CHARACTERIZATION OF TIMBER FRAME STRUCTURAL SYSTEM OF TABIQUE WALLS

Jorge Pinto¹, Sofia Ribeiro¹, Vítor Cunha¹, Nuno Cristelo¹, José Lousada¹, Anabela Paiva¹, José Padrão², Humberto Varum³, Aníbal Costa^3
1 University of Trás-os-Montes e Alto Douro, Vila Real, Portugal
² Polytechnic Institute of Viseu, Viseu, Portugal
³ University of Aveiro, Civil Engineering Department, Aveiro, Portugal

ABSTRACT

Tabique construction corresponds to a relevant Portuguese heritage. Urban and rural tabique constructions exist all over the country. This traditional building technique started to be in disuse after the massive introduction of the reinforced concrete. Therefore, tabique buildings are already aged and, in general, they show signs of degradation. Recent research works led to the conclusion that this degradation stage is essentially caused by the absence of a regular and proper maintenance. The fact that tabique buildings are mainly private property, that there are economical limitations and that there is still a lack of knowledge concerning this traditional building technique, have been the main reasons identified to its degradation. In order to contribute to solve this problem, this paper intends to give an input on the characterization of timber frame structural system of tabique walls. In fact, a tabique building element is a structural system formed by a timber frame coated with an earth render. Technical information related to distinct traditional structural solutions are given and may be helpful in future rehabilitation processes.

Key words: Tabique construction, timber frame structures, traditional timber structures, rehabilitation, safety, durability.

INTRODUCTION

Research has been done in order to study the existing tabique heritage in the northeast of Portugal [1–5]. The fact that there is still a lack of published technical information related to this context and that the actual codes do not contemplate specific design rules for tabique have been the main motivation aspects to perform these works. At the same time, it has also been noticed that this type of construction presents a certain stage of degradation and that the respective main cause is essentially concerned with the lack of a proper maintenance [6]. Thus, frequently rehabilitation procedures are required. This research work may be helpful in order to give guidance for the above mentioned rehabilitation procedures.

Tabique is a traditional building technique that applies natural building materials. Wood and earth are two of these building materials. In fact, wood plays a relevant role in terms of structural behaviour, since the main structural solution of a tabique building is wood based. Therefore, both vertical and horizontal structural elements of these constructions are likely to be timber elements. In parallel, earth is applied as a finishing material. It has the ability of protecting the timber structure contributing significantly for the durability of the construction.

In fact, other building solutions are applied in the context of tabique aiming to assure better durability. Providing the outer face of an external tabique wall with a revetment is one example of these solutions. Metal plates, schist tiles and ceramic tiles are some traditional revetment solutions, which are able to increase the waterproofing of a tabique wall avoiding the deterioration of the earth render and the integrity decay of the timber structure.

Meanwhile, the existence of a Portuguese tabique heritage is itself a fact that assures that this type of construction is durable and safe, considering that most of the buildings are over one hundred years old. Therefore, knowing the technical specificities of this building technique may help future rehabilitation processes and also may give guidance for applying this construction technique in new building. For instance, focusing on the typical timber frame solutions of tabique walls and understanding their structural behaviour is an example. Comparing these structural solutions with similar ones applied in other type of construction may also be technically profitable. Thus, the main objective of this research work consists on attempting of presenting, describing and correlating the traditional timber structural frames of tabique walls with similar type of structures applied in traditional Portuguese barn. Additionally, it is also intended to highlight another interesting building aspect of tabique construction, which is related to the fact that there is a tendency of using agricultural organic products waste as alternative building materials.

Thus, this paper is structured as follows: firstly, the tabique construction is contextualized; secondly, six alternative timber structural frame solutions of tabique walls are introduced and briefly described; thirdly, an attempt of comparing structural timber frames of tabique and barn is done; fourthly, building details based on the application of organic materials including agricultural wastes are highlighted; finally, the main conclusions are drawn.

