Information on the impact wind loads on the back ventilated facade systems with air gap
Agenda: When measuring the parameters of back ventilated cladding systems with air gap, including the entire fastening system, the design version of the wind load is the determining factor.
The current state of the development of technology makes it possible to use the wind load and pressure indicators given in DIN 1055 T4 to calculate the stability factor, however, it should be noted that this section of DIN is applicable only to the airtight firmware of the building. In the supplements to the paragraph 6.1 of DIN 1055 part T4: "To determine the pressure and force indicators, calculations are made on experimental models with a rigidly fixed and airtight cladding."
However, the difference between back ventilated facades consists precisely in the fact that air from outside constantly circulates through the air gap located between the cladding and the layer of thermal insulation; i.e. the shelling of the building with such a facade will be breathable. If the calculations take into account the constant, plane pressure differences, then the pressure difference between the air from the outside and the air circulating in the air gap does not arise (the principle of communicating vessels), as a result, the cladding is not exposed to wind loads. But it must be taken into account that the ratio of the pressure and wind flux parameters are non-constant, depending on the distribution of pressure on the external walls, which in turn depends on the strength of the wind. Thus, the difference in pressure inevitably arises.
Within the framework of the research carried out with the assistance of the German Research Society, under conditions of actual airflow impact, the resulting difference in pressures and associated possible problems with the cladding were obtained; on the basis of this study, the optimal concept of preventing the hazardous effect of wind loads, which was added to EC 1 Part 6, was developed, capable of meeting the requirements of DIN.
Figure 1 shows the process of building exposure to the air
flow. First, as a result of the surrounding air movement, an air mass is formed in front of the building, which causes a complete or partial conversion of the kinetic energy of the air into the energy of pressure. Under the influence of the accumulated air, the incoming air changes its direction, thus encircling the building and creating a vortex (the so-called horseshoe vortex) at the base of the building, which moves in the direction of the airflow along the longitudinal sides of the building.
Figure 1. Encircling wind movements around the building
It is important to take into account the direction of air flow along the longitudinal sides of the building (Fig. 2, part a). The air moving in this manner contributes to the formation of a secondary vortex with a high speed of rotation (Fig. 2, part b), which periodically increases (Figure 2, part c) and subsides. Below the line of air flow diverted from the windward side, an area of increased underpressure is formed. The size of this area increases if the air flow again moves in the direction of the sides of the building (Figure 2, part a). At the moment, within the framework of DIN 1055 T4, calculations are carried out of the indices of air underpressure area of along the edge of the building, taking into account the coefficient cp.
Figure 2.Temporary change in the direction of air flow along the vertical edge of the building
The circulation of air in the gap between the air-permeable cladding and the impermeable outer wall of the building is shown in Fig. 3. The air flow in the air gap influences the distribution of the pressure therein, which differs significantly from the distribution of the external pressure.
Figure 3. The airflow along the vertical edge of the building in the area of a ventilated cladding structure
The index of the final wind load on the outer walls of the building is obtained from the pressure difference between the internal and external wind load (Fig. 4).
Figure 4. Final distribution of wind load in back ventilated cladding system
In order to achieve a reduction in wind load, it was decided to divide the air gap under the vertical edges of the building with airtight layers. In Fig. 5., it is clearly visible how the movement of the air flow changed. To determine the effect of the "wind barrier" presented in Fig. 5, experiments were performed both in the wind tunnel and in real conditions.
Figure 5. The movement of airflow relative to the building with or without a wind barrier in the air gap area
Wind tunnel testing and testing in real conditions to determine the wind load
The determining parameters influencing the equalization of pressure between the outside air and the air circulating in the air gap are as follows:
Breathability of the outer cladding;
Resistance to air flow in the air gap.
The difference in the pressures of the outside air and air in the air gap is equalized if, on the one hand, a high resistance to air flow occurs in the air gap, i.e. if the movement of air in the gap is suspended, and, on the other hand, if the resistance to air flow in the area of the external wall cladding is minimal, i.e. if the outside air can freely enter the air gap.
The resistance of the cladding of external walls to the air flow (air permeability) is largely determined by the size of the rustics. The wider the rustics, the higher the air permeability or the lower the resistance to the flow of air. In order to operate this value, the following ratio was calculated:
А gap – open gap area
А wall - outer wall cladding area
Resistance to air flow in the air gap is largely dependent on its thickness. The thicker the gap, the higher the resistance to the air flow circulating in this gap. To determine the resistance to air flow, the following indicator was introduced:
s- air gap width (from 20 to 50 mm)
a- length of the side of the building (5-10 mm)
Tests inside the wind tunnel were carried out both with the presence of a "wind barrier", passing along the vertical edges of the building, and without it; while the following parameters during the tests changed their values: (see Figure 6)
Figure 6. Coefficients for determining the resistance of air circulating in the air gap
building geometry (h/a (height) and b/a (width);
wind flow resistance
resistance to circulating air in the air gap;
air inflow / direction of inflow.
