Analysis of Air Pressure Load Effect and Structural Bonding Design of Insulating Glass

Abstract: Edge bonding is the only connection of the hollow glass structure. The bending deformation of the glass under the action of wind pressure and internal air pressure will produce non-negligible shear and tensile stress on the edge bonding, and even cause the glass to burst and cause accidents or excessive early degumming and disintegration greatly reduce its thermal insulation function. To ensure the safety of construction projects and prolong the functional life of insulating glass in building energy-saving and emission reduction, this article focuses on the analysis of the air pressure load effect and the correlation with the insulating glass edge bonding size, sealant modulus, and other parameters, and its bearing safety. 

At present, the global annual output of insulating glass has exceeded billions of square meters. It is mainly used for building doors, windows and curtain walls, and plays an important role in improving building thermal resistance and reducing energy consumption. Insulating glass is a unit component composed of two pieces of glass spaced apart and assembled by peripheral bonding and sealing. It is composed of several materials (as shown in Figure 1). 

^{The schematic diagram of insulating glass structure and deflection 1}

When used in construction, the glass specification, sealant brand, and bonding width The thickness and thickness are all given by the architect of the building structure. Edge bonding is the only structural connection of the unit component. Under the action of building wind pressure and air pressure, the flexural deformation of the glass causes the connection to produce a complex force effect. Premature condensation, water seepage, edge opening, and even outer glass falling accidents of building insulating glass are mostly related to insufficient edge bonding durability and bearing capacity. The structural design has fully considered the wind pressure load. This article focuses on the analysis of the pressure load effect and the correlation with the edge bonding size of the insulating glass and the sealant modulus, and proposes suggestions for its bearing safety issues.

1. Gas pressure effect in hollow glass cavity　　

1) The gas state of the hollow glass cavity　　 

The pressure difference between the inside and outside of the sealed cavity causes the hollow glass to flex and deform (Figure 1). The gas in the hollow glass is mainly air (or argon), and its properties are close to ideal gas. It is well known that the volume, pressure, and temperature of gas follow the ideal gas state equation (Claperon equation):

PV = nRT　　

(1) In the formula: p——gas pressure, kPa　　

V——gas volume, m3　　

T——system temperature, K　　

N——the amount of gaseous substance, in mol　　

R——Ideal gas constant 　　

The molar mass n of the gas enclosed in the cavity by the edge sealant is a constant value. The initial state of the gas is determined by the bonding seal assembly temperature T0, atmospheric pressure P0 and the initial volume V0. After the state changes, the linear relationship follows the equation (2):

2) Air pressure difference and voltage stabilization measures caused by high altitude
When insulating glass produced in coastal areas is used in high-altitude cities, because the gas pressure in the unit is higher than the local atmospheric pressure, the glass will flex and deform, or even burst during transportation and installation. The pressure depends on the altitude. 1 Take cities and regions in the United States as an example. List the air pressure difference of typical cities. For example, Colorado is 4.08kpa, Utah is 11.21kpa, and the highest point of the Rocky Mountains is as high as 34.77kpa. In order to balance the destructive effect of the air pressure difference between the inside and outside of the hollow glass, the hollow glass of the doors and windows of railway vehicles running across the interval is often equipped with a device (such as an airbag) to balance the air pressure. The voltage stabilization measures in the building door, window and curtain wall industry are relatively simple. When the insulating glass produced in coastal cities such as New York, Boston, Washington, Miami and other coastal cities is used in high-altitude urban buildings, companies often require the unit to be installed with stainless steel breathing tubes (inner diameter 1.6mm). In order to maintain the internal and external air pressure balance, until the building is installed before bending, this voltage stabilization measure has been well known in the industry.

