The majority of bridges in Amsterdam’s historic city centre are built on timber foundation piles, typically 12 to 15 meters in length, which function as end-bearing elements reaching the underlying stiff sand layer. Currently, many timber foundations have been in service for up to 300 years, raising concerns about their remaining load-bearing capacity and the overall safety of the bridges they support. The timber piles beneath the bridges remain fully submerged, where their outer cross-section is exposed to slow bacterial decay in anaerobic conditions, leading to a reduction in the sound load-bearing core of the piles. Despite decay, the piles maintain their structural capacity for many years, although their load-bearing capacity gradually decreases over time. The two primary risks are attributed to the applied loads exceeding the load-bearing capacity of the timber piles, and progressive damage accumulation over time due to sustained loading, ultimately leading to large settlements or failure. In this context, 201 pile segments were extracted from two bridges in Amsterdam and mechanically characterised, with respect to their amount of biological decay and service durations ranging from 100 to 300 years. Large-scale compression tests were carried out to determine the remaining saturated short-term compressive strength of the piles. Micro-drilling measurements were conducted to assess the amount of bacterial decay, validated with Computed Tomography (CT) scanning. On this basis, this study investigates the load history and current loads acting on the timber foundation of Bridge 30 (De Isa van Eeghenbrug) in Amsterdam, to assess the remaining load-bearing capacity of the historical piles, considering mechanical damage and decay as function of time.

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.

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.

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.

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.

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.

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.

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.

Timber modular buildings are an emerging construction method, due to the environmental and construction speed benefits. However, the inherent discontinuity and limited deformation capacity, hinders their ability to effectively redistribute loads under accidental load cases and thus, their robustness. A method to quantify the robustness of a building is to assess its behavior under notional column removal scenarios. This study numerically investigates the behavior of a hypothetical five-storey timber post-and-beam modular building under accidental damage events represented by four different column removal scenarios. The findings indicate that the structure could develop sufficient alternative load paths to sustain the amplified accidental limit state design load in most cases, primarily through flexural mechanisms. However, due to the limited ductility of these mechanisms, modular connections were optimally redesigned to enhance axial elongation and capacity, enabling the development of catenary action. The most effective strategy for achieving a robust catenary response was the introduction of a fuse element, significantly improving the ductility of the connection and enhancing the overall structural robustness.

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.

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.

Masonry structures occupy a significant share of the current building stock due to widespread material availability and cost-effectiveness. Regions with high seismicity, such as the Himalayas, have often developed a local seismic culture over the centuries. This has led to improved construction techniques providing an enhanced seismic performance, as evident from post-earthquake surveys in this area. In this framework, Bhatar is a building typology found in the greater Himalayan region, which features embedded horizontal timber bands in masonry walls, enhancing the box-behaviour and in turn avoiding their premature out-of-plane failure. This work aims to quantify the improvement of the out-of-plane performance of masonry walls because of the presence of horizontal timber bands. In order to achieve this research objective, numerical analyses were conducted in DIANA FEA finite element software starting from the few experimental results on this building typology available in the literature. These were used to calibrate the properties of masonry, which was represented as a homogeneous isotropic continuum, with nonlinearities taken into account by means of a total strain rotating crack model. Firstly, a U-shaped masonry wall having the same geometry and boundary conditions as the experimental tests was simulated using a 3D modelling approach. Non-linear static analyses were performed and very good agreement was obtained with the results from the literature. On this basis, the calibrated numerical model was then employed to conduct sensitivity analyses considering varying factors, such as masonry material properties, geometry, opening configurations, timber section sizes and properties. The outcomes of this extensive study show a considerable improvement in the out-of-plane response of the masonry walls in the presence of horizontal timber bands. Given the limited research conducted on the Bhatar building typology in the past, this work constitutes a further step towards a better understanding of the behaviour of Himalayan masonry structures under earthquakes, promoting more effective seismic risk reduction strategies. This improved understanding into timber’s role in imparting greater seismic resilience to masonry structures can inform better maintenance, conservation and preservation of heritage and historical masonry structures in the Himalayas.

Reversible retrofitting techniques for protecting architectural heritage against seismic events have found increasing application in existing or historical buildings in the last years. In this framework, the use of wood-based strengthening solutions for both timber and masonry structures has shown promising results, as proved by several recent research studies, highlighting benefits such as reversibility, compatibility, lightness, sustainability, and effectiveness of these techniques. With specific reference to existing timber floors, the superposition of a plywood panels overlay fastened to the sheathing has proved to be an excellent method to improve the seismic response of such structural components. One of the main benefits of this intervention is related to the combination of a great improvement of in-plane strength and stiffness of the diaphragms, with a considerable increase in their hysteretic energy dissipation. In other words, the method does not only improve the capacity of the floors, but also contributes to a reduction in seismic demand, because of the damping effect induced by the yielding of the numerous fasteners. In order to facilitate the design and use of this reversible, efficient and sustainable retrofitting method in practice, this work first presents the derivation of nomograms, based on previously formulated analytical models. The graphs can be adopted by professional engineers as useful tool for a preliminary structural analysis of timber diaphragms strengthened with plywood panels, enabling a more in-depth understanding of the key design parameters and resisting mechanisms. In addition to that, the implementation of a calculation tool is presented, enabling structural engineers to visualize the expected in-plane force-displacement response and energy dissipation of the retrofitted diaphragms. This work can contribute to the promotion of timber-based techniques in the combined structural, seismic, and conservation upgrading of existing buildings belonging to the architectural heritage of seismic-prone countries.

