The interaction between a building’s façade and its structure is frequently ignored in the design phase. Analysis of the structural movements, architectural intent and specified glazing system are required during the early stages of a project design.
Glazing and cladding systems need to be designed to allow for building movement without compromising the performance or safety of the system – failure to provide sufficient allowance for inservice building movement in the connections between façade and structure may result in leaks, cracks, failure of connections, buckling of mullions and breakage of glass.
Due to the extension of structural spans seen in today’s building designs, slab deflection is increasing placing additional demands on the façade design team to accommodate this differential movement between slab and façade. We explore this in more detail over the page.
Early engagement with system specialists is the answer
Analysis of the structural movements, architectural intent and specified glazing system is usually carried out by the specialist façade contractor or system supplier. It is imperative that main contractors and building designers engage with the system specialists during the early planning stage of a project to ensure that the proposed façade system can accommodate the associated building movements and is in line with the proposed site installation methodology. If the specified glazing system is not aligned with the building structure and installation plan at the early stage of a project, it can cause unforeseen delays and costs when it is aligned at a later stage.
Factors influencing façade and building movement
Accommodating building movement and the resultant interaction between the façade and structure is complex and specifiers should consider many different factors examples of which are: Temperature, Moisture. Floor loading, Wind loading, Snow loading, Live Loads, Dead loads, Settlement Creep, Seismic sway.
Selecting the most appropriate glazing system at the early stages in a project is very important and can have a major impact on the intended appearance and lifetime performance of the façade. For example, wider framing members can accommodate more building movement, and in some instances double transom details are required at floor slabs for the accommodation of differential building movement.
Differential slab deflection
One of the main complexities is designing the façade to accommodate differential slab deflection.
These movements caused by post installation dead and live loads are commonly found in the structural engineer’s report which are typically defined by a span ratio. Due to a desire to extend structural spans in modern structures, slab deflection figures based on the span ratio are increasing significantly making façade design and movement accommodation even more challenging. Arguably, floor slabs in real life don’t deflect anywhere near to figures being documented but it is essential to design façades as if they do – including an allowance for manufacturing tolerances and thermal movement.
Determining the limitations of a particular system or offered solution must be understood before complete alignment can take place. Failure to provide sufficient allowance within the system (or to design/specify a system with serious limitations), and failure to correctly design/detail the critical connections between the façade and structure may result in leaks, cracks, failure of connections, buckling of mullions and breakage of glass.
Evaluating the various curtain walling options
Conventional stick curtain wall is widely specified in the UK and can offer a cost-effective façade solution. However, compared to unitised curtain wall, it is very limited in terms of accommodating differential slab deflections.
Kawneer UK – with a long track record of dealing with building movement – has recognised this and has recently developed a movement/expansion joint for their AA®110 ‘stick’ curtain wall system that can accommodate up to +/- 15mm of differential slab movement – so copying the ability of unitised solutions to accommodate movement whilst offering the cost effectiveness of a ‘stick’ solution.
This expansion joint solution comprises a specialised engineered foam which expands and contracts with the movement of the structure. A unique transom profile is used at each floor level which allows the spandrel glass/ panel to slide within the glazing rebate whilst maintaining airtightness, ventilation, and weathertightness of the system. A bespoke breather membrane is used on the bespoke floor transom which is designed to allow any excess water in the glazing rebate to escape – it is also designed to expand and contract with the racking movements of the glass when subjected to the 15mm slab deflection.
One of the key aspects of the development was the performance test methodology to ensure that the system would perform when subjected to this level of movement. Kawneer worked closely with CWCT to develop the test method which was an enhanced version of the sequence B which included a structural movement regime comprising of 3 cycles of +/- 15mm deflection of the structural beams supporting the test specimen.
Help is at hand…and not just a product solution
Kawneer now has a systemised solution tested in accordance with CWCT standards that can be readily tailored to the requirements of each project. Kawneer is also launching its ‘Façade Workshop’ which consists of a team of façade specialists set up to work collaboratively with developers, architects and engineers during the early stages of a project. The aim is to provide a unique design and consultancy service working across every aspect of the façade to ensure that the design intent and every detail is realised.
The thinking of the Kawneer team is early involvement always leads to a successful outcome for the project. Kawneer are committed to providing expert advice not only to solve problems for clients in terms of building movement, but to ensure that façade selection and key design issues are highlighted and resolved with as early as possible.
The Kawneer technical team have a diverse skill set, and understand the challenges associated with façade design and their construction. The holistic approach is set up to classify façades by function, materials, mullion type, glass type, key components, structural safety, thermal performance and future maintenance. Furthermore, we understand architectural trends and features are a key part of the buildings identity – we are committed to turning these features and daring concepts into reality to meet the desired aesthetics of the building.
As stated above early engagement is the answer to reducing these incidents of façade/building conflicts and with its no obligation ‘Façade Workshop’, the company is offering to help refine the design, ensure the scheme is ‘value engineered’ and can be built on-site safely.
This article first appeared in IGS Magazine’s Winter 2018 Issue – Read the full Magazine here for more thought-leadership from those spearheading the industry
Editors note: At the time of publishing this article Danny Birrell was Technical Director of Kawneer UK, however is presently Design Director Hansen Facades UK
Danny Birrell has an M.Sc. in façade engineering and 18 years of experience in the construction and architectural aluminium industry, most recently as technical leader with a specialist sub-contractor. At Kawneer he leads a team of more than 20 consultants, designers, engineers, technicians and trainers and will be part of the Cheshire company’s senior management team led by Managing Director Phil Randles.
High profile projects that he can put his name to include the Olympic village project (Stratford City, London), the former Royal Eye Hospital in Manchester, 123 Victoria Street (London), Winnersh Triangle in Reading and BSKYB Academy in London.
Formerly the founder of his own curtain wall and window installation business at the tender age of 21, and appointed as design leader for Glassolutions aged 26, he is a member of the chartered institute of building service engineers (CIBSE) and studied architectural and façade engineering at Bath University.