Timber pile foundation piles are important structural components of historic buildings and constructions throughout the Netherlands. Many of these piles have been in service for a long time and start showing serious signs of biological degradation that affects their load-bearing capacity. It is essential to preserve these aged timber piles and, in order to make an estimate of the remaining functionality, it is essential to compute their wet mechanical properties. The conventional method for determining the wet compressive strength is by performing large-scale compression tests on segments from the pile. In this project it is researched whether it is also possible to determine the wet compressive strength by performing small-scale compression tests on discs taken from the pile, which could possibly save both time and resources during future investigations.

For this experimental study a total of 6 spruce foundation piles were selected that originated from 1727, 1886, 1922 and 2019. These piles brought forth 45 round wooden discs with each a height of 15 centimeters extracted from the pile head, middle and tip. The discs contained different amounts of bacterial degradation and high concentrations of knots in order to investigate their effects on the strength. The degradation was quantified using micro-drill measurements and the presence of the knots was specified as a knot ratio. From the results of the small-scale compression tests on the discs it can be concluded that the strength of the discs is well correlated with the strength from the large-scale tests. It was observed that the degraded wood hardly contributes to the strength of the pile and that the section with the highest knot ratio governs the strength of the pile. All in all, it was found possible to obtain accurate values for the wet compressive strength of aged timber piles by performing small-scale tests on discs from the piles, as long as the effects of the biological degradation and the knots are taken into account.

This research focuses on comparing Accoya® with unmodified wood and investigates Accoya®’s structural performance in environments with varying relative humidity levels. The wood’s moisture content, and consequently its swelling and shrinking behavior, fluctuates on an annual basis due to these varying relative humidity levels, especially when exposed to outdoor conditions. FEM analyses were conducted to investigate physical properties and the performance of moment-resistant connections, with their implementation aimed at incorporating rotational stiffness into a structural portal frame. Accoya® demonstrates excellent moisture resistance and an increase in dimensional stability of approximately 80%, based on Dutch climate conditions. The reduction in swelling alleviates internal stresses within the connections, enhancing the strength and stiffness of moment-resisting connections. Specifically, a clamped connection and a circular dowel connection were analysed using a linear elastic static FEM model, revealing internal stress reductions of 81% and 52%, respectively. This reduction was observed during the simulated initial annual swelling cycle that the wood may undergo. With the use of Accoya®, significantly less plastic deformation is expected in connections due to swelling issues compared to unmodified wood and reduced deflection in structures is expected. An increase of 219% and 58% in rotational stiffness was observed for the respective cases. To evaluate the impact on overall stiffness, these observed values were implemented in a portal frame structure. A reduction in horizontal displacement was observed ranging from 31% to 66%. This opens up new possibilities in structural wood design, allowing for slimmer and lighter wood constructions. Due to Accoya®’s lower property degradation and more stable structural performance in high-humidity conditions, an adjustment of the kmod and kddef factors is suggested; however, this is not sufficiently substantiated in the current study. Future research could explore long-term performance factors with experiments such as creep and fatigue to validate Accoya®’s structural reliability further.

In the Netherlands, there is a significant housing demand. The Dutch government aims to construct 100,000 houses per year, which presents an enormous challenge. To address this demand and simultaneously tackle the challenge of creating a more sustainable construction industry, timber modular construction emerges as a potential solution. Compared to traditional construction methods, timber modular buildings offer several advantages: lower weight, more sustainable, faster construction, reduced waste, greater efficiency, and higher quality. Currently, most modular components can be prefabricated off-site, except for the stability system. Traditionally, this system involves a concrete core
or a steel frame. To eliminate their need, self-supporting modules can be applied, making the building purely modular and more sustainable. Within self-supporting modules, stability is a critical aspect. While incorporating stability elements along the longitudinal side of the module poses no issue, the main challenge lies in the limited available length for stability walls in the transverse direction. Also, the shear wall located in the module conflicts with the preferred open floor area. The design of the module, and in particular the shear wall, has a significant influence on the transverse deflection. Calculating the deflection is crucial to fulfil the requirements for maximum displacements. Therefore, the
main research question of this thesis is: Is it possible to predict the lateral deflection in the transverse direction of multi-storey timber modular buildings with a calculation method based on proposed equations?. The goal of this thesis is to propose a calculation method that can be used in the design phase, which will give insight in the effect of design choices on the lateral deflection of multi-storey modular buildings.

The approach to answer the main research question consists of several steps. The first step was a literature study that was conducted on modular construction, lateral stability and deformations of modular buildings. Knowledge on the structural design of timber modules and cross-laminated timber was obtained. Also, the deformation mechanisms of multi-storey modular buildings were investigated. The second step was analysing the deformation of individual modules. A general module design and four configurations were established and the numerical modelling method was investigated and validated.
Hereafter, the equations to calculate the deformation of individual modules in the transverse direction were proposed for the four module configurations. Based on the numerical model, the equations for the horizontal displacement and rotation were established. Additionally, the formulas were extended to incorporate various design options like connection design, shear wall thickness and shear wall position. The proposed equations were integrated in a calculation method that predicts the lateral deflection of multi-storey modular buildings in transverse direction. Hereby only the deformation of the modules was taken into account and not those from the inter-module connections. The cumulative effect due to rotation of lower modules is added to the individual module displacement to find the total deflection.

The results showed that the proposed equations accurately calculate the deformation of the individual modules. With differences below 10%, the formulas are able to calculate the horizontal displacement and rotation for various designs. The calculation method was validated with two examples. Example 1 was a four-storey, four-span building where at the top a maximum error of 8.7% was found. Example 2 was an eight-storey, eight-span building where the proposed calculation method resulted in a maximum error of 9.5%, which was on the lowest storey. At the top, a difference of 2.6% was found. Both
errors were lower than 10% and at the safe side, meaning that the calculation method provides accurate and safe results. A case study was performed to verify the practical applicability of the method. The lateral deflection results showed significant differences (45%), which can be assigned to limitations of the calculation method. These limitations ensure the method is not suited for detailed calculations that are necessary in later design phases. For these detailed calculations, a (parametric) finite element model
would be more suited. However, the calculation method offers a solution for early design stages, as it provides quick and easy insight into effects of certain design choices on the building’s lateral deflection.

In recent years, the building industry is experiencing a shift towards timber buildings driven by the need for a sustainable building industry. A popular building typology consists of Cross-Laminated Timber (CLT) panels. There is an ongoing trend for leaving CLT panels exposed, meaning without protective fire-resistant cladding. This enhances the aesthetical appeal of the building but raises questions regarding fire safety.

