a Department of Civil Engineering, HZ University of Applied Sciences, The Netherlands
Managing the vulnerability of the residential stock is today a social issue to include in the European agenda, despite the many barriers and constraints and the complexity of the parameters to consider -
Keywords: seismic retrofit, energy retrofit, social housing, residential heritage, integrated retrofit, housing vulnerability
This paper discusses the building heritage produced after the massive destruction of the Second World War, period that saw especially the emergence of new social housing districts (Turkington et al., 2004). These new districts, accepted positively by the market and the residents, proposed a new way of living, with large building blocks
(Murie et al., 2003) mostly built exploiting prefabrication techniques, in order to provide affordable housing in short time. The new construction methods (Andeweg et al., 2007) were soon supported by applications in many central, east and north European countries, because successful schemes were reproduced almost identically in different social housing estates throughout each country.
It was only during the oil crisis in the 1970s that these estates revealed inadequate performances in terms of energy efficiency, and from the 1980s this problem added to functional, architectural and structural obsolescence. The districts, once popular among the citizens, lost their attractiveness (Power, 1997; Andersen, 2003; Murie et al., 2003, Wassenberg, 2004).
Turkington et al., 2004 have determined three main classes of problems affecting the social housing stock: intrinsic problems related to the building characteristics, management problems, and finally problems caused by a critical socio-
This classification derives from Prak and Priemus's (1986) three "spirals of decay", used to describe the housing decline of the Post Second World War social housing estates. Inadequate initial quality, due to limited construction time, poor materials and lack of experience, activates the first spiral of technical decay, which is further aggravated by the lack of proper management and maintenance. As a result, the population of the estates experiences a detrimental turnover in which dwellers are replaced by weaker socio-
Nevertheless, these estates still accommodate 41 million people in Western Europe and represent a fundamental part of the housing market.
In recent years, social housing estates are the main target of urban policies, at first focusing on demolition and reconstruction (Priemus, 1989). However, since the 1990s the new policies are shifting towards the market of recovery, which appears today as the only horizon for a building sector in continuous decrease (Scuderi, 2016). Today, demolition is not a sustainable solution (Preservation Green Lab, 2011) because it produces waste and consumes resources. A more sustainable solution is to requalify the existing building heritage (Langston et al., 2008) considering that the costs are lower because of the pre-
Additionally, the Buildings Performance Institute Europe (2011) stated that considering the almost irrelevant annual growth rate of the residential sector (Jaretti, 2008), the target of energy saving should be associated with buildings dated from the 1960s.
In seismic prone countries, problems about energy performance are also coupled with structural safety, (Rodrigues & Teixeira, 2006). The key issues connected with seismic vulnerability are the inefficient methods of classification of the territory (Dolce, 2012, Martelli, 2006), the inefficiency in controlling and monitoring the degradation process of the building life cycle (Rodrigues & Teixeira, 2006) and the lack of prevention measures. To avoid dramatic cultural, social and economic losses it is necessary to focus on campaign of seismic retrofit, by recovering the original performances of structures, upgrading them or reducing the seismic response of the building (Fukuyama and Sugano, 2000). Selecting among the different strategies is a complex process which depends on several factors (Thermou & Elnashai, 2006): for instance, the cultural, social and economic importance of the building, the expected damage and the structural typology and technology are some of the key issues to consider (Caterino et al., 2008).
Typically, the most effective retrofit is the one that allows a balance between costs and performance, and this is the reason why current programmes are propending towards integrated approaches, able to solve contemporarily architectural, functional, and structural problems.
From the definition of Caffrey (1988), an "Intelligent retrofit" should provide a "productive and cost-
The money invested in "Intelligent" and therefore integrated retrofits can be capitalized in relatively short time considering the improved level of performance and safety, and also the increased popularity of the estates which can reverberate on the marketability of the dwellings (Egbelakin and Wilkinson, 2008).
2. Managing the built heritage
Tenner and DeToro (2008) introduced the possibility to "manage the quality" of a building through eight factors, namely performance, features, reliability, conformance, durability, serviceability, aesthetics and perceived quality. It is important to mention that those factors are closely interlaced and they influence each other; for instance, technical problems or aesthetical obsolescence often have impacts on users' satisfaction.
