abstract

As a novel type of stone curtain wall, the open back bolted dry hanging stone curtain wall is increasingly used in curtain wall engineering due to its unique structural technology, and has also been recognized by professionals in the construction industry.
With the development of the construction industry, building decoration materials can be described as a hundred flowers blooming and competing for excellence. Stone, with its unique advantages such as naturalness, thickness, and luxury, has given rise to an increasing number of buildings that are dignified, luxurious, pure, natural, or magnificent, all of which create a dignified atmosphere that shakes people’s hearts; Colorful colors, abstractly embodying the infinite changes of nature, can be subtle or romantic. Therefore, stone is widely used in building exterior walls, indoor and outdoor floors, walls, etc.
The stone used in the curtain wall is mainly natural granite. Natural granite is Igneous rock mainly composed of Silicate mineral. Because of its high formation temperature, tight combination of various mineral crystals, hard texture, acid, alkali, salt and other corrosion resistance, and good chemical properties.

Therefore, the application of granite in exterior walls is becoming increasingly common. Due to the inherent defects of wet bonding method (also known as grouting method), dry hanging granite curtain walls have been widely accepted by owners and architects.
In recent years, with the development of curtain wall technology and the improvement of curtain wall technical specifications, the dry hanging stone technology has become increasingly perfect.

There are currently several typical structures:
Steel pin structure, semi circular groove structure, full length groove structure, small unit structure, and back bolt structure.
This article focuses on the open back bolt dry hanging stone curtain wall structure.
Open back bolted dry hanging stone curtain walls are widely used in curtain wall engineering due to their unique characteristics.

Structural principle of open back bolt dry hanging stone curtain wall

The back bolt dry hanging method is to use specialized drilling equipment to drill holes on the back of the stone panel, then install stress free bolts to fix them on the back of the stone, and then connect the bolts to the aluminum alloy pendant.
Dry hang the stone on the curtain wall skeleton through the connection between the aluminum alloy pendant and the skeleton (see Figure 1 and Figure 2). Each stone plate forms its own connection system, and adjacent plates do not transmit load effects.

Figure 1. Vertical Section of Curtain Wall Structure

Figure 2. Cross sectional view of curtain wall structure

The open system originates from the principle of “ventilation and convection”. The open back bolt dry hanging stone curtain wall structure refers to the use of a non adhesive process at the joints of the back bolt dry hanging stone plate. By opening the joints, the air layer behind the stone curtain wall can flow smoothly, which can achieve good insulation and sound absorption effects, and can evaporate condensed water during the air layer circulation process.

B. Performance characteristics of open back bolt dry hanging stone curtain wall

1. There is no gluing operation on the construction site

Traditional curtain walls are often sealed with weather resistant sealants, but during the process of gluing, the properties of the adhesive are greatly affected by surrounding temperature, humidity, and worker skills, making it difficult to ensure construction quality and easily contaminating the stone.
The open back bolt dry hanging stone curtain wall, due to its bolt connection and open structure, does not require gluing operations on the curtain wall. The connection between stone and bolts can be completed in factories with good conditions, ensuring the construction quality of the entire curtain wall. This is also in line with the current overall development trend of the curtain wall that there is no gluing operation on the construction site.

2. Thermal insulation and energy conservation

The open curtain wall structure utilizes the air convection on its back to reduce indoor and outdoor heat exchange, and its inner insulation board further hinders heat penetration, achieving a significant insulation effect. In summer, building cooling costs can be reduced, while in winter, heating costs can be greatly reduced.

3. Seismic performance

The back bolt structure belongs to the third generation product of stone dry hanging technology, and is currently a relatively advanced technology in the world. Its characteristics are to achieve stress free processing of stone, high connection strength of stone, and saving about 30% of the strength value.
Backbolt structure is the independent decomposition of stone panels, with each panel forming its own independent connection system. Adjacent panels do not transmit load, and the plates and skeleton are still designed as movable connections to ensure sufficient displacement and deformation space for the panels. Therefore, theoretically, backbolt structure has better displacement, deformation and seismic performance than other structures.

