Curtain wall is defined and the historical development is presented according to literature reviews. The curtain wall system main classification is presented to make it easier on the decision-maker when choosing the curtain wall system according to their designs. After that, the curtain wall’s main components are introduced. Lastly, the performance criteria of the system are explained.
Definition and Historical Development of Curtain Wall System
The curtain wall term drives from the wall that are thin and hang on the structural frame like a curtain. Most curtain wall panels do not hang from the frame which generates tension, but supported from below at every floor level. In most literature, the curtain wall is defined as the external building envelope, a separation between the inside and the outside environment. It is a non-load-bearing wall, which means that it supports no vertical structural loads aside from its weight. The environmental forces acting on the curtain wall are transferred to the building structure (Vigener, 2016). Standards define the curtain wall (BS EN 13830) as follows:
“Curtain walling designed as a self-supporting construction which transmits dead-loads, imposed loads, environmental load (wind, snow) and seismic load to the main building structure.”
In the late 19th century, the greenhouse structures designed to provide much daylight to the plants contributed to the advent of metal structures in architecture. In 1892 elevators were invented, also the high price of land in Chicago led to the prosperity of tall buildings construction. In addition to the method of frame construction with large spaces without obstruction, pre-manufacturing enabled fast construction as well, this in turn, led to the development of a new type of building, instead of the mega buildings with the external bearing walls.
The first building in this kind was the Reliance building by Burnham and Root (Figure 2.1). This building represents the transition to the modern age where the load-bearing building structure was separated from the facade structure, which marks the beginning of the curtain wall system’s emergence.
Figure 2.1: Reliance building
Several references stated that the first introduction of the curtain wall, forgetting previous attempts, was in 1951 in Crystal Palace design by Joseph Paxton, the whole building consisted of iron and glasses inserted in between.
The buildings designed to have an envelope that was a lightweight non-loadbearing curtain wall frame instead of a large loadbearing wall at the start of the twentieth century. However, aluminium has merged through landmark projects of pioneer architects, such as Walter Gropius, Mies van der Rohe, Peter Behrens. The glass properties such as transparency, translucency and reflection has been among the architects focused beside the architectures went towards the manipulation of light, shadow, view and reflection.
Increasing the glass durability and strength led to the widely use of curtain wall, especially for high-rise buildings. A curtain wall that was first applied on a high-rise building was in Lever house in New York, constructed in 1952. The formal image of those curtain walls still repeated today by the use of modern profile construction.
In 1970’s, there has been a massive demand for energy-saving and sustainable solution in architectural design because of the energy crisis happen at that time. For this reason, in the building’s envelope this precaution had to be considered, so there were multiple attempts to reduce energy consumption by using multiple layers of glass to increase thermal insulation as well as other developments in the glass industry, which led to the emergence of highly reflective glazed buildings all over the world where the sun’s rays are reflected by the outer skin of the building to avoid overheating in the interior spaces.
In the nineties, there was an increase in the awareness to conserve energy and preserve the environment. To achieve this purpose in the curtain wall, double skin façade was started to be used to reduce consumption of energy as well as protection from sunlight and wind in high-rise buildings and also providing natural ventilation by helping the protective windows in the internal part of the facade. The use of double skin facades is usually very little due to the high cost, besides the curtain wall system became more than a barrier between inside and outside and a light façade system, but became an as intelligent system that fulfils the indoor comfort quality requirements.
Classification of The Curtain Wall System According to Construction Method
Curtain wall can be classified by their fabrication and installation methods into three categories: stick system, unitized system and mullion and unit system.
Stick curtain wall system
This system consists of vertical (Mullions) and horizontal (Transom) members that make up the frame, and the opening between the frame members are filled with glass or infill panels as shown in Figure 2.2. The frame members prefabricated and painted in the factory, then installed and connected in the construction site, then the glass and infill, panels attached. This system referred to as “stick built” due to the grid-based construction.
Figure 2.1: Stick curtain wall system
In its installation process, firstly anchorage members are installed at the floor slab and in specific areas, then the vertical members are attached to them, then the horizontal members are connected. To control the wind, seismic and thermal movement, and deflection on the system, the anchorage members should be arranged to allow the movement of vertical and horizontal members. Lastly, the glass and infill panels are installed between the grid system.
The advantages of stick curtain wall system are that most of the stick system are standard, thus it offers low material cost, and it gives the low shipping cost. Beside, this system offers the flexibility to on site adjustment. The main disadvantage is derived from the method of on-site assembly and this may cause slow work and increase the cost of labour in addition to problems related to the quality of the performance. This system cannot be used in constructing high-rise building’s façade or large areas, but is used mostly in the facades of low- and medium-rise buildings.
