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.

Trees exhibit adaptability in response to external loads, which allows them to form an inosculated connection (self-growing connection) with a neighboring tree. Such connections have the mechanical potential to build living tree structures. Although qualitative studies have studied this phenomenon, quantitative analysis of its growth features remains limited. Self-growing connections fused by weeping figs (Ficus benjamina L.) are utilized to study growth features. X-ray scanning and optical microscopy techniques are employed to investigate parameters including density, geometry, fiber structures, and material compositions. Key findings demonstrate that the fused region of a connection has a larger volume and a higher density on the intersected surface. Microscopic analysis identifies that the enlarged wood in the fused area is tension wood characterized by G-layers. The key component that connects trees is referred to as merged fibers, and the pattern of their distribution is found to be mainly in the outer layer of the larger cross-angle of a connection. At the cellular level, crystals within cells are identified in the fused region, implying possible mechanical stresses the interface has experienced. The findings in self-growing connections can serve as inspiration for structural design in living structures, biomimicry, bioinspired structures, and advancements in bioeconomics.

Hundreds of kilometers of timber sheet piles protecting the banks of canals of the Netherlands are nearing the end of their service life and need to be replaced. Even though azobé wood is very widely used, little is known about the current state of the azobé timber sheet piles that have been in service for many decades. More information about the current strength is necessary for planning any intervention, maintenance or reuse of the recovered sheet pile boards. This study aims to characterize the current state of azobé sheet piles using destructive techniques such as four point bending tests, compression tests and non-destructive techniques such as micro-drilling and visual assessment. To achieve this objective sheet piles that were in service for over 57 years were pulled out and tested in the laboratory. The non-destructive tests results indicate that the deterioration of azobé sheet piles is concentrated on the superficial layers of the boards. Visual classification and micro drilling techniques did not yield results that support the findings from the destructive tests. The bending strength and modulus of elasticity of in service sheet piles used in current study was found to be lower by about 25 % and 30 % respectively when compared to new azobé sheet piles reported in literature. Based on average values of measured dimensions, density of the sheet piles in this study was in general, lower compared to new sheet piles. Thus, the lower strength could be due to deterioration, lower intrinsic quality of the recovered sheet piles, or simply fall within the natural scatter of the material. In addition an exercise to classify the samples to a strength class is shown for practicing engineers.

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.

This study explored the most employed techniques for the assessment of the state of conservation of traditional wooden foundation piles in Amsterdam and Venice. The techniques were evaluated for their relevance and effectiveness in assessing decay impact on centuries-old waterlogged wooden piles. The techniques adopted in Amsterdam and Venice were complementary. In Amsterdam, underwater micro-drilling was employed to accurately estimate the amount of decay and the remaining strength of the piles. In contrast, the techniques in Venice were based on microscopic and mechanical testing of small wood samples to provide a detailed decay analysis. The successful use of underwater micro-drilling in Amsterdam, which allows for fast and accurate pile decay assessment, presents an opportunity to enhance the piles conservation database of Venice. Adopting this technique in Venice could support more timely and effective preservation strategies.

The strength degradation resulting from duration-of-load (DOL) effect and bacterial decay poses significant challenges to historical timber piles. Many historical European cities still heavily rely on the infrastructure supported by their original timber foundations. A reliable modelling approach on the structural performance of timber piles is needed to avoid the economic loss caused by closing down infrastructure. In this work, we consider a simplified bacterial decay model and develop a numerical framework to integrate the decay model into a standard DOL model. Two approaches are proposed and compared: one considering the homogenised effect of bacterial decay over the entire cross section, and the other taking into account the localised failure accelerated by bacterial decay and applying stiffness reduction to allow stress redistribution. Although the homogenised failure criterion is found to potentially underestimate the effect of bacterial decay, both approaches are able to capture the designated decay pattern. Ultimately, there is a potential for future extension to more intricate loading conditions and decay patterns.

There is an increasing need for using nature based solutions in protecting canal and stream embankments in the Netherlands and delta areas in general. Vegetation provides additional reinforcement and forms an integral part of many nature-based solutions. However, quantifying this reinforcement in-situ is challenging. This study aims to quantify the root reinforcement of three species prevalent along canal embankments – Salix fragilis L. (SF), Salix purpurea L. (SP), and Crataegus laevigata DC. (CL) – using the corkscrew extraction technique. Furthermore, canal bank stability was analyzed under different bank conditions regarding protection (unprotected, protected by vegetation), bank geometry, and hydraulic conditions.

