Brittleheart, also known as compression failure, is a widespread phenomenon observed in numerous tropical wood species, significantly diminishing their strength properties. According to strength grading standards such as BS 5756, NEN 5493 and EN 16,737, timber exhibiting brittleheart characteristics must be rejected. Oftentimes brittleheart remains undetectable on the outer surface and cross-section of sawn timber. This study focuses on qualitatively characterizing compression failures in tropical hardwood and its mechanical properties. In this context, various non-destructive detection methods were explored. Five grades of compression failures were characterized based on the deformation and displacement of wood tissue. Results demonstrate that CT-scanning shows promising as a technique for detecting these five defined grades. Quantitative assessments of brittleheart on the mechanical properties were conducted to determine bending strength (f_m) and modulus of elasticity (E_m). Multiple regression models were developed to predict the bending strength with a highest coefficient of determination (R²) of 0.778 and a relatively high SEE of 17 N/mm².

By using minerals instead of bioresources in construction, greenhouse gas emissions are nearly doubled. It is vital to transition from this traditional paradigm to a low-carbon model in which woods and bamboos are essential components. To proliferate bio-urbanization, challenges must be overcome in forestry and construction. Our study is a necessary starting point for more sophisticated studies and policies to support the valorization and utilization of bioresources, especially wood and bamboo, in greener construction solutions for a sustainable urbanization. Our main results elucidate examples and benefits of biobased cities and buildings, raise issues and current challenges, and suggest opportune actions to be globally addressed in collaborative proposals. Assertive codes, well-managed resources, resolution of challenges, and clarification campaigns on decarbonization are priority targets in future environmental and societal commitments.

Strength grading is essential to ensure standardized and reliable design processes in timber construction. Current visual strength grading (VSG) standards, such as DIN 4074-1, focus on regulating natural features and grading new sawn timber. Sorting criteria for man-made defects resulting from prior use are lacking. This study examines the potential of VSG for battens processed from salvaged rafters for structural applications and proposes modifications to DIN 4074-1:2012 to make it applicable to battens processed from recovered wood. Battens were visually graded based on natural features, and fastener holes were characterized. Bending strength was tested against EN 338 classes, and the impact of fastener holes was assessed. The results indicate that grades S10 and S13 together account for a total yield of 57%. Clusters of fastener holes reduced bending strength in S10 battens, whereas larger holes affected S13 battens. Modification options for DIN 4074-1:2012 were identified, and their effectiveness was validated. As a result, a new sorting criterion, SRC (strength reducing criteria), was introduced to address the interaction between natural features and fastener holes. Battens processed from recovered wood can serve as an alternative to new timber if classified under the proposed grade W10+.

Traditional "hard" protection systems, such as hardwood timber sheet pile walls, are often used to protect banks of canals and streams, but the tropical hardwood they require is not always locally available. This has led to increasing interest in nature-based, bio-engineered solutions that combine locally sourced wood with vegetation to protect the soil. To assess the behaviour of locally available softwood timber sheet pile walls, a full-scale surcharge loading test was performed under realistic conditions. The test applied a 30 kPa surcharge load, representing the weight of a heavy agriculture machinery, while monitoring the wall's horizontal and vertical displacement, along with its rotation at the top, mid-height, and base of the retained soil. This resulted in a displacement of approximately 1.9% of the one meter retaining height. The potential onset of a failure wedge was observed after an extended loading period. Nonlinear tilt measurements showed peak curvature at mid-depth (0.66° top, 0.71° mid, 0.69° bottom), indicating dominant flexural bending. Additionally, the measured horizontal displacement exceeded the rotational contribution estimated from the tilt. The material properties of the softwood sheet piles were determined through four-point bending tests. A numerical model, calibrated with experimental data, was then developed to simulate the long-term performance (10 years) of decayed sheet piles with both bare and vegetated backfill. The results indicate that vegetated backfills significantly reduce displacement and the bending moment on the wooden sheet pile compared to bare soil.

