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Article Metal Cladding and Roofs WIND LOADINGS – WHAT ARE THEY AND WHY IS IT SO IMPORTANT TO GET THE CALCULATIONS RIGHT? Clare Fenton, Chair of the MCRMA, explains why building envelopes must be designed to resist forces resulting from strong winds, and highlights how the association’s new CPDapproved guidance will support designers and installers to achieve higher standards. Environmental conditions such as wind, and changing environment conditions such as gusting wind, can have a dramatic influence on the structure and roofing and cladding of the building envelope. It must be noted that the design calculations for the structure, and those for the roofing and cladding envelope are completely different, and must not be regarded as the same or interchangeable. It largely comes down to the difference in pressure that can develop on a windy day between the exterior and the interior spaces, and the strong forces which result. When the wind blows on a building, the change in wind speed as the air negotiates the obstruction in its path (the building) may result in either an increase or a decrease in pressure. When combined with changes to the internal air pressure, the result is either a net positive pressure (on windward facing walls and the windward slopes of steep roofs), or a net suction (on leeward facing walls, on walls parallel to the direction of the wind and on roofs generally). The wind pressure and suction can have other effects too, chiefly causing deflection in the wall and roof cladding which may, in extreme cases, cause structural failure of the roofing and cladding profile. The strength of these forces is such that wind suction may also result in failure of the fasteners holding the panels to the building structure if not properly specified. Clare Fenton is Chair of the Metal Cladding and Roofing Manufacturers Association (MCRMA). “Metal building envelopes need to be designed in a way that takes into account the strong loads resulting from the pressure differential caused by strong winds – which we know as wind loadings” strong loads resulting from the pressure differential caused by strong winds – which we know as wind loadings. However, numerous factors affect the wind load values, which means it is impossible to arrive at a single value which can be used for the design of all buildings. variation in wind speed can be additionally affected by a building’s location and its geometry. This means several factors are most influential on wind loadings: • Location – some parts of the country are windier than others Hence, why metal building envelopes need to be designed in a way that takes into account the Common factors affecting wind loadings The UK’s weather is notoriously changeable, but • Altitude – the higher the site, the greater the wind speed 36 TC MAY <strong>2024</strong>
• Distance to sea – locations closer to the sea experience greater wind speeds • Town or country – in built-up areas, buildings may provide shelter from the wind • Topography – topographical features can result in increased wind speed • Wind direction – the strongest winds generally blow from the south west • Building height – taller buildings are exposed to stronger winds Given these factors, the wind loading is different for every site and every building, which is why it has to be calculated for each and every project and each and every area of any given building. But this should not be confused with the area imposed loading on roofs and walls – that is a completely different calculation which should also be carried out at the design stage. How to get the wind load calculations right It is critically important to ensure the wind load calculations are correct for every building. Failure to do so may result in buildings being unsafe in some cases, and uneconomic in others. This is one of the reasons why the MCRMA has published a new guidance document called GD41 ‘Wind Loading for Metal Roofing & Cladding - Protocol for Calculations’. It is designed to give anyone involved in the design of a metal façade a basic understanding of what is involved in the calculations process. The guidance document was written by David Roy of MCRMA member company Roofconsult, and it is also approved as a CPD. David said: “As an independent consultant I recognise the importance of wind load calculations in the selection of systems and fixing for roofing and cladding specifications.” GD41 provides guidance when undertaking a wind load calculation and also assists with the interpretation of results. It introduces the industry standards relating to wind load calculations and Windload diagram what values make up the calculation itself, qp*(Cpe-Cpi). Given the complexity of wind loads, it is standard practice to use a dedicated software program. There are a number of excellent programs available, but it is important to note that whichever one is used, it must comply with the requirements of BS EN 1991-1- 4:2005+A1:2010, and supplemented with the UK National Annex (NA) incorporating National Amendment A1 dated January 2011. Wind load calculation software programs require input data that falls into the following categories: • Location • Roof perimeter zones • Adjacent structures • Roof type • Building height and parapet walls Assuming the correct data is provided in relation to these categories, the software will provide the wind loadings that need to be accommodated by the design. This will inform the building envelope’s design and specification, helping to determine the most appropriate roofing and cladding systems and products, as well as their installation methodology, including which types of fasteners to use and how many. It is here where the wind load data is vitally important. The capacity of the sheeting and the performance required of the fasteners, including their pull-out and/or pull-over strengths, as well as their quantity and installation pattern on each area of the roofing and cladding can only be “Numerous factors affect the wind load values, which means it is impossible to arrive at a single value which can be used for the design of all buildings” developed with confidence if the wind load forces are known. With several of the industry’s leading fastener manufacturers within the MCRMA’s membership, this is an area where the association’s support can be particularly valuable, as they can advise further. It is also important to keep abreast of software updates and capabilities as developments in this area are enabling the wind load calculations process to be more streamlined and accurate. For example, the SCI (Steel Construction Institute) has recently launched an updated program called SCIPHYR (pronounced ‘zephyr’), a cloud-based solution which replaces the BREVe program with added tools and functionality including an up to date location and altitude database. Advances like this from the SCI are extremely beneficial in terms of delivering robust metal building envelopes that stand the test of time. But remember, wind load calculations are only one factor in the design and installation process that requires focus and attention to detail throughout. Guidance document GD41 is available to download on the MCRMA website along with numerous other resources designed to support everyone in the metal building envelope supply chain. Specific technical advice can also be requested from any of the 40+ MCRMA member companies to reduce risk and achieve the highest quality results. Contact MCRMA www.mcrma.co.uk mcrma.co.uk/wind-loadings-guidance/ MAY <strong>2024</strong> TC 37
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