This thesis attempts to model the fire dynamics within a CLT compartment with Fire Dynamics Simulator (FDS), which is a CFD software. Hereby, special attention is given to modelling the pyrolysis and combustion reaction for the fire exposed CLT panels. The ability to model the burning behaviour of wood in FDS is a big step forward to be able to move from large-scale experiments to computational models.

This thesis validates the FDS model by comparing the simulated results with the experimental series by Olivier (2019). This series assessed small-scale cubical compartments with 0,5 m sides and a varying amount and orientation of CLT.

Extensive analyses of the simulated results show that the energy release due to the burning CLT is overestimated by approximately 50%. The simulated burning rates range from 1,15 – 1,33 mm/min, while experimentally measured burning rates are approximately 0,60 mm/min which is more commonly assumed in practice. The fire growth phase is approximated well, however validation of the decay phase is poor.

The simulated gas temperatures vary significantly more locally compared to the experimental data. Simulated gas temperatures in the middle of the compartment are underestimated by approximately 30%. On the other hand, gas temperatures near the burning CLT surfaces are approximated well. It is therefore concluded that the radiation from the burning CLT is poorly approximated and has a further reach in real-life experiments.

Extensive sensitivity analyses have shown that the simulated results in the FDS model are highly sensitive to the mesh cell size and the heat of combustion of wood. It is concluded that the three-dimensional mesh should consist of 20 mm cubical cells positioned within a proximity of 10 – 20 cm from the CLT surfaces. In the remainder of the computational domain 100 mm cubical cells should be used to minimise the number of cells in the simulation while maintaining simulation accuracy.

Conclusively, this thesis provides significant added value by enhancing the knowledge within the CFD modelling research field for FSE. The presented FDS model has potential for serving as a tool to roughly predict the fire dynamics within a CLT compartment. However, further optimisation of the model is required before practical applications in daily engineering can be justified.

Based on previous studies, the Dutch Building Regulation (BBL) is inadequately equipped to address fire safety in timber-structured buildings, particularly when the timber is exposed and in the context of tall buildings. Fire compartments containing large volumes of structural timber suffer from additional fire safety risks compared to traditional buildings without a combustible structure. These extra risks include: a longer fire duration, unpredictable fire scenario, potential second flash-over, failure of timber members due to heat wave-penetration, larger protruding flames from openings and increased smoke production (Brandon et al., 2022). This master thesis focuses on studying fire safety of CLT (Cross Laminated Timber) compartments with properties typical for apartment buildings. The master thesis contains a literature study, divided into material level, structural component level, compartment level and building level. This literature study can be helpful for people who want to understand the fundamentals of fire safety in contemporary CLT timber buildings and want to understand the further master thesis with more ease.
In this master thesis we used three fire behaviour models to analyse compartment fires previously tested in full scale by Brandon et al. (2021a). The outcomes of simulations done with these models were compared to full-scale compartment fire test data (for a compartment of 48 square meters) to get an understanding of the accuracy, and strengths and weaknesses of the models. The study suggests that the ZHM and Brandon models are able to simulate the average char depth in the ceiling. For compartments that have percentage of exposed mass timber and an opening factor equal to that in test 1 of Brandon et al. (2021a), the outcomes of the Brandon and the ZHM models give conservative results for the the maximum char depth in the ceiling. There is a discrepancy between the simulation results and the maximum char depth in the walls when amount of exposed timber and opening factor were increased beyond the values used in test 1 of Brandon et al. (2021a). Suggestions for correction factors based on linear interpolation are given to arrive at a better approximation for the maximum char depth. It is advised to use CFD and/or compartment fire tests for any compartments that differ from tests 1-5 in Brandon et al. (2021a). Targeted design measures were listed to protect the critical areas where maximum char depth is expected.
In order to get a better understanding of the fire behavior models for CLT compartments, I simulated 24 different realistic compartments, with differing sizes, shapes, amounts of exposed timber and opening factors. The parameters were chosen in such a way, that the variations encompass a range of typical CLT apartment buildings. I performed this analysis using three different fire behaviour models. The models calculate the charring depth into the CLT cross-section. We then added the zero strength layer (timber without any structural resistance due to the heat-wave) to the modelled charring depth, resulting in the effective char depth. The larger the effective char depth, the more of the cross-section’s initial resistance is lost. Over the three models, the largest correlation was found to be between opening factor and effective depth. The second largest correlation was found to be between amount of exposed timber and effective depth. Using the results of the models, without any correction for maximum char depth, none of the simulated compartments suffer failure of the ceiling panel according to calculation.
Between the three models, the zone model (Brandon et al., 2021b), is best capable of simulating protection for a finite number of minutes, as the exact number and thickness of the protection can be input in the model.

Timber as a construction material for high-rise buildings is gradually entering the construction industry. Timber is considered a sustainable option, because it is a natural material that absorbs CO2 rather than producing it.
However, using timber as a building material also introduces new challenges, one of which is the problem with lateral stability, due to the relatively low stiffness of the elements and their connections. This research investigates the value of a timber core in a building that has bracings in the façade. The study examines this value in both normal conditions and the accidental limit state in case of a fire situation. The key question being addressed is:
Can a timber core sustain lateral load as a secondary load path, in case of failure of the tability bracing?
First, a literature review is conducted to understand the material and relevant safety mechanisms. A parametric program is used to explore the core’s parameters, alongside a linear elastic 3D FEM (Finite Element Method) program that utilises members and surfaces to analyze the building structure. The considered building is a 10-story rectangular structure (28.8 x 21.6 meters) with glulam beams, glulam columns, and CLT floors. The building was designed on an infinite stiff foundation by using a timber core and timber bracings in the façade. The core consists of cross-laminated timber (CLT) panels, connected with slotted steel plates and dowels. The bracings are glue laminated timber and are connected with two slotted in steel plates and dowels. The CLT panel connections and the bracing connections were calculated by hand and implemented in the FEM model. Which was validated
by hand calculations. Additionally, wind load, variable load and permanent load were applied on the model.
This model was used to answer the following question. What percentage of the lateral force can the core take?
The model with a timber core in the Urban Woods shows an 18% reduction in global deflection compared to the model without a timber core in the ultimate limit state. The deflection of both models where to be within the prescribed limit. Additionally to the deflection reduction, the forces in the bracing are reduced by 33% in the model with a timber core. The core parameters that influence these reductions are the connections between the core panels and the cut-outs in the timber core. A core parameter study has been carried out to answer the following question:
How do the core parameters influence the global deflection?
For the connections, increased stiffness enhances the contribution of the core to reducing global deflection, with reductions ranging from 0% to 22%. Using longer panels results in fewer connections, which makes up for 7% of the global deflection reduction at any given stiffness of the CLT connections. Regarding the cut-outs, a larger cut-out size results in a lower contribution to deflection reduction. The reduction in global deflection for
different cut-out sizes ranged from 5% to 27%. As the number of floors increases, the value of a timber core diminishes.
In the accidental limit state of fire, strong wind and failure of a facade bracing element, the timber core will serve as a sufficient alternative load path. However due to the wind force reduction in the ALS the remaining bracing is also capable of withstanding the lateral wind force. When two elements are removed than the core will be necessary. The unity checks for the CLT elements and connections did suffice.

