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ARQ (Santiago)

On-line version ISSN 0717-6996

ARQ (Santiago)  no.84 Santiago Aug. 2013

http://dx.doi.org/10.4067/S0717-69962013000200014 

 

READINGS

Wood´s Composite Constructions for Free-standing Enclosures

  

Khaled Saleh Pascha *(1)

* Professor, Technische Universität Wien, Vienna, Austria


Abstract

This article understands construction as an exercise that aims to articulate pieces and different materials. From this point of view it presents building components that combine wood with concrete or glass, to be used in horizontal or vertical envelopes.

Palabras clave: Construction, wood structure, enclosures, envelope, building components.


 

Solid or hollow? Heavy or light? Black or white? There exists a utopia called "material purity" in architecture that basically consists of using a single material in the edification. If it is built, it gets mixed up! As we begin to plan buildings we think in different materials, we want to give the building the varied properties only permitted by the combination of different materials. To build means to adapt to the specific conditions of the commission, codes, demand and place using the properties of the available materials.

The association of different materials together to achieve something new and better than the mere sum of each already has tradition, as shows the example of constructions of reinforced concrete. Without a doubt, this combination of concrete with steel creates an extraordinary compound that takes the advantages of a material that works principally in compression and another in tension. But at the same time shows the limits of this construction: basically the upper art of the combined plate works as a compression zone, the rest of the lower transversal section carries the force of tension, and as such the concrete only serves to give a matrix for the frame. The lower part of the slab loads a disproportional dead weight of the whole system.

An alternative would be to execute a slab as an outlined plate of nerves in the lower part resulting in the savings of a certain quantity of concrete material while simultaneously would raise the production cost, and so is only economical in countries with low labor costs. This example shows that no only is the structural analysis is relevant, but also other aspects such as economy, physics and aesthetics in the choice of construction materials are important.

Wood as a construction material has the great advantage of being a light, durable and ecological material. Being a cellulous fiber based material within a matrix made of pectin and lignin (substances that bind the fibers to create a continuous three-dimensional structure), wood combines two important qualities for constructive use. For its density it is a very resistant and adaptable material that can resist extreme forces for a short time without failing. At the same time, its structure based on fibrous tubes give the material its lightness and its performance as an isolating material. Its poor thermal transmittance gives the material excellent isolative properties and helps to reduce the problem of thermal bridges as is the case in its main competitors in the construction area: concrete and steel. However, there are criteria in which wood cannot compete with other materials, particularly in the area of fire protection, acoustic performance and poor thermal inertia (the capacity to activate the mass of heat storage to prevent a rapid heating or cooling effect). The last is a direct result of its lowdensity and poor heat transmittance. Mixing wood together with materials like concrete and stone (as have always existed buildings composed in the history of wood architecture), the thermal mass required can be easily acquired. Above all, this measure improves the behavior of the whole building in the even of a fire as well as its acoustic properties.

A classic historical example of the typology of wooden framework (which in reality is a mixture between load bearing elements of wood with all the advantages that wood carries (high load capacity, high malleability) and a fabric of solid materials (adobe and masonry) that lends other desired behaviors to the mix (thermal mass, acoustical protection, humidity-absorption capacity), things that wood alone is in incapable of providing. Combining materials to achieve a structural, physical and economical optimum is still more important today given the high multi-faceted demands required of a building envelope today.

I. Wood-concrete

An advantageous solution is the marriage of wood with concrete in the matter of slabs, where the lower part of the element is formed by a deck of prefabricated wood panels (made from wooden trusses or solid wood) that works in tension while the compressive forces are taken by the concrete poured over the wood deck of the composed slab. The wood deck provides a completely sealed superior surface (either in a framework structure or heavy construction base); in this way the deck acts like lost formwork for the concrete. The wood-concrete combination for slabs is constituted by surface of wooden pieces (boards over wood beams or simply a wood slab) and over it a layer of fresh poured in place concrete. This way, the properties of each material are positively joined in the structure. The element subjected to tensile forces is the wooden surface below the slab that accounts for 60% of the total height. This wooden tympanum takes the forces of tension while the layer of concrete resists the compressive forces to reach the required net weight –for vibration, noise reduction– (Natterer, Hamm and Favre, 1996). While concrete usually only suffers compressions, only a minimum of reinforcement is necessary to avoid fissures. The sectional forces between the concrete and wood are transferred by means of grooves sawn into the wood surface or through screws or specifically designed metal anchors.

