―　　―31

Structural Design Achieving Floating Artificial Ground and Attractive Office Space

— Nichia Suwa Technology Center —

浮遊する人工大地と魅力的な執務空間を実現する構造デザイン― 日亜化学工業諏訪技術センター ―

* ** *** ****

* Junichi SASAKI: Takenaka Corporation佐々木　淳一：（株）竹中工務店

** Souichirou KUSHIMA, Ph.D.: Takenaka Corporation九嶋　壮一郎，博士（工学）：（株）竹中工務店 *** Hisato OKUDE: Takenaka Corporation奥出　久人：（株）竹中工務店

**** Masayoshi IMAMURA: P.S. Mitsubishi Construction Co., Ltd.今村　雅泰：（株）ピーエス三菱

Contact: sasaki.junichi@takenaka.co.jpKeywords: PCaPC beam, hybrid structure, long span, labor savingDOI: 10.11474/JPCI.NR.2018.31

SynopsisNichia Suwa Technology Center has an open office space that commands panoramic views of Lake Suwa. A dynamic and flexible structural frame floating in the air with a 28.8-m span and 6.4-m overhangs was constructed by applying the Gerber beam concept to a hybrid framing of precast prestressed concrete (PCaPC) and steel. The authors increased the percentage of PCa concrete in the members with the aims of saving labor in framework construction in a cold region and improving quality by ensuring accuracy.

Structural DataStructural Type: PCaPC structure, reinforced concrete (partially PCa) structure, and steel structureScale: 2 floors above ground, 1 basement floor, and 1 penthouse floorBuilding Height: 15mBuilding Area: 2,625.44m2

Gross Floor Area: 4,002.8m2

Owner: Nichia CorporationDesigner: Osaka Main Office, Takenaka CorporationContractor: Tokyo Main Office, Takenaka CorporationConstruction Period: Jan. 2016 – Nov. 2016Location: Suwa County, Nagano Prefecture, Japan

1. IntroductionThis building is a technology center with an open office space that commands panoramic views of Lake

Suwa. The whole office appears to float on the artificial ground constructed of a structural frame system, by which the authors have achieved a façade design that highlights the image of an advanced company while taking the surrounding abundant natural environment into the interior space (Fig.1).

2. DesignThe design concepts in the architectural planning phase were “creating a favorable research environment” and “enhancing intelligent productivity.” The aim was a symbolic work that would afford the company recognition in the surrounding areas (Figs. 2 and 3).

3. Structural PlanningTo match the architectural plan, the authors applied a combination of different types of structures so that

Fig.1 Full view of building

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the right structures might be used in the right places. The substructure is made of reinforced concrete (RC) while the superstructure is made of steel. Some parts of the RC structure have a PCa structure and a PCaPC structure in combination depending on the span (Fig.4).Since only the minimum areas are used for the building uses on the first basement and first (ground) level floors, they comprise two cores, one in the east and the other in the west. Each core has a pure rigid-frame structure consisting of eight PCaRC columns and PCaPC girders, thereby minimizing the number of structural members and ensuring all of flexibility, bearing capacity and stiffness. Using PCa columns and PCaPC girders simultaneously resolved two major issues, namely snow accumulation and a shortage of

construction workers. The second-level floor comprises a steel rigid-frame structure with braces to achieve an open space. The aim was to minimize the member sections and reduce the load on the substructure.

(1) Construction of Artificial GroundThe artificial ground constructed at the second-floor level is a structure that can support the superstructure stably. It also gives the impression of floating by showing a dynamic framework with a 28.8m distance (on-center) between the columns in the east and west cores and 6.4m overhangs from the east and west cores (Fig.5).The 28.8m span was achieved by pin-joining the steel beams to the PCaPC beams hanging over 2.2m from the cores and turning them into Gerber beams (Fig.6). The light-weight steel beams minimized the structure section, and PCaPC beams at the ends increased the stiffness, thereby enhancing the habitability of the office. The maximum height of the steel beams was 1,800mm considering the habitability of the upper habitable rooms. The maximum section of the PCaPC beams is 850mm by 1,500mm, and the specified concrete strength is 60MPa. The PCa members for integrated column–beam joints were fabricated in a

