| 1. |
- Akter, Shaheda T., et al.
(författare)
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A numerical study of the stiffness and strength of cross-laminated timber wall-to-floor connections under compression perpendicular to the grain
- 2021
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Ingår i: Buildings. - : MDPI AG. - 2075-5309. ; 11:10
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Tidskriftsartikel (refereegranskat)abstract
- The use of cross-laminated timber (CLT) in multi-story buildings is increasing due to the potential of wood to reduce green house gas emissions and the high load-bearing capacity of CLT. Compression perpendicular to the grain (CPG) in CLT is an important design aspect, especially in multi-storied platform-type CLT buildings, where CPG stress develops in CLT floors due to loads from the roof or from upper floors. Here, CPG of CLT wall-to-floor connections are studied by means of finite element modeling with elasto-plastic material behavior based on a previously validated Quadratic multi-surface (QMS) failure criterion. Model predictions were first compared with experiments on CLT connections, before the model was used in a parameter study, to investigate the influence of wall and floor thicknesses, the annual ring pattern of the boards and the number of layers in the CLT elements. The finite element model agreed well with experimental findings. Connection stiffness was overestimated, while the strength was only slightly underestimated. The parameter study revealed that the wall thickness effect on the stiffness and strength of the connection was strongest for the practically most relevant wall thicknesses between 80 and about 160 mm. It also showed that an increasing floor thickness leads to higher stiffness and strength, due to the load dispersion effect. The increase was found to be stronger for smaller wall thicknesses. The influence of the annual ring orientation, or the pith location, was assessed as well and showed that boards cut closer to the pith yielded lower stiffness and strength. The findings of the parameter study were fitted with regression equations. Finally, a dimensionless ratio of the wall-to-floor thickness was used for deriving regression equations for stiffness and strength, as well as for load and stiffness increase factors, which could be used for the engineering design of CLT connections.
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| 2. |
- Akter, Shaheda T., et al.
(författare)
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Experimental assessment of failure criteria for the interaction of normal stress perpendicular to the grain with rolling shear stress in Norway spruce clear wood
- 2020
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Ingår i: European Journal of Wood and Wood Products. - : Springer. - 0018-3768 .- 1436-736X. ; 78, s. 1105-1123
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Tidskriftsartikel (refereegranskat)abstract
- The anisotropic material behavior of wood, considered as a cylindrically orthotropic material with annual rings, leads to several different failure mechanisms already under uniaxial stresses. Stress interaction becomes important in the engineering design of structural elements and is often predicted by failure criteria based on uniaxial properties. The prediction quality of failure criteria has been assessed with longitudinal shear stress interaction, though less is known on rolling shear stress in interaction with stress perpendicular to the grain. The study aims at investigating the corresponding mechanical behavior of Norway spruce (Picea abies) clear wood by validating failure envelopes for stress combinations in the cross-sectional plane, based on experimental investigations. For this purpose, a test setup that controls the stress interaction and loading of clear wood along pre-defined displacement paths needed to be developed. Experimentally defined failure states could then be compared to failure surfaces predicted by the phenomenological failure criteria. Material behavior was quantified in terms of stiffness, strength, and elastic and post-elastic responses on dog-bone shaped specimens loaded along 12 different displacement paths. A comparison with failure criteria for two nominal compressive strain levels showed that a combination of failure criteria would be required to represent the material behavior and consider the positive effect of compressive stresses on the rolling shear strength. The findings of this work will contribute to studying local stress distribution of structural elements and construction details, where stress interactions with rolling shear develop.
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| 3. |
- Akter, Shaheda T.