In the research project "The Good House: Horizontal envelope designs for the Wooden dwelling"(1) a study was made of a floor element made of wood with a collaborative concrete topping. One of the objectives was to see if it was possible to develop a prefabricated wooden element that, complementing the slab poured on site, would be able to combine the advantages of concrete (acoustic quality, thermal mass, fire-protection and seismic resistance) to the compound and reduce the costs of logistical forces and diminish construction times compared to a conventional concrete slab (fig. 1).

Fig. 01. Solutions proposed for floors with woodconcrete slabs. Research project "Diseno de envolventes horizontales para la vivienda de madera". The solutions with the concrete slab on top of the wood (EP01, EP03, EP08, EP10) are statically more advantageous yet on the other hand are unable to make use of the thermal mass of the concrete due to the covering of insulating materials on both sides (wood and acoustical isolators). The solutions with the concrete deck below the slab (EP02, EP04) take better advantage of the thermal mass of the concrete but are less efficient structurally.
Author's image

The results of the investigation show that with a construction composed of wood and concrete can support spans of up to 10 meters. It is economically feasible to build these floor elements to extend without supports over beams for more than 7 meters (Fritz and Ubilla, 2012). For example, for a slab with a span of 7 m, 2.0 kN/m2 of live load and approximately 1.0 kN/m2 of dead load, for example, a thickness of 24 cm (14 cm high beams and 10 cm of concrete) is sufficient.

Also, the physical benefits of the construction are generated by the relationship between the materials. The wood deck provides a comfortable environment or atmosphere (high superficial temperature, humidity compensation). For its part, concrete generates the noise and fire protection(2) –up to F90– (fig. 2).

Fig. 02. Photos: Álvaro Carboni.

The acoustical test showed that the noise reduction compared to classic wood floor buildups (platform trusses) is superior. A typical standard slab of wood beams 140/200 mm with a 21 mm osb board on top, has an acoustic isolation factor to aerial noise R´w of 26 dB. The same construction with a 45 mm concrete deck placed on top already has a factor R´w of 51 dB(3), a much better value corresponding to the European code for new buildings in dwellings, schools, hotels etc. Also, the composite constructions of wood-concrete has a much better performance to fire, allowing for if the wood construction fails, the concrete deck remains intact despite major deformations which is acceptable in emergency situations (fig. 3).

Fig. 03. Structural tests for solution EP01 (EP04 in the nomenclature used for tests), a composite slab of laminated pine beams 88 x 185 mm wide, spaced 600 mm apart. Over the beams lies 65 mm H-25 grade concrete slab, reinforced with a ACMA mesh welded C92. The cut keys are concrete trapezoids 50 mm deep and 80 mm wide, and inside carry a 10 mm diameter steel screw between the 20 mm nuts. The results of the bending tests EP01 solution possesses a bending resistance of 1,934 kg/m2, a satisfactory result taking into account that only 23% of the wood was necessary to achieve it in comparison with the previous solution. The amount of concrete in the slab that now works with the wood was augmented slightly to 144%. The respective security factor to failure for the case of total load (fs) is 5.6, while the service moment is 0.83.
Photos: Álvaro Carboni

The technology can be applied, not only in new construction, but also in the interesting field of application of remodeling existing buildings and reinforcing aging woods beams. The new technology has the advantage of being more economical and result in less loss (regarding the original substance) compared to the whole wooden parts replacement. For new construction the physical advantages of fire protection, noise and vibration control stand out along with the advantageous possibilities of prefabrication. The studies and experiences in various countries show that the wood-concrete composite has the excellent property of supporting loads and an excellent response to the deformation problem.

II. Wood-glass

The load bearing glass joined to a metal sub-structure such as the "Structural Sealant Glazing" type, is already widely used and technologically recognized, as in automotive construction, for example, where its principal use is as part of the structural hull of the car body. In construction, its use is generally infrequent, due to the size of the elements, their long term performance and the compressive tensions produced between the metal and the glass when they expand from thermal difference, a much greater effect in construction than in automotive building due to the size of the elements.