The construction industry contributes significantly to global emissions. In response to the growing concern about climate change, new methods are being developed to reduce these emissions. Building with timber is one such approach, as it can be CO2 neutral if reforestation is practiced. Timber is a lightweight and flexible material compared to traditional construction materials like steel and concrete, which can result in high accelerations and deflections, negatively affecting user comfort. Engineered wood products such as CLT and GLT make it possible to construct larger panel sizes and spans, which can lead to more efficient building design. Many buildings utilize core systems to provide lateral stability and facilitate vertical transportation of goods and people. Therefore, replacing a conventional concrete core with a CLT core could be a viable solution to reduce the building’s environmental impact. Timber, being a flexible material, often lacks the necessary stability to meet user requirements for deflection and acceleration in high-rise buildings. An outrigger, similar to those used in concrete and steel structures, can provide the solution to enhance user comfort. The outrigger transfers bending moments from the core to the columns, reducing forces in the core and increasing overall stiffness. However it is unclear what the influence of the addition of a timber outrigger on a CLT core building is and how such a system can be improved. Therefore the purpose of this thesis is to evaluate the impact of a timber outrigger on a CLT core building, and provide guidance for structural engineers on how to improve the structural behaviour of such a system. The impact of adding an outrigger is evaluated through the comparison of different numerical case studies with and without timber outriggers. Additionally a sensitivity analysis is performed in which the influence of core joint design and the impact of outrigger design parameters is evaluated. By introducing a timber outrigger to a CLT core building bending moments are transferred from the core to the column, partially restricting rotation of the core. This will have the following effects on the behaviour of the structure. Firstly, the deflections due to lateral wind loading are reduced due to the increased global stiffness. Secondly, the increased stiffness of the structure will result in a reduction of peak lateral acceleration. Thirdly, by transferring loads from the core to the columns the strength requirements on the connections of the core are reduced. Finally, by introducing a timber outrigger system to a CLT core building horizontal forces are generated in the core at outrigger level, which should be accounted for in the design phase of the vertical core joints. Increasing the stiffness of the outrigger will further reduce deflections, peak lateral acceleration and strength requirements on the core joint, however strength requirements on the vertical core joints at outrigger level will further increase. An exponential relationship has been derived between the lateral deflections and the ratio between the effective bending stiffness of the outrigger and the effective bending stiffness of the core, a similar exponential relationship has been derived for the peak lateral accelerations Additionally it was investigated what the critical design parameters are and how the performance of a CLT core building with timber outriggers can be improved. For the design of a CLT core the connection between the core and foundation showed to be critical, having large effects on the lateral deformation. While increasing the stiffness of the vertical connections showed to have little influence on the interaction between core walls due to their respective high interaction factors. In the design of the outrigger it was shown that a two storey outrigger is able to more than double the reduction in deflections compared to a single storey outrigger, further increasing the overall effectiveness of the system. Finally the effectiveness of the system was found to be significantly influenced by the configuration of the outrigger truss, demonstrating a greater impact than increasing member size or connection stiffness.

The demand for timber buildings is rising because of the need to make the construction industry more sustainable. Eurocode 5 provides guidance in the design process of timber structures. Numerical modelling is generally used for complex structures like multi-storey timber buildings. Currently, there is limited understanding of the serviceability behaviour of multi-storey timber buildings that are stabilized by glulam trusses. This is partly because the load-slip behaviour of large timber connections is not well understood.

The aim of this thesis is to develop a better understanding of the influence of the connections on the serviceability behaviour of timber buildings with glulam stability trusses. In the first part of this thesis, the state-of-the-art of timber connection design is discussed. The second part is dedicated to the development of a finite element model. A modelling approach is developed that allows for the specification of the load-slip behaviour of the truss connections in the end nodes of the modelled members. A sensitivity study focused on the serviceability stiffness of the truss connections is performed using the FE model. Next, variants on the base design are modelled and studied for comparison, which provides insight into the effect that changing geometry has on serviceability behaviour.

This thesis shows that the load-slip behaviour of the truss connections has a significant influence on the serviceability behaviour of a timber building. It is also found that the current design rules are not suitable for predicting the serviceability stiffness of these connections. For timber connections with slotted-in steel plates and dowels, their load-slip behaviour can be idealized by an initial slip followed by its elastic stiffness. The initial slip is caused by the hole clearance in the steel plates in combination with various non-linear effects like densification of the timber. The hole clearance in the steel plates is also responsible for the sequential activation of the dowels, which is considered to be the primary cause for the reduction in elastic stiffness per dowel per shear plane as the number of dowels increases. This so-called ’group effect’ results in a much lower stiffness of the truss connections than the current design formula suggests.

The results from the numerical modelling show that the load-slip behaviour of the connections between the diagonals and columns has a large influence on the serviceability behaviour of the truss. A distinction is made between diagonal connections and vertical components. The initial slip in the connections causes an increase in the maximum displacement of the structure. The elastic stiffness of the truss connections decreases the global stiffness of the structure. This resulted in an increase in maximum displacement, a decrease in the natural frequency, and an increase in peak accelerations.

The serviceability behaviour of large timber connections should be explored further. Current design codes are not suitable for predicting their serviceability behaviour, which can have consequences for the design of complex structures like multi-storey timber buildings with glulam stability trusses. To ensure the proper design of these buildings, our understanding of their serviceability behaviour is critical. This thesis is anticipated to be a valuable contribution to bridging the knowledge gap. It shows the shortcomings of the current design rules and provides practical information about how to predict the serviceability behaviour of a timber building that is stabilized using glulam trusses.

