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Discussion on the bonding design of the hollow glass structure of the hidden frame curtain wall

Abstract: The edge structure of the hollow glass is ∏ three-sided bonding. The stress and load effect are different from the conventional two-sided bonding (H-type). The bonding body has a small displacement under the same force, but when the structure is displaced The resulting stress is high, and there is a tearing force at the junction of the bonding interface. To design the bonding width according to the ultimate load-bearing capacity state, more factors must be considered comprehensively. Under a given bonding thickness, a structural adhesive with greater displacement capability should be selected.

For the insulating glass used for the hidden frame curtain wall of the building, the relevant international industry standards and regulations require that when the hollow glass used for the hidden frame curtain wall of the building is produced and processed, the second seal should be made of silicone structural sealant; for the appearance quality and performance, the relevant international industry standards The regulations should meet the requirements of limiting the second seal bonding width of 5mm to 7mm, and the thickness size can be 6mm/9mm/12mm/16mm/20mm (Figure 1). There is no requirement for the bearing capacity of the bonding body, and the structural bonding design is not It is easy to understand that it only requires the use of silicone structural adhesive.

This article discusses the structural characteristics, stress distribution, load transfer, and bonding design, and checking calculations of insulating glass, and draws the conclusion: insulating glass structural bonding body transfers load, and safety of the structure is the basic guarantee for the product sealing function life. Bonding size and the selection of structural rubber modulus should be designed and checked according to the state of ultimate bearing capacity. 

1. Bonding characteristics and stress distribution of insulating glass structure  

1) The bonding characteristics of the hollow glass structure of the hidden frame curtain wall   

There are two types of bonding body in structural bonding assembly (Figure 1, Figure 2):

Figure 1 and Figure 2 H type-double-sided bonding and ∏ type-three-sided bonding

Figure 1 and Figure 2 H type-double-sided bonding and ∏ type-three-sided bonding

H type—bonding on both sides. The bonding of the hollow glass unit and the two parallel surfaces of the metal frame structurally assemble the glass and the frame system together and form a waterproof seal;  

∏ type-three-sided bonding. Bond the inner and outer sheets of glass and the spacer into a whole. Since the spacers intersect the two glass surfaces perpendicularly to form a three-sided bonding, the junction of the bonding surfaces will be torn when deformed by force. This bonding form is generally not adopted for structural use, and the use of this form for insulating glass is to satisfy control The requirements for visual space, limit excessive displacement of a sealing body and prevent the spacer from "wandering".Once the H-type bonding body fails, it will cause the hollow glass unit to separate from the frame structure; when the ∏-type bonding body fails, the hollow glass will lose its function or even disintegrate, causing the outer glass to fall, while the ∏-type bonding structure will be stretched , Spacer bonding design must fully consider the product structure characteristics.  

1) Tensile properties of structural bonding

The tensile test results of different brands of silicone structural adhesives show that the bonding strength and maximum elongation of the ∏-type adhesive are much lower than that of the H-type adhesive. Doubled (Table 1), the edge of the bonded body showed early tearing. This phenomenon shows:  

The strain of the ∏-type adhesive body under the same stress is much smaller, and a sealing body is not prone to excessive displacement, which is beneficial to the edge sealing of the insulating glass;   

However, the stress of the ∏-type adhesive body is much greater than that of the H-type under the same strain, and the edge is easily torn, which has a significant impact on the bearing capacity of the adhesive structure.

Tensile properties of structural adhesive bonding under standard conditions
Silicone Structural Adhesive Number1234567891011Mean value
Stress at 10% stretch(MPa)H-
Maximum bonding strength(MPa)H0.990.921.301.031.271.300.981.251.090.970.971.10

Elongation at maximum strength(%)H8666766613754781421387310894

2) Stress distribution of structural bonding body  

According to the properties of typical structural adhesives, the finite element analysis of the stress of the bonded body when the tensile deformation is 10% under transverse load shows that the effects of the two bonded bodies are significantly different:  

Calculation model establishment

Constitutive model   

Figure 5 and Figure 6 Stress distribution of bonding body

Figure 5 and Figure 6 Stress distribution of bonding body

The structural bonding body is a viscoelastic body formed by curing silicone structural adhesive, with a Poisson ratio of 0.5, isotropic, and the expression of the strain energy function is determined by the nonlinear relationship described by the stress-strain test curve (omitted).  