It is for this reason that combining glass with wood has a series of advantages that run from a favorable structural performance for having very similar physical properties (a similar thermal dilatation) (Hochhauser, 2011) to the possibility of mutual protection between the materials involved (protection for the wood from the elements using the glass on the exterior side and/or protection from the glass to avoid force increases in the contact, using the flexibility and adaptability of wood). Due to the fiber structure base, wood presents good resistance properties to traction while the glass resists compressive forces well, showing a tendency towards break danger when the forces pass certain limits (Blyberg, Serrano and Enquist, 2012). While with wood, the material acts inversely: under the demands of large loads, the wood tends to deform plastically until it fails completely, maintaining a certain load bearing capacity for a certain time, including in load situations above the calculation studies.

So, the combination of wood and glass results beneficial due to that favorable combination of two materials with very different structural aspects (fig. 4). Lately, composite elements that combine wood and glass have been analyzed within several research projects (Blyberg, Serrano and Enquist, 2012; Weinand, 2007) and a number of final dissertations and thesis (Hochhauser, 2011; Neubauer, 2011; Wellershoff, 2006). As a result, today it is possible to find some building prototypes that include this kind of composite elements (fig. 5a and 5b)(Kreher, Natterer, 2004).

Fig. 04. Wood with glass fixation principals. The transfer points of the loads were developed in such a way that only allow for the introduction of pressure forces in a controlled way, avoiding that traction forces or moments be transmitted to the glass which be harmful to them.
Source: Thomas Edl.

Fig. 05a. Schindler Glass Tower, Germany. Built with glasswood composite boards.

Fig. 05b. Interior view, showing glass panes fixed to the wood frame.
Photos: P. Schober.

The objective of the research project "Elements composed of Glass-wood"(4) of the Pontificia Universidad Católica de Chile is to develop a new composite building element for construction of wood and glass, joining two things that until now have been considered contradictory: to be load bearing and at the same time highly transparent (fig. 6).

Fig. 06. Joint system. Only in the frame angles are there joint elements that resist compression and thus allow the respective forces to the glass that can be transmitted in a diagonal direction along the glass pane. The system does not allow the transmission of traction and as such only one side will be structurally activated at a time. By restricting the introduction of forces at the angles, moments and bending forces are not produced on the glass. The silicone transfers the pushing forces from dilatation up to a certain point.
Source: Author's image.

The goal is to develop industrialized products based in glass and wood with certain criteria and quality standards with constructive and technological concepts of efficiency and which respond to the structural requirements and solicitations, structural requirements, climate demands, fire protection and that resolves problems of humidity, thus opening new markets both at home and abroad for products with added value originating from the Chilean forest sector.(5)

Two different, basic principles have been studied for the installation of the glass panel: first, a line of binding tape runs along the perimeter of the panel and second, a combination of flexible adhesive and rigid blocks in the corners appears to transfer the load to the glass pane. The last variable requires an adhesive that is sufficiently malleable to transfer the force in an efficient way and sufficiently rigid so as to not cause excessive deformations. In this case, the glass panel is used as a compressive element, leaving a compressed diagonal in the glass plate activated during the load application (fig. 7).

Fig. 07. Two introductory principles of glass loads: 1 – pushing: produced in the line of contact between glass and wood when load is applied; moments and torsion forces within the glass pane. 2 - compressed diagonal: upon introducing loads only on the corners of the plate. The idea of the proposed solution is that the pushing component is reduced to the minimum possible (through using a highly malleable silicone adhesive) in favor of a concentration of introduction of the load by means of some dowels in the angled points of the glass plate.
Source: Author's image.

The technology applied for the transmission of forces between the glass and the wood is made through a specially formed frame that guarantees the mounting of elements of large dimensions given that it only needs to be assembled on site, and at the same time simplifies prefabrication which has been done in studio conditions with standards of cleanliness, temperature and clean environment. While permitting a rapid installation without the need for framing, this system maintains the position of the glass panel under loads in a way that allows for the movement from differential thermal expansion, induced load deflection and structure settling (fig. 8).

Fig. 08. Prototype (2010) of a façade with glass-wood composite elements realized in the investigative project "Holz-Glas-Verbundkonstruktionen: Berechnung und Bemessungskonzept", financed by the OFG, 2008-2011 (Holzforschung Austria, Technische Universität Wien, 2010).
Photo: Khaled Saleh Pascha.

