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A way to Sustainable Architecture by new technologies for engineered timber structures

Julius NATTERER * Professor hon. EPFL

Swiss Federal Institute of Technology Lausanne, Switzerland

ABSTRACT Only the use of wood in the construction field can save and renew the forests of the world and motivate people to maintain and plant forests in a sustainable way. Today, ecological concerns become more and more important and wood, under the double aspect of the energy necessary to its production and its ap-titude to store CO2, could be the best-suited building material of XX1e century. However, if these ecological concerns take more amplitude and influence, there is another aspect, the economic concern. Thus each project must pre-sent not only one ecological or architectural value, but also an economical one.

Keywords: massive wood, nailed planks, mixed structures, glued and spatial structures

1. INTRODUCTION

Today, ecological concerns become more and more important and wood, under the double aspect of the energy necessary to its production and its aptitude to store CO2, could be the best-suited building materials of XX1e century. However, if these ecological concerns take more amplitude and influence, there is another aspect, the economic concern. Thus each project must present not only one ecological or ar-chitectural value, but also an economical one.

It is necessary to promote the different possibilities where wood as timber can be used. Besides the utilization of high quality wood for high-tech constructions of halls, wide span covers and bridges, one should further develop the possibilities of using medium to low quality for massive-timber construction for floors, walls and roofs, also in association with other materials like steel, concrete, glass or fiber glass.

2. MASSIVE-TIMBER CONSTRUCTIONS

Round, sawn timber constructions or nailed, screwed, doweled and glued massive plank systems today

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allow us to reach these objectives. The nailed, screwed, doweled and glued massive plank system con-sists of planks, aligned one next to the other, face to face and assembled with nails. Massif elements with a thickness that corresponds to the width of the planks are obtained. With these systems, a hypothetical defect in one plank will not lead to a failure of the whole structure. The stress is then taken over by the adjacent planks through the nailing pattern (fig. 1).

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d = 6-18 cm

Variantes

Nailed planks with visible desk

Figure 1. System effect Figure 2. Variant of cross-section

The advantages of these structures are multiple. They allow reducing the necessary static height in comparison to the traditional joist and improve the acoustic protection as well as the thermal inertia. With this system, during summer, overheating of the buildings is limited and in winter, the solar heating is better distributed during the day. These structures may remain visible, coated or not, or recovered with plaster and wall paper. Different variants of sections can be obtained without high costs (fig.2).

For floors, higher spans can be obtained through the use of mixed systems, where wood is in tension and concrete in compression. Materials are thus used to their best abilities. The connection between the two components is realized through grooves and pre-stressed bolts (fig. 4). Depending on the different loadings and the aesthetic requirements, the wood parts can have differing forms: From round wood for bridges or half-round wood for ceilings without any particular aesthetic demands, to nailed planks for normal buildings or even glue laminated beams in “T” section for high stressed floors. Comparing to a traditional concrete slab, the self-weight is heavily reduced (fig.3) and it is fire resistant from 60 to 90 min.

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Comparison live and dead load

A : Compression zone

WOOD DECK

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CONCRETE DECK

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tige filetée M12

résine époxyde(scellement)

capuchon de protectionécrou M12(précontrainte)

tube plastique(protection)

collerette en acier

Pre-stressed bolt

Figure 3. Comparison of live and dead load Figure 4. Principle of liaison between wood and

concrete

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Tower, Lausanne (CH) 2003 Tower high 36 m. Obser-vation platform at 30 m. Diameter: 12 m at base, 6m at the platform. 24 poles half round are dis-tributed around the spiral staircase made of 20x40 cm Douglas sections. The spiral builds two inde-pendent staircases one behind the other. The up-per platform and the two intermediates one are made of nailed laminated timber

Houses of Residence, Arlesheim (CH), 1999 The grouped residence consists of 72 two-story houses. The floor is built from VNP elements. The double diaphragm separates and stabilizes as a solid element between two houses. A layer of plaster panel also covers the visi-ble facets for satisfying fire resistance criteria. Outside of the building, the VNP elements are in Douglas timber with a section of 30 mm x 30 mm which pro-vides well the whether re-sistance without any chemical treatment. In or-der to optimize the con-struction cost, most of the elements are prefabricated.