Finite element model and determination of material parameters   

Bonded body (flexible body) tensile finite element analysis. The structure of H-type and ∏-type adhesive body is shown in Figure 2. The length of the bonded body is 50mm, the thickness is 12mm, and the size of the bonding surface is 12mm*50mm. The particles of the bonding interface cannot be moved. The finite element model of the bonded body after meshing is shown in the figure below.

Bond stress distribution  

Using structural adhesive products of the same brand, the bonding body is stretched by 10%, and the stress distribution of the bonding body is calculated as follows:   

H-type adhesive body-the stress concentration area is symmetrically distributed on the side, and the maximum stress is 0.21MPa as shown in Figure 5;  

∏ type bonding body-bonding body stress is not completely symmetrical distribution, concentrated on one side, the state is shown in Figure 6. 

The maximum stress of 0.44MPa is twice that of the H-type bonded body.

2. Insulating glass loading and loading transfer

In addition to the wind load, the loads acting on the insulating glass should be considered as permanent loads due to its own weight, shear forces due to thermal displacement, tension due to nonlinear deflection of the glass, bonding shear forces due to curing shrinkage, and structural component displacement. Forces, etc., should also consider factors causing changes in the physical properties of the bonded body during use. The main force and force transmission can be analyzed as follows.Lateral load-the negative wind pressure load acting on the outer sheet of insulating glass produces outward transverse pulling force, which is transferred to the inner sheet of glass through the ∏-shaped adhesive body, and then from the inner sheet of glass to the H adhesive body, and finally to the metal Frame system.   

Under negative wind pressure, the outer glass deflects outward, and part of the force is transferred to the inner glass through the air pressure inside the hollow glass, and the share of the transferred load is related to the thickness of the glass (Figure 7).

Figure 7 Wind load produces an outward pulling force on the outer sheet of glass

  Figure 7 Wind load produces an outward pulling force on the outer sheet of glass

The self-weight produces a tangential permanent load-under the condition of no self-weight support, the glass's self-weight produces a downward permanent shear load. The dead weight load of the outer glass is transferred to the inner glass through the ∏-shaped adhesive body, and then the total weight of the hollow glass unit is transferred to the H adhesive body by the inner glass, and finally transferred to the metal frame system.   

Thermal displacement produces tangential load——Insulating glass is a heat-insulating material. The change of ambient temperature causes temperature difference between inside and outside, and the thermal displacement between the glass produces tangential load acting on the ∏-shaped adhesive body. The tangential load depends on the size of the plate and the temperature change. The temperature difference between indoor and outdoor can have a peak value every day, and there may be several maximum values per year. The H-type bonded body in the room also produces tangential load due to the temperature difference displacement, but the indoor environment temperature does not change much, and the displacement is small.   

Gas expansion produces lateral load-hollow glass can be regarded as a closed container. When the ambient temperature rises, the internal gas expansion produces an outward lateral load on the glass and acts on the ∏-shaped adhesive body (Figure 8). Lateral load peaks with temperature changes every day, and there are several maximum values every year.  

3. Bonding design of insulating glass structure   

The limit state design principle is that the stress of structural members or connections should not exceed the material strength (including fatigue), and should not produce excessive deformation that is not suitable for continued bearing. The relevant international industry standards specification 5.6 requires that the bonding width of the structural adhesive bonding design should ensure that the stress of the structural adhesive under load is not greater than the structural adhesive strength design value f1 (0.2MPa), and the elongation strain (%) at a tensile stress of 0.14MPa is specified as the structure The displacement bearing capacity (δ) of the glue requires that the thickness dimension of the structural bonding body should ensure that no strain (%) greater than the δ value is generated when the structure is displaced, that is, the stress corresponding to the strain is not greater than 0.14 MPa (f1). According to this principle, the stress situation of the two-sided bonding (H-type) is standardized and analyzed, and multiple calculation formulas for the selection of bonding width, thickness and structural adhesive δ value are given. ∏-type adhesive body has different structural characteristics and stress conditions from H-type, so the applicability of these checking formulas should be considered.