This element will form part of the building envelope, acting as a conventional load-bearing element that carries load but tends to be opaque. Its position within the building can vary in the same way; it can be integrated into the façade, the roof, in the interior as a partition wall and even as a floor element (fig. 9).

Fig. 09. Prototype of a glass-wood element with a birch plywood coupling-bar (Petschenig Glastec GmbH, Austria). Based on an investigative project from the Holzforschung (Center of Investigation for Wood) in Austria in which a wood-glass composite element is developed using a line of adhesive that works in base of pushing force. This prefabricated wood-glass element already comes with the woodconnecting element glued to the glass in production. The adaptor chassis is screwed to the load bearing structure on site. This way the panel is interchangeable.
Photo: Khaled Saleh Pascha.

A wood-glass structural composite element is developed in this project for a variety of possible building applications. Possible applications of this product in architecture in Chile could include office buildings, prefabricated homes, greenhouses, load-bearing façade elements, structural elements as part of a system that improves an existing building and of prefabricated dwellings with integrated thermal and photovoltaic energy systems (fig. 10).

Fig. 10. Thermal panel fixation principle.
Source: Khaled Saleh Pascha, Vitalija Rosliakova.

This innovative product can be achieved with a series of demands. From our point of view it will provide new perspectives and applications: -It could lead to an economic solution for small buildings with transparent façades such as single-family dwellings, small warehouses, etc. Here the façade itself carries part of the building loads and also in part gives rigidity to the building. -it could substitute a conventional façade joint with its load bearing structures due to the fact that gluing the glass with the wood simplifies is execution. The wood as a nationally produced material can substitute the importation of specialized metal shapes.

-In the case of traditional constructions, this technology can be used to improve the seismic behavior its structure. The large quantity of older homes built in adobe or other materials that have been responding to loads for long times tend to suffer in seismic situations due to their materiality and building type. Such buildings do not resist the horizontal and traction forces of a seismic event and so developing a simple and effective solution for a second skin based on this technology would reinforce the existing construction and allows the transfer of seismic loads to new areas that would collaborate with the older parts of the construction. A glazed façade placed in front of the existing façade would also protect it from weather while not modifying its appearance.

-It allows the creation of roof coffers and structural facades that contain thermal and hybrid solar panels: here energetically active envelopes are created that capture solar energy and integrate thermal solar collectors and photovoltaic collectors. These elements would come with the glass attributes of impermeability; that is to say as a cover material. The will be structural elements with multiple functions making them more efficient and economically valuable (fig. 11).

Fig. 11. Wood-glass façade element with the integration of solar panels (photovoltaic hybrid element in the laminated glass and solar collector in the coffer cavity.
Source: Khaled Saleh Pascha, Vitalija Rosliakova.

Conclusions

There are profound changes in the demand of building in quantity, quality, size, functionality and flexibility. The technical possibilities have multiplied, and today new materials exist, new construction methods and new knowledge over environmental aspects and sustainability criteria.

Together with this trend, contemporary architecture has oriented toward quality of live and low energy consumption. Also the objective of reducing the co2 emissions and greenhouse gases means that the buildings will have to lead with solar design and ecological materials concepts. The glazed façade with its selective transparency is the most important component to obtain energetic gains with solar energy.

Wood as a sustainable and ecological material has a good opportunity to augment its impact in the world of construction. The excellent seismic and structural behavior of wood in Chile feeds this trend. The combination of wood with glass offers a new opportunity to build seismically resistant buildings with a lightweight, transparent and aesthetically attractive construction.

Today Chile not only has a high quality production of pine (for its optimum climate and soil conditions from the south of the country), but also has a prepared logging industry. The problem/opportunity resides in that this industry has primarily focused on foreign markets with low value-added products, leaving the national market aside. The "Glass-wood composite elements" technology could take advantage of this industrial capacity installed to innovate and improve building in Chile while also opening opportunities for commercialization in other countries in the region with greater value-added products.

The development of the industrialization of these components with good quality products will generate a business impulse with job creation in higher quality and better-paid jobs. Benefits would be added to the wood industry incentivizing the use of pine as well as those generated by energy efficiency that can be attained by wood-glass buildings. This technologically change will have effects on the country, the rise in the product value that will position Chile in the international context as a country producing wood-based composite systems for construction instead of just a timber country.

