Aluminum curtain walls, widely used in modern building envelopes, are inherently sensitive to temperature changes due to their material properties. Aluminum alloys, being non-ferrous metals, have a significant coefficient of linear expansion, meaning they expand along their length as temperature rises and contract as temperature falls. If this thermal expansion and contraction is not properly controlled, it can lead to curtain wall structural deformation, seal failure, and even safety hazards. The installation of expansion joints is a key design feature to address this issue.
The core function of expansion joints is to provide deformable space for the aluminum curtain wall to release internal stresses caused by temperature changes. When ambient temperature rises, the components of the aluminum curtain wall expand. Without expansion joints, the expansion force concentrates at the fixed connection points, causing curtain wall panel distortion, frame deformation, or sealant cracking. Conversely, when temperature drops, contraction force can cause joints to loosen, compromising overall sealability. Expansion joints, by providing pre-existing gaps, allow adjacent components to expand and contract freely within a certain range, thus avoiding stress concentration and maintaining structural stability.
From a materials mechanics perspective, aluminum alloy has a much higher coefficient of thermal expansion than commonly used curtain wall materials such as glass and stone. Without expansion joints, the difference in expansion between the different materials will further exacerbate local stress. For example, in the high temperatures of summer, the expansion of the aluminum alloy frame may exceed that of the glass panel; if the two are rigidly connected, the glass edges may crack due to compression. The introduction of expansion joints separates the deformation paths of different materials, allowing each component to respond independently to temperature changes, significantly reducing the risk of damage caused by material differences.
The design of expansion joints must comprehensively consider the temperature variation range, curtain wall dimensions, and structural form. In extreme climates, such as desert regions with large diurnal temperature variations or cold northern winters, curtain walls need to withstand greater temperature fluctuations, requiring correspondingly larger expansion joint widths. Furthermore, in large curtain wall projects, panels with larger length and width dimensions are more prone to significant displacement due to thermal expansion and contraction. In such cases, it is necessary to decompose the overall deformation into multiple smaller displacements by segmenting expansion joints or using sliding connectors, reducing the impact of a single displacement on the structure. For example, unitized aluminum curtain walls use interlocking upper and lower units and interlocking left and right vertical frames, reserving expansion space between the horizontal and vertical frames to ensure that each unit can move independently when temperatures change.
The structural details of expansion joints directly affect their functional effectiveness. The gap width must be determined according to design specifications, meeting deformation requirements while avoiding excessive size that would affect aesthetics. Specialized inserts, such as sleeve connections, are typically used at the gaps. The insert length must extend a certain distance into both ends of the vertical frame and be fixed with self-tapping screws or rivets to ensure connection strength. The gap is filled with aging-resistant silicone sealant, which prevents moisture intrusion and corrosion of the aluminum alloy while maintaining the gap's elastic deformation capacity. The sealant application process is particularly critical, requiring two applications: the first fills to half the gap depth, curing to form an elastic base layer; the second application covers the surface to ensure complete sealing.
Maintenance and management of expansion joints are equally important. Long-term exposure to the natural environment can cause the sealant to age and crack due to ultraviolet radiation and rainwater erosion, leading to the failure of the expansion joint's function. Regularly checking the integrity of the sealant and promptly repairing or replacing damaged areas are essential measures to maintain the long-term performance of the curtain wall. Furthermore, cleaning dust and debris from the gaps prevents them from hindering component expansion and contraction, ensuring unobstructed deformation space.
The installation of expansion joints also has a positive impact on the seismic performance of the curtain wall. Under earthquake loads, the building structure will experience horizontal displacement. The elastic design of the expansion joints can absorb some of this displacement energy, reducing the rigid impact between the curtain wall and the main structure and lowering the risk of damage. This "flexible connection" concept allows the aluminum curtain wall to self-adjust through the deformation capacity of the expansion joints in response to both temperature deformation and seismic forces, thus enhancing overall safety.
