AA Membrane Canopy in London
The AA membrane canopy provides the terrace constant weather protection. Yet it is so permeable that it maintains views of the surrounding London roofscape and avoids excessive wind pressure. Strong wind loads and the low load-bearing capacity of the terrace ruled out a closed surface: the structure is supported by three existing columns, which are only capable to a small degree of resisting the horizontal loads resulting from torque moment. A system was developed based on composite components that reconcile the performance criteria – allowing light and air to enter, maintaining views and providing protection from the elements – by explicitly defining each component in an overall system. The individual components, developed on the basis of numerous analogue and digital tests, are made of tubular-steel frames (resisting compressive forces) and membrane elements (tensile strength). The material and geometric properties were embedded in a parametric model, which constituted the building block of the subsequent integral design process. The modification of the system morphology then occurs on the basis of the progressive differentiation of the parametric system. In conjunction with tension cables, pre-tensioned membrane surfaces function structurally. Each phase of the development was analysed with the aid of computer fluid dynamics in a feedback process with finite element analysis of the structural behaviour. Here the aerodynamic behaviour of the components influences the adaptation of the respective component’s form in such a manner that protects from air draughts, yet prevents the permeability of the surfaces from producing excessive dynamic pressure. The goal was to minimize the loads on the supporting structure and the cross-sections of the steel elements. The field of view, the system’s permeability and the canopy’s shading behaviour were additional focal points of the design. Both were tested by means of digital simulations. The parametric system overcomes the clear hierarchy of mono-functional systems and unites a variety of functions in one structural system. Different geometric configurations of the membrane system were analysed and the digital model was developed iteratively to attain a finely calibrated relationship of the different criteria by adapting each component locally, a complex task due to the non-linear causalities to be reconciled, as the following observation illustrates: although the problem of large horizontal loads was solved with the differentiated, permeable system, due to the openings in it, the airflow was accelerated locally in a manner that diminished the users’ comfort. In addition, rain was forced through the canopy horizontally. By enlarging the openings, the local acceleration of the airflow was reduced, which, however, impeded continual, gravity-activated run-off of rainwater. Thus, the generative process served as the basis for an overarching integration of the different design criteria.