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Church, Schnerverdingen (D), 2000 The church consists of a free space with a height of 2-stories. The roof sus-pends a platform at the first floor. The casing of the building is erected by VNP element in Oak. The inside bearing structure is built by VNP elements in Pine. Two frames with longitudinal trusses and two transversal frames transmit the load from the platform and the church tower.

Sport hall, Haukivuori (FIN), 1999 Sports hall of 24 x 30 m. The roof is composed of VNP modules between the pri-mary bearing system. This consists of subtensioned beams with a Kerto web. The walls are also made of nailed planks recovered with plywood for stabilization. Due to its easy construction system, unemployed local people can be involved. Before the construction, several tests were made in order to determine the physical characteristic of the nailed planks as well as their fire resistance. Finally, be-fore putting them in place, the primary systems were tested with a load over 50t.

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Schaanwald, 1995 Mul-ti-Family House (CH) The construction consists of five two-family houses. The roof and the floors are single span structures, which are spanning from one partywall to the other. The floors inside one apartment are VNP, visible on both sides. The inter- mediate floors between two apartments are made up of the TCC-system with soundproofing insulation and floating screed. The interior wind bracing walls are VNP, which are covered with wood-particle panels. Due to fire and phonetic related reasons the partywalls were filled with concrete.

Building of residence, Rieselfeld (D) 1999 A 4-story residence build-ing, with vertical and hori-zontal bearing systems in wood-concrete mixed system. The wood-concrete diaphragm consists of two facets in VNP element with 80 mm of thickness and (a con-crete layer with 150 mm thickness in-between due to the 90-minute fire re-sistance was required be-tween apartments). An overhanging gangway is fixed on the front wall of the 4th floor, which continues to the facade of 3rd floor. Braces in compression support these two floors.

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Heavy load bridge, Le Sentier (CH), 1997 Principal purposes were the optimal use of the raw material of a forest munic-ipality and the application of modern civil engineer-ing. The wood concrete cross section consists of eight 13 m long round timber cross sections (∆ 48-72 cm). In order to achieve thereby a parallel limited carriageway slab, this was bilaterally cut in order to have a constant width and provided with discharge slots. The dis-tribution of the wheel loads is made by reinforcement in the grooves of the con-crete composite slab

Wimmis (CH) 1989 Pedestrian and cycle bridge over the "Simme". Main structural system of the bridge of two parallels and over three fields passing trusses with cross beams in the distance of 6.75 m. The horizontal stability of the bridge is obtained by a bracing composed of the lower chords of the truss, the transversal beams and crossing round steel di-agonals. Center zone: 54 m, side fields: 27 m, width: 4.5 m

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3. SPACE STRUCTURES

Besides a quantitative use, high-tech wood constructions like bridges, wide span covers require a detailed planning to obtain a "light" looking structure. Different criteria have to be respected to optimize the structure. First the choice of hyperstatical systems, which transfer the stresses toward higher quality el-ements, allow to make uniform the woods characteristics. Further, the adjunction of members in com-pression reduces the span of the primary bearing system and transfers the stresses from bending, which requests big sections, to normal stresses. If those members are spatially distributed, they allow stabilizing directly the structure. Finally, details are of particular importance because of their costs. It is therefore necessary to reduce their number or simplify them. For instance, compression members can be joined only by contact.

Space structures like shells, ribbed or suspended shells fulfill the first two criteria. They are highly hyperstatical and stressed mainly by axial stresses under permanent loads. However, the high number of nodes necessary for their realization has during a long time limited the construction of such structures. But these nodes, if they are numerous, can also be realized in a easier way. In the screwed plank shells, the joints are made only by planks alternately continued in each direction with filler elements in the other di-rection and screwed to obtain composed elements. All these techniques request a close and excellent collaboration between architects and engineers to best profit of the diversity of forms and texture that wood constructions can give.

Lüterkofen, Sports-Hall (CH) 1993 Only the lower entrance area is a truss grid (span: 16x16 m, grid: 2.3 x 2.3 m) with load distribution in 2 directions. The roof of the sports hall it-self contains main- and sec-ondary truss structures.

Suspended shell, Wien (A) 1981 Suspended roof shell of 170m diameter. The shape of the ribs has been de-veloped to achieve symetrical loads in a state of tension. The covering layer of counter direction-ally aligned planks assume the shear resistance forc-es.

