On-site detection method and application of safety hidden danger of insulating glass structure.

Introduction

The development of high-efficiency energy-saving glass is one of the important ways to reduce building energy consumption. As the most widely used energy-saving glass, the energy-saving effect of insulating glass will decline or even fail over time, and its durability is difficult to match with the life of the building. The reason is that organic materials (structural glue, polysulfide glue, etc.) are used on the edge of the insulating glass, and the insulating glass market in some developing countries is relatively chaotic. Even with inferior structural adhesives, these phenomena occur from time to time, causing the insulating glass to be prone to interface degumming and air leakage, and poor durability; in addition, the physical aging of the structural adhesive itself after the insulating glass is used for a certain period will also weaken the bonding interface strength. The cyclic load of the upper environment will cause the degumming of the insulating glass interface and form a permeability leak.

^{Figure 1 Schematic diagram of the load-bearing performance of insulating glass in sealed and leaked states} 

When permeable gas leakage occurs in the sealing unit of insulating glass (such as degumming of the secondary sealant, glue breaking, etc.), in addition to the failure of the heat insulation function, the bearing performance of the insulating glass will also change. Figure 1 shows a schematic diagram of the load-bearing of the insulating glass sealing unit under sealing and penetrating cracks. In the sealed state, under the action of external load (wind load), due to the sealing and air leakage state shown in Figure 1, the bearing performance of the insulating glass is shown in Figure 1. The gas in the hollow layer of the insulating glass has the effect of transmitting the load, and the inner and outer pieces of the insulating glass can withstand the load at the same time. load action. However, once the gas in the hollow layer of the insulating glass is transparent to the outside world, the gas in the hollow layer no longer has the function of transmitting the load. Especially for the hidden frame glass curtain wall, when the outer sheet of insulating glass is subjected to negative pressure, the negative pressure acting on the glass panel will be transmitted to the sealing layer of the edge of the insulating glass, and the sealing unit will be subjected to tensile force. Cause the insulating glass outer sheet to break or fall off as a whole. Therefore, detecting whether there is penetrating gas leakage in the sealing layer of insulating glass is very important for evaluating the safety performance of insulating glass, and it is also an indirect detection method to predict whether the insulating glass has a structural failure and brings hidden risks.

1. The detection principle and calculation basis

Based on the change of the load-bearing deformation performance of the insulating glass hollow layer before and after the failure of the permeability seal, by applying a load P to the center of the insulating glass plate online, and measuring the thickness change of the hollow layer in the center of the insulating glass plate, it is possible to identify whether the hollow layer of the insulating glass appears transparent. Air leakage phenomenon, and then evaluate its structural safety performance. When the concentrated force P acts on the center of the plate, the deflection equation of the simply supported rectangular plate with four sides is:

(1)Where: m、n is odd, m=l, 3, 5, ... , n=l, 3, 5, ...; D is the stiffness of the glass plate, D= Eh3/12 (bv2), E is the elastic modulus of the glass, v is the Poisson's ratio of the glass, h is the thickness of the glass; P is the concentrated load acting on the center of the glass plate; a, b are The short and long sides are long. If the hollow layer of the insulating glass is completely sealed, then under the action of the concentrated force P, the pressure change of the hollow layer of the insulating glass is set to 4P, then the outer sheet of the hollow layer is deformed under the action of the forward (the direction of the concentrated force) P and the reverse 4P. The inner sheet is deformed under the action of positive 4P. According to formula (1), the volume deformation of the outer glass under the action of P is:

(2)When the four sides are simply supported and the uniform load acts, the deflection calculation formula of the glass plate is as follows:

(3)Among them, qo is the uniform load acting on the glass plate. According to formula (3), the volume deformation of the inner and outer sheets of glass under the action of the uniform load 4P is:

(4)Then the total volume change 4V of the hollow layer under the action of the concentrated force p is:

Because the air in the hollow layer of insulating glass satisfies the gas equilibrium state equation, namely: Outer children = (island must be p) (several w) (6)

In the formula: Po is the initial pressure of the hollow layer, usually 1 standard atmospheric pressure (1.013*10⁵ Pa); Vo is the initial volume of the hollow layer. After substituting Equation (5) into Equation (6), the equation has only one unknown, △P, so the value of △P can be solved. At this time, according to the load of the inner and outer sheets of glass, according to the four-sided simply supported plate theory, the deflection value of the inner and outer sheets of glass can be obtained, to reversely push the value of the thickness of the hollow layer at the center of the insulating glass plate after loading.

If the permeability and sealing of the hollow layer of the insulating glass fail, then only the glass on the loading surface is deformed, and the change in the thickness of the hollow layer in the center of the plate is the calculated value of the formula (1).According to the above calculation method, when the insulating glass sealing unit is completely sealed and permeability leakage occurs, the calculation of the thickness of the hollow layer at the center of the plate is shown in formula (7) and formula (8) respectively:

In the formula, 

a is the dimension of the long side of the insulating glass; b is the dimension of the short side of the insulating glass; the force is the thickness of the insulating glass directly bearing the panel; t₂ is the thickness of the insulating glass indirectly bearing the panel; h0 is the initial thickness of the hollow layer of the insulating glass; Po is the standard atmospheric pressure, taking The value is 1.013*10⁵Pa; E is the elastic modulus of the glass, which is Pa; v is the Poisson's ratio of the glass, which is 0.24; m and k are the coefficients, and the values are shown in Table 1.

