This paper is the second in a series on the behavior of closed cavity facades (CCF) under termo-mechanical loading excitations. CCF are a novel trend for sustainable high quality double skin façade solutions, characterized by a minimal maintenance demand. The air cavity is designed according to stringent air tightness requirements, provided with a small dry air flow that preserves the relative humidity, minimizing the risk for condensation and dust ingress. This extremely reduced permeability, with respect to typical double skin design, gives a scenario in conflict with the general prescriptions for the structural calculations of double skins under wind and climatic loading. This paper is the second of a series of three documents that aim to identify the structural shortcomings in the current codes and to propose efficient calculation methods and modifications to the current calculation strategies in order to overcome a critical design paradox in double skin and in particular CCF aim to reach the highest sustainability performance, by means of outstanding thermal and acoustic insulation efficiency and providing the cavity with a clean and dry environment, suitable for a significant upgrade of the façade life expectancy. On the other hand, an overly conservative calculation approach is generally against the optimization of the sustainability performance indices, determining an excessive use of materials and then impacting the overall life cycle cost under several perspectives. In particular, the objective of this second paper is to describe in a measurable way the permeability behaviour of a CCF, defining statistical variability and nonlinearity effects measured during dedicated experimental testing. The permeability functions represent a fundamental input for the proposed assessment tool introduced by the first paper and it will be seen that an accurate quality control during the manufacturing can ensure the robustness of an optimized calculation approach, based on the load sharing mechanism between the skins. In the third paper it will be shown that the proposed tool, fed by the experimental permeability functions, renders adequate to predict the façade structural behaviour under the superimposition of thermal loading, wind loading and dry air flow effects, when compared with measurements collected during outdoor testing on a façade specimen.
This paper is the first in a series on the behavior of closed cavity facades (CCF) under termo-mechanical loading excitations. CCF are a novel trend for sustainable high quality double skin façade solutions, supplying high performances in terms of thermal and acoustic insulation and providing valuable benefits in terms of maintenance cost reduction. However, existing calculation methods adopted for double skin façade glazing verification under wind and climatic loads do not seem accurate to capture the particular behaviour of the CCF. In particular, the permeability of a CCF air cavity is several orders of magnitude less than the permeability of a ventilated double skin façade and current standards, such as EN1991-1-4, do not comprise any guideline for the optimized design of impermeable double skins. Both skins are recommended to be designed based on the full net pressure, or only discounting the internal pressure portion, without any provision for the load sharing effect. Moreover, climatic loads are conservatively applied ignoring permeability effects, overestimating in several cases the safety and serviceability behaviour under the daily and yearly temperature cycles. It is clear that the absence of a validated calculation approach enforces the demand for extremely safe assumptions, giving a paradox that a sustainable façade is currently designed by a non-sustainable calculation approach. In order to solve the shortcoming at hand, a comprehensive coupled pressure equalization-thermal-mechanical model has been developed and validated by means of experimental measurements. In this paper, the analytical basis of the model is described, starting from the scenario of a completely impermeable cavity and comparing the model outcomes with a reference software for the analysis of insulating glazing units. It will then be shown that the model can be extended to permeable cavities by accounting for a proper pressure equalization scheme through the openings. Finally, a sensitivity analysis of the major façade geometry variables was conducted with the numerical model, considering a wide range of possible applicative scenarios in term of permeability and skin stiffness.
This paper is the third in a series on the behavior of closed cavity facades (CCF) under termo-mechanical loading excitations. CCF are a novel trend for sustainable high quality double skin façade solutions, supplying high performances in terms of thermal and acoustic insulation and providing a valuable benefit in terms of maintenance cost reduction. Current structural codes, in particular the EN1991-1-4, contain incomplete and very conservative assumptions about the wind load sharing design for this type of multiple skin although current sustainable design objectives require glass thicknesses to be optimized. In addition, in most codes glazing skins are too often investigated as separated structural glass elements. However, it is more appropriate to include the structural interaction between the two skins coupled by means of the confined air cavity. Indeed, the key feature of the structural behaviour of a CCF is the air cavity response to wind loads and the way the cavity air supply is regulated in order to preserve low relative humidity. An optimal design of the outer and inner glass skin should take in account the superposition of the actions of wind and dynamic temperature and therefore the variable mass within the cavity itself. The mass variation becomes fundamental, as it is governed by two counteracting effects: on one side dry air is pumped into the cavity in order to avoid the risk of condensation, on the other side the mass flow through the skin openings that connect the cavity with the external and internal environment. During the last years Ghent University and Permasteelisa have conducted a theoretical investigation and developed a numerical calculation procedure in order to provide a unique assessment tool for the structural design of mfree-S CCF facades. The tool has been validated by means of an extensive experimental campaign and the different relevant effects have been described. Several mfree-S CCF elements have been exposed to on-site wind loads, solar thermal imposed loads and controlled laboratory quasi static and cyclic loading pressures (with and without dry air supply). As such, the response to the natural climatic loading has been collected in order to constitute a representative model of the mfree-S CCF working conditions during its entire life. This paper is the last of a series of three documents that summarizes the outcomes about the research on the CCF panels. In the first paper the basics of the pressure-equalization model has been discussed and verified, describing in particular the extreme case of a fully closed cavity. In the second paper the permeability functions of the CCF have been derived as fundamental input for the dynamic simulations under variable cavity temperature conditions, which are the major objective of this third and last work. The experimental results and the numerical simulations are demonstrating the need for an improvement of the current codes in a direction of a more sustainable design. In particular, the simulations allow to account for a reliable load-sh...
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