3.0 The importance of correct information and calculation procedure

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1 Item Wind load design requirements for flat roofing. Evaluation of the affect of change from BS to the new BS EN calculation method. Contents 1.0 Introduction 2.0 Scope 3.0 The importance of correct information and calculation procedure The new BS EN wind loading calculation method initial comparison between the two calculation standards Analysis of calculations taken using both calculation methods Conclusion on the effect of using the new BS EN calculation method 5.0 References A Wind load calculation request form B Comparison of wind velocities in BS and BS EN Introduction The calculation of wind load on roofs is an essential part of the design process. The designer must ensure that the installed roof system is able to contend with the wind load requirements imposed by a building's location and that the correct calculation procedures are followed. Until recently the main calculation method of wind loading in the UK was undertaken using BS , however a new calculation method for wind loading BS EN is now available following the withdrawal of BS in April This report will highlight the main affects of this change in calculation method. 2.0 Scope The report will highlight the effects of the introduction of BS EN with respect to wind load calculations and current methods of insulation and membrane restraint. It will focus specifically on Flat roof applications using polymeric single ply membrane. 3.0 The importance of correct information and calculation procedure The chosen method of attachment of the insulation and membrane layers should be appropriate to the resulting wind load values derived by calculation. Incorrect calculation of the insulation and membrane restraint requirements can lead to premature failure of the construction. It is therefore essential that the contractor and designer have correct calculation data to ensure that the chosen method of restraint is suitable and fit for purpose. It is essential that those undertaking wind uplift calculations must have a thorough knowledge of the calculation procedure and are trained in the use of the appropriate software, with coverage from appropriate PI insurance. The person undertaking the calculation is responsible to ensure that the correct calculation procedures are followed and that accurate values of wind load for designated roof corner, perimeter and field areas are provided. It is the responsibility of the person requesting the calculation (roofing contractor / designer or others) to ensure that the information supplied is correct and accurate with respect to the building location, height, dimensions and all other required parameters. Incorrect data could well affect the overall wind load design provision with the consequence of roof failure. (See check list information required for undertaking a wind uplift calculation Annex A). 1

2 4.0 The new BSEN wind loading calculation method It is important to distinguish the resulting differences between the two calculation standards. 4.1 Initial comparison between the two calculation standards BS EN in comparison with BS , has a much wider scope which includes high rise structures such as Chimneys, masts and bridges. The Standard Procedure of BS used by many to calculate wind load currently will be replaced by the new Standard Method in BS EN There are, however, a number of significant differences between the EN and UK current practice, especially in the calculation of the wind speed profile and the local external pressure coefficients. Basic wind speed Vb is replaced by fundamental basic wind velocity Vb and the figure in BS EN is based on a 10 minute mean wind speed, whilst BS is based on a mean hourly wind speed. See Appendix B for comparison of wind velocity data. BS EN must be used in the UK with the accompanying National Annex. Each EU State will have its own National Annex to be applied accordingly. The values for external pressure coefficient within BS EN are different from those referenced by BS The BS data is widely regarded as the best self-consistent set. As a consequence, the UK National Annex advises that the BS co-efficients are still used to preserve the current level of safety in UK design. Changes to the UK National Annex are expected before the end of Please check the SPRA website for details. 4.2 Analysis of calculations undertaken using both calculation methods SPRA has undertaken calculations based upon the existing BS EN 6399 and BS EN methods, for the same building criterion located in different parts of the UK. Analysis of the data indicates that the resulting comparisons show significant variation particularly when the BS EN method is used without the BS external pressure coefficients. Wind Calculation Comparison London BS EN1991-4(UK Annex) EN1191-4(Rev UK Annex) Wind Calculation Comparison Glasgow BS EN1991-4(UK Annex) EN1191-4(Rev UK Annex) kn/m Fie ld Perimeter Corner kn/m Fie ld Perimeter Corner Zone reference Zone reference The above two graphs indicate the effect of the different calculation methods upon wind load in two separate UK locations. BS EN (Rev UK Annex) is the resulting wind load using the BS external pressure co-efficients. For the London example, whilst a differential of 20% can be seen in the resultant wind load, when the minimum requirement identified by best practice of two membrane fasteners per square metre is taken into account, the there is no significant effect upon fastener density. 2

3 For the Glasgow example, wind loads are significantly higher, exceeding the minimum two membrane fasteners per square meter best practice values. However the differential between the resultant wind uplift calculations is only in the region of 6% and when factors such as decking corrugation centres and available widths of membrane are taken into account, again there may be no significant effect. For adhered or ballasted membrane and insulation applications the reduction in wind load values as a consequence of the new BS EN method may result in an 'apparent' increase in safety margin. 4.3 Conclusion on the effect of using the new BS EN calculation method. Comparison between the BS and BS EN calculation methods undertaken by SPRA have shown significant inconsistencies leading in some cases to serious erosion of safety margins. However with the adoption of the BS co-efficients, expected during the latter part of 2010, the resulting differentials will be reduced. It is SPRA's opinion that the BS calculation method should be maintained until such a time that the new BS EN calculation method is modified to reflect UK climatic requirements. This is expected later in 2010 with the introduction of an amended UK National Annex with new external pressure co-efficients. Climatic change is underway in the UK and future trends may include increases in mean sea level, and frequency and magnitude of storms. To reduce the current safety margins in wind load calculation methods at such a time will unnecessary increase the risk of storm damage to roofs and thereby the financial liability of designers and contractors as a consequence of such actions. 5.0 References [1] BS 6339: Part 2: Loadings for buildings. Code of practice for wind loads [2] NA to BS EN : 2005 UK National Annex to Eurocode 1. Actions on structures. General actions. Wind actions [3] SPRA Design Guide 3

4 APPENDIX A Wind load calculation request form 1.0 Project information 1.1 Project name 1.2 Project Address 1.3 Street 1.4 City 1.5 County 1.6 Post code 2.0 Roofing Contractor details 2.1 Company name 2.2 Contact name 2.3 Tel no 2.5 / Fax 3.0 Project type 3.1 New / Refurbishment 4.0 Building location 4.1 Exposed / Not exposed 5.0 Construction details 5.1 Open (building with dominant openings) Y/N 6.0 Building Geometry. Drawing enclosed (Required in all cases) 6.1 Roof type (please tick appropriate box), Flat Mono pitch Duopitch ridge Duopitch valley Curved / barrel vault roof 6.2 Curved roofs only: rise height (in metres): curve span in metres: 6.3 Length (metres) 6.4 Width (metres) 6.5 Height (Metres) 6.6 Roof pitch (degrees) 6.7 Internal / External gutter 6.8 parapet height (metres) if applicable 7.0 Roof Construction 7.1 Substrate type (Metal / Concrete / Timber / Other) 7.2 Detail total Roof build up (including insulation) thickness (mm) 7.3 For metal deck substrates only: metal deck profile type Direction of laying (dwg) Crown to crown dimension (mm) Deck thickness (mm) Steel or Aluminium 8.0 Fastener details 8.1 Manufacturer 8.2 Type / Reference 8.3 Pullouts undertaken? Y/N 4

5 APPENDIX A Wind load calculation request form (continued) 9.0 Membrane details 9.1 Manufacturer 9.2 Type / Reference 9.3 Preferred width (mm) 9.4 Thickness (mm) 10.0 Additional information Note: Complete a separate form for each roof area 5

6 APPENDIX B Comparison of wind velocities in BS and BSEN Appendix B: Comparison of wind velocities in BS and BS EN BS BS EN Based on a mean hourly wind speed Based on a 10 minute mean wind speed 6