The first fundamental step is to recognize the loss of performance and the consequent degradation as a natural and expected part of the life cycle of a construction (Gruis at al., 2006). The objective of interventions is then an ideally endless extension of the life span of a building, throughout modifications of its physical, functional, architectural and environmental characteristics (van der Flier and Thomsen, 2006; Thomsen and van der Flier, 2008; Thomsen, 2011).
Another key point is that the life span of the different elements of a construction greatly differs, going up to 300 years for structural elements and up to only 20 years for envelopes. This means that interventions can often be directed towards specific components of the construction, still allowing the overall life-
Many procedures can lead to different results (Priemus, 2005). When speaking about retrofit, in general the word indicates an intervention focusing on upgrading the building to current technical, functional and architectural standards, in opposition with refurbishments which restore the initial performance of the building.
In general terms, when retrofit is considered the best operative option, the next steps have to be undertaken in order to assess the condition of the building:
a. Selection and design of retrofit strategies.
The selection of an option depends on the available technical expertise and inconvenience during the intervention. The strategies can be grouped under global and local strategies. The final scheme should be cost effective.
b. Verification of the retrofit scheme.
The scheme should be ensured through analyses of the retrofitted building and that the selected scheme satisfies the identified objectives. The scheme should be viable in terms of costs and execution.
The effectiveness of the retrofit scheme greatly depends on the quality of construction. Hence, the construction as per the suggested details and specifications is imperative.
d. Maintenance and monitoring.
2.1 Introduction to seismic retrofit
The purpose of seismic retrofit is to enhance the structural capacity of a building acting on its strength, stiffness, ductility, stability and integrity in order to upgrade its performance to the current standards.
Tierney (2005) showed how the seismic retrofit can lead to risk mitigation and disaster prevention, fundamental since earthquakes are more and more random in place, time and intensity (Nuti & Vanzi, 2003). Stevens and Wheeler (2008) underlined that improving the quality of the building heritage guarantees the sustainability of the construction industries and of the cities.
However, while many ordinary buildings are considered unsafe, financial implications are always fundamental in order to select the best retrofit campaign. Nuti and Vanzi (2003) declared that in order to make retrofit a good investment, the ratio between costs and reduction of the risk should be calculated.
When the campaign is considered economically viable, it is possible to select global and local strategies, which are not always mutually exclusive: a global retrofit targets the performance of the entire building, while local retrofit targets the seismic resistance of a member, without necessarily affecting the overall resistance of the building. In many cases, it may be necessary to combine both local and global retrofit strategies under a feasible and economic retrofit scheme. In general, when a building is severely deficient in withstanding seismic forces, a global retrofit strategy is recommended to strengthen and stiffen the structure. Consequently, if deficiencies still exist in the members, local retrofit strategies are to be selected. Beyond this recommendation, it is not prudent to prescribe a retrofit strategy as a generic application, since each one has merits and demerits in relation to the project (Scuderi, 2016).
Some examples of retrofit goals are: increasing the lateral strength and stiffness of the building, increase the ductility of the structural elements, improving the continuity of the members, eliminating or reducing the effects of irregularities, or enhancing the redundancy of the structural elements resisting to seismic loads.
In Table 1, some of the most common global retrofit strategies with their advantages and disadvantages.
The effectiveness of the retrofit is monitored by the operationalization of the demand to capacity ratio for both vertical and lateral loads, and by the reduction of the inter-
Consequently, in recent years, considerable attention has been paid to research and development of seismic dissipative devices, which can provide passive, active or semi-
Among the possible solutions, many studies have been conducted in order to assess the practical feasibility and economic convenience of using smart materials (SMA) to implement these dissipative devices (Dolce and Marnetto, 2000) however the high material costs prevent extensive applications in the field of civil engineering so far (Alam et al., 2007). A viable solution is to use the alloys in smaller devices or in selected regions of the structure (Janke et al., 2005). With reference to bracing systems for framed structures, apparently the cost of SMA is negligible with respect to the cost of the dissipative device. Consequently, the cost of SMA braces is practically identical to the cost of steel braces.
Additionally, the cost of shape memory alloy has decreased significantly, due to increased demand and improved in manufacturing (Frick et al., 2004) and it is realistic to assume that the price will further drop in the next years, as new applications in the civil engineering field develop (DesRoches et al., 2004).