Self-tapping screws are popular tools used in timber engineering. The axial withdrawal strength of self-tapping screws could be used in timber structures as an active or passive application. An example of an active application is the use as a fastener in wall to wall connections. An example of a passive application is the use as reinforcement perpendicular to the grain, to prevent cracking along the grain. Existing methods to calculate the withdrawal strength of self-tapping screws are mainly based on tests done on softwood. Some screw manufacturers offer calculation methods for the use of their screws in hardwood from temperate regions in so called European Technical Assessments (ETAs). But the Eurocode and the ETAs offer no calculation methods for high density tropical hardwood species. Tropical hardwood species are generally used in The Netherlands for hydraulic and outside structures such as lock-doors, bridges, weirs and jetties. Tropical hardwood species are used because of their strength and durability in outside conditions. There are no well-established methods to calculate the withdrawal strength of self-tapping screws in tropical hardwood perpendicular to the grain and the purpose of this thesis is to gain more knowledge on this topic. The central question of this thesis is: What is the withdrawal strength of self-tapping screws in tropical hardwood species perpendicular to the grain and is it possible to develop a verification model for engineering purposes?
To find answers to the main question first a literature review has been conducted. Literature on timber anatomy, self-tapping screw properties and available calculation methods have been reviewed. This is followed by a literature review on possibilities to drive self-tapping screws into high density woods. This is problematic because screws could fail already during insertion or cause splitting of the timber. Screw modifications such as cutter tips could be used to reduce the insertion moment, but also pre-drilling of the timber could be done to avoid torque failure of screws or the splitting of timber. The literature study is finalized with an investigation to the influence of material properties on the withdrawal strength. Tropical hardwood species are often used in outside or wet structures so literature on the influence of moisture content has been investigated. In research done by Ringhofer et al. [22] it was found that if the moisture content increases above 12 % the withdrawal capacity significantly decreases. Pre-drilling of the timber helps to insert self-tapping screws more easily into timber, but this also removes material and could therefore reduce the withdrawal strength. Research done by Brandner, Ringhofer, and Reichinger [5] found that if the pre-drilling diameter is smaller or equal to 80 % of the nominal screw diameter pre-drilling had no influence on the withdrawal strength. The mentioned research however has been done on temperate hardwood and the question is whether this is the case for tropical hardwood as well. According to EN 1995-1-1:2011 pre-drilling is necessary if the characteristic density of the timber is larger than 500 kg/m³. With increasing timber density the withdrawal strength also increases (Hubner, Rasser, and Schickhofer [13]). Regarding the screw diameter, with increasing diameter the withdrawal strength (f_ax, MPa) decreases as was found by for example Hubner, Rasser, and Schickhofer [13]. With increasing diameter the withdrawal capacity (F_ax, kN) also increases. With increasing effective length of self-tapping screws inserted in wood the withdrawal strength (f_ax, MPa) does not increase or decrease (Xu et al. [30]). The withdrawal capacity (F_ax, kN) however increases, a linear relationship between the effective length withdrawal capacity can be observed in research done by Xu et al. [30].
Findings in the literature study were the basis of how the test series were designed. EN1382:2016 describes how to test and determine the withdrawal strength of screws in timber. This standard also gives rules on the dimensions of the test piece. The width of the available tropical hardwood pieces was to small according to EN1382:2016 which states that a minimal edge distance of 5d has to be used. Methods from the EN 1995-1-1:2011 and ETA-11/0030:2020 from Rotho Blaas have been used to make a selection of screws that will be used in testing. The goal is to have withdrawal as main failure mode. The following screw diameters have been selected: 6, 8 and 11 mm. The diameter 11 mm screw has been tested in a pre-testing series with pre-drilling diameters: 0,7d and 0,8d; with insertion depth 90 mm; Also three tropical hardwood species with the highest density were selected to test the limits of the materials. It was found that the smaller edge distances did not cause any problems. Further because of pre-drilling no damages occurred on the timber or screw, and there were no problems during screw insertion. No significant difference in withdrawal capacity was found between pre-drilling 0,7d and 0,8d so this was changed to pre-drilling 0,8d and 0,9d in the main test series. Also screw failure was observed for insertion depth 90 mm, this was changed to a maximum of 70 mm in the main test series to make sure screw withdrawal is the main failure mode.
For the main test series 6 tropical hardwood species were used (Kanda, Lati, Longhi, Tali, Limbali and Mukulungu), and also the softwood Spruce as a reference material. The diameter 6 mm, 8 mm and 11 mm screws were used with pre-drilling diameter 0,8d and 0,9d. Conditioning of the wet test pieces has been done by putting the timber blocks underwater. On every test piece or timber block four tests were done: two withdrawal strength tests (according to EN 1382:2016) and two stiffness tests (according to EN 26891:1991). Also the insertion moment was measured. After testing the density of every test piece was determined together with the moiture content.
The main test series provided a broad range of data. The following was measured: the density, the moisture content, the withdrawal capacity, the insertion moment and slip modulus. All results can be found in annex A. The test piece geometry was not according to EN 1382:2016, in general larger edge distances were required. Nevertheless did this not lead to any problems, no cracks or screw failure occurred during the insertion of the self-tapping screws nor was it any problem to insert the screws at all. This had all to do with the pre-drilling diameters of 0,8d and 0,9d what was used. Because of this very low insertion moments were measured. Also during and after withdrawal tests no significant damage was observed. Even in extreme situations that were tested during pre-testing there was no evidence that a larger edge distance was needed. In the analysis it was found that with increasing density the withdrawal strength also increases, this is in line with literature. It was also found that the species does influence the withdrawal strength, as was found by calculating the withdrawal strength to density ratio per species. With increasing screw diameter the withdrawal strength decreases as was found in this analysis, and also in line with literature. Only in the case of pre-drilling 0,9d for the diameter 8 mm screw this was not the case when comparing it to the diameter 6 mm screw, this probably was a result of pre-drilling accuracy. Regarding the influence of pre-drilling it was found that on average the withdrawal strength is reduced by a factor of 0,68 when the pre-drilled hole is 0,9d instead of 0,8d. In literature it was found that the effective screw length does not influence the withdrawal strength. No clear influence of effective length has been found, except for screw diameter 6 mm and 8 mm when pre-drilled 0,9d. The diameter 6 mm and 8 mm screws are HSBH screws with a larger inner diameter and it might be that the withdrawal strength is not optimal for these types of screw when inserted in 0,9d pre-drilled holes. During testing wet and dry pieces were tested. It was found that the moisture uptake rate is very different for the various tropical hardwood species that were tested. It was also found that as expected the withdrawal strength is negatively influenced by an increased moisture content. First of all it has to be mentioned that the scatter in data is larger when compared to the test data from withdrawal tests, this has to be taken into account. In the analysis it was found that with increasing density the slip modulus also increases. Also the timber species influences the slip modulus just like in the case of the withdrawal strength. No clear influence of screw diameter was found on the withdrawal strength. Just like for the withdrawal strength is the slip modulus influenced negatively when pre-drilling 0,9d has been used instead of 0,8d. This negative influence increases with increasing diameter. With increasing effective screw length also the slip modulus increases, this would be expected and was also found in the test results. Also a higher moisture content has a negative influence on the slip modulus, this was also as expected. A larger influence was observed in the case of timber species with the highest densities measured (Tali and Mukulungu), while for these species the lowest moisture content was measured in wet conditions compared to other species.
Finally two models have been proposed to better predict the withdrawal strength of self-tapping screws perpendicular to the grain in tropical hardwood. For the first model withdrawal strength $f_{ax}$ parameters are determined per species, per timber condition and per pre-drilling diameter. The second model is one formula based on the density, screw diameter and effective screw length for all tested tropical hardwood species. Both proposed models show a good fit to the test data. 