Fig.2 Concept diagram

Fig.4 Building section (upper) and Floor framing plan, Level 2 (lower)

Floating

Lake Suwa

Mountain

View Ventilation Sunlighting

Artificial ground

上部建物外形

ピロティ

実験室ゾーン

執務室ゾーン来客ゾーン

２階平面図（メインフロア）

１階平面図

Pilotis

Shape of superstructure

Floor plan, Level -1

Floor plan, Level -2(Main floor)

Laboratory Zone

Office ZoneVisitors Zone

諏訪湖Lake Suwa

N

Fig.3  Plan of each floor

3,80

06,

000

3,00

0

PHFLRFL2FL

1FLB1FL

Core（West） Core（East）

Steel structure

PC structureRC structure

2,60

0

6,400 16,000 28,800 16,000 6,400

Artificial ground2.2 2.23.2 3.2

Half PCa void slab(Reverted beam)16,000 16,00028,800 6,4006,400

Steel beam

Existing RC beam

3,20

03,

200

8,80

08,

800

8,80

0

PCaPC beam PCaPC beam ：PC pressure bonded

Vertical brace

Fig.5 View looking up at 6.4 m overhang

2,200

納まり改善

Fig.6 Joint between PCaPC beam and steel beam

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many members as possible were made in the same shape, thereby enabling the same forms to be used for fabrication. For the PCa floor members, the cantilever floor members at the corners are large, measuring 6.4m by 3.2m, and two sides at the end of each overhang have rises. Special fixtures were used for form removal to prevent the members from being inclined. The same fixtures were used when erecting the members (Fig.11).

factory and then erected and tensioned on site.A 6.4m overhang was constructed by integrating a half PCa void slab into a 3.2m overhanging PCaPC beam in a reversed beam style. This style facilitated the design of an exposed slab on the underside of the slab, an under-floor air-conditioning system on its top side, and a structural design that looks like a 6.4m overhanging slab.Combining these Gerber beams, PCaPC beams, and large-scale overhangs helped balance the long-term stressed condition of the artificial ground, leading to construction of stable ground (Fig.7).

(2) Construction of Open Office SpaceThe office space above the artificial ground was made an open space with a uniform column diameter of 139.8mm by applying all the horizontal forces to the vertical braces (Fig.8). The vertical braces are concentrated, well balanced, and aligned with the core positions of the substructure, thereby minimizing the sections of the horizontal brace members that ensure the roof stiffness. Moreover, the authors used a combination of a pure rigid-frame RC substructure and a steel superstructure with braces to balance the story stiffness ratios of the whole building.

4. Construction(1) Fabrication of PCa MembersBecause the project site is in a cold region, as many RC members as possible were precast to improve their workability. Fig.9 shows the shapes and layout of the members and Table-1 lists their quantities.The PCa column members were designed to have projections in the east and in the west (Fig.10). Therefore, concrete was cast only in the north direction.The PCaPC beam members were designed so that their column–beam joints might be integrated. As

