Oct 11 Wind load design pt 2
Martyn Holloway, Chairman of SPRA Technical Committee and Flat Roof Applications Manager at SFS Intec Limited, introduces the second part to his series on wind loading (view for Part 1).
Assessment of the effect of wind load is a fundamental requirement of the design process for any roof construction. This may sound obvious, but in a world of constantly changing design standards and construction methods, important issues can be overlooked.
Designers must ensure that the correct calculation procedure is followed. For the UK this is BS EN 1991-1-4:2005 + A1:2010: This standard must also be used in conjunction with The UK National Annex: NA to BS EN 1991-1-4:2005 + A1:2010: UK National Annex to Eurocode 1. Note that the previous standard, BS 6399-2 has now been withdrawn.
It is essential that all components specified and their method of installation, are able to contend with the calculated wind loads. Incorrect installation of restraint to the deck, insulation and membrane can lead to premature failure of the construction. It is therefore imperative that the designer and contractor are aware of the attachment requirements at each interface, as determined by the wind load calculations undertaken. These will vary with the method of application.
For mechanically fastened membrane applications the correct membrane fastener frequency is determined by the wind load requirement and fastener design load. For new build steel decks for example, where these parameters are known, the fastener frequency can be specified by the SPRA membrane manufacture. For refurbishment and concrete decks however, pullout testing should be undertaken by the fastener supplier/manufacturer to determine by calculation the correct fastener design load. Insulation should also be fixed correctly. For example to comply with BRUFMA requirements, the minimum number of fasteners per insulation board should be determined in accordance with the calculated wind load requirements.
There is one mechanical fastening issue that needs reiterating. In recent years the use of steel stress plates and long fasteners (which create a thermal bridge) has given way to plastic tube thermal break plate/tube components with short fasteners because the latter do not require a compensatory increase in insulation thickness. All SPRA-member insulation manufacturers assume the use of thermal break components in their calculation of required insulation thickness. Thus, although metal components are not contrary to Building Regulations, if they are to be used it will be necessary to check that this compensation has been allowed for in the thermal design or target U-value may not be achieved in practice.
For adhered membrane and insulation applications, the selected system must be suitable to resist the calculated wind load at each interface. Whatever the means of adhesive attachment it is a SPRA requirement that mechanical restraint is installed at the roof perimeter, at changes of slope and around details to ensure that any tension in membrane in the roof field or upstand is not transferred to the other as peeling action.
Ballasted systems should have sufficient mass to meet the requirements of the wind load calculations. This also applies to green roof applications where the "dry" weight of the system may require the membrane to be additionally restrained mechanically or by the use of adhesive to resist wind uplift.
- ‘Quality control and use of adhesives for the attachment of vapour control layers, insulation and single ply membranes in flat roofing’ (SGD6/08), which includes checklists.
- ‘Wind load design requirements for flat roofing - requirements for insulation and single ply membrane’ (SGD/10/11), which highlights specific issues to be addressed by the contractor and designer, including references for further detailed guidance.