3) Air pressure difference caused by high temperature and voltage stabilization measures

The hollow glass installed in the building has visible optical distortion, especially on the hidden frame curtain wall. This phenomenon is related to the relative deflection of the panel caused by the pressure (decompression) of the gas in the hollow glass caused by the temperature change. When the gas temperature Tt, (k) in the insulating glass is higher than the packaging temperature T0, (k), if the gas volume (V) remains unchanged, the ambient atmospheric pressure is p0 (such as 100kpa), the pressure at which the glass will bend The difference ΔPt can be checked by the following formula (3): 

^{The temperature and deflection of insulating glass 1}

(3) In the formula: pt——gas pressure at Tt temperature (kpa).　　

Prevent the hollow glass from flexing and deforming due to gas expansion and pressurization at high temperature. The known measure is to set up a breathing tube connected to the atmosphere to exhale part of the gas at high temperature and inhale gas at low temperature. The American Morton Company conducted the corresponding condition test. The test piece specification is (6+12+6), and the size is 356mm×508mm hollow glass. The deflection of the test piece at a temperature of 50℃～80℃ is shown in Table 2, Figure 2. The test pieces were bonded with two kinds of sealants, 5#～6# were not filled with desiccant, and 1# and 3# had breathing tubes (stainless steel pipe diameter 0.5mm, length 300mm). The test results show that the specimen without snorkel exhibits flexural deformation, the maximum deflection is 0.17mm; the deflection of the specimens bonded by different sealants is different; the deflection of the specimen with desiccant is slightly greater than that of the specimen without desiccant. It can be seen that the breathing tube can effectively balance the air pressure.

^{The condition of insulating glass specimen 1}

2. The effect of breathing tube on the functional life of insulating glass　　 

Breathing tube can reduce the air pressure generated by the air in the hollow glass during altitude changes, but the air pressure caused by the temperature fluctuation of the enclosed gas cannot be avoided after bending. The breathing tube connected to the atmosphere can balance the air pressure in the insulating glass with the ambient air pressure, but the breathing tube becomes a channel for breathing atmospheric moisture, accelerating the desiccant saturation and moisture absorption speed, and affecting the durable insulation life of the insulating glass. Morton Company proposes life expectancy model 1: 　　

(4) In the formula: L——the life of the hollow glass with breathing tube, a　　

E——the life of hollow glass without breathing tube, a　　

C——Climate coefficient (mg/a.m³) )　　

D——The saturated moisture absorption capacity of the desiccant in the compartment (mg/cm)　　

P——The length of the space frame filled with desiccant (cm)　　

V——gas volume in hollow glass (cm³)

Take the Los Angeles area (C=0.86 mg/a.m3) as an example. The insulating glass for buildings has a lifespan of 20 years (size 600mm×1200mm×19mm), and the predicted lifespan after the breathing tube is installed is only 13.34 years. The main influencing factors are the gas volume and climate coefficient in the hollow glass. The climate coefficient is related to geographic location, glass orientation, air humidity, and daily temperature difference. The data shows that the climate coefficient of shaded/sunny cities in the United States is the largest in the southern Miami and Brownsville areas, which are 1.67/2.27 and 1.76/2.45 (respectively). mg/a.m3).

3. Insulating glass edge bonding design and sealant selection analysis　　 

The building environment temperature has a peak every day and a maximum value every year, and the air pressure in the hollow glass will change cyclically accordingly. If the initial volume (V0) remains unchanged, the internal and external air pressure difference (ΔPt) at Tt temperature can be calculated according to formula (3). The pressure acts on the glass, and the edge bond produces tensile stress (F) and elongation (ε, %), will change the thickness of the hollow glass cavity to expand the volume proportionally (ΔV, %), and decrease the gas pressure proportionally (ΔPv, %) (Figure 3). The initial deformation stage (within 25%) of the sealant with Poisson's ratio 0.5 can be regarded as an elastic body, and the tensile stress (f), elongation strain (ε, %) and the initial elastic modulus (E0) have a linear relationship, then:

ΔPv=ΔV=ε=f / E0 (%)　　

(5) The pressure difference between the gas in the hollow glass and the atmosphere after the sealant is stretched can be calculated as follows:

(6) If sufficient bonding width is set to ensure that the stress of the edge sealant at the Tt temperature is the strength design value f1s, the insulating glass bonding width Cs should conform to the following formula according to the ultimate bearing capacity: 