 

Notes

1. FONDEF D06i1034 "La Buena Casa. Diseno de envolventes horizontales para la vivienda de madera". Pontificia Universidad Católica de Chile School of Architecture. Centro de Innovación y Desarrollo de la Madera CORMA-UC (CIDM).

2. Structural tests for the solution EP09 (EP03 in the nomenclature used for tests), a floor composed of solid wood and a concrete slab. The test is composed of two prefabricated pieces 600 mm wide, made up of serrated pine wood 1 1 x 6 (30x138) and 1 1 x 4 (30x92) joined by formaldehyde adhesive. Over the wood deck exists 200 mm of wool insulation, 0.2 mm polyethynol, an acoustic isolator of 5 mm and over this a H-20 grade concrete slab 45 mm thick, reinforced with mesh to protect from cracking from the retraction of the concrete. Differing from the original proposal, the composite is no collaborative, that is to say that the concrete works independently without connective elements between the two materials. This test consists in apply point loads along the thirds of the floor. In this way a zone of the floor subjected to the pure bending moment is achieved (the central third) and two zones subjected to uniform loads and linear variable moments (the two exterior thirds). To load the prototype two 20 ton jacks loaded over two steel beams. The beams were supported by posts and two 6 x 6" section oak pieces that rest on the concrete slab, located approximately at the thirds of the span, 380 cm between supports. The deflection was measured by 4 transducers in the midpoint of the span of the floor. In each test, a load cycle was realized up to the service level, followed by a load cycle up to failure. The tests realized, supervised by uc professors Hernán Santa Maria and Rafael Riddell, have permitted the evaluation of the security factor to the failure (resisting moment / service moment). The solution possesses a resistance to the bending moment of 2.395 kg/m², a value 10 times higher than the temporary loads considered for the use of the floor for a bedroom, office, etc. of 200 kg/m². The respective security factor for the total load (FS) is 6.5, while the service moment is 0.87.

3. Leonardo Meza, professor at Universidad Católica, calculated the acoustic performance of EP01 solution (see fig. 3) regarding aerial noise and acoustic pressure levels using INSUL v 6.0 software.

4. CORFO, Line 1, I+D Aplicada Profile, 12IDL1-13097 – Glass-wood composite elements. For Pontificia Universidad Católica de Chile professors Khaled Saleh Pascha, Waldo Bustamante, Claudio Vasquéz, Francisco Chateau and Christian Bartlau (scientific collaborator) participated. Coexecutors from the industry are infodema and Solimpeks/Vicmar. Chile is part of an international team that gathers academic institutions, technical investigation institutions and the main glass and wood producers in Europe (Austria, Germany, Sweden, Slovenia, Turkey) and Brazil, under the name "URBAN WOOD based construction for multi-storey buildings. The Potential of Application of Timber-Glass Composite Structures for Building Construction" (Joint Call of the ERANET WoodWisdom-Net 2 y ERANET Bioenergy Programm).

5. As the European members of the project based their efforts in the technological development of the components of the composite element –for example, in the Technical University of Dresden are the engineering experts for glass adhesive, in Sweden those for glass and Austria, those for wood; in the case of Slovenia, in the University of Liubliana, are the seismic engineering experts–, the Chilean part in the project development will consist in large part in the development of possible products based on this technology.

 

Referentes

BLYBERG, Louise; SERRANO; Erik y Bertil ENQUIST. "Adhesive joints for structural timber/glass applications: Experimental testing and evaluation methods". International journal of adhesion and adhesive Vol. 35. Elsevier, Philadelphia, June 2012, p. 76–87.

FRITZ, Alexander and Mario UBILLA. (eds.). Manual de diseno. Construcción, montaje and aplicación de envolventes para vivienda de madera. Centro de Innovación y Desarrollo de la Madera PUC-CORMA, Santiago, 2012.

HOCHHAUSER, Werner. A contribution to the calculation and sizing of glued and embedded timber-glass composite panes. Civil Engineering Doctoral dissertation directed by Professor W. Winter, Technische Universität Wien, Vienna, 2011.

KREHER, Klaus, NATTERER, Julius and Johannes NATTERER. "Timber-Glass-Composite Girders for a Hotel in Switzerland". Structural Engineering International SEI Vol. 14 No 2. International Association for Bridge and Structural Engineering, Zurich, 2004, p. 149-151.

