Despite the growing popularity of wood and wood-based products in the construction industry, there has been insufficient focus on assessing the condition, preservation, and potential reuse of existing timber. While numerous standards evaluate the quality of freshly sawn timber, there is currently no standardized system for assessing the strength properties of aged and reused timber. The lack of these guidelines is also one of the reasons the results obtained in numerous research are often fluctuating, and we cannot draw clear conclusions. The matter is further complicated by the lack of data on old in-situ wood and its exploitation, which would help to evaluate its condition. Consequently, there is a real practical need to assess the condition of old timber to avoid unnecessary demolition and the loss of valuable and structurally sound building material. What sets this study apart from others is that, in addition to destructive testing, the 4-point non-destructive (ND) bending tests were conducted on all four faces of test specimens. This provided an opportunity to assess the wood visually and then find connections to associate external characteristics with real properties. This methodology aimed to determine whether it is feasible to visually assess the most practical way to use wooden elements in construction. If this question arises, which face of the beam would be better suited for the tension side and which for the compression side? The old timber used in testing originated from an old library building located on Vaksali Street, Tartu, Estonia and is estimated to be about 120 years old. This paper investigates and compares the collected data with a Nordic standard for grading fresh-sawn timber and two established Italian standards for visually assessing aged timber. This comparison contributes to developing a standardized framework for future visual assessments. ND and destructive four-point bending tests were performed to validate and find appropriate visual characteristics to determine the strength and stiffness of the timber elements. The primary goals of this study were first to compare the results obtained from existing ND methods with actual results and secondly to provide guidelines for better visual grading of wood in the future, based on Nordic Standard INSTA 142 (2010) and Italian standards UNI 11119 (2004) and UNI 11035 (2010) Contrary to previous research conclusions, the visual assessment results yielded unexpected outcomes. The results show that the grading standards significantly underestimated the real strength of the wood, and even more, none of the visual assessments overestimated the real strength of the specimens. Therefore, based on prior research and the findings derived from this study, there is evident a substantial potential for extensive development and optimization within this field.

The number of tall buildings combining both a visible mass timber structure and large open floor plans is growing rapidly introducing new fire safety challenges. One risk is that of very rapid flame spread in the ceiling, originating from a severe but localized fire, resulting in fires where the majority of large compartments burn simultaneously. Such phenomena have been observed in both tests and accidents, but knowledge of effective mitigation without the use of sprinklers is scarce. In Europe, this problem is commonly addressed in construction by complying to prescriptive rules of reaction-to-fire classification of linings. The reaction-to-fire classification, primarily based on the single burning item (SBI) test of EN13832, characterizes the material’s contribution to a fire in the very initial phase of the fire. Treatments can be used to improve the reaction-to-fire class of mass timber, which will reduce the risk of substantial fire development. Fires can, however, develop and grow large even without the contribution of lining materials. For this reason, and in light of the recent findings of research of large open floor plan compartments, it is of interest to assess the effectiveness of treatments to reduce the risk of rapid flame spread. Therefore, eight tests in 18.0 × 2.3 × 2.2 m3 compartments were performed. Six had exposed timber surface with a clear coating or impregnation in the ceiling, complying with a reaction-to-fire class B and two served as untreated timber and non-combustible reference tests. The fire source, representing a fire in moveable fuel, was severe enough (3 - 3.7 MW) for flame impingement on the ceiling. The rate of at which wood ignited from the heat in the ceiling, the temperature development at different heights, as well as external flaming were assessed and were used as indicators of performance. Additional indicators were the estimated tenability and ceiling char depths throughout the compartment. The untreated timber and the non-combustible ceiling represented the two extremes for most indicators with the class-B treated timber surfaces falling in between. Close to the fire source, the test indicators for treated timber surfaces performed similar to those of the untreated timber surface while the non-combustible ceiling performed significantly better. With increasing distance from the fire source, indicators from treated timber tests more resembled the non-combustible ceiling. This behavior was noticed for all types of indicators. With increasing distance from the fire source, the fire exposure is naturally less severe and thus, more similar to the small burner exposure used in SBI-testing which the treatments were developed against. Both final charring depth and temperature developments for ignition and tenability were clearly improved by the treatment, but the SBI test results (FIGRA and THR600s) did not correlate well to the compartment test indicators (Figure 92 andFigure 93). Nevertheless, using treatments assessed by SBI is a common strategy to mitigate fire spread in newly constructed mass timber buildings and practitioners should be aware that while the treatments have significant effects on the flame spread they are not to be treated as incombustible. We propose that addressing the ceiling spread problem requires an additional indicative test with more severe exposure than the SBI test setup. The impregnated timber experienced loss of integrity due to substantial shrinkage of the timber during the severe exposure. Such phenomena were not captured in the SBI testing. Comparisons of performance of the impregnated specimens indicates that it can be beneficial for the performance to implement more impregnation than needed for reaction-to-fire class B. Whether this holds for all treatments cannot be concluded.

