Certainly, not all buildings can be demolished due to, for example, restrictions regarding urban design and the evident value placed on the preservation of the select buildings. Hence, a solution to renovate existing buildings to minimise environmental effects and energy consumption whilst minimising construction costs and maximising return on investment is certainly needed.
Designing new buildings has ―no‖ limitation on the design whereas the original design of the existing building may be highly restricted with respect to energy renovation. The design of both new buildings and renovation of existing buildings may be restricted by economy. These limitations suggest the need for a method to optimise the economic aspects when considering energy renovation of buildings.
2.7 Design methods
The retrofitting of old buildings includes the risk of changing a well performing structure into one with a critical moisture balance. Moisture, temperature and time are probably the most dominant causes of failure in renovation projects. Especially buildings worthy of preservation have these issues when e.g. inside insulation is the only applicable retrofitting measure combined with a structure containing wooden parts. New retrofit measures should, naturally, be long-lasting and not cause collateral damage to the existing structure. Thus investigation of the long-term durability of the energy saving measures is needed when planning a holistic energy renovation of a building.
Design methods can be any procedure, technique, aids or ―tools‖ for designing.
Drawing is probably the most common design method. However, design methods represent a number of distinct kinds of activities that the designer might use and combine into an overall design process. Creative methods such as brainstorming stimulate creative thinking by removing, so-called, mental blocks. These blocks inhibit both creativity and a widening of the possible solution area. Rational methods encourage a systematic approach to design. Cross (2000) presented seven methods including Quality Function Deployment (QFD) which helps transforming the customer needs into engineering characteristics. Furthermore, hazard identification techniques are common support tools for designers developing new products. Hazard identification techniques are a proactive way of dealing with risks in product development. Gould et al. (2005) identified 40 hazard identification techniques pointing out their strength and shortcomings. The risk level is related to the severity of the risk and the likelihood of occurrence (Smith 1999). Thus managing the likelihood of occurrence is the best way of controlling risk.
2.7.1 Failure Mode and Effect Analysis
One of the identified techniques managing likelihood of occurrences is Failure Mode and Effect Analysis (FMEA) (Stamatis 2003, Stunell 2003). In Figure 2.7, the three general steps of the FMEA process are shown.
2 Background 2.7 Design methods
22 Technical University of Denmark
Figure 2.7: Failure Mode and Effect Analysis processes based on work by McDermott et al. (2009) and Stamatis (2003). S: Severity; O:
Occurrence; D: Detection.
FMEA uses the brainstorming in a systematic bottom-up approach that identifies and corrects the potential failure modes during the design stage. The Risk Priority Number (RPN) of a failure mode is calculated based on the multiplication of the severity (S), occurrence (O), and detection (D). The severity, occurrence and detection are ranked on an arbitrary, subjective scale, e.g.1-10. The failure modes with the highest RPNs are the ones to take action on. Calculation of RPN neglects the relative importance of the severity, occurrence and detection. This implies that high-risk events may be unnoticed (Gilchrist 1993). Furthermore, the RPN cannot measure the effectiveness of proposed corrective measures (Puente et al. 2002). FMEA considers only the effect of one failure mode. Thus, combined hazards from coherence of multiple effects of failure modes are not considered. Finally, durability evaluation is not part of FMEA.
A state-of-the-art study was prepared by Talon et al. (2006) regarding FMEA research for and application to the building domain. The collection of research work underscored the usefulness of the FMEA and FMECA (―C‖ for criticality) in the building domain. Some examples of the uses of:
FMECA has been employed in the maintenance management of building components (Talon et al. 2008).
FMEA has been applied in the process of predicting service life of building materials and components (Hans, Chevalier 2005) by employing the factor method (ISO15686 2000).
2.7.2 Factor method and Limit states method
The factor method, which is a service life format, is one of two approaches provided by (ISO13823 2008) checking structures for durability. The other approach
2.7 Design methods 2 Background
is the limit states (LS) format. The factor method determines the estimated service life (ESL) based on a known or assumed reference service life (RSL) and corrective factors (ISO15686 2000), see Eq. 2.4.
ESL RSL A B C D E F G (Eq. 2.4)
where, A: quality of material or component; B: design level; C: work execution level; D: indoor environment; E: outdoor environment; F: in-use conditions and G:
maintenance level.
According to Marteinsson (2003a, 2003b) one of the key issues regarding the factor method is the consideration whether it is trustworthy taking the probabilistic of the field service life planning into account. Another issue is the difficult determination of the factors and the consequences of changing these factors (Listerud, Bjørberg &
Hovde 2011).
Contrary to the service-life format that is the factor method, the LS format evaluates the performance of a component or structure against various limit states.
Figure 2.8 gives the framework of the LS format.
Figure 2.8: Limit states method for durability (ISO13823 2008).
The limit states are divided into the following two categories (ISO2394 1998):
Ultimate Limit States (ULS). This corresponds to the maximum load-carrying capacity or, in some cases, to the maximum applicable strain of deformation.
Serviceability Limit States (SLS). This concerns the normal use.