Unitized curtain wall system
The units are manufactured and assembled at the factory, then shipped to the construction site for installation. Usually, the units can be one story high by a modular width.
In the installation of unitized curtain wall system according to (BCI (Architectural Glazing and Building Enclosures Company)), firstly the anchor’s points are identified following the grid line made by the architect. The cast in a concrete channel inserted into the place due to the grid line, and the curtain wall brackets placed over the channels. Moreover, T screws or anchor screws are inserted into the holder, and then securely fastened with anchor bolts.
Then the unite is installed, and because its heavyweight, each pane is attached to a crane or davit located on a higher floor, the crane lifts the panels into place, workers will guide into the assigned plan, as shown in Figure 2.3. Then the panel hung from the brackets are installed on the building and adjusted following the next panels, then bolted into place and joints are sealed with silicone, gaskets and caulking. Figure 2.4 shows a unitized curtain wall system.
Advantage of unitized curtain wall is speed of completion because most of the work is done in the factory. This system is used to construct high rise building’s facades because the unit’s fabrication is done in factories under controlled condition, so quality efficiency is higher, the number of workers is less, therefore the cost is less. Design capability to accommodate in-plane movements due to sways and seismic events.
The disadvantages of this system are that the units are more sophisticated to handle and raise on site. Also, protection during transportation and before installation should be more expensive. Because modules are based on an interlocking mullion, it can be difficult to leave openings in the interface for purposes of access unless the number of units can slide or be sideways or if special joints are provided details in the access panel. Replacement of the damaged unit can also be a problem.
Mullion and unit curtain wall system
This system is a combination of stick and unitized types. This system was usually used when the design of mullion is a deep member, but it is rarely use today. In is installation firstly, the mullion members with one story height or more installed, then preassembled frame units are installed between them. The frame section may be either full floor height or divided into vision panel and spandrel units.
Advantages of this system is the same as the unitized system. With the addition of opening, temporary access can be provide more easily. Its disadvantages are the requirement of more labour on site, beside the additional connection linked to mullion members.
Classification of The Curtain Wall According to Glazing Method
Curtain wall can be classified into three categories related to the glazing method:
Structural silicone glazing system, pressure plate system and point-fixing system.
Structural silicone glazing system (SSG)
Structural silicone glazing system is a glass bonding method to the aluminium frame utilizing a high-performance and strong silicone sealant. The bond between glass, frame and silicone provide structural resistance to lateral load. This is one of the curtain wall types used primarily for high rise buildings. One of the advantages of this system is, when used to bonding glass to frame without any necessity of mechanical retention such as beads, clips, or bolt fixing, it provides provisions for the entire system.
Besides wind loading out off the plane, another available option with the SSG is the design of the glass panel as a diaphragm to resist the forces in the plane. The small, continuous bead of structural sealant provides great shear and tension values that move the shear forces edge directly inside the plane, and this feature of SSG is especially useful in designing corner conditions in curtain walls.
According to glass and metal curtain wall best practice guide, there are two types of SSG, 2-sided which means two edges of infill adhere to the framing with structural silicone sealant and two edges are usually mechanically fixed using Pressure plates. While 4-sided means the four edges of infill adhere to the framing with structural silicone sealant, the infill materials’ dead load supported by metal fin and setting block.
Pressure plate system
The pressure plate system is the most famous architectural system and widely used. The frame building technology is the method to construct this system. According to the glass and metal curtain wall best practice guide, it depends on the use of pressure plate, stating that “Pressure plates must provide adequate compression to the glazing gaskets through the proper tightening of pressure plate screws. At least three weep slots should provide to drain the glazing cavity”.
The initial version consists of mullions, transoms, glass parts with pressure plate and cover caps. In modern architecture, there are three types of this system: standard type, two-sided type and four-sided type.
The standard type consists of both horizontal and vertical cover cap; the cover cap gives the linearity impression to the façade, the cover cap installed above the pressure plate for hiding the pressure plate and the fasteners, and more functionally important, it acts as a barrier to the drainage holes in the horizontal pressure plate. The two-sided type is also called mixed type, and in this type the cover cap is used in one direction either horizontally or vertically and silicon used in the opposite direction.
Lastly, in the four-sided system silicone is applied in the vertical and horizontal direction, the same way as the structural silicone glazing system gives the impression of a flat face view. This diversity gives the designer the ability to create various façade designs with aesthetic appearances.
