Insulated glass units have particular properties when it comes to transfer of climatic load. Each modification of gas temperature in a gas-filled gap or external atmospheric pressure creates a load on component glass panes and causes their deflection (Figure 5a, 5b). As a result of glass panes deflection, the gas in a tight gap changes its volume and pressure, partially compensating for the changes of pressure and temperature, however that doesn’t change the fact that all changes of weather conditions, pressure and temperature are unfavourable for those structures. For example, deflections of glass panes are visible as the image seen in the light reflected by the glazing is distorted. There are attempts undertaken to reduce the loads of climatic influence by using devices equalizing gas pressure in the gas-filled gaps with the atmospheric pressure [11, 12]. Those solutions are in the testing phase now and are not mass produced.

Figure 5 Typical deflections of insulated glass units: a) increase of external pressure or decrease of gas temperature in the gaps, b) decrease of external pressure or increase of gas temperature in the gaps, c) wind pressure.

In the case of applied surface load, for example wind pressure (Figure 5c), the changes of gas parameters in the gaps have a beneficial influence on the load distribution, as it is distributed on all glass panes in the unit.

The distribution of static values - resultant load per area *q* [kN/m^{2}], deflection *w* [mm] and stress *σ* [MPa] of component glass panes in a unit is therefore a result of momentary balance between external loads and parameters of gas in tight gaps: pressure, volume and temperature.

To determine a resultant load applied to each of the component glass panes, it is necessary to calculate the gas operating pressure, at which the system is in equilibrium. For a double-glazed unit appropriate calculation models are specified, among others, in [13, 14, 15]. In the article [16] the author has presented his own model allowing for estimation of gas operating pressure for a unit with any number of gas-filled gaps. It was assumed in this model that gas in the gaps meets the general gas equation.

$$\frac{{p}_{0}\cdot {v}_{0}}{{T}_{0}}=\frac{{p}_{se}\cdot {v}_{se}}{{T}_{se}}=\text{const}$$(2)where:

*p*_{0}, *T*_{0}, *v*_{0} – initial gas parameters in the gap: pressure [kPa], temperature [K], volume of the gap [m^{3}], obtained during the production process,

*p*_{se}, *T*_{se}, *v*_{se} – service parameters, respectively.

It was also assumed that each of the gas-filled gaps changes its volume due to deflection of the limiting glass panes.

$$\mathrm{\Delta}{v}_{j}=\underset{0}{\overset{b}{\int}}\underset{0}{\overset{a}{\int}}w\left(x,y\right)\text{dxdy}\phantom{\rule{thinmathspace}{0ex}}={\alpha}_{\text{j}}\cdot {q}_{\text{j}}$$(3)where:

*Δv*_{j} – change of gas-filled gap volume caused by deflection of one of the glass panes limiting this gap [m^{3}],

*w*(*x,y*) – function of deflection, [m] dependence of the deflection value on the coordinates (*x,y*) of any point located on a glass pane with width *a* [m] and length *b* [m],

*α*_{j} – proportionality factor [m^{5}/kN]; it is a change in volume with unit resultant load per area of the glass pane,

*q*_{j} – resultant load per area on a glass pane that limits a gap [kPa].

The assumption that the deflection dependence on the load is linear is a sufficient approximation in case if the deflection value does not exceed the thickness of a glass pane [17].

On the basis of formulas (1) and (2) it is possible to create an equation for each gas-filled gap that describes its operational volume in equilibrium condition, with a determined load. The solution of this equation (for doubleglazed units) or simultaneous equations (for multi-glazed units) allows for determining operational pressure in the gas-filled gaps, as described in detail in [16]. The resultant load for each of the component glass panes is defined separately on basis of pressure difference between the gaps or between the gap and its environment, with regard to external surface loads, *e.g*. wind pressure. The knowledge of resultant load allows for calculating maximal deflection and stress for each glass pane by means of dependencies known in Kirchoff-Love theory of plates, for example in accordance with [18].

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