While the majority of the buildings I worked on are non-curvy tall high-rise buildings, I had a fair share of working on challenging geometry projects. An amazing thing about being a façade engineer is to be able to drive variety to your job to keep yourself enthusiastic. It is therefore a good fit to people who like variety in project tasks. It is also a good fit to people who like to see things built. Sometimes façade engineering projects appear to be unbuildable at first glance. So without the drive to make the seemingly unbuildable real, it is easy to resort to traditional technologies that sometimes limit architectural freedom.
Luckily, technology is also constantly progressing. We learn about new options on conferences, exhibitions, but also, we define new type of demands to manufacturers, until a materialized response is generated.
Introducing On-site Cold Folding
I personally worked on Opus since 2007 and followed up until into its most recent full handover; the hotel wing opened end of February 2020. The project is a cube with a freeform void. The modulation of the void is defined by the slab lines and by a regular grid “melted” onto the freeform surface, necessitating bent surfaces to follow the geometry.
At the same time back in 2008, I also worked on Shining Towers, a project in Abu Dhabi with identical floor plates rotated in steps as we go up. A third project, a tower in Abu Dhabi I could mostly describe as beetle-shaped (remains unbuilt), was also on my desk. Cold bending seemed to be a good solution to introduce cost efficiency but allowing for warped geometry. For this, flat units were to be produced and a corner would be fixed in an offset position during installation. This was 2008, 12 years ago, when the topic was a relatively new entity and less researched, not documented, and design guidelines were not available. Panels cannot go on buildings without proper verification.
Such verification is traditionally a two step process: engineering calculations and a physical mock-up test to verify the model. We had one little problem here: uncertainty on actual behavior and method of modelling. It is a composite system under low, but permanent stress, acting on the glass corner connection, structural and IGU sealants.
Additionally, the concept is only buildable if the assumed geometrical restrictions prove to exist, namely the sides of the units must stay straight while on-site cold forming happens. So to the benefit for each project, a third step was introduced: a mock up test to verify the theoretic behavior prior to designing and calculating the project conditions. For these tests, with industry support, 4 full units were manufactured for research purposes.
The test carried out was published at several conferences and in a number of publications and served as a design basis foundation for projects of this type. The process is actually incorrectly called cold-bending, it should be defined as On-site Cold Folding. What really happens is that the connections are designed to take the stress and the unitized panel folds along its diagonal. This keeps the sides straight and allows easy interlocking with adjacent panels during installation, which arrive as fully flat panels. The vertical side engages and the new panel is site folded by pulling in the other bracket hook while lowering down to its final position. Furthermore, it reduces stress on the sealants.
The clear advantage of allowing the possibility of Site Folding is to be able to produce and use flat panels with straight extrusions and flat glazing.
There are many other methods to create warped surfaces. The contractor of the Opus utilized a range of solutions, including the use of hot-formed double curved glass or the use of curved framing members with silicone curing while the glass was forced to follow the curvature by mechanical clamps.
The difficulty of producing double-curved glazing
Glass is tempered (heated up and cooled down rapidly) to increase its strength. For multi-axial curved shapes, a mold is used with the semimolten glass sheet. This does not easily match with the traditional method of linear wor flow furnaces used for tempering. As a result, double bent glazing is traditionally annealed.
Unfortunately, as our stress reading tests in 2016 reconfirmed, the surface limit stress values of double bent glazing produced with a mold slumping method are not improved when compared to that of annealed glass.
This heavily limits the applicability of such glazing, as at certain climate and orientation conditions the induced thermal stress may cause fracture. The risk is greatly contributed by the heat absorption properties of the glazing (- a body tinted glass would absorb more, increasing the risk). While tempered mold based double curving has long been in
existence in the automotive industry, it has not been implemented in architectural glazing for its inefficiency when it comes to reusability of molds. However, the process is not impossible. While on Opus and other buildings we reached out to have double curved tempered glazing on the building, this comes with very high cost, and very high risk of shape intolerance and anisotropy presence.
Demand for iconic architecture
We live in interesting times, in an accelerated world where information travels fast and achievements are global.
