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Analysis and suggestion on the safety of glass curtain wall

  The glass curtain wall has become the best choice for the outer envelope of modern buildings. The glass curtain wall adds decorativeness on the basis of the exterior skin lighting, heat insulation, wind shielding and rainproof functions, and integrates the architectural function and architectural aesthetics into In one, it becomes a beautiful scenery of a city.

  In recent years, with the rapid development and widespread application of architectural curtain walls, accidents such as safety and quality have also occurred, which has had some negative impacts on society. This article is based on the author's decades of construction design experience, some views on the safety of the curtain wall, and discuss with you.

  一、The safety of glass curtain wall

  In recent years, security incidents on the glass curtain wall have occurred continuously. The glass on the curtain wall shattered and hurt people, causing people to regard the glass curtain wall as an "air bomb". The industry and society have caused great repercussions. Is the glass curtain wall really unsafe? What caused the glass to fall and fall?

  The curtain wall safety incident that people usually understand is the glass falling, and the curtain wall glass falling is just the appearance. In addition to the fall caused by the self-explosion of tempered glass, there are many factors that can cause the curtain wall glass to break, such as the processing quality of glass, boundary conditions, curtain wall structure, etc., and the curtain wall structure and wind pressure resistance are extremely important factors.

  1、 Processing quality of glass

  Glass is a brittle material. One of the reasons why glass is widely used in building exterior windows is that it has high compressive strength and high hardness. The theoretical strength of glass is about 1010~1.5×1010Pa, but the actual strength is less than 108Pa, which is much worse than the theoretical strength, and the tensile strength is also smaller than the compressive strength.

  It can be seen from Table-1 that the actual strength of the bulk glass is much lower than the theoretical strength, which is 2 to 3 orders of magnitude different from the theoretical strength. The reason why the actual strength of bulk glass is so low is due to the brittleness of the glass and the existence of micro-cracks and internal uneven regions and defects in the glass, which cause stress concentration. In order to overcome the effect of surface micro-cracks and improve the strength of glass, architectural glass is usually quenched (physical tempering) or surface ion exchange (chemical tempering) to obtain a compressive stress layer to strengthen the surface and increase its strength.

  The glass will have residual stress during the production process, especially the unevenly distributed residual stress, which greatly reduces the strength. After the glass is tempered, it produces uniform compressive stress on the surface and uniform tensile stress inside, which greatly improves the mechanical strength of the glass. The tempered glass is 5 to 10 times more resistant to mechanical shock and thermal shock than well annealed glass.

  2、Design defects of curtain wall structure

  The glass curtain wall is composed of panels and metal frames, and its deformability is relatively small. When the structure moves laterally under the action of horizontal earthquake or wind load, the glass curtain wall of the curtain wall component cannot withstand excessive displacement by the panel and the metal frame, and the impact of the excessive lateral movement of the main structure can only be avoided by the elasticity of the connecting member Due to the difference in the deformability between the curtain wall glass and the rod, a certain compressive stress will inevitably be generated on the glass. The glass curtain wall has strict regulations on the structural design of the glass tank. There must be a certain gap between the installation of the glass and the horizontal frame to ensure that the glass will not be squeezed due to squeezing when the interlayer displacement occurs broken.

  3、Curtain wall structure design

  The rigidity of the curtain wall rods is the key to the safety of the curtain wall structure. For vertical glass curtain walls, the wind load is the main effect. The seismic effect is relatively small compared to the wind load effect, usually not exceeding 20% of the wind load effect. Is the main consideration.

  The anti-wind pressure performance of the glass curtain wall is a whole concept, which is determined by the strength and rigidity of the glass panel and the rod. The glass curtain wall mainly bears the wind load, and then passes through the glass to the curtain wall rod. Curtain wall glass and rods are bound to deform under wind load. In the deformation design of curtain wall glass, the theoretical calculation of stress is applied to the four-sided support glass panel with thin elastic deformation thin plate, but glass is a typical brittle material. In order to correct the reduction factor, it still cannot fully and accurately reflect the actual stress and deformation of the glass panel.

  The deflection of curtain wall glass is also related to the boundary conditions of glass installation. The curtain wall glass is installed in the frame slot, the curtain wall frames are assembled together by connecting pieces, the curtain wall vertical frame is a unidirectional bending rod member, and the deflection deformation occurs only in the horizontal wind load direction, and the curtain wall horizontal frame is bidirectional bending The rods are flexed and deformed in the direction of gravity and horizontal wind load. After the vertical frame and the horizontal frame are deformed, the edges of the glass panel will not be in the same plane, but in a twisted state, especially in the horizontal frame and the When the connection of the vertical frame adopts a connection method with a larger degree of freedom (such as a steel pin connection), the relative distortion of the horizontal vertical frame will be greater, and under extreme weather conditions, the glass panel will be broken due to uneven stress.

  二、Safety awareness must be raised

  As a building envelope, the glass curtain wall should abide by three basic principles: safety, applicability and economy. Safety is paramount, and an appropriate and reasonable structure is a must. The limit state of the component includes not only the limit state of the bearing capacity but also the limit state of deflection (deformation), which already contains some concepts of safety and applicability.

  The structural design method has gone through the development process from allowable stress method, failure stage design method, limit state design method to probabilistic limit state design method. The design of the building curtain wall is derived from the design of the steel structure, which is also based on the limit state design method of approximate probability theory. The structure that meets the functional requirements is designed, that is, the effect of the external effect on the structure is compared with the resistance of the structure itself to achieve the structure. The design is both safe and economical. Specifically, it is to determine the cross-sectional size of the structural member according to the functional requirements and meet the structural requirements under the predetermined role and material performance conditions. However, the limit state design does not mean that the design of the bearing capacity of curtain wall members tends to the limit value.

  Safety means that the curtain wall rods must be in a safe bearing capacity and stability, not in the limit state. The limit state is the critical state where the bar is in the failure state. If the ultimate bearing capacity state is adopted in the design, the curtain wall member will be placed on the dangerous edge. Once there are short-term extreme climatic conditions, excessive wind force will lead to the instability of the rods, distortion and deformation resulting in damage to the curtain wall, which may cause personal injury and death and a lot of property damage.

  Applicability means that the component has good working performance under normal use conditions. For the curtain wall, it should also include the matching between the curtain wall structure and the main structure. Today's curtain wall design is tailor-made according to specific building structures. Some good curtain wall design projects are suitable for their own structures, and may not be applicable to other building structures. They should be analyzed and considered comprehensively based on actual conditions.

  The reliability of the structure is the general term for the safety, applicability and durability of the structure. We must consider these three factors in the design of the curtain wall. We advocate bold innovation, and only innovation can lead to development. But innovation does not mean brutality. As an old expert in the industry said: The innovation of architectural design must be completed by logical structural design, and boldness should not be arbitrarily carried out; it can be scattered over a large area without being able to withstand the natural storm, which is definitely not equivalent to accidental broken and scattered; Careless design and construction in front of safety will inevitably lead to disasters that are difficult to manage.

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