CONTEXT

Tabique is a Portuguese traditional building technique that had been applied intensively until the early twentieth century. The north region of the country is rich in tabique heritage. According to Cardoso [7], tabique construction can be categorised as urban and rural. In the city centres, tabique buildings tend to have more floors. For instance, the tabique buildings shown in Figure 1 have three and four floors, respectively. These buildings are located in the city centre of Amarante that is a small town in the north of Portugal. In this case, the partition walls are in tabique as well as the external walls of the upper floors. The tabique building highlighted in Figure 1 was under a rehabilitation process in which maintaining the original building solution was a design priority.

Fig. 1. Examples of tabique dwellings

As it was stated earlier, several studies have been done in order to characterize the existing tabique heritage in the northeast of Portugal [1–5]. The fact that a dwelling of two floors is a current tabique building solution and that tabique buildings tend to present a generalized degradation stage because of the lack of maintenance procedures, are two conclusions that have been highlighted in these studies.

A tabique building element such as a wall is composed by a structural timber frame covered with an earth render. Thus, wood and earth are building materials that play an important role in this context. The wood works as structural material and earth works as a finishing material. The earth render has the ability of protecting the timber structure from actions such as fire or insect attack. Therefore, this building solution has adequate durability for normal applications under normal conditions. Applying earth or earth mixed up with a small amount of lime are currently applied solutions in the typical earth renders of tabique constructions [1–5].

Tabique walls are preferentially built in the uppers floors, Figure 1, It is uncommon to find tabique walls placed at the ground floor, because  the direct contact of a tabique wall with the ground may proportionate decay and insect attack scenarios of the wood. Therefore, on the ground floor the walls are built with granite or schist. .

In the context of tabique construction, the stone masonry and the tabique walls are the main vertical structural buildings elements, and the timber floors are the main horizontal structural ones. Thus, it is a construction technique deeply wood orientated.

SOME TIMBER FRAME SOLUTIONS OF TABIQUE WALLS

Different timber frame solutions of tabique walls have been identified so far.Six distinct solutions are presented in Figures 2, 3 and 4, respectively. The totality of these solutions are related to single tabique walls. There is also the possibility of having double tabique walls. This last building scenario is not visualised in the above identified figures. Moreover, the tabique timber frames presented do not contain the earth render.

a. b.

Fig. 2. Structural timber frame solutions of tabique walls: a) Solution 1, and b) solution 2

a. b.

Fig. 3. Structural timber frame solutions of tabique walls: a) Solution 3, and b) solution 4

a. b.

Fig. 4. Structural timber frame solutions of tabique walls: a) Solution 5, and b) solution 6

A current applied timber frame solution (Solution 1) of tabique walls (both external and internal) is based on a sequence of vertical timber boards linked to each other by a set of horizontal timber battens (laths) placed on both faces of the vertical timber boards, Figure 2a. Vertical and horizontal timber elements are nailed to each other. Usually, the timber boards present an irregular geometry and a rugged finishing surface, Figure 2a.

Another alternative solution (Solution 2) of timber frame was found in a tabique dwelling, Figure 2b. In this case, the main structural elements correspond to timber boards displayed sideward. Laths are also applied in both faces of the wall to connect the main timber elements and also to increase the adherence of the earth render to the timber structure.

Complementarily, an elaborated solution (Solution 3) consisting on presenting a complex arrangement of oblique timber boards is presented in Figure 3a. The set of laths is also applied in both sides of the all for the same reasons indicated above. This solution with oblique timber elements is more suitable to face up with horizontal loadings such as wind and earthquakes.

In Figure 3b, another solution (Solution 4) of timber frame of a tabique wall is introduced. This solution is quite similar to Solution 1, Figure 2a. In fact, in this case, an oblique timber board is incorporated in the timber frame formed by a mesh of vertical timber boards and laths which characterizes Solution 1. The added oblique timber board has an approximate 45º slope and it is placed in the plane of the wall. Thus, the vertical boards have to be sectioned in the intersection with the oblique boards. Laths are also applied as described above.

Solution 5 also presents some similarities with Solution 1, because the regular timber frame made of vertical timber boards and laths are applied. However, in this case, this regular timber frame is reinforced with a stiffer complementary structure. This complementary timber structure is formed by one vertical and two oblique boards that are applied in the plane of the wall as shown in Figure 4a.

