External Wall Insulation (EWI) cladding detachment and falling from buildings: Wind Loads, Storm Damage, Building Defects Investigations.
External wall insulation cladding systems have to be designed to withstand the worst wind loadings likely to be anticipated in the United Kingdom without the risk of the cladding failing by becoming detached and falling from the building. The safe performance of the external wall insulation cladding depends on the standard of care taken not only in the design of the structural fixing system used to secure the system to the supporting structure but also to the care and skill taken during the manufacture and the installation of the various materials and components which comprise the system.
High-Rise Tower Blocks
Fig 1. External Wall Insulation
Fig 2. External Wall Insulation
Wind suction forces together with rain water ingress are two of the greatest potential causes of damage to external wall insulation cladding and can also severely damage the building structure. Recent media reports of storm damage contain vivid images of wall claddings becoming detached and falling from tall buildings onto traffic below.
The accurate assessment of anticipated wind loads likely to be encountered on external wall insulation claddings depends on a number of factors including:
The height of the building – low, medium and high-rise tower blocks
The geographical area, coastal locations, inner city, wind speed, etc, and
The general topography of the area
All of the relevant factors to be taken into consideration when designing an external wall insulation system to resist wind loads are detailed in BS EN 1991 1-4:2005 + A1, European Standard for wind actions on structures.
The wind loading resistance of an external wall insulation cladding system is determined by several factors including but not limited to the following:
The type of supporting structure (brickwork, blockwork, no-fines concrete, dense concrete, structure timber-frame, structural steel-frame, etc)
The performance characteristics of the mechanical fixings to be used to secure the external wall insulation to the supporting structure
The fixing arrangement to be used
The spanning characteristics and spacing of the studs/rails in framed structures
The strength, thickness and stiffness of the sheathing panels in framed structures
As illustrated by the above, careful calculation of the structural design of the external wall insulation system is a critical aspect of the design process. Therefore, prior to installing any external wall insulation cladding, it is essential that a detailed structural survey is carried out by a fully qualified Chartered Structural Engineer member of IStructE for each individual property to confirm not only the condition and suitability of the overall structure and each elevation of the structure, but also to confirm its ability to support the additional load and all imposed loads including the additional load of the EWI system and, in particular, in the case of cavity walls, structural frame-walls, etc., to also confirm the structural adequacy of the outer leaf tied back to the inner leaf/structure to support the additional loads.
It is common practice for the structural engineer with overall responsibility for the structural design of the external wall insulation system to calculate the building wind loads as part of the design process to determine the suitability of the fixing and arrangement to be used to secure the external wall insulation system to the structure of each individual building to be clad.
It is generally the responsibility of the main contractor to verify calculations for wall cladding have been carried out and approved by an independent chartered structural engineer with adequate public indemnity insurance cover.
Building owners and, in particular, owners of high-rise multi-storey tower blocks, should periodically check the condition of all external wall insulation EWI systems as part of their regular maintenance and management plan and in relation to structural matters should seek expert advice from a suitably qualified Chartered Structural Engineer. In relation to external wall insulation EWI systems with render and brick slip masonry finishes it may also be necessary to seek advice from a suitably qualified Chartered Building Engineer experienced in the design, installation and long-term performance of these systems in use.
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