EXTERNAL ENVELOPE High rise not high risk!

Danny Birrel of Kawneer advises on how to design curtain wall facades to accommodate building movement


and-pressed developers are increasingly turning to high-rise buildings to deliver best footprint

value but the interaction between the facade of a building, particularly a tall one, and its structure is, sadly, frequently ignored in the design phase.

Analysis of the architectural intent, structural movements and specified glazing system is required during the early stages of a project design so that glazing and cladding systems can be designed to allow for build- ing movement without compromising the performance or safety of the system. Failure to provide sufficient allowance for in-service building movement in the connections between facade and structure may result in one or more issues, such as leaks, cracks, failure of connections, buckling of mullions or breakage of glass. Due to the extension of structural spans seen in contemporary building designs, slab deflection is increasingly placing additional demands on the facade design team to accommodate this differential movement between slab and facade. The solution to this is simple – early engagement with system specialists. While analysis of the structural movements, architectural intent and specified glazing system is usually carried out by the system supplier or specialist facade contractor, it is imperative that main contractors and building designers also engage with the system specialists during the early planning stages to ensure the proposed facade 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 stages of a project, it can cause unforeseen delays and costs when it is aligned at a later stage. Accommodating building movement and the resulting interaction between the facade and structure is complex and there are many factors to consider. These include tempera- ture, moisture, floor loading, wind loading, snow loading, live loads, dead loads, settle- ment, creep and seismic sway.


Selecting the most appropriate glazing system at the early stages in a project is vital as it can have a major impact on the intended appearance, and more importantly, lifetime performance. For example, wider framing members can accommodate more building movement, and in some instances double transom details are required at floor slabs for the accommoda- tion of differential building movement. One of the main complexities is designing the facade 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 facade design and movement accommodation even more challenging. Arguably, floor slabs do not deflect

anywhere near the figures being documented but it is essential to design facades as if they do, including an allowance for manufactur- ing 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 and/or specify a system with serious limitations, and failure to correctly design/detail the critical connections between the facade and structure may result in the issues mentioned earlier. Evaluating the various curtain walling options should be a first-case scenario as although conventional stick curtain walling is widely specified in the UK and can offer a cost-effective solution, compared to a unitised curtain wall it is very limited in terms of accommodating differential slab deflections.

Kawneer has recently developed a movement/expansion joint for ‘stick’ curtain wall systems that can accommodate up to +/- 15 mm of differential slab movement, so mimicking the ability of unitised solutions



Determining the limitations of a particular system or offered solution must be understood before complete alignment can take place

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