Climate change and global warming are causing a growing number of countries to seek strategies for reducing the use of fossil fuels for energy and the resulting emission of CO2. In the construction industry, wood has low production energy requirements compared with concrete and steel building materials, and wood building materials have capacity for long-term carbon storage. The processing of by-products for biofuels can replace the use of fossil fuels. Using sustainably managed forests for timber production enhances capacity of forests to absorb CO2 from the atmosphere, as mature trees are replaced by young trees. Therefore, timber is gradually becoming accepted as the most suitable building material to be integrated with strategies for addressing climate change. In a number of countries, this recognition has helped to revise building codes and provide new options to timber construction in segments, which were prohibited in the past, while also boosting the development of new technologies for engineered timber products and construction methods for new building applications, including multi-storey timber construction.

The advantages of multi-storey timber construction are in expanding timber construction from low-rise buildings to multi-storey buildings, while providing the option of replacing construction methods, which cause heavier loads on the environment. In addition, the fact that multi-storey timber construction requires different structural and technical solutions than low-rise timber buildings creates national economic opportunities as a by-product of the construction itself. Often, countries that have recognised both the national environmental and economic opportunities strive to expand timber construction to the segment of multi-storey buildings.

The overall purpose of this dissertation is to understand how multi-storey timber building could better be developed in order to expand its use in Japan. The study focuses on residential building because this merges environmental and economic opportunities of timber construction. In multi-storey construction, the residential segment has the largest share of the construction industry. Consequently, promoting multi-storey timber residential building offers the largest growth potential for multi-storey timber construction. To maximise Japan's full opportunities from the promotion of multi-storey timber construction, the larger segment of residential construction needs to be well progressed.

While promoting multi-storey timber construction, it is important to demonstrate that timber material has both advantages as a construction material and several disadvantages; it burns, has low density and low ductility that can cause acoustic and earthquake resistance problems, and as a biological material it changes and can decay. Modern construction, however, as a result of extensive research works demonstrates that, with correct use of technical solutions, safety and quality requirements can be addressed and multi-storey timber building can achieve high building performances, equal to any other modern building. In other words, for multi-storey timber construction, building performances should not be a reason for hindering expansion.

This study examines the progress of multi-storey timber residential building in Japan. The methodology includes reviewing and analysing constructed buildings and the motives driving construction in the country. This study compared Japan with several European countries, which, like Japan, have a relatively short history of construction in this segment. The results show that in Europe, multi-storey timber residential projects are built in large housing markets, and have similar or larger built floor area and construction budgets to non-timber housing projects. They often have better building performances compared with common reinforced concrete buildings and high architecture value, due to design competitions. In general, the segment enjoys a good image among decision-makers and the general public. New technological developments of engineered-timber products, structures and construction methods were invented. Projects are built by financially sound mid-size or large construction companies. Clients are from the public and private sectors, even though the buildings are about 10% more expansive than conventional buildings. Several specific downsides were also found, but while they need to be addressed, they are not critical obstacles for further expansion.

The study found that that the segment has made good and steadily progress as a result of numerous, usually closely co-ordinated, supportive measures undertaken by three groups of key players: the government, research institutes and forest industries, which see a construction segment with large growth potential.

In Japan, the review and analysis of multi-storey timber residential building found that there has been little progress since construction was permitted by Building Standard Law in 2000. The study found that construction is limited in several ways: nationwide expansion is narrow; the segment occupies a narrow market niche of low-grade one-room rental apartment projects; and only small number of buildings has been built. Moreover, building performance has not achieved the sound insulation target grade, the number of available construction methods and the type of construction companies are limited; and there are no public clients, which could influence the type of projects built. Cost effectiveness motivates the segment's construction. All multi-storey timber residential apartment projects built in the period examined in this study, used timber-frame construction, which costs about 20% less than equivalent concrete buildings. The method was developed as a cost-effective construction method and it was shaped by the needs of its current market niche.

A review of the activities, conditions, and approaches of the three groups of key players, namely, the forest and construction industries, national and large construction companies' research institutes, and the government found that relatively little has been done to promote multi-storey timber residential construction in Japan. Two timber housing builders associations, most importantly the Japan 2x4 Home Builders Association, are behind efforts to move the segment forward. However, the associations' limited capability and wherewithal, due to the size and types of member firms, hinder significant progress.