The goal of the thesis is to investigate two main questions for the purpose of modelling fractures in timber with a finite element method. The first question is to evaluate the use of digital image correlation during a three point bending test to obtain mean values for tensile strength and fracture energy, and the follow up question is if it is possible to replicate the tests with these mean values. The final result should be a guide to help modelling fractures in timber and how to obtain the necessary data.

The demand to build with wood and other bio-based materials has increased rapidly the last decade, due to the ambition to build more with sustainable building materials. One of the major challenges in building with wood, is to account for the lower strength and stiffness perpendicular to the grain. Wood is an anisotropic material, meaning the material has different properties in different directions. Loading in the perpendicular to the grain direction is sometimes unavoidable and fracture often occurs in a brittle manner. Unfortunately, our understanding and knowledge of the perpendicular to the grain fracture is limited. This is especially the case for hardwoods, for which the material properties and the parameters important for the fracture process are often unknown.

The goal of this thesis is to reduce this knowledge gap on the material properties and the relevant parameters in mode-I (pure tension) fracture process of (tropical) hardwood. The knowledge gained in this research provides the answer to the main research question of this thesis: Which parameters should be considered when describing the constitutive model of a (tropical) hardwood in mode-I tension?

A single-end notch three-point bending (SEN-TPB) test demonstrated a strong influence of the orientation of the growth rings on the perpendicular to the grain fracture process of azobé. The value for the peak load and post-peak behaviour change significantly depending on the orientation in which the sample is placed. Furthermore, visual observations and results from a profilometer revealed that there is an inconsistency in the roughness of the cracked surface area between the different series. From the microscopic analysis is concluded that this inconsistency in general mechanical behaviour and the roughness of the cracked surface area is linked to the composition of ray and parenchyma cells. Moreover, the direction of the ray and parenchyma cells with respect to the fracture plane is different depending on the orientation of the sample. When the ray cells are orientated perpendicular to the crack plane, a higher strength is obtained. The thin-walled parenchyma cells significantly reduce the post-elastic stiffness if these cells are orientated perpendicular to the crack plane. The influence of the composition and orientation of both cells is reflected in the numerical value for the modulus of elasticity, the tensile strength and the fracture energy. This research showed that the composition of the cells in a wood specie and the orientation in which the growth rings are placed significantly influence the mode-I fracture process of a hardwood. This influence is reflected in the modulus of elasticity, the tensile strength, the fracture energy and the shape of the tension softening curve perpendicular to the grain. The results of this research provide new insight into the fracture modelling of hardwood. Both in numerical modelling and design considerations the variation in the material properties because of the orientation of the growth rings, should not be neglected.

Concrete and Steel are the materials with the largest market share in the construction industry and have been for a long time.
With environmental awareness increasing, timber is regaining popularity due to the potential for carbon neutral and even carbon negative construction.
Use of structural wood elements in bridges, however, is often limited to foot- and cycle bridges.
In this thesis, key aspects with regards to the design of heavy traffic bridges incorporating timber members are identified.
A fully steel bridge and an equivalent bridge, combining timber members with steel are designed within boundaries set by a case study. These designs are developed to a level sufficient for an adequate comparison of the bridges.
The basis on which the bridges are compared are laid out, followed by the conditions the bridges are subjected to. These are based on typical conditions found in an urban Dutch environment.
Analytic equations are automated, by way of python scripts, for the analysis and optimization of the steel bridge longitudinal dimensions under simplified ULS loading. After the optimization is complete, these initial bridge dimensions are verified with a 2D plate element model in SCIA engineer. The full loading for the bridge during utilization, save for accidental loading, is then modelled and the bridge dimensions are adapted in order to meet ULS, SLS, and fatigue conditions.
Several potential versions of a bridge with timber members are considered. Following this, a bridge with a mostly timber superstructure, supported by a self-anchored cable system is further worked out. For this, a SCIA model, with 1D elements and subjected to the same loads as its steel counterpart, is produced. The incompatible combination of 1D elements, thick cross sections, and surface loads is addressed by the use of connector elements (“dummy members”) and individual load panels per member.
After the global optimization of the bridge dimensions with regards to ULS and SLS, the connections are designed with a combination of detail 2D element FE models and analytic equations. The forces and support conditions of the connections follow from the global bridge design. A fatigue check is then run on the timber members of the bridge.
Subsequently, the durability and eco- costs of the bridges are computed. The data for the durability estimation of the steel bridge is based on experience within IV- Infra and the durability of the timber bridge is estimated using the RISE factor method. The eco- costs of the bridges are computed using the IDEMAT database.
The results from the analyses are discussed based on this.
Finally, aspects of relevance in the design of timber bridges are synthesized and recommendations for the application of bridges and further research are given.

Inner–city quay walls fulfil a number of very important functions where the retention of water is the most important one observed from a structural perspective. Despite their importance, not all of these structures are very well maintained, especially the older ones. Each municipality has their own maintenance regime, depending on the allocated budget. Deficient quay wall management of a few municipalities in the Netherlands resulted already in several quay wall failures, but there are also a lot of those structures still in a good condition. The differences in quay wall compositions and internal force transfer mechanisms could be one of the reasons why some structures do not show signs of deterioration where others are in a very deteriorated state. This hypothesis is the starting point for the following research question: “What is the influence of structural deficiencies to the overall strength and stability of a wooden foundation system under a quay wall structure?”. The geotechnical and material resistances of the quay wall elements are used as a basis for the study to the influence of quay wall defects on the internal force transfer mechanisms. Five quay wall structures with different geometries in Rotterdam and The Hague are selected for the structural calculations. With the help of assessment reports, a few common defects are chosen which are likely to occur for these kind of structures. First, the displacements and internal forces of the pile cap beam in the middle cross-section are calculated when the structure shows no signs of structural declination. The obtained results are then compared with the same base quantities where one of the structural defects is applied. In this way, the effect of each defect can be determined. Although there are large differences between the composition of the various quay wall structures, a general conclusion can be drawn from the performed calculations. A larger number of piles under a quay wall structure has a positive influence on the redistribution of internal forces from a weakened spot to an unaffected part of the structure.