Quantity of roots and Root Area Ratio (RAR) generally decreased with depth for all species. While root breakage was observed in most samples, all species exhibited increased ductility with higher root densities, except for CL at two depths. SF showed higher root reinforcement at shallower depths (≤ 250 mm), while SP demonstrated greater reinforcement at deeper depths. Results demonstrate that the corkscrew extraction technique is a quick and minimally invasive method for measuring root reinforcement in riparian environments.

Bank stability simulations revealed that vegetation significantly increases the stability of canal banks. Notably, when considering measured root reinforcement, the factor of safety improved dramatically from 1.08 to 2.46, even under analyzed worst case conditions. However, the analysis suggests a limiting root reinforcement beyond which further increases in root reinforcement have minimal impact on stability. Monitoring using the corkscrew apparatus and future design approaches could aim to achieve this minimum reinforcement.

This study examines the spring-back effect and residual stresses in curved glued-laminated timber (glulam) beams during the manufacturing process. In addition to curving along the longitudinal axis (X), a cup-deformation develops in the transverse direction (Z) due to the Poisson's effect. This deformation, combined with the glue-hardening process and the release of pressure, leads to the development of residual stresses in all three axial directions, as well as shear stresses within individual boards and at their interfaces. Besides the well-known factors such as longitudinal elasticity, board thickness, and inner radius, the study reveals that the number of layers (n) and Poisson's ratio (ν_LT) significantly influence the magnitude of residual stresses. However, aside from longitudinal stresses, the impact of n and ν_LT, as well as other residual stresses, have been scarcely studied and are not adequately addressed in current design standards. A Monte–Carlo analysis of the growth-ring effect is conducted, taking the pith location of different board layers as the input random variable. Strong influences can be identified on the residual stresses in both radial and tangential directions, with intensified maximum values and more scattered distribution inside the cross-section. The time- and moisture-dependent relaxation analysis using the rheological wood model shows a significant influence of the temperature and relative humidity.

The lack of strength values for wooden foundation piles in the design standards for timber (Eurocode 5) hinders their proper engineering design and assessment. In order to fill this gap, an extensive experimental campaign was conducted to characterize the mechanical properties of large-scale, water-submerged spruce (Picea abies L.) and pine (Pinus sylvestris L.) piles. This was achieved through the execution of axial compression tests on 253 full-scale pile segments. Wet compressive strength and stiffness values were derived for both spruce and pine piles, applicable to the whole pile and/or its parts: head, middle-part, and tip. The quality variables that most influenced the wet compressive strength of the piles were density, knot ratio (KR), number of annual rings (age), and growth rate. Based on this, characteristic strength values were derived for piles with the following grading limits: KR < 0.5, age between 20 and 100 years, and a growth rate <5 mm/year. These variables were used as key parameters to develop prediction models for the wet compressive strength of spruce and pine piles. The saturated compressive strength values and grading boundaries presented in this study contribute to the engineering design of timber piles and support the integration of reliable design values into future versions of Eurocode 5.

Wood-steel hybrid (WSH) elements are gaining popularity in the construction industry due to their reduced environmental impact and high load capacity. However, fire resistance remains a crucial challenge for advancing wood as a construction material. The proposed WSH slab consists of a trapezoidal steel profile sandwiched between two laminated veneer lumber (LVL) beech panels. This research aims to numerically predict the fire performance of the proposed WSH slab element by generating heat transfer models that consider convection, radiation, and conduction. The objectives are to predict the temperature profile of the system's components, assess the charring rate of the LVL panels, and validate the results with experimental fire tests. Computed Tomography (CT) scanning was additionally used to detect the material density variation in the remaining LVL layers after fire tests. Simulations reveal that the size and shape of the internal cavity significantly influence heat flow within the system. Analysis of different thicknesses and heights of the steel sheet shows a substantial impact on the charring initiation time of the upper LVL layer. Temperature profiles of the components from numerical analysis exhibit similar behavior to that observed in the experiments. The experimental charring rate averages between 0.88—1.00 mm/min, while the numerical rate averages between 0.95—1.06 mm/min, with a 5–8% average deviation attributed to conduction interaction between LVL and the steel sheet. This variation may also be caused by the definition of generic thermal properties of wood according to EN1995-1-2, which may not accurately represent the behavior of the LVL element under fire.

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.