Finally, the last structural timber frame solution of tabique wall described in this paper is designated as Solution 6 and it is presented in Figure 4b. A net of vertical and horizontal timber boards reinforced with oblique timber boards characterize this solution. The vertical and horizontal timber boards have similar stiffness and they are applied sparser. The horizontal boards are only applied in one side of the wall.

To have more than one type of timber frame structural solution in the tabique walls of the same building is a common scenario.

Based on our experience, there is a huge size variability of the structural elements which form the different solutions described above. Having timber boards placed sideways also increases the stability of a tabique dwelling to horizontal loading resulting in buildings much more robust and safe. These structural solutions have several timber structural elements linked to each other making a traditional structural system. This structural solution is adequate because the structural vulnerability decreases and also the risk of vulnerable failure scenario. Therefore, they contribute for robust constructions [8–9]. In addition, typical building details related to the connection between tabique walls, and tabique walls and masonry walls are delivered in [4].

All the structural timber frame solutions of tabique walls shown in Figures 2, 3 and 4 were collected from old buildings. In general, most of them present a very good conservation level, which means that they have adequate durability. We believe that this achievement is mainly due to the fact that the timber structure was covered with an earth render and kept dried during time. For instance, the tabique wall featured in Figure 2b is partially suffering from decay due to water infiltration. Problems of the waterproofing ability of the roof allowed a continuous direct contact of the wall with the rain water resulting in this pathology. In contrast, the other examples of timber structural frames are very well preserved.

ANALOGY BETWEEN STRUCTURAL TIMBER FRAMES OF TABIQUE AND BARN

There are interesting structural similarities between the timber frame system solutions of tabique walls and traditional Portuguese barn. In both cases, the main structural element corresponds to a regular timber frame panel system. Vertical and horizontal timber elements nailed to each other are used. Simplicity, symmetry and regularity are some adjectives that characterize these systems.

Taking into account that the main function of a barn correspond to store corn cereal, adequate natural ventilation is required. Therefore, the timber frame of the walls does not tend to be filled or covered, Figures 5 and 6. In contrast, the timber frame of a tabique wall requires to be covered in order to fulfil its purposes, which include partition, thermal and sound insulation functions.

a. b.

Fig. 5. Some examples of structural timber frame solutions of traditional Portuguese corn granary: a) Solution 1, and b) solution 2 (Key: I – Vertical element; II – Horizontal element; III – Diagonal element; IV – Beam; V – Column; VI – Brace)

a. b.

Fig. 6. Some examples of structural timber frame solutions of traditional Portuguese corn granary:a) Solution 3, and b) solution 4 (Key: I – Vertical element; II – Horizontal element; III – Diagonal element; IV – Beam; Column; V – Brace)

In general, a traditional Portuguese barn construction is formed by columns that support the beams. Timber framed walls are supported on the beams and they support the roof structure. The roof may be covered with ceramic tiles, metal plates, timber, straw or stone (e.g. granite or schist). The barn constructions presented in Figures 5 and 6 feature some of these building details.

Focusing on timber frame system solutions of the walls of barn construction it is verified that there are different alternatives. This fact also occurs in the tabique context and as it was described in the previous section. Figures 5 and 6 highlight four different alternative solutions of these types of systems. Solution 1, Figure 5a, is related to a regular timber frame formed by vertical boards linked to each other by horizontal battens. On the other hand, Solution 2, Figure 5b, corresponds to a timber frame essentially formed by vertical timber boards connected to each other by three horizontal and two oblique boards.

ORGANIC MATERIALS

As it was stated earlier, a tabique element is mainly based on raw materials such as wood and earth. Only the nails are industrialized because they are steel alloy, [7] and [10].

