The five types of curtain wall systems can be further narrowed down into two main categories based on the method of installation and fabrication: unitized (or modular) systems and stick systems The unitized curtain walls are comprised of large units that are put together in the factory, after which it is transported to the site and assembled on the building.
Horizontal and vertical mullions of the modules are fixed together with the adjoining modules. Predominantly, modules are designed to be one story tall and one module wide and which may include multiple modules. A standard unit is built within five and six feet wide. Whereas, in the stick system, the frame of the curtain wall otherwise mullions, and the panels, either in glass or opaque are bound and adjoined together individually.
Both such curtain wall systems are versatile to function in the interior or exterior glazed systems. However, they do have their respective advantages and disadvantages. For an example, in interiors using glazed systems, glass panels either translucent or opaque are installed onto the opening of curtain wall from the interior of the building. This installation method is mainly designed for spaces with minimal obstacle toallow for adequate access to the interior of the curtain wall. In low rise construction with easy access to the building, external glazing is commonly used. The same way, interior glazing is often the option in high-rise construction because of the ease of access and logistics during repair or maintenance work. On the other hand, in exterior systems glass and opaque panels are installed from the exterior of the curtain wall.
In exterior glazed systems its necessary to have scaffolding access for maintenance works to the exterior of the curtain wall. Though there are systems that can be glazed using both the interior or exterior methods, either way like any other system, has its ups and downs in the application of these curtain walls. Laurea (2011), in his article on Seismic Behavior of Curtain Wall Facades, states that a Curtain Wall Façade is “a non-structural element attached to the building structure by a brackets system that anchors it allowing necessary tolerances of installation and building natural movements.” However a seismic occurrence can expose the structure to extreme levels of ground basement acceleration, this is the case even for very normal or low-intensity seismic reactions. Clearly, an earthquake leads to a basement acceleration that is surely horizontal but could also possibly be vertical to a smaller degree. When in play with non-structural systems, basing on their typology, fixing system and position inside the building, it will be critical to assess the worst possible load condition that could be applied to every single element: displacements, accelerations or forces. A non-structural element itself by nature is not required for the building to withstand the force and not collapse.
However such structural element should not be in any case of danger to its inhabitants, be it from the interior or exterior of the building. Not only that, in case of failure, it shouldn’t directly or indirectly be an obstruction to the functionality of another non-structural system, for example relating to user safety such as a fire-extinguisher system. Lastly, in case of normal and ordinary earthquake magnitude, or any other mild events, non-structural elements should stay functional, safe and permit, if required, the possibility of not immediate but postponed substitution.
That being clear, it is highly important to study carefully the behavior of a system or a non-structural element to compute the failure and collapse process that predominates, to ensure the possibility to take proactive prevention measures and to intervene during its design. However whether or not a mechanism goes wrong does not solely depend on the element itself, but also on the type of load that acts on it. This, as mentioned previously, could be either an acceleration, a displacement or a force. Therefore, based on the non-structural system and its characteristics, it will be necessary to evaluate which is (or are in the case there were more than one) the worst loading condition and proceed to verify it.
In the case of specific consideration of non-structural curtain wall façade, and primarily on the unitized and panelized system, we can start making some preliminary observations Sandesrs, Hargrove, et. al, (2012) believe that the open design of a suspended curtain wall allows for more vulnerability towards the spread of fire. Therefore firestops are mandatory at the perimeter of each floor as a preventive and safety regulation in the prevention of any possible fire occurrence in the building.
The use of firestop components in material, incorporated into hollow areas between floor slabs and curtain walls is to ensure a contained space at each story. Knock-out glazing panels are another safety measure, being made from tempered glass it permits relatively safe emergency access in the event of a fire. Sanders and Hargrove (2012) also state that, despite all the challenges faced with the curtain wall systems, laminated and insulating glass has a quality that acts as a sound filter, allowing for some level of noise reduction, similar to an airtight construction.
Another major point they state in their article is the high maintenance charges of curtain walls. The replacement of glazing is costly, in addition to this is the used heating and cooling equipment overtime to combat solar heat gain which all adds to the cost factor of this alternative. A major problem which can be seen only after installation is that poorly designed windows can create a lot of stress looking at a computer monitor, and such situation hardly allows for daylight energy savings especially when glare results in having to draw blinds losing daytime lights. The correct glazing, as Sanders and Hargrove states, creates for ample views, adequate but not overly excessive natural light, and pleasant indoor temperatures. Vistas Mehta published an article relating to the use of glass curtain-walls in tropical climates in the year 2015 (http://www.btsquarepeg.com/sustainable/energy/glass-curtain-walls).
In one specific part, he mentions the two reasons causing glass curtain wall to be unsuitable for a tropical climate similar to India for example: 1. Using a glass facade causes the inside to be subjected to an enhanced greenhouse effect. As glass results in the penetration of short wavelength light through its façade and upon reflection off objects in the room, it is of a longer wavelength which then is blocked by glass. Resulting in the internal temperature of the building rise as the heat stays tapped indoors. 2. This trapped heat is in some way to be released somehow and due to not having any significant air ventilation in a glass building it results in the excessive use of air-conditioning which is environmentally expensive.
Not forgetting also that living and working in a fully enclosed environment results in what is known as a “sick building syndrome”. The last but definitely not the least, the facet of curtain walls consumption he mentions in this article may not be highly energy saving but is definitely worth the attention. Migratory birds get very distracted due to the glass and often die or endure critical injuries when they bang against the massive transparent panes. This factor is also given emphasis in the article Sam Lubell wrote in 2017, Vikings Stadium: Reflector of Light, Murderer of Birds. (The article can be retrieved on https://www.wired.
com/2017/03/vikings-stadium-reflector-light-murderer-birds). “Last year, during the fall migration period between August and November, members of the Society’s local chapter, the Roberts Bird Sanctuary, and Minnesota Citizens for the Protection of Migratory Birds photographed 74 birds that were either killed (60) or stunned (14) after colliding with the stadium’s glass windows.” Another significant factor that requires being highlighted in the design of buildings with the use curtain walls is that the designing should try to respect and emulate the integrity with the adjacent buildings and the surrounding neighborhood. A high-rise building with glass curtain walls gives the inhabitants access to the whole view of the street and neighboring buildings. Depending on the height and condition of the other building the curtain wall can either be a pleasing aesthetical feature or result in psychological breakdown. This can be seen evidently in Charsoo building, a high-rise building partly designed with curtain walls in Tehran.
The topmost floor is mainly designed to hold multiple movie halls, which is meant to be artistic, inspiring with a pleasant view. However, the view that the adjacent buildings offer, with their roofs used as the poorly conditioned storehouse, is an eyesore giving the impression of the designing as a work of decontextualization.