Adhesives state the essential prerequisite for manufacturing large timber construction elements from rigidly bonded solid wood boards of growth and processing bound limited dimensions. In the first two decades after the invention of glulam up to the 1930s, adhesives based on natural organic substances like blood and proteins were used. Such adhesives can have high dry strength but are weak when applying water or temperature. These adhesives were then replaced by synthetic ones, firstly in the early 1930s by (phenol )-resorcinol-formaldehyde (RF/PRF) adhesives and then by urea-formaldehyde (UF) adhesives. Numerous tests have shown that the boiling water resistant duroplastic RF/PRF adhesives are very stable at high temperatures up to/beyond the charring of wood (Dorn and Egner, 1967; Klippel 2014). Contrary hereto, the UF adhesives later classified in Europe as type II adhesives have significantly reduced water resistance (e.g. Raknes (1997) and are less temperature stable and fire resistant, although the latter was not communicated sufficiently. The RF-, PRF- and UF- adhesives were exclusively used up until the 1980s when the presently existing timber standards for “cold” and fire design were being developed. From the 1980s onwards, adhesives with various chemical compositions have been added to the market. Firstly the duroplastic melamine-urea-formaldehyde and pure melamine formaldehyde (MUF/MF) adhesives, followed in the mid-90s by the moisture-hardening one-component polyurethane (1C-PUR) adhesives, then followed by the emulsion-polymer isocyanate (EPI) adhesives. In order to speed up curing times, being of utmost high economic importance, significant amounts of polyvinyl acetate (PVAc) have been added to the hardeners of MUF adhesives with drawbacks on temperature stability. Each of the developed adhesives has its advantages and disadvantages regarding strength, water and/or temperature resistance, application robustness and price. According to EN 1995-1-2:2004, chapter 3.5, the behaviour of a bond line in fire may not be considered explicitly if the bond line is made of phenol-formaldehyde and aminoplastic, Type I adhesives, according to EN 301. Regarding the general principle that adhesives shall produce joints of such strength that the integrity of the bond is maintained in the assigned fire resistance period, a footnote hints at the point that some adhesives show softening considerably below the charring temperature of wood.

Wooden I-joists, being factory-made ultra-light and highly optimised products, are typically used as the load-bearing elements in timber frame assemblies (TFA), which consist of a combination of material layers – sheeting boards (claddings) and cavities which may be partially or completely filled with insulation. The materials used in conjunction with a timber member in a TFA affect the heating of the timber member. The joists consist of flanges (made of sawn wood, LVL or glulam) and a web (made of a wood - based board). Fire resistance of such wooden structural products is a complex matter. However, the current European design standard for timber structures in fire – Eurocode 5 Part 1-2 (2004) provides no guidance for I-joists. The fire resistance of wooden I-joists has been previously investigated by König (2006) and Schmid et al. (2011), who developed calculation models to analyse the load-bearing capacity of wooden I-joists exposed to fire for floors. There have been significant changes in the variety and types of materials used in conjunction with I-joists. Therefore, the application of these models is limited nowadays. Additionally, they focus on the reduced properties method. Only the effective cross-section method will be included in the revised Eurocode 5 Part 1-2. This report describes the unified model for wooden I-joists in both wall and floor assemblies which follows the philosophy of the effective cross-section method. Additionally, it should be used with all types of cavity insulation and fire protection systems. The unified I-joists model aims to be introduced to the new revised Eurocode 5 Part 1-2. Two phenomena have to be considered according to the ECSM: charring and mechanical resistance. It is assumed that the charring of wood is a material characteristic which is not dependent on the orientation of the structure (wall or floor). The charring of the flanges is primarily dependent on the cladding material and thickness. After the failure of the claddings, the charring is influenced by cavity insulation. Gypsum plasterboards as cladding and stone and glass wool as cavity insulation have been tested and analysed. A large number of thermal simulations have been analysed to investigate the influence of various factors (e.g. flange size, cavity insulation material, protective boards) on the charring behaviour of the fire-exposed flange and the web. The combination of different materials and the slender nature of I-joists makes their fire resistance a complicated issue. The thin web is very sensitive to elevated temperatures and charring. Additionally, adhesives used in finger joints in the flanges and the joint between the flanges and the web influences the load-bearing capacity. The loss of strength and stiffness were seen in wood at elevated temperatures is considered in the ECSM by a zero-strength layer. The zero-strength layer (ZSL) is an additional reduction of the cross-section to compensate for the decrease in strength and stiffness properties. The development of the expressions to calculate the ZSL depths for compression elements is discussed in this paper.