Fig.7  Long-term stresses on artificial ground

16,000 16,00028,8006,400 6,400

3,2003,200

24,4002,200 2,200

PCaPC beam PCaPC beamSteel beam

Fig.8  Office interior

SG4A

SG3

SG1

SG3

SG1

SG4

SG2

SG4A

sb58M

CPB3

PB1

PB1

PB1

PB1

CPB1 CPB1

CPB1

CPB1

CPB1

CPB1

CPB4A

CPB2A

CPB2

CPB2A

CPB4 CPB2

CPB2

CPB2A CPB3

G1 G1

G1

G2 G2G2

G3 G3

G3

PG1A PG1A

CPG2

CPG2

PG1

CPG1

CPG1

CPG1A PG1A CPG2A CPG2A PG1B CPG1B

CPG2A

PG1

PG1

FG6

FG1

CPG2A CPG1BCPG2A PG1B

PG1A

CPG1B PG1B

PG1

G1

G2

G3

CPG2A

CPG2

CPG2

CPG1

CPG1

CPG1A CPG1A

C1 C1

C2 C2

F2F1

F3 F2a

2FL

RFL

PHFL

1FLGL

平均GL

2FL

RFL

PHFL

1FL

B1FL

3200

8800

8800

8800

3200

6400 6400 9600 9600 9600 9600 9600 6400 6400

1800

6400 6400 9600 9600 9600 9600 9600 6400 6400

73600

32800

73600

1695

6000

3800

2600

100

14095 3800

2600

3000

6000 15400

A

B

C

F

D

E

1 2 4 5 6 8 9 1073

1 2 4 5 6 8 9 1073

Fig.9 Layout of members

factory and then erected and tensioned on site. A 6.4 m overhang was constructed by integrating a half PCa void slab into a 3.2 m overhanging PCaPC beam in a reversed beam style. This style facilitated the design of an exposed slab on the underside of the slab, an under-floor air-conditioning system on its top side, and a structural design that looks like a 6.4 m overhanging slab. Combining these Gerber beams, PCaPC beams, and large-scale overhangs helped balance the long-term stressed condition of the artificial ground, leading to construction of stable ground (Fig. 7).

(2) Construction of Open Office Space The office space above the artificial ground was made an open space with a uniform column diameter of 139.8 mm by applying all the horizontal forces to the vertical braces (Fig. 8). The vertical braces are concentrated, well balanced, and aligned with the core positions of the substructure, thereby minimizing the sections of the horizontal brace members that ensure the roof stiffness. Moreover, the authors used a combination of a pure rigid-frame RC substructure and a steel superstructure with braces to balance the story stiffness ratios of the whole building.

4. Construction (1) Fabrication of PCa Members Because the project site is in a cold region, as many RC members as possible were precast to improve their workability. Figure 9 shows the shapes and layout of the members and Table 1 lists their quantities. The PCa column members were designed to have projections in the east and in the west (Fig. 10).Therefore, concrete was cast only in the north direction. The PCaPC beam members were designed so that their column–beam joints might be integrated. As many

members as possible were made in the same shape, thereby enabling the same forms to be used for fabrication. For the PCa floor members, the cantilever floor members at the corners are large, measuring 6.4 m by 3.2 m, and two sides at the end of each overhang have rises. Special fixtures were used for form removal to prevent the members from being inclined. The same fixtures were used when erecting the members (Fig. 11).

North

West East

Projection

Fig. 9 Layout of members

SG4A

SG3

SG1

SG3

SG1

SG4

SG2

SG4A

sb58M

CPB3

PB1

PB1

PB1

PB1

CPB1 CPB1

CPB1

CPB1

CPB1

CPB1

CPB4A

CPB2A

CPB2

CPB2A

CPB4 CPB2

CPB2

CPB2A CPB3

G1 G1

G1

G2 G2G2

G3 G3

G3

PG1A PG1A

CPG2

CPG2

PG1

CPG1

CPG1

CPG1A PG1A CPG2A CPG2A PG1B CPG1B

CPG2A

PG1

PG1

FG6

FG1

CPG2A CPG1BCPG2A PG1B

PG1A

CPG1B PG1B

PG1

G1

G2

G3

CPG2A

CPG2

CPG2

CPG1

CPG1

CPG1A CPG1A

C1 C1

C2 C2

F2F1

F3 F2a

2FL

RFL

PHFL

1FLGL

平均GL

2FL

RFL

PHFL

1FL

B1FL3200

8800

8800

8800

3200

6400 6400 9600 9600 9600 9600 9600 6400 6400

1800

6400 6400 9600 9600 9600 9600 9600 6400 6400

73600

32800

73600

1695

6000

3800

2600

100

14095 3800

2600

3000

6000 15400

A

B

C

F

D

E

1 2 4 5 6 8 9 1073

1 2 4 5 6 8 9 1073

Fig. 8 Office interior

φ139.8

16,000 16,00028,8006,400 6,400

3,2003,200

24,4002,200 2,200

Fig. 7 Long-term stresses on artificial ground PCaPC beam PCaPC beamSteel beam

Table 1 Quantities of members

Fig. 10 Shape of column member projection

Floor level -Work area Member Qty.