(7) In the formula: a——the short side size of rectangular insulating glass, mm 

^{The schematic diagram of volume change of insulating glass cavity 1}

Example: Insulating glass sealed and bonded in coastal cities (P0=100kpa), the size is 1000mm×1500mm, the interval thickness is 12mm, and the packaging temperature is 15°C. During the use of the local building, the gas temperature in the cavity can reach 50°C. If the sealant is complete Rigidity, keeping the volume of the inner cavity unchanged (ΔV≈0), according to formula (3), the pressure difference between the inside and outside of the cavity ΔPt can reach 12.2kpa, which is much larger than the wind load of the known building: 

If there are three grades of sealants to choose from, the elastic modulus E0 is 7Mpa (10% tensile stress 0.70Mpa), 2.3Mpa (10% tensile stress 0.23Mpa), 1.4Mpa (10% tensile stress 0.14 Mpa), the strength design value of the sealant f1s=0.14Mpa, respectively check the volume change rate of the hollow glass inner space ΔV, the air pressure difference with the environment ΔPv, and the minimum limit of the insulating glass elastic sealant bonding width Cs when the gas temperature is 50℃ , The verification results are listed in Table 3.

The results of the example verification show that the air pressure inside the insulating glass increases with the increase of the elastic modulus of the sealant. In order to ensure sufficient bonding bearing capacity, the bonding width must be increased; low modulus can reduce the internal air pressure and reduce the bonding width , Reduce the amount of glue used for sealing. The calculation conditions of the example have not considered the fatigue effect of cyclic load, nor the combined effect of the instantaneous variable wind load of the building.

4. Frameless support insulating glass bonding width setting and material selection

The hollow glass supported by the frameless building is shown in Figure 4. In addition to the internal air pressure, the negative wind pressure pulls the glass panel to the edge sealant. The setting of the joint width and the selection of the applicable modulus of the silicone sealant have become an important part of the bonding design.

1) Standard value and design value of bonding strength of edge sealant

According to the application experience, the European architectural glass structure bonding system certification specification stipulates that the edge bonding tensile strength standard value is statistically selected according to the test results. This value is considered the most reliable value. The value method is consistent with the GB 50068 building structure reliability design The statistical method stipulated by the standard is consistent, that is, based on the test data, the probability of 95% tensile bond strength is taken as the strength standard value formula (8) with 75% confidence, and the probability of the product greater than this value is 95%. This value method is conducive to the performance characterization and application of different products.
σR,5 =σX,23℃ －ταβ. S　　

(8) In the formula: ΣX, 23℃ ——23℃ bond tensile test strength average value;

S ——The statistical standard deviation of the test results of n specimens:

Ταβ ——75% confidence level 5% deviation factor, the value depends on the number of test pieces n, such as:

N=5,ταβ=2.46 σR,5=σX －2.46×S

N=10, ταβ =2.10 σR,5=σX －2.10×S

^{The frameless support of insulating glass 1}

The design value of the bonding tensile strength of different brand sealants, the European architectural glass structure bonding system certification code (CE) is checked and set according to formula (9): f1s ≤σR,5 /6　　

(9) Compared with the empirical value (140 kpa) adopted in the United States and Japan, this value method is conducive to the reasonable selection of sealant for bonding design.

2) Design and calculation of insulating glass bonding width Cs　　

A) Bonding width under instantaneous wind pressureThe stress distribution of rectangular hollow glass under the action of instantaneous wind load (W) is similar to trapezoid. The edge bond bearing capacity of the single element is checked according to the ultimate bearing capacity state, which should conform to formula (10), and the result of checking calculation should be compared with formula (7), and the reliable value is taken : 　　

^{Design Checking Calculation of Insulating Glass Bonding Width 1}

Cs≥ 6 mm　　

(10) In the formula: β——partial coefficient of wind load　　 

When d1≦ d2, β≈1/2, then β≡1/2　　 

When d1> d2, β> 1/2, then β=1　　

B) Setting of the bonding width of the edge of the unit without self-weight support　　 

When the bottom is not supported by its own weight, the edge bonding body will bear the permanent load of the outer glass, and its width Cs should conform to the following formula (11): 

^{The setting of the bonding width of the edge of the unit without self-weight support 1}

(11) In the formula: Wg——the weight of the outer glass;

Fws——The design value of the adhesive shear strength of the sealant under permanent load (take the test value without creep under permanent load);

H ——The width or length of the outer glass.

