This thesis investigates the global stiffness of cross-laminated timber (CLT) buildings. A case study is conducted on the 10-storey Urban Woods building in Delft, in which the lateral load-resisting system consists of façade bracing supported by a central CLT core. The study evaluates the accuracy of early-stage analytical and numerical models in predicting global stiffness and examines the influence of uncertainty in the slip modulus of dowel-type connections. The objective is to improve the understanding of the applicability of early-stage modelling approaches for estimating the global stiffness of timber structures, and to quantify the impact of uncertainties in connection stiffness on the global structural response.

Assessing the global stiffness of timber buildings is critical, as their lower stiffness and self-weight compared to conventional concrete, masonry, and steel structures make them more susceptible to global deformations. The global stiffness is governed by both the configuration of the structural system and the stiffness of its connections, particularly within the lateral load-resisting system.
During the preliminary design phase, engineers must estimate global stiffness and the associated horizontal deformations of a proposed structure. However, due to the relatively limited number of timber high-rise buildings and the associated lack of practical experience, these estimations remain uncertain and need to be supported by complex numerical models. In addition, recent studies have reported discrepancies between experimentally obtained slip moduli and analytical predictions according to Eurocode 5 for dowel-type connections. As these fasteners are widely used in timber lateral load-resisting systems, this introduces additional uncertainty in predicting global structural deformations.

The translational stiffness of the connections was determined using the analytical serviceability approach prescribed in Eurocode 5. These stiffness values were implemented in both analytical models based on the method of Virtual Work and numerical models developed in RFEM. The models were subjected to self-weight and wind loads according to serviceability limit state combinations. To assess the influence of connection stiffness, the translational stiffness values were varied, and the resulting maximum horizontal deflection was used as the primary response parameter.

The results show that analytical models based on the method of Virtual Work systematically overestimate the global stiffness of timber structures by approximately 10–40% compared to numerical models developed in RFEM, primarily due to simplifying the structure. A key difference between the modelling approaches lies in their sensitivity to changes in connection stiffness. For moderate reductions in connection stiffness (up to approximately 50–60%), the overestimation remains relatively constant. However, beyond this threshold, a significant divergence emerges: the numerical models exhibit a non-linear decrease in global stiffness, whereas the analytical models predict a linear reduction. For the global structural model, a reduction of 60% in connection stiffness results in an average decrease in global stiffness of approximately 20%. These findings are supported by literature, which reports that analytical estimates of connection stiffness based on Eurocode 5 can overestimate stiffness by up to approximately 60%, indicating that such reductions are feasible in practice. Therefore, it is advised to increase the estimation of the horizontal deflection by up to 20%, if the lateral load-resisting system is dependent on the stiffness dowel-type connections.

The results further indicate that the accuracy of analytical and numerical models depend on the structural complexity, with better agreement observed for simpler configurations. In addition, the importance of form stability in the lateral load-resisting system is highlighted: structures with a higher degree of form stability are less sensitive to the stiffness of connections. For systems made out of simple and form stable configurations an analytical model can provide reliable estimations of the global stiffness. However, numerical models tend to perform better and are therefore preferred for the analysis of more complex and non-form stable structures.
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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. ... Read more

Summary

Numerous ancient historical constructions worldwide depend primarily on an extensive array of wooden foundation piles, as they are subject to loading conditions governed by the superstructure above. Wooden foundations transfer loads through a combination of compression and lateral resistance. The inherent strength of wood handles compressive forces, while stiffness and soil friction counteract lateral loads. Proper arrangement and maintenance ensure even load distribution. Careful design, wood quality, depth, and protective treatments are essential for longevity and load-bearing efficiency.

Amsterdam, the Netherlands' capital city, renowned for its rich artistic heritage, intricate canal infrastructure, and slender architectural dwellings, originated as a modest fishing hamlet that underwent remarkable development into a prominent global European city. During this urban transformation, less visible engineering elements, such as wooden foundation piles, were overlooked, despite their critical significance. In Amsterdam's historical core, the majority of structures including buildings, bridges, and quay walls, rely on these wooden supports. Noteworthy, the city estimates that 12 million such piles are still active. These structural components have consistently demonstrated economic efficiency and reliability. Nonetheless, the aging process affecting these foundations, with some dating back up to 500 years, introduces complexities when assessing their current load-bearing capacities and the ensuing reliability of the structures they support.

The lack of knowledge and inspection techniques of the mechanical and physical properties of these timber piles hinders a proper evaluation of the remaining life span of the foundations which could lead to possible irreplaceable structural damage to these structures. This body of research evaluates the physical and mechanical properties such as the actual moisture content, density distribution, compressive strength, and modulus of elasticity through the cross section of Spruce (Picea abies) foundation piles. Therefore, the overarching research question has arisen:

“How do the variations of mechanical and physical attributes manifest across the cross-sectional profile of both degraded and non-degraded spruce foundation piles and how can micro-drilling techniques be utilized to assess these characteristics?“

This will be achieved by means of small-scale compressive experimental testing of five prisms extracted from each cross-section (3 separate locations along the length of the pile) of foundation piles never driven into the soil and piles that were retrieved under bridges in the historical centre of Amsterdam that were planned to be demolished. These aforementioned retrieved piles had a service life between 100 years and 300 years, always under the water table, presenting mechanical degradation due to loading over time and in addition possible bacterial degradation of the cross-section peripheral regions.

Initially, micro-drilling techniques were employed to ascertain the drilling amplitude. This step served to assess the initial quality of the wood under examination. Additionally, it aided in identifying specific points of interest for specimen extraction, including degraded wood in the peripheral regions, sound wood in the internal section, and the pith. Subsequently, the acquired data underwent thorough analysis. This analysis, combined with the micro-drilling measurements, enabled an assessment of the potential applicability of drilling amplitude in predicting the mechanical and physical properties of the pile. This sequential approach ensured a systematic and scientifically rigorous evaluation of the wood's characteristics and its implications for pile performance. The investigation was conducted to enhance the understanding of the structural performance and material characteristics of spruce foundation piles, while also evaluating the applicability of micro-drilling methods as a predictive tool in engineering assessments...


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