The opportunities for Japan in promoting multi-storey timber building are in forest economy and health; national economy in cumulative ways; and environment. Compared with a few years ago, there is greater recognition today of the opportunities in expanding multi-storey timber construction in Japan. This study concludes that a clear policy is required with defined objectives aimed at large-scale expansion. In addition, the government, research institutes, and industries need to provide strong and well-coordinated supportive measures. As a by-product of this process, timber residential building would be promoted because of its large share in construction industry, which cannot be ignored. A strong and expanding multi-storey timber construction industry could achieve full economic and environmental opportunities for Japan.

提出された学位請求論文「A study on multi-storey timber residential building in Japan- Contemporary applications and potential opportunities (日本における多層木造集合住宅の研究- 応用の現状と潜在する可能性)」は,多層木造集合住宅について技術的・法的・生産組織的観点からその全体像を把握し,ヨーロッパ諸国との比較検討に基づいて日本における発展の可能性について論じたものであり,全8章からなっている.

第一章「Introduction (序章)」では,研究の背景,目的と方法,論文の構成,既往の研究の整理および本研究の対象とする範囲を明らかにしている.本論では,木質構造の中でも特に集合住宅というひとつのビルディングタイプを対象としているが,その理由および意義についても述べられている.

第二章「Advantages of Timber Buildings (木造建築の利点)」では,主に環境的な側面から木造建築を建設することの意義について述べられている.木材利用による二酸化炭素固定及び,国内の森林資源の有効利用について国内だけでなく諸外国の既往研究をもとに考察を行っている.更に,木質構造で多層の集合住宅を建設することに伴い,新しい産業を誘発できる可能性についても論じられている.

第三章「Disadvantages of Timber Material in Construction and Technical Solutions (建築材料としての木材の問題点と技術的な対応策)」では,木造建築の防火・防音・耐久性・耐震性について,包括的に論じると共に著者が日本及びヨーロッパ諸国で収集した資料をもとに具体的な解決方法と事例を紹介している.その結果,最新の技術を適切に利用することにより多層の木質構造は他の構造種類と比較しても遜色ない性能を確保することが可能であるとしている.

第四章「Progress of Multi-Storey Timber Buildings in Europe (ヨーロッパにおける多層木造建築の進展)」では,ヨーロッパの中でも近年比較的多くの木造建築が建てられているオーストリア・フィンランド・スウェーデン・スイス・イギリスを対象に,多層木造集合住宅の実情に関する現地調査および聞き取り調査の内容をまとめている.その結果,近年ヨーロッパ諸国で多層木造集合住宅が建設される主たる目的は,環境的な配慮とともに国内産業の誘発と発展であり,各国の経済活動の中で森林産業が占める割合が高いためこれらの企業が政府の林業および木質材料関連の政策を積極的に後押ししていることが明らかにされている.著者は,政府・研究機関・森林産業を3つのKeyPlayersと位置づけ,これらの効果的な連携が極めて重要であることを述べている.

第五章「Progress of Multi-Storey Timber Residential Buildings in Japan(日本における多層木造集合住宅の進展)」では,日本国内で建設された多層木造建築21棟(うち集合住宅は11棟,136戸)を対象とした調査結果をまとめている.各建物について,図面や聞き取り調査の結果をもとに具体的に解説するとともに,戸数・床面積・性能・構法等について考察し,第4章で示したヨーロッパ諸国の事例や傾向との比較検討を行っている.その結果,日本国内で建てられている多層木造集合住宅は,ヨーロッパのそれと比較して計画・規模・性能・採用構法等,全ての面で格段に限定されていること,その理由として前述の3つのKeyPlayersが十分効果的に連携していない可能性が指摘されている.

第六章「The Role of the three key players in Japan(日本における3つのKeyPlayersの役割)」では,第5章の結論を踏まえて国内の3つのKeyPlayersの現状について,文献調査および聞き取り調査に基づいて述べられている.更に,政府・研究機関・森林産業それぞれについて,今後期待される役割と活動の方向性について,ヨーロッパの例をもとに述べられている.

第七章「Opportunities and Potential for Japan(日本での実現および発展の可能性)」では,多層木造集合住宅を日本で建設することの可能性と想定される利益について,環境的・経済的な観点から定性的かつ定量的に述べられている.

第八章「Conclusion(結論)」は,以上の内容をまとめると同時に,今後当該分野の発展のために必要な研究の方向性について述べられている.

以上、本論文は国内外の膨大な資料による綿密な文献調査および,現地調査に基づき,国内の多層木造集合住宅の現状および可能性について論じられている.更にヨーロッパの事例との比較検討を通じて,今後当該分野を発展させるための方向性に関する考察を行った論文であり,建築学の発展に寄与するところが大きい。

よって本論文は博士(工学)の学位請求論文として合格と認められる。