| 1. |
- Ekevad, Mats, 1956-, et al.
(författare)
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Mechanics of stress-laminated timber bridges with butt end joints
- 2017
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Konferensbidrag (refereegranskat)abstract
- A number of variants of single span and three-span stress-laminated timber bridge decks have been studied via finite element simulations and experiments. Glulam beams in the decks were in general shorter than the total length of span which means that there were butt end joints in the decks. The butt end of each beam in a joint was not connected to the other beam which means that each butt end joint reduced the strength and stiffness of the whole of the deck. Results for deflection and stresses were examined for the studied variants in the form of reduction factors for strength and stiffness relative to a deck without butt end joints.Factors are shown in diagrams as function of ratio butt end distance/beam width and also butt end distance/span width. Comparison of achieved results with existing Eurocode rules shows that Eurocode rules are not totally appropriate.
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| 2. |
- Huber, Johannes A. J., 1989-, et al.
(författare)
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A method for generating finite element models of wood boards from X-ray computed tomography scans
- 2022
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Ingår i: Computers & structures. - : Elsevier. - 0045-7949 .- 1879-2243. ; 260
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Tidskriftsartikel (refereegranskat)abstract
- A method is presented for reconstructing the geometry, the pith, the knots and the local fibre orientations in timber boards, based on X-ray computed tomography scans. The local fibre deviations around knots were found by a new algorithm, based on image analysis. The experimental data comprised tomography scans, eigenfrequency measurements and four-point bending tests of 20 Norway spruce boards. 3D and 1D finite element models of the pure bending zone of the bending tests were created, accounting for the exact board geometry and the reconstructed fibre deviations. A purely density based, a purely eigenfrequency based, and a mixed constitutive law were compared. Model estimations showed a high coefficient of determination (R2) for global modulus of elasticity (MoE) (R2⩽0.93), local MoE (R2⩽0.87), bending strength (R2⩽0.83), and the location of initial failure. Constitutive laws accounting for eigenfrequency showed the most accurate results. In the future, adapting the method for logs could enable analyses of boards before sawing.
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| 3. |
- Huber, Johannes A. J., 1989-
(författare)
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Numerical Modelling of Timber Building Components to Prevent Disproportionate Collapse
- 2021
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- An increasing number of multi-storey buildings are being constructed with engineered wood products, such as glulam or cross-laminated timber (CLT). Multi-storey timber buildings can be safely designed for foreseeable loads, but knowledge is limited concerning their ability to survive unforeseeable events, e.g. accidents, natural disasters or terrorism. Multi-storey buildings with many occupants are required to be able to resist a disproportionate collapse due to an unexpected event. Collapse resistance consists of three lines of defence: I) decreasing the probability of the event, II) decreasing the structural vulnerability and III) increasing the structural robustness. The focus of the present thesis is on defence lines II and III, since they can be affected by engineering considerations.Robustness requires the availability of alternative load paths (ALPs) after an initial structural damage, e.g. the removal of an element. The activation of an ALP, e.g. catenary action, usually happens as the result of a larger displacement than that for which the components are designed, and with the participation of the surrounding structure. Physical tests of removal scenarios are expensive and they are often unable to represent realistic building situations. Numerical models can replace physical tests, e.g. by introducing parameter variations or changed boundary conditions, and can deliver an insight into the underlying mechanisms. Vulnerability depends on the ability of individual components of the structure to withstand loads greater than their intended design loads. To reduce vulnerability, so-called key elements can be be made overly strong. If the uncertainty concerning the material properties is high, e.g. for timber, both nominally stronger and larger amounts of material are required, resulting in inefficient material utilisation. Automated strength grading of sawn timber can narrow the uncertainty, but, even with the current technologies, the variations in the graded material remain large.The predictive power of computerised models for sawn timber offers a great potential for integration with traditional strength grading based on testing combined with statistical models. So far, surface data of sawn timber has been used for numerical models, but X-ray computed tomography (CT) scanning equipment now being installed in sawmills has made it possible to measure the inner structure of logs. Using CT data could make it possible to develop high-fidelity numerical models for predicting the mechanical properties of sawn timber, possibly even before sawing, and this could reduce the uncertainty for structural components and enable the production of high-strength timber. However, attempts to develop CT-based models for timber have been scarce.The objective of