The infrastructure industry currently deals with two issues: the deterioration of (highway) bridges, and the urge to reduce emissions by the construction sector. Rijkswaterstaat must repair or replace hundreds of bridges in the upcoming decades. At the same time, the impact of human behaviour on climate change becomes more visible and must be reduced. The most structural solution to diminish the human impact on the environment is switching from a linear economy to a circular economy (CE).
This research aims to develop a system for circular highway bridges by both constructing with timber and extending the lifespan. The first contributes to a lower environmental impact as timber is a renewable material that captures carbon during growth. For an outdoor timber structure, protection is crucial to prevent the timber from deteriorating due to weather influences. The lifespan extension is obtained by applying three circular principles: (1) Design for Material Efficiency (DfME), (2) Design for Adaptability (DfA), and
(3) Design for Disassembly (DfD). Variant studies on typology, connections and material optimise for material efficiency. A flexible structure enables DfA: converting in function and expansion in length and width is possible. Furthermore, a modular system is developed to include DfD: connections between modules are demountable to disassemble, adapt and reuse the system. In summary, four design strategies are defined: efficient, protected, adaptable and demountable.
This research aims to provide an alternative for concrete highway bridges with a lower environmental impact.The timber bridge, a circular concrete bridge and a traditional concrete bridge are compared. The comparison includes the production (A1‐A3), construction (A4‐A5) and end of life (C1‐C4) stages. Timber results in a lower carbon footprint for all reference periods and lifespans. The study considers the third scenario most reliable, as the technical lifespans are most substantiated. In this scenario, reductions of 18% and 47% for respectively the circular and traditional concrete bridge are established.
This study concludes that Dutch highway bridges suit replacement with a modular timber alternative. The developed modular system covers 58% of the highway bridges stock owned by Rijkswaterstaat that were built in 1950‐1980. Moreover, timber shows substantial environmental benefits compared to concrete. The alternative reduces the carbon footprint by 47% compared to a traditional concrete bridge. Furthermore, this study observes additional benefits when one considers carbon emissions in time, as the structure
sequestrates carbon during use. Consequently, building with timber lowers the carbon concentration in the atmosphere, reducing the global warming effect. This study recommends Rijkswaterstaat to invest in the development of circular timber highway bridges. By changing the ’business as usual’ from concrete to a more sustainable alternative, the circular and climate goals can be obtained. Furthermore, the recommendation is made to clarify the regulations on quantification of environmental impact, both on data and methodology level.

This master thesis is on post-tensioning cross-laminated timber stability cores for multiple story buildings. When designing a CLT core, significantly larger core sections will be needed than when designing a stabilizing core in concrete. This is for one part due to the limited stiffness of the CLT compared to concrete. For another part it is due to the limited stiffness of connectors in CLT. Sliding and uplift can occur in connections in CLT loaded in tension and shear respectively. The CLT panels behave like rigid bodies, with most of the displacement occurring at the connections. In addition, cooperation between flange and web may be limited, depending on the stiffness of the corner connection and the occurrence of shear lag. Post-tensioning is suggested as a solution to diminish uplift and sliding in the connectors. In this way, with the same core section, a taller building may be realized compared to the non-post-tensioned case. In the thesis also the long-term effects on the prestress level is assessed, as estimating these effects is important for the safety of the system.
This thesis adds to the body of knowledge on post-tensioned CLT structures. Firstly, previous studies on post-tensioned CLT focus on individual shear walls and on seismic design situations. This thesis explores how beneficial post-tensioning is from the perspective of serviceability limit state governed design. Furthermore, though post-tensioning as a prestressing method has been applied often in concrete structures, prestressing of CLT is a novel research subject. Especially the estimation of long-term force loss is a topic that still requires research. This thesis provides the designer with a straightforward calculation method (using python) for estimation of prestress force loss in the long-term.
The research was carried out with a literature study and a case-study. The literature research comprised of studies on structural design with CLT loaded in-plane; the effective flange of a CLT core; stiffness of connections in CLT; prestressing of CLT; a design approach for post-tensioning; time dependent losses in post-tensioned CLT. The case study was based on a fictitious floorplan including a “minimal core”, and at expressing the benefit of post-tensioning in terms of height gain.
The degree to which the flange and the web cooperate showed highly dependent on the connection between flange and web and the core height. In the case study, the effective flange width showed to depend highly on the height of the core and the stiffness of the connection between flange and web.
In the case-study, without post-tensioning, approximately half of the displacements could be attributed to the connections. With post-tensioning, the uplift and sliding displacements in the horizontal joints was eliminated. Consequently, the attainable height was significantly increased: from 5 storeys in the un-post-tensioned case, to 8 storeys in the post-tensioned case. Long-term effects on the prestress loss were considerable. In the case-study, approximately 40% loss of post-tension force in the lifetime of the building was predicted and included in the design. Largest causeof force loss was due to changes of moisture content during construction. The remaining lateral displacements after post-tensioning were due to bending and shear.
Post-tensioning of CLT cores is a powerful method for reducing lateral displacements in cases where uplift and sliding are dominant contributors to the lateral displacements. This is especially the case in light-weight buildings. Uplift and sliding displacements can be eliminated altogether with post-tensioning. The designer should realize that post-tensioning does not increase the bending and shear stiffness of the core. The thesis also concludes that with the post-tensioning of CLT walls, the compressive strength of the CLT in the so-called “compression-toe” might be exceeded. It is an important check in design. Furthermore, depending on the decision to re-tighten the rods at some point or not, the post-tension force loss should be calculated and included in finding the right prestress level. For this estimation of the moisture level of the CLT proved to be an important but difficult step. It is likely that the 40% force loss in the case-study is on the conservative side, since a large change in moisture content has been assumed. In practice, the moisture content can be measured on site. This can help verify the assumptions on the moisture content used in force loss calculations. This can help in assuring the structure is safe in the long-term.