NATTERER, Julius; HAMM, Jan and Pierre-Aimé FAVRÉ. "Composite wood-concrete floors for multi-story buildings". Proceedings of The 4th International Wood Engineering Conference. Omnipress, Madison, 1996, p. 431-435.

NEUBAUER, Georg. Entwicklung und Bemessung von statisch wirksamen Holz-Glas- Verbundkonstruktionen zum Einsatz im Fassadenbereich. Doctoral dissertation directed by W. Winter, Technische Universität Wien, Vienna, 2011.

WEINAND, Yves and Klaus KREHER. Holz-Glas-Verbund als großflächige Scheibensysteme zur Gebäudeaussteifung. Final report 06/07 – Research project buwal No 2005.05. École Polytechnique Fédérale de Lausanne, Lausanne, 2007.

WELLERSHOFF, Frank. Nutzung der Verglasung zur Aussteifung von Gebäudehüllen. Doctoral dissertation directed by M. Feldmann, Rheinisch-Westfaelische Technische Hochschule Aachen. Shaker Verlag, Aachen, 2006.


1. Khaled Saleh Pascha. Master of Engineering Dipl.- Ing. Arch., 1995 and Doctor of Engineering Dr.-Ing. Arch., 2004, Technischen Universität Berlin. His research focuses in the building area, specifically wood science, bioclimatic architecture and façades systems. He has written several articles and chapters of books on wood construction, energy efficiency and theory of architecture; he has been published in England, Austria, Germany and Chile. He is currently director and responsible researcher of the CORFO research project "Diseno y aplicación de elementos estructurales compuestos en base a vidrio y madera para la construcción de edificios" 12IDL1-13097; he is assistant professor at Pontificia Universidad Católica de Chile and Technische Universität Wien, at the department of Wood Science and Structural Engineering.

BLYBERG, Louise; SERRANO; Erik y Bertil ENQUIST. "Adhesive joints for structural timber/glass applications: Experimental testing and evaluation methods". International journal of adhesion and adhesive Vol. 35. Elsevier, Filadelfia, junio de 2012, p. 76-87.         [ Links ]

FRITZ, Alexander y Mario UBILLA. (eds.). Manual de diseño. Construcción, montaje y aplicación de envolventes para vivienda de madera. Centro de Innovación y Desarrollo de la Madera PUCCORMA, Santiago, 2012.         [ Links ]

HOCHHAUSER, Werner. A contribution to the calculation and sizing of glued and embedded timber-glass composite panes. Tesis para optar al grado de Doctor en Ingeniería Civil, profesor guía W. Winter, Technische Universität Wien, Viena, 2011.         [ Links ]

KREHER, Klaus, NATTERER, Julius y Johannes NATTERER. "Timber-Glass- Composite Girders for a Hotel in Switzerland". Structural Engineering International SEI Vol. 14 No 2. International Association for Bridge and Structural Engineering, Zurich, 2004, p. 149-151.         [ Links ]

NATTERER, Julius; HAMM, Jan y Pierre-Aimé FAVRE. "Composite wood-concrete floors for multi-story buildings". Proceedings of The 4th International Wood Engineering Conference. Omnipress, Madison, 1996, p. 431-435.         [ Links ]

NEUBAUER, Georg. Entwicklung und Bemessung von statisch wirksamen Holz-Glas-Verbundkonstruktionen zum Einsatz im Fassadenbereich. Tesis para optar al grado de Doctor, profesor guía W. Winter, Technische Universität Wien, Viena, 2011.         [ Links ]

WEINAND, Yves y Klaus KREHER. Holz-Glas-Verbund als großflächige Scheibensysteme zur Gebäudeaussteifung. Reporte final 06/07 - Proyecto de investigación buwal No 2005.05. École Polytechnique Fédérale de Lausanne, Lausanne, 2007.         [ Links ]

WELLERSHOFF, Frank. Nutzung der Verglasung zur Aussteifung von Gebäudehüllen. Tesis para optar al grado de Doctor, profesor guía M. Feldmann, Rheinisch-Westfaelische Technische Hochschule Aachen. Shaker Verlag, Aachen, 2006.         [ Links ]

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