Several samples of elements of the timber frame of tabique walls have been collected and tested. The wood species identification has been one of the tests performed. It has been concluded that different types of wood species can be applied in the tabique context. These wood species are generally autochthonous ones. They can be made of hard or soft wood. Generally, the timber elements are preferentially applied without any treatment, which facilitates the adherence of the earth render. This technological particularity also gives strength to the idea that tabique may be considered as a low technology building technique. In [6], a summary of the obtained results related to some experimental work done concerning the wood specie identification of timber elements of tabique walls existing in the north of Portugal is presented. This summary indicates that the main wood species identified are by order of incidence importance the Pinuspinaster, 65% of incidence, the Castaneasativa Mill, 25% of incidence, the Populussp, 7% of incidence, and the Tíliacordata, 3% of incidence. Pinuspinaster is clearly the most applied wood in this context. These species are autochthonous of the north of Portugal. Other wood species types are likely to be found in other parts of Portugal.

Additionally, as it is reported by Cardoso [7] and Paiva [11] different alternative types of organic products have also been applied as a filling of the existing gap between the main boards of the timber structural frame of tabique walls. The organic products already identified are straw (Fig. 7a), wood shaving (Fig. 7b), dried onion foliage (Fig. 8a) and corn cob (Fig. 8b). Wood shaving, dried onion foliage and corn cob may be considered as organic waste products and therefore these traditional building applications may be interesting in the perspective of alternative sustainable building solutions.

a. b.

Fig. 7. Some examples of organic filling solutions of the timber frame of tabique walls: a) Solution 1– straw, and b) solution 2 – woodshaving (adapted from [7])

a. b.

Fig. 8. Some examples of organic filling solutions of the timber frame of tabique walls: a) Solution 3 – dried onion foliage (adapted from [7]), and b) solution 4 – corn cob

CONCLUSIONS

The existing tabique construction in the northeast of Portugal was briefly described and some specific technical aspects were introduced. The fact that it is a traditional construction highly dependent of raw building materials (e.g. wood, stone and earth) was empathized.

Six alternative structural timber frame solutions of tabique walls were presented and described in brief. These timber frame solutions are made of a regular net of timber boards. In general, they are based on a regular frame formed by vertical timber boards and laths. In this regular frame, the vertical boards are the main structural elements taking into account that are stiffer than the laths. The laths are applied on both faces of the wall guarantying the connection between the vertical timber boards and also facilitating the application of the earth render. The structural timber frame solutions introduced tend to differ among them mainly in terms of the inclusion of oblique timber boards. This technical aspect is important considering that the ability of a tabique wall to support horizontal loads such as wind or earthquake loads increases sharply by considering oblique structural elements. The robustness of tabique buildings and also their safety may increase by considering the incorporation of oblique timber elements. Additionally, this technical information concerning the richness of alternative structural timber frame solutions available indicates the versatile attribute of the tabique construction technique.

The fact that these types of timber structures are covered with an earth render and that they are kept dried justifies their high durability.

Some examples of traditional Portuguese barn constructions were also presented. Four alternative timber frame system of wall solutions of this type of construction are identified and described in brief. An attempt of correlating these solutions with the similar systems applied in the tabique context was also done. It leads to the conclusion that these types of systems tend to be regular, symmetric and simple. The application of oblique timber boards seems to be a current applied technical solution aiming to achieve a robust construction and dealing with horizontal loads.

It has been noticed that the application of organic products such as wood as structural material, and wood shaving, straw, dried onion foliage and corn cob as filling materials are building techniques applied in the tabique context. The applied wood species are mainly autochthonous. Additionally, there is evidence that these species may vary Meanwhile, some of these organic products are agricultural waste such as dried onion foliage and corn cob. This practise may correspond to a sustainable, affordable and low technology building technique option, that can be applied in new construction and rehabilitation.