Point fixing system
This system is used to provide the largest possible transparent area by taking advantage of the transparency of the glass with minimum structural elements. Many types of structural systems can be used to collect the point support location such as spider fitting, cables network and tension structure, and the support points are designed to absorb all the static and dynamics loads then distribute them to the supporting.
Curtain Wall System Components
The curtain wall system is a complex system consisting of several interconnected parts; some to perform the function of protecting the building from external conditions and some transferring the loads to which it exposed to the building’s structure. Therefore, it can be said that the curtain wall system consists of structural components that include the frame and anchors, infill components usually glass for transparency, and opaque parts consisting of stone or different materials.
It is the main part of the curtain wall, since the loads are transferred directly from the glass to the frame, they must be strong enough to carry these loads to the building’s structure. The frames are consisted from a vertical and a horizontal part usually made from aluminium or steel. The selection of the material type depends on the aesthetics choices, module spacing, load and grid considerations. Conventional curtain walls integrated aluminium framing with the glazed opening.
The framing members contain not just the load-carrying mullion and transom but also the pressure plate, snap cap and stiffening components. The aluminium framing can be found in many depths for vertical and horizontal elements. One of the critical architectural advantages of using aluminium framing is that multiple profiles can be easily streamed to develop specially shaped framing files or distinct configurations, the depth of the system is mainly determined by the requirements for wind load.
By the facade technology development, members of the aluminium curtain wall system can be produced lighter to reduce the weight of the curtain wall system. Moreover, the uses of the smaller aluminium section will reduce the load of curtain wall on the building beside it can be more economically solution. Aluminium has many advantages when used as a curtain wall framing material, but it has the obvious drawback that it is three times more deformed than steel.
Usually, deflection is the ruling criteria in curtain wall design. So the building set a deflection limit for perpendicular (wind-induced) and in-plane (dead load-induced) due to limit the deflection of glass to ensure that it does not come out of its pocket in mullion as well as to ensure that it is not broken under massive deflection.
In the case of steel framing, a steel frame can be implemented as individual elements, such as the “sticks” of the frame, or as an assembly as trusses. Horizontal and vertical supports are used. Horizontal supports can be used to support the stone panels by developing module units, and vertical trusses, such as bow truss or tubular pipe truss, can be used to support glazing. Other types of steel are steel cable which are used in cable net system. This system is used to provide much transparency with minimal framing, but the deflection allowed is significantly greater than it is for traditional framed systems.
Glass is an essential architectural component of the curtain wall system due to its three main properties; transparency, translucency and reflectance. As its primary function, it helps to penetrate light into the interior in addition to providing transparency to the occupant inside the building. So that it helps to see the outside, that is, it works to communicate between the inside and outside, in addition to the idea of energy efficiency, safety and acoustics that have become an essential role in choosing the type of glass.
There are other requirements such as heat and solar control, noise and fire protection, which affects the total performance of the curtain wall system. There are many types of glass according to the particular requirements of the building and the types used in the curtain wall system need to be of high efficiency, strength and durability because they are exposed to external conditions as the influencing factors such as wind load, earthquakes. The main types of glass used in curtain wall system are insulating glass, safety glass (laminated, toughened), coated glass (low emissivity, solar control) and opacified glass
The anchors are used to connect the curtain wall frame system to the main structure of the building. Anchors can be divided into two groups: gravity and lateral load anchors (fixed) and lateral load anchors (slotted). Every anchor consists of two parts: an embedded component that is cast in or drilled at the top of the edge of the floor slab edge; and a mounting lug assembly that fixes the embed and the framing. The attachment mode depends on the wind load at the specified location. Beside, the wind load can be different at the different parts of the building that leads to anchor design which can be different in the same project.
There are anchors of different shapes and sizes, but the most effective one to resist the additional load is channel anchors. Usually, the installation is done by casting into a concrete member of the building structure. However, in Turkey, most of the contractors welded it or bolt it to the slab instead of the cast into the concrete because it took much time or the difficulties, although this method was considered the most effective against the earthquake load.
The traditional approach to connect the curtain wall to the building floor slabs is by using mechanical or chemical post-installed anchors or steel plates with stud welded anchors. This method changed nowadays by using the mechanical friction bolts, and it has many advantages such as no time consuming, efficient, adjustable installation. According to the MBY engineering, the maximum load on each anchor should not increase by 20 kN(KiloNewton), and the thickness of the bracket depending on the load (6-15 mm).
For high-rise buildings “there should be almost no deviation of the anchor points in the curtain wall. Otherwise, flaws in application details turn out to be a big problem when the height of the interface becomes longer.”