Calibration report for Eurocode 1 on wind actions
BSI has produced a report on the calibration of the Eurocode for wind loading (BSEN 1991-1-4)
and its UK National Annex against the current code (BS 6399: Part 2). NBS Information Specialist Michael Smith takes a look at the report content.
Eurocode BS EN 1991-1-4 Actions on structures – General actions – Wind actions was published in the UK in 2005. The UK National Annex to BS EN 1991-1-4 (NA) was issued for public comment in July 2006. The relevant BSI committee has finalised the NA taking into account the comments received. BS EN 1991-1-4 together with the NA will replace BS 6399:2 no later than 2010.
Modern structures tend to be much lighter than earlier construction, and are more sensitive to wind loading. Successive editions of wind codes have therefore had an impact on both the safety and economy of design. For this reason industry will need to be confident that adoption of the new wind code and its National Annex will result in safe and economic buildings.
The DCLG commissioned calibration study focuses on key parameters that have a significant effect on design wind loads on structures. These issues are:
- Wind speeds at the centre of 10km inland squares and at the centre of 10km coastal segments
- Wind pressures for sites in country and in town when orography1 is not significant
- Wind pressures for sites in country and in town when orography is significant
- Overall loads on typical building types in seven sites across the UK
- Altitude factors and dynamic magnification factors
- External pressure coefficients for walls and roofs and comparison of cladding loads for typical building types in seven sites across the UK
- Size effect factors.
In each case the results of the procedures given in BS EN 1991-1-4 and its UK draft National Annex and those obtained by using BS 6399 Part 2 are compared. The proposed changes to the new wind actions code are outlined below.
The wind speed map in BS 6399-2 gives hourly mean wind speeds at 10m above flat open country at sea level, based on an analysis of some 50 meteorological stations. For the most part, the stations used for the new code are identical; however the opportunity has been taken to replace poorly-exposed stations with new stations.
- Scilly anemometer – replaced by Camborne
- Lynemouth anemometer – replaced by Boulmer
- Tummel Bridge – discarded
- Exposure corrections for Coltishall have been reassessed
- Calibration factor of 1.06 applied to derive the 10 minute mean map for the new EN code.
A calibration of the gust wind pressures predicted by both standards was carried out. Ratios of the design pressures (before applying pressure coefficients) predicted by these were obtained for several cases, covered by 125 spot checks where orography is not significant, and 225 checks where orography is significant.
Generally, where orography is not significant there is some reduction in pressures compared to BS 6399:2 for low rise structures. Part of the explanation for this lies in the adjustment to the fetch factor model now used. The differences are more evident at low heights and short fetches. Where orography is significant there is an interaction between many parameters, however the comparison reports that it is difficult to detect any specific trends.
Overall wind loads have been compared for five structure types in seven site locations. A study of the basic parameters suggested that it would be sufficient to consider wind direction normal to the long face of the buildings only. Further, it is assumed that orography is not significant.
The calculation results show:
- Overall loads derived using net pressure coefficients are always smaller than those derived using front and rear face pressures
- For low-rise buildings the EN procedures lead to a reduction in loads
- For medium-rise buildings the overall loads are approximately similar in the two codes
- For tall buildings, particularly with h/d > 5, the overall load in the EN will be greater than that obtained from the BS.
Altitude and dynamic factors
BS 6399: Part 2 contains an altitude factor that depends on the altitude of the site above sea level, assuming that topography is not significant. This factor is applied to the calculation of the hourly mean site wind speed, being originally calibrated empirically against measured data over sites of varying altitude.
The proposed formulation for altitude factor in the National Annex of BS EN 1991-1-4 is more rational than that in BS 6399:2. Unlike the formulation in the BS it reduces with height above ground level, with the magnitude of the reduction increasing both with site altitude and height above ground level.
Both BS 6399: Part 2 and BS EN 1991-1-4 include a factor to allow for the dynamic augmentation of response that occurs under the buffeting action of wind turbulence. In BS 6399: Part 2 this is called the dynamic augmentation factor and is applied by factoring static loads. In BS EN 1991-1-4 it is called the dynamic factor and is applied by factoring static loads combined with a size factor. However, the UK National Annex allows these factors to be separated for other calculation and design purposes.
External pressure coefficients
BS EN 1991-1-4 gives two sets of external pressure coefficients for walls and roofs. The values are intended for the design of elements and fixings with an area of 1m² or less, or of 10m² or greater. This latter value is essentially equivalent to that given in the BS code. BS EN 1991-1-4 has a procedure for calculating the external pressure coefficients on loaded areas based on logarithmic interpolation.
The EN values generally give an 8% reduction in maximum external suctions in side zone A for all building configurations. The critical design case for wall cladding design is often the maximum suction in this zone.
On the larger side face zones B and C the EN gives identical values to BS 6399: Part 2. The EN gives a 6% reduction in positive pressures on the windward face for tall and mid-rise buildings, although for low-rise buildings there is a 17% increase.
On the leeward face the external suctions are 40% higher in the EN for tall buildings and mid-rise buildings, and 40% lower for low-rise buildings.
Other data comparisons given in this section include:
For flat roofs with sharp eaves BS EN 1991-1-4 will give reductions of up to 14% in external wind pressure coefficients for loaded areas.
For flat roofs with parapets the EN will give reductions of up to 31% in edge zone B.
For curved eaves the EN values are up to 9% lower in the highly loaded corner zone but generally very similar to the BS values for other roof zones.
Mono pitch roofs
The EN generally gives external pressure coefficients on mono pitch roofs within ±25% of those given in the BS. However, there are some pitch angles and roof zones where the difference is greater than ±100%, offering potential safety or economic implications.
Duo pitch roofs
The EN generally gives external pressure coefficients on duo-pitch roofs which are within ±25% of those given in the BS, though there are a small number of cases where the EN values are up to about 50% larger than those in the BS. There are ten cases where the EN gives pressure coefficients which are as low as a quarter of those given in the BS.
Other roof forms
The EN and BS both include pressure coefficients for hip roofs, and free-standing canopies. However, on this occasion a detailed comparison of these values has not been carried out.
Internal pressure coefficients
The treatment of internal pressures for enclosed buildings without dominant openings in BS 6399-2 is simplistic and very limited in scope, while the BS EN1991-1-4 procedure, though more difficult to use, is more flexible and allows internal pressures to be determined for a very wide range of building forms. The values it gives are likely to be far more realistic.
BS 6399-2 and BS EN 1991-1-4 both allow for the non-simultaneous occurrence of surface gust pressures through ‘size effect’ factors. However, the approaches used in these standards are different.
In the BS 6399-2 method, the size effect factor is determined separately for internal and external surfaces. In BS EN 1991-1-4 the size effect factor is combined with a dynamic factor. The agreement between the resulting size effect factors is surprisingly good, especially as the calculation methods are both empirical. The difference between the EN method and BS one is generally within about 10% for panel areas up to 20,000m² although this will increase for larger areas. In practice, it is unlikely that these will need to be considered.
The National Annex to BS EN 1991-1-4 is expected to be published during 2009, and together with BS EN 1991-1-4 will replace BS 6399:2 no later than 2010.
BS EN 1991-1-4: 2005 Eurocode 1: Actions on structures. General actions – Wind actions
1 The study of the physical geography of hill and mountain ranges
Related NBS information:
Written January 2009
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