Table 1 Values of m and k

In the field inspection, when the measured value of the center deflection of the insulating glass plate after loading is closer to the calculated value of formula (7), it means that the insulating layer of the insulating glass is well sealed, and the insulating glass structure has no potential safety hazard. When the measured value of the center deflection of the insulating glass plate after loading is closer to the calculated value of Equation (8), it means that the hollow layer of the insulating glass has permeability and air leakage, and the insulating glass structure has potential safety hazards. To save the above tedious calculation process, Jinan LIJIANG Glass has compiled automatic calculation software. As long as the test parameters and test results are input into the corresponding positions, the software can automatically judge whether there are hidden safety hazards in the insulating glass structure. The software interface is shown in Figure 2.

^{Figure 2 Software interface for automatic calculation of hidden safety hazards in insulating glass structures}

2. On-site detection steps for hidden safety hazards of insulating glass structures

2.1 Loading device

To achieve stable loading, it is recommended to use lever-removal code loading and decay for on-site testing, as shown in Figure 3. An elastic loading ball is arranged at the top of the adjustable length lever, and the contact radius between the loading ball and the glass after loading is 5~15 mm; a suction cup is arranged at the bottom end of the lever, and the suction cup is fixed on a rigid object with a smooth surface; One end of the code is applied to remove the code.If it is difficult to use the lever-removing code loading device on-site, it can also be loaded by hand. When loading, use the hand to push the loading head and record the loading force value (Figure 4).

^{Figure 3 Lever f to the code loading device}

^{Figure 4 Photos of the on-site loading and measurement of the thickness of the insulating glass layer by hand}

The thickness of the hollow layer of the insulating glass can be measured by a glass thickness gauge (the thickness of the glass and the hollow layer can be measured at the same time), and the detection accuracy is accurate at 0.1 mm.

2.2 Detection steps

1) Measurement of the size of the insulating glass and the initial thickness of the hollow layer

As shown in Figure 5, a measuring tool with an accuracy of 1 mm is used to measure the long and short sides of the rectangular insulating glass, and a measuring tool with an accuracy of 0.1 mm is used to measure the glass thickness of the insulating glass and the initial value of the hollow layer at the geometric center A of the insulating glass plate. Thickness and initial thickness of the hollow layer at B at 50~100 mm from the midpoint of the long edge of the insulating glass. In the figure, A is the center measuring point of the hollow glass plate, B is the measuring point of the long edge of the insulating glass.

2) Install the loading device

Install the concentrated load loading device as shown in Figure 3, align the loading ball with the center of the insulating glass directly bearing the panel, and use a tape measure to measure the distances of H, d, L, and S shown in the figure, accurate to 1.0 mm.

3) Apply the weight

The applied weight mass M should be within the following range:

M_min≤M≤M_max

The maximum allowable mass M_max and the minimum mass M_min of the applied weight is calculated according to formula (9) and formula (10)

Among them, M_max is the maximum allowable value of the loaded weight, kg; t is the glass thickness of the insulating glass bearing surface, mm; H is the vertical distance from the loading point to the lower end of the lever, mm; d is the horizontal distance from the loading point to the lower end of the lever, mm;L is the length of the lever, mm; S is the length from the hanging point of the weight to the lower end of the connecting rod, mm; b is the short side dimension of the insulating glass, mm; β  is the coefficient, see Table 2 for the value; g is the local acceleration of gravity, m/S² 

Among them, M_min is the minimum weight of the loaded weight (N)

Table 2 β  values

4) Concentrated load calculation

According to the moment equation, the concentrated load P applied to the insulating glass by the ball at the top of the lever after applying the weight is:

5) Thickness measurement of insulating glass hollow layer after loading

Measure the thickness h of the hollow layer at point A of the insulating glass after loading.

6) Result in judgment

According to the measured value of the thickness of the hollow layer, compare it with the calculated value of formulas (7) and (8), calculate the approximate value respectively, and then determine whether there is a safety hazard in the insulating glass.

3. On-site inspection cases

A glass curtain wall adopts a hidden frame structure, and the insulating glass is directly bonded to the aluminum profile attached frame with a silicone structural sealant. There is no supporting device under the insulating glass, and the self-weight load of the outer glass of the insulating glass is all borne by the two sealants. During the use process, a part of the insulating glass outer sheet suddenly fell as a whole without the action of external force. To understand the safety status of the remaining curtain wall glass in use, the method given in this paper is used to measure the height of the insulating glass and the thickness of the insulating glass of the curtain wall is measured by the insulating glass thickness meter. Determine the surface curvature of the insulating glass and the thickness of the initial spacer layer. By pushing the loading equipment manually, a concentrated load is applied to the center of the insulating glass plate, and the magnitude of the applied load and the thickness of the spacer layer before and after the center of the plate is loaded are recorded. A total of 12 curtain wall insulating glass permeability leaks were tested on-site, and the results are listed in Table 3. The test results show that there is no permeability and air leakage in the insulating glass sealing unit of the tested curtain wall.

Table 3 On-site detection results of air leakage of insulating glass

4. Conclusion

In the state of gas leakage in the hollow layer, the gas loses its carrying capacity, the external load is directly carried by the force-bearing surface of the insulating glass, and the other side of the glass is not subjected to the load. At this time, the overall carrying capacity of the insulating glass is significantly reduced. It is also easy to cause the outer sheet of the insulating glass to be broken or fall off as a whole.
The precursor of the detachment of the outer sheet of the insulating glass is the permeability and air leakage of the hollow layer, and the bearing deformation performance of the insulating glass changes obviously after the air leakage of the hollow layer fails. Therefore, this paper proposes to apply a concentrated load to the insulating glass and measure the thickness of the hollow layer in the center of the plate at the same time, compare it with the theoretically calculated failure and non-failure values, and then determine its failure criteria and then evaluate its potential safety hazards. The method is simple to operate and has been tested in the laboratory and in the field to demonstrate its accuracy and effectiveness.

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