991-1-3 and bs … · 2012-01-24 · the scope of bs en...

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