（pieces）

Memberweight（t ）

Grossweight（t ）

B1F‐East Column 8 7.75-10.04 63.96

1F‐West Column 8 14.16-15.05 115.71

1F‐East Column 8 9.52-10.16 78.72

1F‐East Girder 12 9.91-20.05 162.42

2F‐West Girder 12 15.80-24.13 236.26

2F‐West Beam 8 13.70-19.34 130.22

2F‐East Girder 12 15.80-24.13 236.26

2F‐East Beam 9 4.6-19.34 135.55

2F‐East Slab 11 5.06-8.51 63.22

2F‐West Slab 11 5.14-8.51 68.63

2F‐South Slab 12 4.73-6.27 60.21

2F‐North Slab 12 4.73-6.27 59.89

Table-1 Layout of members

North

West East

Projection

Fig.10 Shape of column menber projection

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概　要　日亜化学工業諏訪技術センターは，諏訪湖を一望できる魅力的な執務空間を有する技術センターである。

　構造躯体で構築した人工大地により執務室を浮遊させることで，周囲の豊かな自然環境を室内に取り入れる

と同時に，先進的な企業イメージをアピールする外観デザインとしている。

　メインの執務空間を支える人工大地は，PCaPC 造と S 造を適材適所に組合わせることで，居住性と安全性の

確保に加え，28.8m ロングスパンや6.4m 跳出しといったダイナミックな架構を実現し，空中に浮かせた。さ

らに，構造部材の数と断面を最小限とすることで，フレキシブルな空間を実現した。

　施工面においては，躯体の PCa 化率を高めることで現場作業を削減し，寒冷地での施工効率・品質の向上

と，省人化による労務職不足の克服を同時に実現した。

(2) Erection and Tensioning of PCaPC BeamsBecause the withstand load of the carry-in route was 25t, the following erection program was incorporated in the design phase: splitting the PCaPC beams into three, carrying them on site, and then pressure-bonding the split members on site. The PCaPC beam members split into three were supported by timbering for installation. The PC cables were tensioned prior to injecting the joint mortar into the column capitals so that prestressing might not cause statically indeterminate stress. Because the authors were able to predict that the column capitals would move 3.3mm inside due to the deformation caused by prestressing, the capitals were shifted 3mm outside prior to installation.Steel liners were used at the joints between the PCa column capitals and the lower surfaces of the PCaPC beams, thereby reducing friction and ultimately preventing constraint of the deformation caused by prestressing. The authors also took care so that the timbering supports installed at the joints of the PCaPC beams might not constrain the deformation caused by prestressing during tensioning work. The tensioned members were therefore erected as accurately as ± 5mm or less.Because the tensioning work took place prior to the floor installation, the stress conditions differed greatly between the loads in the construction phase and in the design phase. The authors considered a tensioning sequence that would not induce excessive tensile stress at the pressure-bonded joints (Figs.12 and 13).

5. ConclusionNichia Suwa Technology Center has a dynamic and flexible structural framework that appears to float in the air. Its 28.8m span and 6.4m overhangs were achieved by using a hybrid structure that combines the features of both PC and steel structures.For the construction, the authors increased the percentage of PCa members and were thereby able both to save labor in structural work in a cold region and to improve quality by ensuring accuracy. The building was also well received by its owner, who noted that it provides a good research environment, taking in the surrounding abundant natural environment.Consequently, the authors have realized the architectural plan that was aimed at a symbolic work that would suit two purposes: an open office space commanding panoramic views of Lake Suwa, and a building that could establish the company in the surrounding areas. This project won the JPCI (Japan Prestressed Concrete Institute) Awards for Outstanding Structures and the JSCA (Japan Structural Consultants Association) Kansai Structural Design Presentation 2017 Award of Excellence.

Fig.11  Fitting used to hoist PCa floor member

Pressure bondedFixed end

Fixed end

16,000 3,2002,200 3,20028,800

PCa column

PCaPC beam Steel beam

Steel liner

PC single sliding Pressure bonded

Pressure bonded Pressure bonded PC single sliding

Fig.12 Framing structure of PCa columns and PCaPC beams

Fig.13 Installation of PCa columns and PCaPC beams