These experimentations about innovative materials and technologies open to a wide range of new possibilities for the construction field, a sector that is still firmly grounded in the tradition.
2.2 Introduction to energy retrofit
Today, the building stock produced after the Second World War is generally characterized by poor energy performances (Itard, 2008) and it can a huge resource towards the targets of energy saving and reduction of carbon dioxide emissions, especially considering that the housing sector is responsible of approximately the 40% of the energy consumed in EU.
Retrofitting the social housing stock could be an important step to ensure a sustainable development, reduce the depletion of fossil fuels and mitigate the effects of climate change.
Hermelink and Muller (2011) studied that a deep retrofit, adopting an integrated set of measures and achieving savings between 60% and 90% has the potential to reach the EU decarbonisation targets for 2050.
Despite these considerations, there are still no European guidelines to address systematically the energy retrofit and most of the implementations are still undertaken as a case by case operation (Nemry et al., 2010). This approach dramatically decreases the overall impact of the retrofitted buildings into the urban environment, providing only local solutions.
Another limitation within the energy retrofit process is that, among the different parameters to consider, energy evaluations are typically calculated at the end of the design, in the form of regulatory or voluntary certificates (Dakwale et al., 2011). This means that the performance evaluation comes after the strategy has been developed, without influencing the decisions made (Attia, 2012).
A shift in the process is needed, and the energy retrofit must be designed in the early stages of the design process, when it can be integrated with other measures -
Recent researches focused on trying to address these issues in relationship to the building envelope, which is considered to be the most relevant component in terms of energy demand and consumption.
3. Barriers and Constraints
The "large and unused cost-
The report "Europe's Buildings under the microscope" (BPIE, 2011) tried to classify the barriers and constraint to the up taking of extensive retrofit campaigns: financial barriers, institutional/administrative barriers, barriers related to awareness, advice and skills, and finally the separation between expenditure and benefit.
Among those classes, financial barriers are the most relevant since the lack of funds prevents investment on retrofit programmes. Long term incentives are necessary to stimulate the market towards integrated retrofit programmes.
Administrative issues are also particularly evident in case of private social housing, which requires higher incentives and residents' consensus, while for public stock control and coordination is generally easier (Waide et al., 2006).
The third class of barriers is related to the lack of technical expertise in the field of retrofit, which requires completely new technical, social and managerial skills and new company organization. This also applies to designers, developers, commissioners and governments, whose knowledge must adapt to new requirements for design, management and monitoring (Thomsen, 2011).
In addition, already in the 1991, Sandivo identified four main constraints to retrofit: time, space, information and environment. These constraints affect the performance of the project in terms of schedule, budget, and scope of work.
Constraints and barriers can both be removed through careful front-
On the other hand, many professionals still see as a challenge to apply whole-
Finally, end users need to be educated on behavioural issues and sustainability issues, especially in context where social housing is privately owned. In fact, studies proved that neighbourhood satisfaction depends mostly on housing satisfaction and on the reputation of the area, factors that should be considered in a retrofit programme through residents' involvement (Wassenberg, 2004).
The "Broken Window Theory" (Wilson and Kelling, 1989) is a criminological theory of the norm-
A neglected and vulnerable property is the signal of a neglected and vulnerable community. "Vulnerable buildings are a manifestation of the vulnerability of an urban system, since the response of the city can be interpreted as the sum of the single responses of its composing elements but also because they are the expression of the attitude of the community towards disasters and unexpected events" (Scuderi, 2016; D'Amico and Currà, 2014; Gargiulo and Papa, 1993; Fistola and La Rocca, 2009).
Integrated retrofit is first of all a social issue to include in the European agenda, despite the many barriers and constraints and the complexity of the parameters to consider -
While a real estate market failure is more and more probable, the building sector is still not able to find a recipe to transform the new challenges into opportunities.
Towards this direction, Druot, Lacaton and Vassal proposed the concept of "economies of scale", within which "light" techniques based on sustainable solutions can be applied to "mass retrofit" of the housing built after the Second World War.
Another relevant reason to adopt these "economies of scale" is that most of the technological solutions are today still too expensive and require long-
while decreasing the dependence of various countries on imported fossil fuels. Solar energy is abundant in the Mediterranean region and the use of solar-
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