This report presents the four large-scale fire tests performed within the FIRENWOOD project. The aim of the tests was to verify the improved fire design models for the I-joists and crosslaminated timber. The results of the loaded floor test with cross-laminated timber were also compared with results from an unloaded model-scale test with similar lamella thicknesses and adhesive. The aim of the compartment fire test was to study the behaviour of I-joists in physically based fire compared to the behaviour in standard fire. The second aim was to compare the fire behaviour of the compartment made of timber frame assemblies with I-joists and the previously performed similar compartments made with CLT. All large-scale tests reported here were performed with engineered wood structures using adhesive No.9

The aim of the tests was to evaluate the potential reduction of the moment capacity of I-joists caused by bondline integrity of the finger joints. Each I-joist was made with 11 different adhesives in the finger joints in tension flange. All the adhesives were from Firenwood selection. Finger joints in tension may show a lower load-bearing capacity in the fire situation depending of the bond line integrity in fire. I-joists were loaded in bending with the tension flange being closer to the fire. I-joists were protected with gypsum plasterboard during the entire fire test. Cavities of the test assembly were filled with stone wool.

The aim of the fire tests presented in this report was to measure the mass loss and the charring depth of CLT (cross-laminated timber) elements with different types of adhesives when exposed to fire from below. CLT elements may heat delaminate when exposed to heat. If this occurs, it depends, among other things, on the temperature of the adhesive and on the adhesive’s ability to retain its adhesive properties. Eleven adhesives are used in FIRENWOOD project in order to compare properties in fire. Adhesives originate from four different chemical backgrounds and represent the state of the art of adhesives used in timber structures. Adhesives are manufactured by 4 leading European adhesive manufacturers, and all 11 adhesives have passed requirements in European standards for load-bearing timber structures. Adhesives are marked with numbers 1 to 12 (adhesive no 10 is not included in this Work Package). Not all adhesives were tested for this method and is therefore not presented in the results.However, some additional adhesives were used in the tests presented in this report and are marked 21, 22, 23 and 24. The same numbers are used for all adhesives in all tests throughout FIRENWOOD project.

Wood construction is growing rapidly and provides a substantial contribution to the development of a more sustainable construction sector. Several modern wood-based building systems are developed with a focus on tall wooden houses and industrial production, where glued products are an important part. Fire safety is important, but the adhesive properties in fire conditions are not fully understood. This applies in particular to new adhesive systems but also to existing ones that exhibit poor load carrying capacity in a fire. The problem has been noted by the FSUW (Fire Safe Use of Wood) global network, which formed a sub-group of “Glue-line failure of engineered wood products” with representatives from Australia, NZ, Canada, USA, France, Italy, Switzerland, Sweden, and Germany. The global network has gathered knowledge and experience from known cases of fire testing of glued wood components (especially glulam, finger joints, and CLT) and has defined research needs. The results highlighted by this group relate to the delamination of glued bonds in a fire which can cause increased charring of glued wood products, especially for CLT. The results show that the temperature during standard fire testing increases continuously without cooling phase and with delamination of CLT until a collapse of the structure occurs. Thicker CLT may be required to reduce delamination risks or to protect the wood material. This can lead to increased costs and greater weight of the construction as well as reduced possibility of using visible wood. Therefore, it is of the utmost importance to find methods for evaluating the adhesive bond properties in a fire. The hypothesis is that different adhesive systems have different behaviour in fire, and especially that delamination behaviour can be avoided by choosing a suitable adhesive system. The best method for the evaluation of fire delamination is a full-scale test, but considering the high costs of such full-scale tests, a smaller-scale test needs to be developed. The intention of FIRENWOOD project is that such small-scale methods should give the same results as full-scale tests. A new, smaller-scale method for classifying adhesives concerning fire properties would also simplify the planning of full-scale tests. In Work Package 3 of FIRENWOOD project, some small-scale fire testing methods for adhesive bonds were evaluated at RISE, and this report includes results from small-scale fire tests of adhesive bonds in finger joints, CLT and GLT. The report is organized in three chapters based on the three different products tested. The common for all three tests was that the same eleven adhesive systems were used in all adhesive bonds.

FIRENWOOD is an Era-NET cofund Forest Value project (2019-2022) dealing with the fire resistance testing and design methods of engineered wood products at elevated temperatures and fire. As one part of the project, various adhesives, allowed for load bearing timber structures, are tested in finger joints in small scale and medium scale fire tests. The paper will provide a description of these tests and an overview and analysis of the results. Based on the test data of both types, a good agreement regarding the adhesive performance between the tests can be shown. The design model for wooden I-joists is described and values for the depth of the zero-strength layer are proposed for different adhesive performance levels in finger joints.