In the historic city centre of Amsterdam (NL), the most widespread foundation system consists of wooden piles. Since these foundations are fully below the water table, they are mostly subjected to bacterial decay. This biodegradation phenomenon proceeds slowly over time, and usually involves the less durable sapwood, with heartwood remaining sound. Hence, obtaining an estimate of sapwood and heartwood proportions in wooden piles can provide information on how deep in the cross section bacterial decay is expected to proceed. This is relevant, for instance, when developing service life models, since the remaining sound cross section of a pile can be estimated. Thus, the present work involves a comprehensive investigation on sapwood and heartwood proportions in spruce, pine and fir wooden foundation piles from different construction periods, ranging from 1727 to 2019. The amount of sapwood and heartwood was determined with computed tomography (CT) scans on 49 wet discs retrieved from the piles. Such measured sapwood width was then compared with that predicted with an empirical model from literature, based on the number of annual rings and growth rate, obtaining a successful validation. Micro-drilling measurements were also conducted on the discs to identify decayed portions, which appeared to always affect (part of) the sapwood only. Finally, this outcome was further validated against a broader dataset of micro-drilling measurements taken on over 200 pile segments, for which the sapwood widths were predicted with the aforementioned empirical model, and were found to be overall greater than the corresponding decayed portions, even in wooden piles having been in service for 300 years.

Laminated bamboo can be produced in sizes that are similar to glued laminated timber. Asaresult, large connections with multiple dowels and slotted-in steel plates are similarly possible with bamboo. MOSO bamboo was used in this study, withadensity of around 660 kg/m³, potentially creating connections having higher load carrying capacity than softwood. A large experimental campaign was set-up in order to determine the mechanical properties of connections with various ratios of dowel diameter to bamboo thicknesses and with single and double steel plates. Furthermore, influences of the density of the material, related to the embedding strength for fasteners, as well as the splitting sensitivity with multiple fasteners inarow are playing crucial roles with respect to the load carrying capacity. Therefore, multiple test series on large bamboo connections have been performed in order to study various possible failure modes, as dependent on embedding strength, steel grade, number of fasteners inarow, and the influence of multiple steel plates. The various failure modes have been analysed analytically with the Johansen equations, similar to the design equations proposed for the upcoming version of Eurocode 5 for multiple steel plate connections, confirming their applicability to bamboo and its similarity with wood.

Humidity fluctuations are a leading cause of damage in wooden constructions. In the case of glulam products, the multitude of possible layups concerning pith locations, diverse material properties across wood species, and the high computational cost associated with multi-field analysis have constrained many research efforts to focus on one specific glulam layup, consequently limiting the generalizability of the findings. To address this challenge, Monte Carlo simulations were employed to assess the significance of various factors. Based on which, two levels of simplification are proposed. The first level reduces the multi-layer problem to a single-layer one by applying appropriate boundary conditions. It substantially reduces the simulation costs and consequently facilitates sophisticated damage analysis, revealing the varying damage pattern across different board types. The second level of simplification further reduces the problem to a single-element model, enabling an analytical estimation of moisture stress. This level of simplification elucidates how factors such as moisture difference, material rotational angle, and other material properties influence the moisture-induced stress. Most importantly, it facilitates a rapid estimation of the critical moisture fluctuation range and the preferred sawing location of boards for different wood species, which can provide guidance to the production of higher moisture resistant glulam.

Enhancing urban tree stability is critical for public safety and infrastructure protection. This study evaluates a nature-based method for improving tree stability using inosculations to form interconnected tree systems. These systems establish biomechanical connections through inosculation, offering both biological and mechanical support. The research focused on lime trees (Tilia Cordata Mill.), comparing parallel and cross connected tree systems with the single tree to evaluate their mechanical performance. The mechanical performance of the interconnected tree systems was evaluated by pulling tests in different directions to simulate wind loads. The study spanned a two-year growth period to investigate the effects of growth on mechanical behavior, with the analysis supported by finite element modeling. The results showed that growth-induced changes increased the overall rigidity of the tree systems and reduced deformation, rotation, and local elongation. Cross connected trees exhibited notable bracing effects in the connected plane, which improved lateral resistance. In a parallel connected tree system, the basal stiffness increased due to the connection between the lower region. Compared to the single tree, interconnecting tree systems can provide additional support and reduce deformation caused by lateral loads, making it a promising strategy to improve tree stability under horizontal loads.