The increased loading and decreasing strength of historical quay walls calls for remedyand has drawn the attention of many Dutch municipalities, Rotterdam being oneof those. In order to set up proper assessment methods that result in an economicaland ecological policy for renovation or replacement of these structures the Port of Rotterdamhas asked Delft University of Technology to assist them. This thesis focusses onthe elements perhaps most prone to decay and hardest to inspect: the timber foundationsimplemented in quay walls with relieving platforms from the late 19th and early20th century. The main research question of this thesis is What is the (remaining) capacityof the timber foundation of a quay wall with relieving platform? and focusses onthe Maaskade case. The aim is the provision of a structural analysis and appointmentof weakspots on a representative quay section build-up using measured material characteristics.This quay has partially collapsed and has been replaced as of May 2020. 45Timber piles from the site have been investigated in the Rotterdam harbour to derivedensity and (dynamic) Young’s modulus measured by a Timber Grader MTG. A selectionof these piles was then further tested in pieces (21) at the TU Delft structural laboratory(Stevin II) to derive the Young’s modulus and compressive strength. Using the correlationfound between the compressive strength parallel to the fibre ( fc,0) and the dynamicYoung’s modulus parallel to the fibre (E0) the compressive strength values of the entire45 pile group have been derived. This results in fc,0,k = 12.64 MPa and E0,mean = 11.3GPa under 50% moisture content, the saturated state the wood is expected to be in inthe quay. The resulting design strength fc,0,d = 10.7MPa is very much in line with the advisedstrength of 10.8 MPa in NEN 8707 . Missing characteristics necessary for structuralassessment are used from bending class C22 (sawn timber) from EN 338 . A FEMmodel of section 4-1 of theMaaskade under different historical build ups has been set upin Plaxis 2D, and an Ultimate Limit State computation has been carried out on the finalbuild-up before replacement under four load cases. The found normative location thatcan be seen as the weakspot is the joint between the raking pile and the adjacent cappingbeam. A structural assessment based on sectional forces results in a negative outcome ofthe structure even on the self weight governed loadcase as unity checks > 3 are given for thecapping beam. These results do have the shortcomings due to simplifications from 3D to2D and a modelling with linear elements with no volume in the model and no allowancefor further load distribution beyond elasticity. A definite answer on the capacity in termsof additional external loads cannot be given and the need for a model that incorporatesthe exact lay out and positioning of the loads is stressed. Further recommendations forpractice are to find out if the capping beam- pile connections are expected to be able totake tension and to name their state after an inspection.

Amsterdam and many other cities consist of a network of old quay walls, which sometimes have been constructed over a century ago. A large uncertainty exists concerning the current safety of these quay walls and their remaining service lifetime. In the current framework for the assessment of old urban quay walls, the influence of timber creep in the structure is being omitted. It had been expected that a part of the excessive deformations occurred are part of this timber creep as opposed to progressive failure. Hence, this research studied the influence of wood creep on the structural behaviour of old urban quay walls, using a case study based on the Herengracht. For the modelling of the quay walls, use has been made of the Embedded Beam Row (EBR) elements in Plaxis 2D. As this element type has no material model which takes into account creep,
use has been made of pseudo-elasticity. The behaviour over time had been modelled by reducing the elastic stiffness of the EBR elements with increasing creep factors. To force Plaxis to perform calculations, this stiffness reduction has been introduced as a reduced strength in the form of a custom moment-curvature diagram. At the time of writing, the Embedded Beam Row had not yet been validated for cohesive soils. Hence, a verification has been executed of the EBR using a full-scale load test performed on a pile group in Salt Lake City. In
this case, a 3x5 pile group driven in multi-layered cohesive and non-cohesive soils had been laterally loaded. The results included deformations, total horizontal load and bending moment distributions over depth. It
has been found that the EBR provides reasonable results when modelling laterally loaded pile groups. The force-displacement curve from the field test was only slightly stiffer compared to those obtained using Plaxis 2D. Maximum bending moments obtained using the EBR had been found to be 20 to 30% compared to the experimental data. The group efficiency of the pile group was initially lower for small displacements, which was likely caused by the lack of installation effects of driven piles in the Plaxis model. It has been observed
that the Interface Stiffness Factors (ISF) that are used to calibrate the EBR behaviour have a limited range. Increasing the ISF values beyond a certain limit will no longer affect results, as the interface connecting the EBR to the soil will have become practically rigid at this point. Nevertheless, it has been concluded the EBR can be used in this case to model laterally loaded pile groups. The influence of the timber creep on the structural behaviour of quay walls has been studied using a model based on the Herengracht in Amsterdam. A maximum creep factor has been applied of Φ=1.6. Two methods have been used to apply the final creep factor. With the "Direct" method, the maximum creep factor was
applied in the same phase as the load. With the "Indirect" method, the creep factor has been applied incrementally in steps of 0.1. It has been observed that the results from these methods deviate significantly. With the Indirect method, larger creep displacements have been calculated, as well as lower maximum compressive stresses in the piles. The creep behaviour has been studied in more depth using the Indirect method. In this case study, creep displacements of 2.22 times the initial displacement have been calculated. When plotted against the increasing creep factors, it has been observed a power function could be fitted to the data. In addition, a stress reduction of 0.590 times the initial maximum compressive stress has been observed in the front two pile rows. This stress reduction was achieved at φ=0.4. In the most landinwards pile row, stresses
continued to decrease, with a reduced rate beyond φ=0.4. A sensitivity analysis has been performed to study which parameters have significant influence on the creep behaviour. Conclusions were drawn based on the relative change in horizontal displacement and maximum compressive stress in the front pile row. It has been concluded that the shear strength parameters of the top layer, the elastic modulus of the timber, surface load and the geometry have the largest influence on the creep behaviour. Furthermore, it has been discovered
that the size of the creep factor steps influences the final stress and displacement. With decreasing stepsize, convergence occurred in the results. From the results it has been concluded that the structural behaviour of
old urban quay walls is significantly influenced by timber creep. The inclusion of timber creep resulted in large displacement increases, but also in stress reduction. The results suggest that excessive deformations of quay walls does not mean that an ultimate limit state has been reached. It is recommended to include the timber creep in the modelling and assessment of existing quay walls.

