# Adaptability of Buildings: A Critical Review on the Concept Evolution
## Content Analysis
### Enablers and Promoting Models
#### 3.3.2 Shearing Layers and Building Decomposition  
A building system consists of a variety of functions and materials, all integrated in one closed entity of permanent status [65]. The rigid integration of these elements into structural systems to perform functions in particular space and time [66] ignores the level of durability pertaining to each component or building system [65], which often has a shorter lifecycle than the building itself [67]. The complexity of building systems resulting from the dynamic interactions and common dependencies between materials and components of different lifecycles makes it rather difficult for a building to change, meaning that any type of adaptation or alteration would be disabled as it entails ramifications on the overall performance of building structure [6]. Nevertheless, buildings are expected to last long despite all the changes that might occur during their service life, influencing their quality of response to emergent needs of users and other contextual factors. The dynamic interactions and functional dependencies among building systems and components have been widely discussed in the literature of adaptability [4; 49; 54; 68; 69] as they largely influence its feasibility and interrupt its mechanisms. Multiple methods have been introduced by scholars to analyze a building composition and explore the interrelationships and interdependencies among its components, systems, and layers. Analyzing the complexity of systems considering the distinct lifespan of their components gives insights to investigating mechanism for change by identifying what elements should change and what types of dependencies would enable or interrupt this change. The model of building layers is probably the most common approach in the literature, being an important design enabler of adaptability [39; 49; 68; 69]. It assumes that a building system is made up of several layers, each defined by elements and functions of similar lifetime. [PERSON] [68] was the first to discuss the model of building layers as he considered the general perception of a building as rigid constant object to be invalid. [PERSON] identifies four layers of longevity of building components: shell (structure, 50 years), services (e.g., heating, plumbing, 15 years), scenery (fittings, decorations, 5-7 years), and set (e.g., furniture, daily). [PERSON]'s early categorization of layers was later extended to six by [PERSON]'s approach of \"Shearing Layers\" (site, structure, skin, services, space plan, and stuff). [PERSON] [69] introduced in his book, _How Buildings Learn_, a building composition of different hierarchal layers with different timescales that make it a dynamic system that transforms itself overtime. For instance, [PERSON]'s model suggests that components of a particular lifespan should have no direct interaction with longer life elements neither with shorter life ones. This separation guarantees facilitated and frequent changes and replacement of some elements without interrupting the general performance of the building. Further layers of different lifespans were later added by other studies using the same principle. The most recent identification of layers was done by [PERSON] and [PERSON] [70] who extended the model to nine layers: surroundings, site, structure, skin, services, space plan, stuff, space, and social. Multiple scholars have built on the model of layers, proposing other methods of system's decomposition and categorization of elements. [PERSON] and [PERSON] [6] argued that establishing specific decomposition characteristics of buildings defines the potential reuse of components and materials. Buildings therefore should be designed for a configuration that defines the relationships and levels of interdependencies and exchangeability among components allowing three-dimensional transformation: structural, spatial, and material. The transformation capacity of a building can be measured using performance indicators that correspond to the three dimensions.