Experimental Validation of a Numerical model for Closed Cavity Façade Glass Structural Calculation under Dynamic Cavity Temperature, Dry Air Flow and Wind Loads Effects

Abstract

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-sharing between inner and outer panes, leading to thinner glass panes. Note that this depends strongly on the airtightness levels of inside and outside, and manufacturing airtightness scatter should be accounted for ensuring a robust calculation approach.