C) Selection of sealant suitable for elastic modulus　　

Considering the cyclical change of the ambient temperature, the internal air pressure of the hollow glass is cyclically acting on the glass, which comprehensively acts on the cyclic tensile stress effect of the sealant. From equation (7), the initial elastic modulus of the sealant applicable to the check modulus can be derived:

^{The choice of sealant suitable for modulus of elasticity 1}

5. Edge bonding structure form　　 

The analysis of the effect of air pressure on the edge bonding in this article is based on two-sided bonding, while the current hollow glass edge structure is mainly three-sided bonding (Figure 1). Studies have shown that the deformation of three-sided bonding is smaller than that of two-sided bonding. The deformation of the bonded body is restricted by the third side bonding (to the spacer frame) and changes its stress distribution. Therefore, an effective way to control the structural stress level is to use a more flexible seal Glue and not the other way around.

ASTM C1249 "Standard Guide for the Application of Secondary Sealing of Insulating Glass Bonded by Structural Sealant" requires that "The edge bonding and sealing design of hollow glass unit is similar to the design method of unit and frame system bonding. It is a bonding system that meets the ASTM C1184 standard. The minimum requirements for structural sealants", "During the life of the building, the second sealing body of the hollow glass unit must be flexible enough to adapt to all the loads imposed on the hollow glass unit", "the hollow glass unit should be The stress of the sealant is limited to 140kpa." Insulating glass structural sealant product standard (ASTM C1369) stipulates in Importance and Application, "This standard does not specify the modulus of the sealant, because the applicable modulus is a function of the sealant being used in a specific insulating glass system (ie The requirements for the modulus of the glass bonding system vary with the specifications of the hollow spacer, the size of the glass, the installation state and the bonding structure and other conditions). Therefore, the product standard should not specify the limit index of the modulus, but should be required to report the initial stress-strain curve and the modulus value to meet the structural design requirements for the selection of bonding materials.

The bonding width set by the bonding design should have sufficient size, the modulus of the sealant should not be too low, and it should be ensured that the displacement of the glass under load does not exceed the bottom support. A seal is a plastic viscous fluid, and the continuity of the sealing layer can be better maintained when the glass is relatively displaced. In order to prevent excessive displacement from destroying its continuity, follow-up spacers, flexible spacers containing desiccant, etc. can be used at the edges. 

Figure 5 Edge structure

^{The U-shaped spacer frame displacement follower 1}

6. Suggestions　　 

The 15-year life span of insulating glass products is proposed in the revision of GB/T11944, but the verification test shows that the compliance rate of the samples submitted for inspection is still less than 70%, and the life span of the products in actual construction use may be shorter. Reliable bonding of the edges of insulating glass is an important guarantee for its functional life. European door and window curtain wall specifications set comprehensive technical requirements for insulating glass and edge bonding materials based on a life span of 25 years. Digesting and absorbing foreign experiences is beneficial to improve edge bonding. Durability and reliability. If the life of insulating glass can be prolonged by one year, its benefit is at least equivalent to the output value of the annual installation of insulating glass in buildings in the country; it is recommended that the engineering technical specifications fully consider the internal pressure load effect of insulating glass and bond the edges Design requirements; it is recommended that the product standard of insulating glass sealant for doors and windows and curtain walls improve the index of moisture permeability resistance, and specify the necessary elastic modulus, strength standard value, design value and other technical indicators and consistency requirements for structural design to ensure coordination with building codes, To promote the technical progress of sealant and the continuous stability of quality.