In the historic city centre of Amsterdam (NL), the predominant foundation system is comprised of wooden piles. Due to their placement below the water table, these foundations are susceptible to bacterial decay. This study aims to investigate and compare various methods for characterizing decay patterns within the cross sections of piles retrieved from two bridges in Amsterdam. The examined piles span different construction years: three originate from 1727, four from 1886, and two from 1922. Following extraction, the piles were transported to TU Delft Stevin II Laboratory, where they underwent further subdivision into three segments, each representing the head, middle, and tip, resulting in a total of 27 segments. The effects of bacterial decay were characterised by performing micro-drilling measurements, small-scale material and compressive tests on prismatic samples extracted from the segments' cross sections, computed tomography scans, and light microscopy observations. Microscopic examination revealed severe degradation in all segments dating back to 1727, extending 20–50 mm from their surface. This outcome was also confirmed by the other adopted methods: the corresponding prisms had large moisture contents and poor mechanical properties, while low basic densities and drilling amplitudes were obtained from CT scans and micro-drilling measurements, respectively. On the contrary, the internal sections of the 1727 segments exhibited no evidence of decay and demonstrated properties consistent with those observed in sound segments from 1886 and 1922. Finally, the observed gradients of density, strength, and stiffness were well correlated with micro-drilling measurements, which can therefore be reliably used as on-site assessment method to reconstruct the properties of the piles.

The majority of the bridges in the historic city centre of Amsterdam are supported by wooden foundation piles. Most of these were constructed 100–300 years ago, currently raising concerns about potential safety issues. The wooden piles under the bridges remain entirely under the water table, potentially subjected to bacterial decay in anaerobic conditions. Bacterial degradation proceeds at a slow rate, allowing the piles to perform their function for many years, although causing a reduction of their load-carrying capacity over time. To this end, a large experimental campaign was conducted to characterize the material and mechanical properties in relation to biological decay of 60 spruce and fir piles, dated back to 1727, 1886 and 1922, retrieved from two bridges in Amsterdam. Large-scale compression tests were carried out on 201 pile-segments extracted from head, middle-part and tip of the piles to determine their remaining short-term compressive strength. Micro-drilling measurements were conducted on each pile and analysed with a TU-Delft-developed algorithm, aimed at determining the soft shell – the width of the decayed outer layer of the piles’ cross section. Micro-drilling allowed to accurately assess the remaining sound cross section of the pile, which resulted to be well correlated to its mechanical properties. The extent of decay throughout the cross-section of the piles was assessed within sapwood and heartwood through experimental models from literature and validated with Computed Tomography (CT) scanning. This allowed to identify that bacterial decay was only present in the non-durable sapwood, even in very degraded piles. Moreover, the soft shell resulted to be rather uniform along the piles’ length. The analysis of decay, supported by micro-drilling and CT scans, allowed to develop experimental equations to predict the remaining short-term compressive strength along the pile length. The micro-drilling technique is now used on a large scale in Amsterdam, supporting the assessment of the remaining load carrying-capacity of wooden foundation piles in the city, aiding in the planning of conservation, maintenance and preservation strategies.

Wooden pile foundations are present in many historic towns in Europe and beyond. The technology of making such foundations was developed primarily in southern Europe, but rapidly spread to other countries. The foundations themselves are made with wooden piles of up to 15m, on top of which horizontal wood members are placed acting as interface between structure and piles. Assessment of the state of wood and its mechanical properties is fundamental for a thorough structural analysis, whether for an existing structure or for re-use. In both cases, the type and amount of degradation needs to be addressed. For wooden structural elements, a fundamental analysis is required regarding the mechanical degradation because of long term loading (duration of load effect), in combination with an assessment of the size and severity of biological or physical decay. These effects are responsible for the remaining load carrying capacity and consequently, also for the decision-making process whether the foundation can be re-used. The assessment of this remaining load carrying capacity is done using an integral damage accumulation model, taking into account the severity and type of degradation, combined with the mechanical load components causing the duration of load effect in wood. As such, the structural analysis approach is different from current design standards for new timber structures, and in-line with the principles laid out in ISO standard 13822 for the assessment of existing structures.