This contribution aims to increase the understanding of the complex mechanical behavior of wood through a framework for simulating mixed-mode failure. Based on physical properties assessment, appropriate constitutive laws, and experimental validation, a generally applicable numerical strength prediction tool for wood from different species and with various natural imperfections is introduced. The 3D orthotropic elastic plastic non-local CDM model considers the local fiber orientation and is implemented as material subroutines in the commercial software Abaqus. Herein, orthotropic Hill-plasticity with exponential hardening represents the plastic behavior in compression. Separated stress-based gradient-enhanced transient non-local damage represents the brittle material behavior in tension and shear. The methodology is validated with experimental data on tensile veneer tests, shear- and compression tests. Moreover, the methodology is applied to four-point bending tests of boards with heterogeneities. The numerical results demonstrate that the proposed model is able to reproduce different crack patterns observed in the four-point bending tests. Detailed investigations of the impact on the strength of the boards can be performed with this method to optimize species-independent strength prediction and engineered wood products. Further combination with other material laws e.g. moisture is possible.

Trees can adapt to external loads and form inosculations (self-growing connections), where stems or branches naturally fuse together. However, a limited understanding of biomechanical features of connections hinders their practical applications. This study used connections formed by Ficus benjamina L. to investigate their mechanical properties at different growth levels. Two parameters (fusion degree and interface curvature) were identified to describe growth levels. Customized tensile tests were designed to measure mechanical properties perpendicular to the interconnected surface. Growth levels of studied connections ranged from initial formation to almost fusion of piths, which provided a range of tensile strength of 0.23 to 1.38 MPa. Two primary failure modes (failure at the interface and failure across the stems) were found to be linked to growth levels. The fusion degree, at approximately 15%, contributed to distinguishing failure modes. The average diameter of a connection had the most significant effect on its tensile strength and stiffness. Moreover, the interface curvature correlated negatively with mechanical properties. Average diameter, interface curvature, and fusion degree were effective predictors of connections’ tensile strength. Regarding Ficus connections, dry connections were stronger than wet connections. These findings provide evidence for nature-based design using self-growing connections under different moisture conditions and growth levels.

Extending the service life of building components is essential for a circular economy. Wood, as a renewable raw material, plays due to its mechanical properties and ease of processing a crucial role in this process. Most studies focus on the reuse of building materials. However, it is essential to detect and investigate the use cases in which reuse is impossible due to changing dimensional requirements or damages. This study examines the bending properties of recovered wood, particularly battens with cross-sectional dimensions of 30x50 mm², which were processed from rafters originating from a roof truss deconstructed in southern Germany. The bending tests were performed and interpreted based on the damages of the prior use and the lumber pieces' background information. The visual observation resulted in many fastener holes, mainly derived for battens from the built-in upper layer of the rafters. Even though fastener holes contributed to or were the single cause for the failure of the battens, bending strength around the mean value or even higher was yet achieved for some battens. Developing unique sawing patterns for each rafter by taking into account the location of the pith and the arrangement of knots can enhance the yield. Additionally, introducing a third grade, S7, alongside the existing S10 and S13 grades - similar to the approach used for joists and boards in DIN 4074-1:2012 - could further optimize yield. Although it has been concluded that knots remain even for recovered wood the key sorting criteria, fastener holes, can additionally influence the mechanical properties and, therefore, need to be considered in a standardized strength grading.

Wooden piles are the most common foundation system in the historic city of Amsterdam (NL). The piles are fully submerged below water table and subject to bacterial decay. This study investigated sapwood and heartwood proportions in spruce, pine, and fir piles from different construction periods, in relation to their degradation. X-ray computed tomography scans on 49 wet discs were performed to measure the piles’ sapwood width, which was then validated against an empirical model based on annual rings and growth rate. Degraded areas, identified with micro-drilling measurements, were found to affect sapwood only. These outcomes were further validated on 201 pile segments, with the predicted sapwood widths being greater than or equal to the decayed portions, even in 300-year-old piles. Therefore, estimating sapwood width can contribute to determine the remaining sound cross section of the piles, providing useful input for service life models for planning timely maintenance interventions.