In order to evaluate the data obtained during testing in a wind tunnel, large-scale tests were carried out on the territory of the Eternit Company plant. According to Fig. 7, it can be seen that the data obtained during testing in a wind tunnel practically do not differ from the results of tests in real conditions.
Figure 7. Comparison of test results in a wind tunnel and in large-scale tests
In addition, in these tests, the efficiency of applying insulation to the vertical edge of the building was assessed. Fig. 8 shows that the application of thermal insulation has a significant effect on the degree of wind load.
Figure 8. Coefficient of load with or without insulation layer.
After analyzing the results of tests in a wind tunnel, it is possible to determine the design load for a stand-alone building without taking into account the dynamic load according to DIN 1055. For this, it is necessary to observe the following conditions:
air gaps in the vertical section of the building should be separated by airtight vertical insulation;
relative permeability of the cladding ε> 0.75% should be correlated with the side surface of the building;
ratio of the thickness of the air gap and the end of the building should be s/a ≤ 0.005;
test results also apply to ventilated cladding supports, Fig. 9.
given pressure ratios are valid only for rectangular objects with pointed ends of the building;
it is necessary that the air circulating through the air gap, which is inside the roof overhang, can freely exit and enter inward (through open rustics).
It turned out that to calculate the wind load, the vertical division of the building area into the middle area and two side edges is not necessary (in spite of the corresponding requirement of DIN 1055). However, in order to calculate the increased load that falls on the bottom of the building, a horizontal risk is placed on its upper edge parallel to the roof overhang. The width of such risks should be 10% of the height of the building.
Figure 9. Separation of the air compartment by means of vertical load-bearing profiles.
To determine the wind load (air rarefaction - wind pressure), the following simplified version of the calculation can be used:
Table 1 to Fig. 10: wind load calculation option
Data in Table 1 is again shown in Fig. 10 Pressure factors for ventilated cladding structures are also established in the draft of EC 1 European Norm, part 6. Reduced values of cp are also given here, in case the corner insulation layer is applied to the edges of buildings. When calculating the values given in the table, first of all, the effect of wind permeability (depending on the width of the rustics) is taken into account, and not the resistance of the air flow within the gap. At the moment, it is planned to include the presented version in the European Norm EC 1-part 6.
The given load factors for ventilated cladding support are much lower than those given in DIN 1055 part 4 for airtight walls of the building. First of all, this concerns vertical marginal risks, the air dilution factor for which is cp = - 2.0.
Fig.10 recommended values of cp for calculations on ventilated cladding support and their anchoring.
Formation of an "air barrier" on the vertical edges of a building
The "air barrier" on the vertical edges of the building can be as follows:
expanding foam materials with a width of at least 10 cm, and the degree of compression of the interlayer should be K≥1:3 (Fig. 11);
Fig.11: vertical "air barrier" of the expanding foam material, passing along the edges of the building.
a strip of insulating material made of mineral fiber 50 cm wide (≥ 70 kg/m3), which should be in a sufficiently compressed state between the outer wall and the cladding (Fig. 12);
Foil (film) as a "wind barrier", Fig.13.
Fixing of thermal insulation in the system of ventilated rainscreen facades with air gap When fixing heat-insulating panels on the supporting wall, it is necessary to verify the reliability of their fixation. The final load per unit area of thermal insulation is obtained from the pressure difference on the upper and lower sides of the thermal insulation layer. The resulting load value depends on the resistance to the air flow of the thermal insulation material ("air permeability"). When applying thermal insulation made of mineral material, resistance to air flow is so insignificant that it can be expected to obtain a pressure coefficient cp = -0.05. In the case where airtight heat-insulating materials are used, for example, pressed polystyrene, it is necessary to fix the heat-insulating panels on the wall with glue. - Such gluing or flat fixation with a dowel is recommended to be made in case of using mineral fiber insulation, on the one hand, to prevent cold air from entering from outside, and on the other hand - "lowering" the insulating material into the air gap, which will disrupt normal circulation air in this gap. The gluing process must be carried out constructively; it is necessary to take into account that when fixing the insulation panels with dowels n ≥ 4 dowels/m2.
Previously, the dimensions of the back ventilated facade with an air gap were determined according to DIN 1055 part 4. However, this norm of DIN is applicable only to airtight claddings of buildings. In the framework of the research project and conducted tests, lower wind load indicators were obtained. Such results can significantly reduce the costs associated with installing a system of hinged facades with air gap.