REFERENCES

1. Gonçalves C., Pinto J., Vieira J., Silva P., Paiva A., Ramos L., Tavares P., Fernandes L., Lousada J., Varum H., 2010. Tabique construction in the municipalities association of the Terra Quente Transmontana. [in:] 3rd WSEAS International Conference on Cultural Heritage and Tourism, Corfu, Greece, 235–240.
2. Pinto J., Varum H., Cepeda A., Tavares P., Lousada J., Silva P., Vieira J., 2010. Study of the traditional tabique constructions in the Alto Tâmega region. The sustainable world, 299–307. WIT Press, Southampton.
3. Pinto J., Cardoso R., Paiva A., Cunha S., Cruz D., Vieira J., Louzada J., Varum J., 2011. Caracterização de paredes tradicionais de tabique. [Characterization of traditional Tabique walls.] [in:] Conference “Paredes Divisórias 2011”, Porto, Portugal, 25–35 [in Portuguese].
4. Cardoso R., Paiva A., Pinto J., Murta A., Varum H., Nunes L., Ramos L., 2011. Building details of a tabique dwelling in Trás-os-Montes e Alto Douro region. [in:] 12th International Conference on Durability of Building Materials and Components, Porto, Portugal, 729–736.
5. Pinto J., Varum H., Cruz D., Sousa D., Morais P., Tavares P., Lousada J., Silva P., Vieira J., 2009. Tabique Construction Characterization in Douro North Valley, Portugal: A First Step to Preserve this Architectural Heritage – 2nd WSEAS International Conference on Urban Rehabilitation and Sustainability (URES'09) – Environmental Science and Sustainability – Proceedings published by WSEAS Press (printed and in CD), Editors: Manoj Jha, Charles Long, Nikos Mastorakis, Cornelia Aida Bulucea, Book Series: Energy and Environmental Engineering Series, 48–53 (ISi-JCR).
6. Pinto J., Gülay G., Vieira J., Meltem V., Varum H., Bal I.E., Costa A., 2014. Save the Tabique Construction. Structural Rehabilitation of Old Buildings. Building Pathology and Rehabilitation. Volume 2. Editors: Aníbal Costa, João Miranda Guedes, Humberto Varum. Springer Berlin Heidelberg, 157–185.
7. Cardoso R., 2013. Construção de Tabique – Construção Sustentável [Tabique Construction – Sustainable Construction.] PhD thesis in civil engineering. Beira Interior University. Covilhã. Portugal [in Portuguese].
8. Pinto J., Blockley D., Woodman N., 2002. The Risk of Vulnerable Failure. Structural Safety, volume 24.
9. Murta A., Pinto J., Varum H., 2011. Structural vulnerability of two traditional Portuguese timber structural systems. Engineering Failure Analysis.
10. Pinto R., 2014. Caracterização das paredes de tabique do centro histórico de Viseu. [Characterization of the Tabique walls of the historic center of Viseu.] Master thesis in civil engineering. IPB. Viseu. Portugal [in Portuguese].
11. Paiva A., Pinto J., Boaventura-Cunha J, Lanzinha J., Pereira S., Briga-Sá A., Varum H., 2012. Materiais e sistemas de construção sustentáveis. [Sustainable materials and construction systems.] CINCOS'12 – Congresso de Inovação na Construção Sustentável, 20 a 22 de setembro, Centro Habitat, Aveiro, Portugal, pp. 213 a 224 [in Portuguese].

Accepted for print: 01.03.2015

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Jorge Pinto
University of Trás-os-Montes e Alto Douro, Vila Real, Portugal

email: tiago@utad.pt

Sofia Ribeiro
University of Trás-os-Montes e Alto Douro, Vila Real, Portugal

email: sofia91ribeiro@hotmail.com

Vítor Cunha
University of Trás-os-Montes e Alto Douro, Vila Real, Portugal

email: vcunha@utad.pt

Nuno Cristelo
University of Trás-os-Montes e Alto Douro, Vila Real, Portugal

email: ncrsitel@utad.pt

José Lousada
University of Trás-os-Montes e Alto Douro, Vila Real, Portugal

email: jlousada@utad.pt

Anabela Paiva
University of Trás-os-Montes e Alto Douro, Vila Real, Portugal

email: apaiva@utad.pt

José Padrão
Polytechnic Institute of Viseu, Viseu, Portugal

email: jpadrao67.viseu@gmail.com

Humberto Varum
University of Aveiro, Civil Engineering Department, Aveiro, Portugal

email: hvarum@ua.pt

Aníbal Costa
University of Aveiro, Civil Engineering Department, Aveiro, Portugal

email: agc@ua.pt

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Responses to this article, comments are invited and should be submitted within three months of the publication of the article. If accepted for publication, they will be published in the chapter headed 'Discussions' and hyperlinked to the article.

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