Within the context of utilizing alternative materials to promote sustainable construction in the building industry, Living Architecture is a concept that uses living organisms as the building materials. As a rather unconventional approach, it possesses benefits such as low cost, not requiring considerable workforce or industrial material, carbon-free, and its ability to return to nature when no longer in use. As an essential part of Living Architectures, scientists and engineers have recognized the importance of fusion between trees. For instance, in projects such as the Baubotanik Tower and the Living Tree Pavilion, a design premise is that the structure becomes ready when the fusion between trees could provide sufficient strength to support the structure. However, little research has been conducted in terms of the fusion processes and the mechanical behaviours of tree connections, which is an essential step prior to designing Living Architecture. With crosswise tree connection as the main focus of this thesis, the following research question is formulated: What are the mechanical behaviours of a self-growing crosswise tree connection when utilized as load-bearing elements in a building structure, and how can such connections be modelled during the preliminary design phase of a building structure? To help answering the research question, literature studies are conducted. In chapter 1, from a botanical perspective, it is concluded that, in the event of two stems contacting with one another, the two stems would gradually fuse together. Such an event is triggered by the abrasion on the bark due to the rubbing between the stems; later, due to the secondary growth, fibers from both stems deviate and join together to form common growth rings, such event can also be verified by the micro-CT scans that have been conducted on the crosswise tree connections. Additionally, due to the nature of the connection, a certain eccentricity from the stems' piths exists, and it is important to acknowledge that the length of such eccentricity stays constant from the start of the fusion process. To investigate the influence of tree growth on the mechanical properties of trees and tree connections, which is needed for design purposes, chapter 2 investigates the growth model and the tapering geometry of living trees. With the help of the Urban tree growth model published by the United States Department of Agriculture and a series of simple tapering equations, the growth parameters of a growing tree can be computed. Part-II of the thesis explores deeper in terms of the mechanical behaviours of such crosswise connections. A simple analysis of two trees connected in a crosswise manner is first conducted in chapter 4. It is concluded that, under loading, the connection can be subjected to tensile stress perpendicular to the grain and rolling shear stress. As two of the weakest strength properties of wood, the crosswise connections should be treated with care by future designers. As a part of an entire building structure, it is essential to determine the rotational stiffness and strength of a structural connection; therefore, in chapter 5, an experimental design is proposed for such purpose. Due to the irregular geometry of the crosswise connection, the experiment makes use of digital image correlation so that the rotational stiffness of the connection under loading can be obtained. Since conducting the actual experiment falls out of this thesis scope, a finite element modelling analysis with similar boundary conditions is conducted in chapter 6. With the results obtained from FEM, it is found that, due to the nature of the connection, complex torsional behaviours occur to the connection. Additionally, with the connection under loading, it is found that non-uniform stress distribution and stress concentration occurs along the interface between two stems. Comparing with the strength properties of wood, it is discovered that the first incident that causes failure is tension perpendicular to the grain, which is the weakest strength property of wood. To conduct preliminary structural design and verification, wireframe modelling approach is often utilized. In chapter 7, it is concluded that it is most suitable to model the crosswise connection with a separate beam element that connects the two stems, for its ability to capture the complex torsional behaviours described in chapter 6.
Lastly, after investigating the local behaviours of crosswise connection, Part-III
investigates the feasibility of conducting a preliminary structural design and verification with such connections. With the case study analysis conducted in chapter 8, it is concluded that, by appropriately capturing the load effects on growing trees and tree connections, designers are able to predict when the structure would reach sufficient strength to be in service. This thesis positions itself as a part of a broader spectrum that examines the feasibility of utilizing tree connections as the load-bearing elements in structures, which can be seen as a step towards sustainable construction.

Timber as a renewable source has been extensively applied among European countries in the construction field. Among more than 1000 available wood species which could be potentially applied as engineered wood, tropical hardwood takes up a significant portion and has advantages of high mechanical resistance and remarkable biological durability against micro-organisms compared to coniferous species and European grown hardwood. Okan has been part of this development, as a hardwood species mostly from the West African regions.The objective of this thesis is to investigate the strength influencing features, calculate corresponding characteristic values and establish the correlation between the influencing factors and the bending strength with the least deviation and create a representative Okan strength predicting model. Abstracted from literature and dataset, geographically speaking, climate, precipitation, soil quality, water sources are expected to have varying degrees of impacts on mechanical properties. From the material perspective, knot ratio, fiber orientation (slope of grain & ring angle), moisture content, density, Modulus of elasticity (dynamic, static) are important factors. Several experiments about hardwood bending strength have been conducted. However, none of those focus on the growth ring angle and the 3D effect of the fiber orientation. To dive deep into Okan, a series of laboratory tests are conducted, including non-destructive inspection, a four-point bending test on 20 beams (10 stored in the dry condition, 10 stored in the wet condition) with the assistance of digital image correlation technology [DIC]). Except for the regulated experimental procedure, two innovative visual inspection methods based on image processing via Matlab were conducted and verified, which turns out the automation method has preferable efficiency and precision. However, this method could hardly identify compression failure. To avoid the compression failure, beams with the dynamic modulus of elasticity less than 18500 N/mm^2 are supposed to be checked again by the inspector. All possible influencing factors are well determined by the above-mentioned testing.Influencing factors of the fracture section form could be concluded from the observation of damaged samples. The slope of grain and growth ring work together to determine the form of the governing crack section. From the experimental outcome, it is clear to observe the linear correlation between mechanical properties and moisture content grouped by the slope of grain. Constant k= 0.13 for bending strength is found, k = 0.05 is found for dynamic modulus of elasticity. The experimental results yield the following adjusted characteristic values: bending strength 63.56 N/mm^2, dynamic modulus of elasticity 21134.55 N/mm^2, density 866.58 kg/mm^3. Based on 20 beams, this batch of Okan could be graded to D55 which is higher than D30-D35 yielded from the dataset. Besides, Okan beams from Gabon could also be graded into D55. Current European standards advise that the reasonable slope of grain range for tropical hardwood is 0 to 0.1. Through the calculation of the theoretical slope of grain of the dataset and experimental samples, increasing the threshold of the slope of grain to 0.3 should be considered in the testing program. Further promotion to 0.2 doesn’t improve the grading outcome and even worsen it.The linear regression result of modulus of elasticity and bending strength is more preferable than density and bending strength in literature and experimental results, which proves the modulus of elasticity is a good indicator of bending strength. Large scatter happens in the regression of the Hankinson formula. In the combination of two basic models, the new proposed model has an optimized coefficient of determination (R^2 = 0.685). The distribution function of the bending strength of the dataset has preferrable overlapping with the theoretical bending strength calculated from the new strength model. To keep the model on the safe side, a safety factor y = 0.9 is applied.It is still unclear if the growth ring angle has a clear numerical relationship with mechanical properties. However, the growth ring angle does bring apparent influences on the form of the fracture section. The 3D effect of fiber orientation needs further investigation.

In this thesis the geotechnical bearing capacity of old timber piles in Amsterdam is studied. This is necessary because many structures such as quay walls, bridges and houses in the (historic) city centre of Amsterdam are founded on timber piles with ages ranging between 80 and 300 years old. The assessment of the foundation is currently done using many assumptions, these are needed because the piles are not easily accessible and old construction drawings are not always available or accurate. Furthermore, the effect that bacterial degradation of the wood has on the interface friction between soil and wood has never been studied in detail. In the assessment it is currently assumed that degradation has no effect on the geotechnical bearing capacity (Kalt and Dusseldorp (2018)), while in the old Dapperbuurt tests it was found that old piles had a reduced shaft capacity of 40% (Korff (2013)). The study is split into two parts; firstly a laboratory study into the effect of bacterial degradation on the interface friction between sand and wood has been performed, and secondly a sensitivity analysis into the geotechnical bearing capacity of timber piles in Amsterdam soil has been carried out.