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+.

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.

This study investigates the fatigue behavior of European ash (Fraxinus excelsior) with density range of 580-620 kg/m3 under relatively short-period cyclic compressive loading. The aim is to understand its mechanical behavior, such as: strain development, recovery, and internal damage mechanisms. Mechanical testings at varying frequencies (0.1, 1.0, and 10 Hz) were performed and the samples were scanned with micro-computed tomography (Micro-CT). Multi-scale assessment of fatigue effects was performed. Strain behavior revealed progressive increases in both viscous and viscoelastic components, demonstrating time- and rate-dependent deformation. For the same loading time, higher loading frequencies resulted in consistently lower accumulated strain; however, no significant differences in strain recovery were observed between frequency groups. Strength and stiffness showed minimal change after up to 2,000 loading cycles at 78 % stress level, highlighting relatively high fatigue resistance of wood under compression. Micro-CT imaging detected internal microcracks in samples pre-loaded with fatigue, inferring that cyclic loading induces more microstructural damage than static conditions. These findings enhance the understanding of the fatigue mechanism in European ash and highlight Micro-CT as a valuable non-destructive tool for internal damage assessment under fatigue loading.

Phragmites australis L., a widespread vegetation in riparian zones such as rivers and canals, is extensively studied for its ecological benefits such as nutrient removal and hydraulic retention. However, its direct contribution to bank stability through root reinforcement, a key factor for its use in soil bioengineering techniques, has received limited attention. This study investigated the root reinforcement provided by P. australis and its root traits at a soil bioengineering test site on a canal bank in the Province of North-Holland in the Netherlands. Direct measurements of root-soil composite strength were performed using a corkscrew extraction technique at two distinct distances from the canal. Concurrently, root distribution parameters, including Root Area Ratio (RAR) and Root Length Density (RLD), were quantified from extracted soil plugs. Root reinforcement was also indirectly estimated using biomechanical models, incorporating measured root tensile strength and root distribution parameters as inputs. A total of 12 excavations, each 0.25 m2, were conducted for comprehensive root trait analysis at both locations. Direct measurements revealed substantial root reinforcement (max 36 kPa; avg 6–19 kPa). RAR showed effective stabilization values between 0.03 and 0.65 %, peaking at 0.65 % in the area close to canal. Root systems were dominated by fine roots (<0.5 mm diameter), comprising >80 % of total root length and creating dense reinforcing networks. Corkscrew measurements yielded conservative values. Modeled estimates significantly exceeded these field measurements, which is consistent with conventional shear testing. The extensive root surface area (>3.9 m2 m−2) further demonstrates the species' soil-binding capacity, with higher values occurring in hydrologically favorable zones. While the ecological implications of using this widespread species must be contextually considered, its pronounced mechanical reinforcement makes it a highly effective biotechnical tool, particularly in managed environments like canals.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.