1) Setting of bonding width under wind load

Insulating glass-type bonding body produces tensile stress and local tearing stress under the action of lateral load. The stress distribution and stress change are determined by the load size, brand structural adhesive material, spacer size and shape, environmental conditions, and other factors, which are variable. There are many factors and uncertain unknowns, and it is difficult to accurately determine the actual stress and stress distribution. Considering the factors of reducing the replacement frequency and maintenance cost of insulating glass during the service life of the building, the bonding design should ensure that there is no condensation, water seepage, and durable sealing. Considering the above factors, the design value of the structural adhesive strength should be appropriately reduced to alleviate the worries about ∏-type bonding and viscosity changes in the bonding design2.Considering factors such as the shape and size of the hollow glass unit, the load transfer effect of the non-linear deflection of the glass is related to the thickness of the inner and outer glass sheets (Figure 7), and the role of a sealant is not considered. The bonding width of the hollow glass structure (Cs, Mm) can be checked according to formula (1):

2) Setting of bonding width under permanent load   

A hidden frame curtain wall is not recommended to install hollow glass under the condition of no supporting device. If it must be installed in this way, sealant factories, glass manufacturers, and professional designers should evaluate the design details. The self-weight of the outer glass plate of the insulating glass installed in this way will be supported by the ∏-shaped structural bonding body, and the design value of the structural glue strength f2 should be limited to no more than 7kpa, to avoid the shear stress caused by the self-weight of the outer glass plate to make the two glass plates misaligned The displacement of a sealant will cause condensation and water seepage in the inner space, or a smaller f2 value (such as 3.45kpa) should be used 2,6. Regardless of the contribution of a sealant, the bonding width Cs of the ∏-type bonded body can be calculated as follows:

Large; to reduce the safety risk, f2 should be set to 345kPa, and the bonding width calculation result is 14.8mm.

The results show that the flexibility (δ) of the selected structural adhesive should not be less than 10%, that is, the modulus of strain 10% should not be greater than 0.14 MPa. However, this calculation result may not be sufficient to meet the bonding requirements of the insulating glass structure, because the stress of the ∏-type bonding structure under the same displacement is far greater than 0.14MPa (see Table 1), and the changeability δ value (compliance) must be selected. To ensure that the stress during displacement is not greater than the design value of strength, the above calculation is only to meet the minimum requirements for the selection of materials for insulating glass structures. 2•. In addition, other displacement factors may also be considered for the insulating glass structure adhesive body, such as the displacement caused by the deflection of the inner and outer panels of the insulating glass caused by gas expansion at high temperature (Figure 8). The magnitude is related to the temperature rise and the panel size.   

Compliance (modulus) characterizes the mechanical properties of silicone structural adhesives. Depending on the formulation and preparation process, the compliance (modulus) test value of the European standard product compliance requirement should not exceed 75% of the average6. GB 16776 mandates the stress-strain relationship (stress at 10%, 20%, 40% elongation) of structural adhesive product inspection reports to characterize its δ value. China Curtain Wall.com publishes and compares the stress-strain characteristics of more than 20 domestic products, The compliance (δ) range of brand products is about 3% to 11%, which can be selected for structural bonding design. It can be seen that the value of δ is not arbitrary and must correspond to a certain brand of glue. For example, the value of δ can be randomly selected for bonding calculation = 40%, although the bonding thickness can be greatly reduced, there is no structural adhesive that meets the requirements, and no adhesive is available. Curtain wall engineering requires structural adhesives with different high, medium, and low modulus (flexibility) to meet different engineering design requirements. Therefore, product standards specifications require product reporting. 

Some people think that only high-modulus structural adhesives can guarantee the airtightness of insulating glass, and it is required that the tensile modulus of 10% must be higher than 0.14MPa. This misunderstanding may cause some rigid structural adhesives to enter the construction project, resulting in a high stress in the bonding structure. State, induce early failure of bonding 3. Therefore, the choice of sealant modulus (compliance) is very important.  

4. Conclusion

The occasional accident of the outer sheet falling due to the failure of the hollow glass structure bonding may be related to the poor consideration of the structural bonding design and material selection. The structural bonding characteristics and the analysis of the influence of related factors should be conducted through experiments to improve the rationality of the structural bonding design sex. This article puts forward the discussion opinions on the structural bonding design method for the industry's reference. I sincerely ask my colleagues for corrections. 

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