4. Easy to maintain

During maintenance, simply move the upper panel up by about 5mm, and then lift the damaged panel up by more than 10mm to remove it. The same method is also used for installation.
This can facilitate the replacement of stone materials, thus maintaining the integrity of the curtain wall for a long time. Generally, each section of the curtain wall structure is connected through adhesive, making it difficult to disassemble independently and avoiding the problem of adhesive interface processing.

5. Open style, beautiful and elegant

The curtain wall structure adopts an open structure, and the stone board joints are not sealed with sealant to make them visually deep, with a strong sense of assembly and three-dimensional effect. The exterior decoration effect is good, and due to the absence of sealant, there is no glue oil that can penetrate and corrode the stone and adsorb dust, keeping the surface of the curtain wall clean for a long time and improving the exterior decoration effect.

C. Key points for technical control of open back bolt dry hanging stone curtain wall

1. External wall waterproofing

In external curtain wall engineering, the treatment of rainwater leakage is very important. The key to the waterproofing of the open back bolted dry hanging stone curtain wall structure is the facing waterproofing. In engineering, the method of combining anti drainage and organizing multiple drainage systems is often used.
The open back bolt dry hanging stone curtain wall cannot be used as the outer protective structure of a building. It needs to be made separately for the exterior wall. The technology of waterproofing on ordinary masonry or board exterior wall materials has long been mature, and the outer dry hanging stone surface layer can block direct sunlight, avoid direct damage from debris, and avoid direct erosion by rain and snow.
In this way, there is even more room for choosing waterproof materials. You can choose either roll type waterproof materials or film type waterproof materials, depending on the appearance, construction conditions, and cost requirements of the engineering building.
After the waterproofing of the base layer is completed, after the installation of the stone keel is completed, water retaining slopes should be set every 4m to 8m from the top and bottom of the outer wall. The water retaining slopes should be set on the curtain wall framework or the concrete structure of the outer wall, which is beneficial for waterproofing. The specific method can be to bend a 1.2mm thick alumina plate into a slope shape, with one end riveted to the concrete structure of the outer wall and the other end led to the joint of the stone plate, Seal the aluminum plate with weather resistant adhesive between the wall and the aluminum plate.
In this way, even if there is rainwater entering the interior of the stone curtain wall, it can be discharged to the outside of the stone curtain wall through a sloping aluminum plate.

2. Stone installation

The quality of stone installation directly affects the overall facade effect of the curtain wall. At the same time, the existence of natural stone color difference issues will also have an impact on the external appearance of the curtain wall. Therefore, controlling the quality of stone installation and controlling stone color difference is also the focus of our control.
Firstly, select the color of the stone and pre arrange and number it for use during formal installation. If the Color difference is large, it shall be installed at the secondary position with the consent of the supervision unit, design unit and construction unit, so that even if there is some color difference, the decorative effect of the entire curtain wall facade will not be affected.
Fix the aluminum pendant on the anchor bolt behind the stone according to the design requirements. The installation of stone should be carried out from bottom to top, and the horizontal line should be pulled through according to the elevation of the stone horizontal joint to control the levelness of the stone curtain wall.
Place the nuts of the aluminum alloy pendant in place, check for flatness, verticality, seam width, etc. If the requirements are not met, adjust the aluminum alloy pendant and nuts until they meet the requirements. The specific control requirements are shown in Table 1:

                                                                  Table 1. Allowable Deviation for Installation of Stone Curtain Wall

After the installation of the stone material is completed and passes the “self inspection, mutual inspection, and special inspection”, the gaps can be cleaned and trimmed, mainly to remove residual debris between the gaps and try to control the gaps as uniformly as possible.

4. Summary

As a novel type of stone curtain wall, the open back bolt dry hanging stone curtain wall structure technology is one of the ideal technologies to solve the problems of early traditional stone curtain walls.
It is a brand new structural technology in stone installation, with advantages such as three-dimensional adjustment, convenient disassembly and assembly, easy maintenance, good seismic performance, and good appearance effect.
However, curtain wall structures are constantly changing and there are many types. An ideal curtain wall structure should be flexibly selected based on the specific characteristics of the project and should not be mechanically copied.