the work presented herein was to advance the research front regarding the prevention of disproportionate collapse in multi-storey timber buildings. The work has focused on numerical modelling aspects and on subsystems and components, rather than on entire buildings. The goals were: 1) to describe the state of the art regarding the prevention of disproportionate collapse and its application in timber buildings, 2) to develop models to identify and quantify the ALPs in subsystems and components of CLT buildings, and 3) to develop models of sawn timber based on X-ray CT scanning data, to reduce the uncertainty regarding the mechanical properties of the timber.For goal 1, the literature was reviewed and a survey was conducted among practitioners and researchers in the field. The results provided an extensive overview of the topic and the status quo in the industry, and identified a scarcity of guidelines for multi-storey CLT buildings.For goal 2, non-linear finite element (FE) models were developed for quasi-static pushdown analyses. A study of a platform joint first validated some modelling assumptions. The ALPs in single storeys in a corner bay of an 8-storey CLT building were then studied after the removal of bottom-storey walls. In subsequent parameter variations, the full bay was studied in dynamic analyses. The results identified six different ALPs, which were dependent on the connection capacities and the shear capacity of the floor panels, and indicated that collapse was likely after a double wall removal, but unlikely after a single wall removal. Furthermore, the ALPs in a platform-type CLT floor system were studied in parameter variations of calibrated FE models. The results showed how three different ALPs can develop, depending on the storey, the floor geometry and the connectors.For goal 3, a method was developed for the generation of continuum and FE models from CT scanning data of sawn timber, in which the knots, pith and local fibre orientations were reconstructed. The models gave realistic impressions and they could predict the bending stiffness, strength and initial failure location for Norway spruce sawn timber. The predictions improved, if the eigenfrequency of the sawn timber was also considered for modelling.
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| 4. |
- Huber, Johannes A. J., 1989-, et al.
(författare)
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Simulation of Alternative Load Paths After a Wall Removal in a Platform-Framed Cross-Laminated Timber Building
- 2019
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Ingår i: CompWood 2019. - : ECCOMAS: European Communityon Computational Methodsin Applied Sciences. ; , s. 33-33
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Konferensbidrag (refereegranskat)abstract
- An increasing number of multi-storey timber buildings use cross-laminated timber (CLT) for their bearing structure. Platform-framed CLT buildings consist of vertical repetitions of floors resting upon one-storey tall walls, squeezing-in the floor panels between the walls. Tall buildings need to be structurally robust because many lives would be at stake in case of a disproportionate collapse. Robustness is the ability of a system to survive the loss of components. For collapse resistance, it poses the last line of defence, after an unforeseen exposure (e.g. accident, terrorism) has already occurred and after the exposed components could not resist failure. A robust building offers alternative load paths (ALPs) which come into action when a part of the bearing structure has been removed [1].Many alternative load path analyses (ALPA) have been conducted for tall concrete and steel buildings using the finite element method (FEM), but for timber, ALPA are still scarce. ALPs depend on the behaviour of the connections after a loss [1]. Studies on timber so far have accounted for connections in a simplified manner by lumping their aggregate behaviour into single points. Our goal is to elicit the ALPs after a wall removal in a platform-framed CLT building, study their development and quantify their capacity, to determine whether they can prevent a collapse.We investigated a corner bay of an 8-storey platform-framed CLT building (see Figure 1) and removed a wall at the bottom storey. We studied the ALPs of each storey by pushing down the walls above the gap in a non-linear quasi-static analysis in the FE software Abaqus. We accounted for contact and friction, considered plastic timber crushing, and accounted for brittle cracking in the panels. We modelled single fasteners with connector elements which simulated the elastic, plastic, damage and rupture behaviour. We recorded the force-displacement curves, i.e. pushdown curves, for each storey and used them to conduct a dynamic analysis of the entire bay in a simplified model, as suggested by [2].The results show that the structure could engage the following ALPs after a wall removal: I) arching action in the outer floor panels, II) arching action of the walls, III) quasi-catenary action in the floor panels, and IV) hanging action from the roof panels. The ALPs were limited by various parameters, but they sufficed to resist a collapse of the bay. We observed that the inter-storey stiffness influenced the load-sharing among storeys, which affected the structural robustness. In the compressed connections, friction, and not the fasteners, transferred most of the horizontal loads. Future research should test the squeezed-in platform joint experimentally, to quantify its capacity for transverse shear loads. We also advise to assess the inter-storey stiffness to estimate the capacity for load-sharing among storeys.
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| 5. |
- Larsson, Maria, et al.