Large parts of banks of canals in the Netherlands are protected by azobé timber sheet piles. Many kilometers of sheet piles in the province of Noord-Holland, are planned to be replaced or to undergo maintenance. Yet, there is insufficient knowledge on the current state of the azobé sheet piles and their residual service life. Based on this, a series of investigations on azobé sheet piles after 57 years of service were performed. Visual inspections showed surface deterioration on the water-exposed side for all boards. Nondestructive testing using micro drilling technique showed no signs of internal deterioration. A maximum reduction in thickness of 17% and an average thickness reduction of 6.7% of original thickness were observed. CT scanning showed that the remaining cross sections of the azobé boards were intact and had comparable density of new azobé boards. An exponential damage accumulation model was used to predict the residuals service life of the timber sheet piles subjected to earth stress. Conservative estimates based on physical measurements and residual bending strength indicate that the sheet piles have an additional service life of 22–43 years from the current state.

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.

Fatigue failures pose significant challenges across various engineering disciplines. Wood, due to its low carbon emissions and high strength-to-weight ratio, has been gaining attention in engineering applications. The fatigue behavior of wood is complex due to its heterogeneous, anisotropic, and viscoelastic nature. This research explores essential insights into the fatigue behavior of wood, with a focus on S–N curves, stress–strain behavior, and failure mechanisms. Due to often varying failure criteria and test settings, direct comparison of S–N curves across different studies can be challenging and inconclusive. A closer look shows that wood in fatigue shows both irreversible and recoverable strain components that are delayed. However, there have been conflicting reports about residual stiffness changes under fatigue loading. Theoretical fatigue life models based on S–N curves or duration of load theory have shown limited applicability. Efforts to develop progressive damage model based on stress–strain behaviors have been challenging and largely unsuccessful due to the lack or inconsistency of data. Understanding the microstructural failure mechanism is crucial in order to build a more trustworthy fatigue modeling technique. Further work is suggested to monitor the microstructural deterioration during high-cycle fatigue loading.

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.

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.

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.

The lack of strength values for timber foundation piles in the current Eurocode 5 hinders their appropriate engineering design and assessment. Timber piles, often submerged for their entire service life, endure high moisture levels, highlighting the need to define strength parameters of round wood under fully saturated conditions. To ensure reliable material properties, a large-scale study was conducted on 70 European softwood piles, determining strength and stiffness through axial compression tests on saturated segments extracted along the pile. Mean and characteristic wet compressive strength and stiffness values were derived, applicable to the whole pile and/or its parts. The mechanical properties of the piles were analysed in relation to grading parameters that may influence the saturated compressive strength, leading to the classification of three strength classes for visual grading. Additionally, two regression models were developed-one based on the most influencing visually graded parameters, and the other on the dynamic modulus of elasticity. The saturated compressive strength values and grading boundaries presented in this study contribute to the engineering design of European softwood foundation piles in the context of a new circular construction ecosystem, and support the integration of reliable design values into future versions of Eurocode 5.

Reliable finite element simulation of orthotropic-dependent failure mechanisms is crucial for understanding the mechanical behavior and optimizing engineered composites and fiber-based materials. Such materials behave brittle under tension and strongly depend on the orthotropic material orientation. Existing non-local models can reproduce brittle fracture for isotropic materials but, in most cases, they are based on the equivalent strain concept for damage initiation, which is unsuitable for orthotropic materials. This contribution introduces a stress-based non-local damage model enhanced with an implicit gradient formulation of the failure criteria. A localizing non-local length is assumed to avoid any pathological broadening of the damage band. The methodology introduces direction-dependent damage variables driven by non-local stress-based damage criteria and can thus distinguish different failure modes. The verification and validation are shown on numerical and experimental benchmark examples. The implicit gradient-based non-local damage approach allows mesh-independent results. Furthermore, it does not require a priori known crack paths and makes it possible to simulate complex failure modes. Perspectively, its effective implementation in the commercial software Abaqus and combination with other constitutive laws, e.g. to account for plasticity or moisture, make it an attractive tool for describing the mechanical material behavior of orthotropic materials, such as wood and fiber-composites.

This paper presents an experimental study on predicting the remaining short-term compressive strength of timber foundation piles in Amsterdam using micro-drilling. A large-scale investigation was conducted on 201 pile segments fromtwo bridges, dating back to 1727, 1886, and 1922.Microdrilling measurements, supported by a TU Delft-developed algorithm, successfully assessed decay by calculating the degraded portion of the cross-section—the soft shell. Piles from 1727 exhibited approximately 50% strength reduction due to significant, consistent decay along their length.More recent piles from 1922 and 1886 exhibited a 10-15% strength reduction despite lower decay levels. The correlation between decay and strength led to the development of an experimental prediction model for the in-situ short-term strength of the pile head, middle section, and tip. These findings aid the city of Amsterdam in estimating the remaining service life of its timber pile foundations.