Renovation of timber floors with structural glass: Structural and environmental performance

Highlights

• A new strengthening technique for timber floors of cultural interest is proposed.

• CLT panels and structural glass strips are considered for strengthening.

• Structural analysis and life cycle assessment are performed.

• A significant increase in the load-carrying capacity can be achieved.

• Proposed strengthening technique has lower GWP and non-renewable CED.

Abstract

A common occurrence in building renovation is the modification in the usage category that can lead to higher imposed loads, and consequently the need for structural improvements to the existing structure or to a structural element, such as timber floors. Strengthening practices oftentimes affect the originality of the building or a structural element under consideration. However, the preservation of the originality of timber floors having historical, architectural and cultural interest can be of high importance. This article provides insight into the field of timber floor strengthening techniques with regards to the requirements for conservation of the wooden built heritage. Moreover, a new strengthening technique using structural glass as a strengthening element for timber floors is presented, as reinforcing the bottom side of timber floors without compromising the appearance of the floor is an important objective of a successful intervention. In order to evaluate the performance of the newly proposed strengthening technique from a structural and environmental point of view, structural analysis and a life cycle assessment on a timber floor strengthened with cross-laminated timber panels and structural glass strips are performed. The structural analysis shows the possibility to use structural glass and still achieve a significant increase in the load-carrying capacity of the timber floor. Furthermore, the proposed strengthening technique has a lower global warming potential (GWP) and non-renewable cumulative energy demand compared to renovation and replacement with a cross-laminated timber panel, and a lower GWP compared to replacement with a reinforced concrete slab. This study not only represents the first holistic approach to evaluate the structural and environmental performance of the proposed strengthening technique, but it also addresses the aesthetically-aware design and technical limitations in the utilization of glass for the renovation of timber floors and thoroughly presents the possibilities to overcome these limitations.

Introduction

Timber floors form a significant part of the existing building stock. Their structural improvement can be required for numerous reasons, e.g., a modification in the usage category may lead to higher imposed loads on building floor components as well as on the entire structure. Another example is wood decay, which also requires the strengthening of timber floors. At the same time, the originality of the timber floor should be preserved as much as feasible, either because of architectural heritage requirements or because of the individual wishes of the architect, owner, or financier. The timber-concrete composite (TCC) section has gained importance as a strengthening technique in recent years [1], as it strengthens the “out of plane” and “in-plane” behavior of the floor at the same time. The disadvantage of TCC is a high amount of moisture introduced into the building (by pouring concrete), together with the fact that it is considered an irreversible strengthening method. In some cases, cultural heritage criteria must be taken into account; for example, in Italy, the timber-concrete approach for strengthening old timber floors is often rejected by responsible authorities due to the irreversibility of the technique [2]. In addition, concrete production is considered a significant consumer of energy and natural resources and a substantial source of greenhouse gas (GHG) emissions. When the environmental aspect is considered, reinforced concrete (RC) is recognized as the construction material that makes the largest relative contribution to the environmental impact of buildings [3]. In order to avoid the use of concrete as part of the TCC section, additional timber elements in the form of cross-laminated timber (CLT) can be used. This solution uses a dry technique to form composite cross-sections, which is preferred for timber floors of historical or cultural interest [2,4]. Furthermore, it is also recognized in the scientific community that structures constructed from renewable bio-based construction materials such as timber have lower GHG emissions compared to structures made of concrete, steel or bricks [[5], [6], [7], [8], [9]]. Renewable bio-based construction materials have the potential to act as carbon sinks in the renovation of existing buildings [10]. The strengthening approaches of TCC and CLT often require that the thickness of the renovated floor element is kept as small as possible in order to minimally alter the original floor and room height as well as to assure the functionality of the space. In the case that the increase in floor height is not acceptable, additional tensile reinforcement components such as carbon fiber reinforced polymers (CFRP) or glass fiber reinforced polymers could be attached to the underside of the timber joist [11,12]. The aesthetics of the timber floor is commonly understood to be the aesthetics of the underside of the timber floor, as the upper side is topped by a floor covering. Therefore, the classic tensile strengthening can have a negative influence on the aesthetics of the timber floor joists. In order to meet the aesthetic and functional requirements in terms of transparency, strength, and stiffness, structural glass elements could be applied in the context of the renovation of existing timber floors. Lowe et al. [13] introduced the use of structural glass elements in the renovation of the historic Menokin House. Oikonomopoulou et al. [14] and Bristogianni et al. [15] investigated the application of glass blocks in the renovation of masonry walls. Konrad et al. [16] investigated the use of timber-glass beams in the renovation of historical buildings and emphasized the importance of glass transparency in ensuring visibility of architectural heritage and access of light. However, from an environmental perspective, glass can have a significant environmental impact due to glass production and processing [17]. Since the construction sector is responsible for a large part of global energy consumption and accounts for a significant share of global GHG emissions [18], it is also important to take environmental aspects into account when renovating buildings. Building renovation is a process that is carried out to improve the existing structure [19]. It assumes a new phase in the life cycle of existing buildings by using components and structures already built, with the addition of new materials, and thus offers a unique opportunity to reduce the impact of such an improvement by assessing its environmental performance. The LCA methodology is widely used and accepted for assessing the environmental performance of buildings or building elements and is also at the heart of existing standards for assessing the sustainability of buildings [20]. A few LCA studies [21,22] have evaluated and compared different floor systems for new buildings. However, studies on the renovation of building floors, which would improve knowledge of the environmental impact of different floor renovation techniques and materials, are not yet documented in the scientific literature. Therefore, the aim of the present paper is to evaluate the overall performance of the newly proposed strengthening method with structural glass in a holistic way, both from a structural and environmental point of view, by applying structural analysis and LCA. Furthermore, the strengthening of an existing timber floor with structural glass strips (SGS) is discussed as part of timber floor renovation when both aesthetic and functional requirements have to be met. The newly proposed strengthening technique includes the use of CLT panels to ensure a diaphragm-like behavior of the timber floor and to strengthen the compression zone of the timber joists and the use of SGS to strengthen the tensile zone of the timber joists without affecting the original appearance of the floor. As the research studies also recognize the environmental preference of building renovation over demolition and new construction [23], our study also assesses a scenario common in practice where a new structure replaces the old timber floor. The LCA evaluates two scenarios:

• The replacement of an original timber floor with an RC slab and CLT floor structure to achieve the load-carrying capacity of 5 kN/m² for the imposed loads.

• The renovation of the existing timber floor by strengthening only with a CLT panel and CLT-SGS to achieve the load-carrying capacity of 5 kN/m² for imposed loads.

The environmental performance of the CLT-SGS timber floor strengthening technique is also examined to determine whether the proposed technique can be considered as carbon and energy-saving technology. The following subsection introduces the application of structural glass in load-carrying structures. In the second section, the structural analysis is described and performed to determine the load-carrying capacity of the proposed strengthening technique on an exemplary timber floor. The LCA is performed in the third section, and the LCA results are presented and discussed. Finally, the conclusion is given in section four.

The term “structural glass” is a source of potential confusion as it sometimes refers to point-fixed glazing systems and often merely to glass for structural applications such as beam or column elements [24]. The latter definition is adopted in this article, as the development of glass implementation has led to the point where glass can no longer be considered as a cladding material only. Besides its use as cladding, glass is more often used as a single supporting element, see Ref. [25], where glass beams are presented, and [26] where glass columns are studied. Another application are composite beams, where glass is combined with other materials to achieve increased redundancy. The most known examples are steel-glass composites, as presented in Ref. [27] and timber-glass composites, as presented in Ref. [28]. Beams with fixed GFRP, CFRP, or reinforcing bars, as presented in Ref. [29], are a relatively recent development. Besides timber glass beams, there are several other examples of timber-glass composites, e.g., timber-glass wall panels, as shown in Ref. [30] and timber-glass ceiling and roof panels as shown in Ref. [31]. The investigations [32,33] possibly represent the first research experiments with timber-glass panels. In those studies, the glass plates were placed on the timber joists, i.e., in the compression zone of the bending cross-section of a simply supported joist. In the research of timber-glass elements by Cruz and Pequeno [31,34], the glass plates were in the compression and tensile zone of the bending cross-section. The examples of timber-glass panels can be considered as a starting point for the technique proposed in this article, but the glass sheet on the timber joists is replaced by a CLT panel, and the bottom glass sheet width is reduced to the width of the timber joists. The proposed technique represents a possibility for the transparent strengthening of existing timber floors, but a similar technique could also be applied to other timber structural elements (beams, columns, frames). In summary, the additional application of structural glass in the future could be the strengthening of existing structures, which represents a significant potential future market. Of course, thorough experimental studies must be carried out to validate this idea.

The presented timber floor numerical example could also be transferred to timber columns since the calculation of the effective bending stiffness (necessary for the evaluation of the buckling resistance) is practically the same. It is important to note that the chosen adhesive in this study was not transparent. This means that at least a semi-transparent adhesive must be used for a successful application. Another possibility would be the formation of small bonding areas (unfortunately not in our example where the entire contact area between SGS and timber joist was defined to be adhesively bonded). However, a common requirement for overhead glazing in the case of breakage is to retain the shattered glass fragments in place to prevent injuries. This is normally achieved with laminated glass. In the case of a glass strip whose entire flat surface (on one side) is bonded to a timber joist, the laminated glass is not required because the shattered glass segments remain bonded to the joist in a large degree. This article, therefore, addresses the necessity of a (semi-)transparent structural adhesive and presents the motivation for future research on different adhesives for timber-glass bonds.