Lisø PhD Dissertation Manuscript - NTNU
Lisø PhD Dissertation Manuscript - NTNU Lisø PhD Dissertation Manuscript - NTNU
Lisø, K.R./ Building envelope performance in harsh climates: Methods for geographically dependent design dependent on these categories. As an effect of the introduction of the coefficient the snow load on a sheltered roof becomes twice as large as the snow load on a windswept roof. Norway is a mountainous country with main settlements in valleys and on the coast (see Figure 2). This basically results in two characteristic type of wind climate near settlements. Inland settlements experience a wind climate governed by valleys, resulting in main wind direction along the valley and a decrease in strength due to topographic effects. The coastal area suffers a higher frequency of extreme winds due to less topographic effects. During snowfall the presence of wind will make the snow load on the roof differ from the snow load on undisturbed ground. Redistribution may naturally also occur in periods without snowfall. Finally, the properties of snow are an important aspect to be considered when trying to assess redistribution of snow on a roof due to the effects of wind. The investigation of the suitability of the proposed exposure coefficient reveals that the coefficient does not reflect the actual effects of wind exposure on roof snow loads in Norway. The main reasons for this can be attributed to oversimplifications in the definition of the coefficient, but also to the extremely varied climate of Norway. The country has areas with high snow loads and high frequency of wind, areas in which the exposure coefficient is expected to achieve its lowest value. These areas are however not pointed out as areas where wind blows snow away from roofs when using the definition as given in the standard. The definition is also based on oversimplifications of snow transport theories. It must be revised and improved to serve as an applicable tool for calculating design snow loads on roofs. These results clearly illustrate the need for methods allowing for climate differentiation in design guidelines for Norway. Further work will focus on developing a definition reflecting the physical processes more correctly, including the influence of the length of snow accumulation on the properties of the snow cover. 22
Lisø, K.R./ Building envelope performance in harsh climates: Methods for geographically dependent design 3 Concluding remarks The presented work is a first step towards methods and approaches allowing for geographically dependent climate considerations to be made in the development of design guidelines for high-performance building envelopes, and also approaches to assess the risks associated with the future performance of building envelopes due to climate change. Approaches to risk assessments associated with the potential implications of climate change on building envelope performance are presented, identifying main areas of vulnerability. For large, complex building projects there is an established tradition of using risk analysis methods. It is shown that there are benefits to be gained from the introduction of risk management strategies also in small-scale building. A way of analysing the building economics of climate change is also proposed. The performed analyses of empirical data from process induced building defect assignments clearly illustrate the vulnerability of building envelopes under varying climatic exposure. New and improved methods for geographically dependent design of building envelopes are proposed, enabling both historical weather data and scenarios for future climate change to be considered. A method for assessing the relative potential of frost decay or frost damage of porous, mineral building materials exposed to a given climate is presented, as well as a national map of the potential for decay in wood structures. Detailed scenarios for climate change for selected locations in Norway are in the latter method used to provide an indication of the possible future development of decay rates in wood structures. A method for assessing driving rain exposures based on multi-year records of present weather, wind speed and direction is also developed. These and other indices, with quantified relations between climatic impact and material behaviour or building performance, can be used as a very suitable tool for evaluation of changes in performance requirements or decay rates due to climate change under global warming incorporating data from regional- and local-level climate change scenarios. Historical records of climate data have also been used to illuminate challenges arising when introducing international standards at the national level, without considering the need for adjustments to reflect local climatic conditions. The work will also contribute to the pre-normative research for the continued development of Norwegian and international standards. This is particularly important with respect to the preparation of additional Norwegian appendices associated with the various types of climatic impact. The ongoing establishment of an electronic building defects archive, initiated as part of this work, will be an important tool in both the continuous efforts towards higher quality in the construction industry and the development of strategies aiming at learning from experience. The archive will also be an important element in the continuous development of more accurate criteria and Codes of Practice regarding the design and functionality of critical elements of buildings in the Building Research Design Series. And, the building defects archive will be an important educational tool in the establishment of knowledge on building defects amongst academic institutions and different actors in the construction industry. 23
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<strong>Lisø</strong>, K.R./ Building envelope performance in harsh climates: Methods for geographically dependent design<br />
dependent on these categories. As an effect of the introduction of the coefficient the<br />
snow load on a sheltered roof becomes twice as large as the snow load on a<br />
windswept roof.<br />
Norway is a mountainous country with main settlements in valleys and on the coast<br />
(see Figure 2). This basically results in two characteristic type of wind climate near<br />
settlements. Inland settlements experience a wind climate governed by valleys,<br />
resulting in main wind direction along the valley and a decrease in strength due to<br />
topographic effects. The coastal area suffers a higher frequency of extreme winds<br />
due to less topographic effects. During snowfall the presence of wind will make the<br />
snow load on the roof differ from the snow load on undisturbed ground.<br />
Redistribution may naturally also occur in periods without snowfall. Finally, the<br />
properties of snow are an important aspect to be considered when trying to assess<br />
redistribution of snow on a roof due to the effects of wind.<br />
The investigation of the suitability of the proposed exposure coefficient reveals that<br />
the coefficient does not reflect the actual effects of wind exposure on roof snow<br />
loads in Norway. The main reasons for this can be attributed to oversimplifications in<br />
the definition of the coefficient, but also to the extremely varied climate of Norway.<br />
The country has areas with high snow loads and high frequency of wind, areas in<br />
which the exposure coefficient is expected to achieve its lowest value. These areas<br />
are however not pointed out as areas where wind blows snow away from roofs when<br />
using the definition as given in the standard. The definition is also based on<br />
oversimplifications of snow transport theories. It must be revised and improved to<br />
serve as an applicable tool for calculating design snow loads on roofs. These results<br />
clearly illustrate the need for methods allowing for climate differentiation in design<br />
guidelines for Norway. Further work will focus on developing a definition reflecting<br />
the physical processes more correctly, including the influence of the length of snow<br />
accumulation on the properties of the snow cover.<br />
22
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