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Open Air Stage, Altusried (D) 1998 The roof structure covers an area of 30 x 100 m.. The 4 highly stressed truss can-tilevers (cantilever 30 m) in a distance of 25 m are in selected round wood trunks. The suspended roof structure that spans between the truss is made up of nailed boards. The different slabs as well as the inclined terraces where made of mixed wood con-crete construction. The wood parts made from round conical trunks (base 32cm top 16cm) thus al-lowing to follow the hemi-spherical form of the trib-une.

Hall for Galley, Morges (CH) 1995 The hall is a shell structure made of screwed planks; it was erected with the help of unskilled workers. The transverse loads due to wind action are supported by external timber trusses, The dimensions of the structure are: Length 60 m, width 20 m and height 12 m

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Sports Hall, Arlesheim (CH) 2000 Sports Stadium 35m X 54m. The roof consists of a barrel form structure sup-ported on the posts. The laminated curves beams are arranged on the roof in order to transfer the hori-zontal force from the vault to the supporting points. The horizontal reactions are beared by the arched binding beams in laminat-ed wood. The arcs placed transversally take the asymmetrical force apply-ing on the structure.

DOME SHELL The following projects are built in grid structure - using the same technique described in the above barrel roofs. Here we have the same advantages and construction methods, with the only difference being the shape of the shell -now it is curved in two directions.

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Polydôme, Ecublens (CH) 1991 The spherical dome structure made of screwed planks construction has the following dimensions: radius: 27.5 m on a ground surface of 25 x 25 m, ridge height: 6.8 m. In the inter-section points planks are bolted to each other. The planking, screwed on the ribs in a diagonal direction, has the function of the bracing in the roof plane The whole structure needed 32 m3 of wood (planks).

Health Center Hall Uzwil (CH) 2004 Kugelförmige Holzhalle als Umfeld für heilende Schwingungen. Die mit einem zylindrischen Anbau ummantelte Kuppelstruktur wurde mit einer Vertikal-schalung verkleidet. Die sich kreuzenden Rippen bestehen jeweils aus sechs Lamellenlagen, die mittels Schrauben ver-bunden werden.

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ExpoDach2000, Hannover (D) For the central meeting area of the EXPO 2000 a wide spaced roof con-struction of 10 square „umbrellas“ was con-structed serving as the main event location. Each „umbrella“ covers an area of 40 m x 40 m and is about 26 m high. It consists of four double curved shell surfaces, having been constructed as partly glue-laminated timber ripped shells. The shells hang over about 26 m and hang on four cantilevers. In the middle of the „umbrel-la“ a big steel structure transfers the forces onto a tower construction. The „umbrellas“ are connected at the outer bending edges of the shells and the ends of the cantilevers. The structural components were mostly manufactured in the plant and in an ex-hibition hall near the site. Big cranes assembled the completely pre-manufactured con-struction parts step by step.

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4. CONCLUSIONS The ecological challenges cannot be taken up without respecting the economic value of the forest,

allowing to cover their costs. The ancients have protected their forest not because of their altruistic spirit, but because the forest gave them more than just some logs to burn. The ability of the forest to resist to the natural elements as well as their protective functions must also be taken into account. But those rules are not bound with financial resources. Using wood for construction assists the chance to save the forests world-wide as it represents a noble use of their product and allows them to be maintained and replanted. Use of wood as an energy source cannot reach this objective.

The role of the forest of the future will have to play for mankind and environment cannot be assured only

through environmental protection- as little as the role of future cities can be granted through the sole protection of monuments.

In the future, the rarefaction of fossil energies and raw materials will mean a growing role for the forest

through wood production and CO2 regulation. It is therefore important to further develop the research on wood, in parallel to the teaching at the 2nd and 3rd cycle, to rapidly diffuse widely these results. The so-ciety must rediscover the privileged bonds it had with wood not only in a nostalgic way, by plagiarizing the traditional construction, but instead being inspired by their concepts. That means a perfect harmony be-tween form and function and the right choice of the materials to be able to respond in a competitive way to the wishes of the modern architecture.

The use of wood is not a “proof” for good architecture. It is, however, an important contribution to the environmental conservation, even if it needs more concentration in the planning phase

“We do not inherit the ground our parents, we borrow the ground of our children” Antoine de Saint-Exupéry “Perfection of means and confusion of goals seem to characterize our age” Albert Einstein

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