There is no better building describing the swift change in architectural concept trends than – as its name suggests- The Museum of the Future. The Museum is built with trueshape double curved panels, typically 9 meterstall by 2.2 meters wide, made of Glass Fibre Reinforced Plastic (GRP) resin faced by stainless steel and completed with layers of insulation and gypsum boarding. The resin contains fire retardant chemicals. The building is iconic and meaningful in many ways. Even before practical completion, it already received global notoriety and I am frequently asked about the technology behind enabling it.
For the Museum, computer instructed CNC machines carved single use foam molds, to the exact shape derived from the 3D model. With this method, repetitive design or rectilinear shapes bring no cost benefit at GRP panel production. The panels were cast to the exact geometry, faced with the exact laser cut stainless steel sheets, and fitted with shaped glazing units. The glass panels follow a small vertical modulation of 1 meter, and kept each piece flat. The faceted glazing was iterated by a computer script to fit in the curved line of the cladding zone depth of the panels.
The underestimated consequences
It may be misleading to think however that these technologies enable premium-free construction of freeform surfaces.
Engineering and documentation of these geometries take a significantly larger effort than that of cladding for rectilinear masses. 3D modulation studies and surface optimization introduces an additional necessary step to the design process. Also, local and global regulations and practice guidelines rarely cater for unique shapes. Finding the correct approach could necessitate additional testing and verification. As an example, climatic load stress calculated as described in the DIN standard can be largely underestimated as it does not consider the adverse effect of rigidity gained from the curved geometry.
All unique pieces require the fabrication to be tightly synchronized with the site programme and delivered in sequence, with appropriately coded labels and well implemented logistics procedures. Out of sequence delivery may halt installation as units are not interchangeable.
During installation, a higher skill level may be needed to ensure that the building envelope performance remains uncompromised at each geometry condition. These factors will increase not only the cost but the risk when dealing with freeform buildings.
Non-trivial sources of complexity
Curved geometry is not a pre-requisite for manufacturing and installation complexity. The KAPSARC building is designed with straight lines. Yet each panel differs in size and geometry. The material used for the cladding is Glass Fibre Reinforced Concrete, with high fabrication tolerances. Materials like this can only be successfully installed in a tolerance sensitive arrangement if the substructure allows for individual fine-tuning of the cladding elements.
The MOL HQ office building has an elegant tower that widens towards the base and transforms into the podium mass. The fully glazed building is 130 meters tall and the transition happens over two floors only. This excludes the possibility of faceting the glazing panels and approximating the curve, leaving the application of double bent glazing the only option.
Crowne Plaza BB is a very simple box shaped building. The fully glazed façade is enriched with differing depth perpendicular glass fins, simple detailing that contributes interesting undulating architectural effect. The waviness is reflected by the canopy, the only truly freeform element of the façade, built from aluminium mesh. The modulation of the canopy needed a separate study to ensure it is buildable from standard width sheets and can be unfolded to flat. As the canopy forms an inclined line across the elevation, it needs subdivisions above the unitized panel modulation lines.
Predicting future trends
The industry has been gearing up to handle anything that computer based design can provide as output. Newly acquired machines in factories are all computer instructed and can mass produce the non-repetitive. Logistics moves into digital platforms. Tasks are getting integrated under BIM or other collaboration models. Quality control readings are fed back and compared in real time. All sections of the design, manufacturing, handling and installation processes are capable to process larger volume data than before. But there needs to be more investment, more research efforts, more innovation efforts to enable more contractors and more manufacturers to update technology or to attempt moving away from traditional methods.
With global economy getting tighter, the general volume of worldwide construction is forecasted to decrease and along with it, architects need to follow demand. The next few years will be a real trial for recent technology, to see whether it can be competitive enough to sustain complexity in construction.
This article was originally published in IGS Magazines Spring 2020 Issue: Read the full Magazine here for more thought-leadership from those spearheading the industry
Author: Agnes Koltay, CEO, Koltay Façades
Agnes Koltay has a Masters degree in Architecture (Hungary, USA) and in Façade Engineering (UK) and worked with award-winning architects and multidisciplinary engineering companies before founding Koltay Façades, a boutique façade engineering consultancy firm, in 2011. She is the lead façade consultant on a number of prestigious high-rise projects, such as Skyviews, Burj Vista, The Opus, The Address Boulevard Hotel.