(författare)
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Procedural texturing of solid wood with knots
- 2022
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Ingår i: ACM Transactions on Graphics. - : Association for Computing Machinery. - 0730-0301 .- 1557-7368. ; 41:4
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Tidskriftsartikel (refereegranskat)abstract
- We present a procedural framework for modeling the annual ring pattern of solid wood with knots. Although wood texturing is a well-studied topic, there have been few previous attempts at modeling knots inside the wood texture. Our method takes the skeletal structure of a tree log as input and produces a three-dimensional scalar field representing the time of added growth, which defines the volumetric annual ring pattern. First, separate fields are computed around each strand of the skeleton, i.e., the stem and each knot. The strands are then merged into a single field using smooth minimums. We further suggest techniques for controlling the smooth minimum to adjust the balance of smoothness and reproduce the distortion effects observed around dead knots. Our method is implemented as a shader program running on a GPU with computation times of approximately 0.5 s per image and an input data size of 600 KB. We present rendered images of solid wood from pine and spruce as well as plywood and cross-laminated timber (CLT). Our results were evaluated by wood experts, who confirmed the plausibility of the rendered annual ring patterns.
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| 6. |
- Mpidi Bita, Hercend, et al.
(författare)
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Prevention of Disproportionate Collapse for Multistory Mass Timber Buildings: Review of Current Practices and Recent Research
- 2022
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Ingår i: Journal of Structural Engineering. - : American Society of Civil Engineers (ASCE). - 0733-9445 .- 1943-541X. ; 148:7
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Tidskriftsartikel (refereegranskat)abstract
- The expanding height of multistory mass timber buildings makes the development of guidance to prevent disproportionate collapse an increasingly important area of engineering design; however, there is no guidance available in building codes or design standards on how to prevent disproportionate collapse of multistory mass timber buildings. In this paper, practical project-specific solutions applied in constructed prominent multistory mass timber buildings to prevent disproportionate collapse are presented. Various approaches have been successfully used for different construction types of multistory mass timber buildings to decrease the potential risk of disproportionate collapse, such as using walls above supports as deep beams and using columns to carry tension forces and hold the floor below in case a supporting column or wall is damaged. Then, recent and ongoing research in the field of prevention of disproportionate collapse of multistory timber buildings is reviewed. Such research is mostly based on numerical simulations of element removal scenarios and pushdown experiments on floor systems and connections and provides valuable insights and guidance to designers on important structural aspects regarding the prevention of disproportionate collapse in multistory mass timber buildings. Finally, the draft provisions for an upcoming standard related to the prevention of disproportionate collapse are reviewed, and the conclusions regarding the state of the art are presented.
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| 7. |
- Tannert, Thomas, et al.
(författare)
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Untersuchungen zur Prävention von progressivem Kollaps von Holzhochhäusern
- 2021
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Ingår i: Bautechnik. - : John Wiley & Sons. - 0932-8351 .- 1437-0999. ; 98:S1, s. 3-11
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Tidskriftsartikel (refereegranskat)abstract
- Ohne weiterführende Entwurfsüberlegungen zur strukturellen Robustheit von Gebäuden aus Holz kann das Versagen eines einzigen Bauteils zu einem progressivem und/oder unverhältnismäßigen Gebäudekollaps führen. Die bestehenden Anforderungen in den internationalen Richtlinien zur Prävention von progressivem Kollaps sind unwirtschaftlich für mehrstöckige Gebäude aus Brettsperrholzplatten (BSP) im Plattformbau. Dieser Beitrag fasst aktuelle Forschungsergebnisse zusammen, um die strukturelle Robustheit solcher Gebäude zu gewährleisten. Eine verbesserte Methode wird vorgestellt, um die Mindestanforderungen an Zugankerverbinder zu quantifizieren und um alternative Lastwege unter Verwendung vereinfachter linearelastischer Ansätze sicherzustellen. Es wird gezeigt, wie in einer nicht‐linearen, sogenannten Pushdown‐Analyse eines Plattformhochhaussegmentes zur Charakterisierung der Widerstandsmechanismen vier verschiedene alternative Lastpfade ermittelt wurden. Danach wird anhand linearer dynamischer Analysen das Tragwerksverhalten bei Bauteilversagen eines zwölfstöckigen Hochhauses mit BSP Geschossplatten und Wandsystem und eines neunstöckigen Hochhauses mit BSP Geschossplatten und Brettschichtholzstützen untersucht. Die vorgestellten Ergebnisse tragen dazu bei, den Entwurf mehrstöckiger BSP Gebäude im Plattformbau zu verbessern.
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