The majority of bridges and quay walls in the inner city of Amsterdam rely on wooden foundation piles. Most of these were constructed 100–300 years ago, implying several challenges for the assessment of the current residual load-carrying capacity and their reliability. In Amsterdam, the wooden piles supporting bridges and quay walls remain entirely under the water table, which means that only bacterial decay can occur. Bacterial degradation proceeds at a slow rate, allowing the piles to perform their function for many years, although causing a reduction of the load-carrying capacity over time. To this end, the municipality of Amsterdam started a large project where non-destructive micro-drilling measurements were employed, with the goal of capturing the in-situ level of decay and the remaining strength of wooden foundation piles. The applicability of micro-drilling was studied on 60 wooden piles with various decay levels, driven between 1727 and 1922, and retrieved from two bridges in Amsterdam. An algorithm was developed for analysing the micro-drilling signals, aimed at determining the decayed outer layer of the pile (soft shell). The micro-drilling approach was validated with the results of mechanical testing on the piles. This study contributes to reliably assessing the decay and remaining load carrying-capacity of wooden foundation piles utilizing in-situ micro-drilling measurements.

This case study explores the utilization of distributed fiber optic sensors (DFOS) in wooden foundation piles, for assessing and monitoring the stress distribution along their length. Three spruce and three pine foundation piles instrumented with DFOS were driven into the soil in a testing field in Amsterdam and axially loaded in compression. Since DFOS provided strain information, calculating the stress distribution in the piles required knowledge of their stiffness properties, which inherently vary from the head to the tip. Consequently, the piles were extracted and their overall wet dynamic elastic modulus (E_c,0,dyn,wet) was determined through frequency response measurements. Subsequently, the piles were segmented, transported to the TU Delft Laboratory and subjected to mechanical testing. For each segment, the mechanical properties were determined and their variability along the pile was studied, in particular for the static modulus of elasticity (E_c,0,stat,wet). This enabled a comprehensive assessment of the actual in-situ stress distribution (Δσ_actual,stat and Δσ_actual,dyn) along the length of the piles, calculated with DFOS strains and the pile stiffness (E_c,0,stat,wet and E_c,0,stat,dyn). Given the novelty of the DFOS application to timber piles, a validation of the accuracy was conducted on 3 pile segments equipped with DFOS. These segments underwent laboratory compression testing, allowing for a direct comparison between DFOS strain readings and strains measured with linear potentiometers attached to the pile segments. The results revealed good accuracy of DFOS in controlled lab conditions, with a maximum stress deviation of 0.65 MPa. Since the testing field featured a 6-meter-deep predrilled layer, where negligible shaft friction was mobilized, the no-friction stress (Δσ_no-friction) approximately aligned with Δσ_actual,stat on the piles. At pile tips, the maximum applied 300–350 kN compressive load (i.e. Δσ_no-friction = 20–26 MPa), resulted in Δσ_actual,stat = 4–7 MPa, highlighting shaft friction effect. The calculated Δσ_actual,dyn with a single E_c,0,stat,dyn for the whole pile, led to 3 MPa stress overestimation at pile tip. Although this calculation is conservative, the detailed knowledge of the variation of stiffness properties along the pile would result in a more efficient structural use.

The majority of bridges and quay walls in the centre of Amsterdam are supported by 100–300 years-old wooden foundation piles subjected to bacterial decay. Bacterial degradation proceeds at a slow rate, allowing the piles to perform their function for many years, although causing a reduction of their load-carrying capacity over time. In this study, micro-drilling measurements were employed to capture the amount of decay and remaining short-term compressive strength of the historic wooden piles. The applicability of micro-drilling was studied on 60 wooden piles with various decay levels, retrieved after 100–295 years of service life. An algorithm was developed for analysing the micro-drilling signals, aimed at determining the decayed outer layer of the piles’ cross section, and validated with the results of mechanical testing on the piles. The micro-drilling technique is now used on a large scale in Amsterdam, supporting the assessment of the wooden foundation piles in the city.

The region of Groningen (NL) has experienced increasing human-induced seismicity caused by gas extraction in the last decades. The local building stock, not designed for seismic loads, consists for more than 50% of unreinforced masonry buildings with timber diaphragms. In this context, a detailed seismic characterization of timber and masonry structural components has taken place, and a retrofitting technique for timber floors activating their energy dissipation has been developed. Besides, specific analytical and numerical modeling strategies for as-built and retrofitted timber floors have been formulated. This work presents a design approach for creating strengthened dissipative timber diaphragms, and maximizing the seismic capacity of existing masonry buildings through this retrofitting method. The results from the performed numerical analyses prove that the proposed design approach for timber floors can increase the energy dissipation capacity of masonry buildings, while improving the box behavior at both damage and near-collapse limit state.