(författare)
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Experimental characterization and numerical modeling of compression perpendicular to the grain in wood and cross-laminated timber
- 2022
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Compression perpendicular to the grain (CPG) of wood is a typical loading situation in timber structures. It has been an extensively studied research topic for decades, due to the highly ductile behavior of wood under such loading, the large variations in mechanical properties, and the relevance of these properties in structural design. Among others, the main influencing factors for CPG properties are stressed volume, load and support configurations, and annual ring orientations to the loading direction. After the innovation of the massive, engineered wood based product, cross-laminated timber (CLT) and its application in high rise buildings, CPG of wood has gained further importance. The development of a non-homogeneous, undesired and combined stress state under CPG in solid wood, due to the material anisotropy in the radial-tangential plane, can build up a complex multi-axial stress state in CLT. As a comparatively new product, the study of the influencing factors for CPG properties of CLT, and an understanding of the local material behavior under such loading, is essential for product characterization and for the development of design guidelines to ensure safe and efficient design.The main aim of the doctoral thesis is to establish a relationship betweenthe anisotropic behavior of clear wood in the transverse plane and the structural response of CLT under CPG loading. Both experimental and numerical studies were adopted herein, to enhance the understanding of the basic material behavior and the product and structural behavior. On the clear wood scale, the focus was on developing a test setup for uniaxial and biaxial loading in the radial-tangential(RT) plane. The potential of the developed test setup for the biaxial testing in the transverse plane was exploited for the investigation of the moisture and time dependent behavior of clear wood under radial compression and rolling shear loading. For data acquisition, in addition to the force and displacement data measured by the internal actuators of the testing machine and an external load cell,a contact-free digital image correlation (DIC) system was used in the experimental investigations. A numerical model was developed, which can describe the elasto-plastic behavior of wood under compression in the transverse plane and predict the structural behavior of solid wood and CLT. For that purpose, a novel Quadratic multi-surface (QMS) failure criterion and a simplified Hoffman failure criterion were implemented in a user-subroutine in the finite element software Abaqus®, and their suitability was compared with the Abaqus implemented Hill’s criterion.The validation of the material models was based on the experimental investigations of failure behavior of clear wood under stress perpendicular to the grain with rolling shear interaction. The material models were further utilized to predict the structural response of solid wood and CLT wall-to-floor connections under CPG loading. The predicted response of CLT connections under CPG by using the above-mentioned material models was compared with experiments, which investigated the influences of different connection types, wall and floor thicknesses, positions of walls, and outer deck layer orientations. The models were then applied to investigate the influence of the pith location in the boards, the number of layers and the thickness of walls and the floor on the stiffness and strength of CLT connections. Moreover, the CLT connection’s rotational rigidity as a consequence of compressive force from the upper floor in a multi-story building was studied by means of finite element calculations.The DIC measured strain fields from the experiments on clear wood confirmed the dependence of strain field on the curvature of the annual rings. As regards the material models, Hill’s model resulted in significantly higher force carrying capacity than experiments on clear wood, whereas Hoffman’s and QMS models predicted reasonably well the force-displacement relationships as found in experiments. The Hoffman’s and QMS models predicted stiffness was about 5–10% higher than corresponding experimental results on clear wood, and about 25% higher for CLT connections. The higher difference in the latter case is due to the difference in material properties of clear wood and structural timber, and the contact behavior between the structural members. The results from CLT wall-to-floor connections revealed a strong influence of loading and supporting configurations, wall thickness and pith locations on their stiffness and strength. A compressive loading on the CLT wall showed a positive effect on the rotational stiffness of CLT wall-to-floor connections, which considerably reduces the CLT floor mid-span deflection in comparison to a simply-supported floor.The thesis work contributes to an enhanced understanding of the anisotropic material behavior of wood in the RT-plane and of its effects on structural timber and CLT under CPG loading. The outcomes of the thesis are beneficial to the product design and standardization of CLT and can be applied in further product development and in optimized structural design.
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| 4. |
- Akter, Shaheda T., et al.
(författare)
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Failure envelope for interaction of stresses perpendicular to the grain with rolling shear stress in wood
- 2018
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Ingår i: 6<sup>th</sup> European Conference on Computational Mechanics (ECCM 6), 7<sup>th</sup> European Conference on Computational Fluid Dynamics (ECFD 7), 11 – 15 June 2018, Glasgow, UK.
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Konferensbidrag (refereegranskat)abstract
- The orthotropic material property in combination with ductility in compression, brittleness in shear and tension, very low shear modulus in radial-tangential (RT) plane etc. requires anisotropic stress failure criteria, as well as their evolution with increasing strains. Three- dimensional failure criteria have been proposed for this purpose, but their validation in the RT plane with interaction of rolling shear stresses has attracted less attention. Corresponding stress interactions are however important for modelling of engineered wood-based products under compression perpendicular to the grain when taking into account influence of the annual ring structure.The work aims at defining failure envelopes for stresses perpendicular to the grain with rolling shear stress interaction based on experimental investigations performed on Norway spruce. The experimental set-up was realized in a biaxial testing frame and consisted of stiff steel plates to transfer load from the testing machine to wood specimen. Mechanical grips prevented rotation and uplifting of the specimen in case of pure shear and tensile loading, respectively. In addition to conventional linear variable differential transformers, a digital image correlation system was used to measure strain fields on the surface of wood specimens and steel plates. Measurements of dog-bone shaped specimens were carried out along different stress interaction paths by displacement controlled loading.The experimental dataset was then compared with commonly used phenomenological failure criteria, namely Tsai-Hill, Tsai-Wu [1], Norris [2] and Hoffman, as well as with regression equations from previous works [3].Experiments revealed that the stress-strain relationship under compression, shear, and biaxial loading differs in radial and tangential directions. None of the three-dimensional stress failure criteria provided good prediction of experiments under compression and rolling shear, but experimental data was closer to the regression equation proposed in [3].
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| 5. |
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| 6. |
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| 7. |
- Akter, Shaheda T., et al.
(författare)
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Moisture and short-term time-dependent behavior of Norway spruce clear wood under compression perpendicular to the grain and rolling shear
- 2023
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Ingår i: Wood Material Science & Engineering. - : Taylor & Francis Group. - 1748-0272 .- 1748-0280. ; 18:2, s. 580-593
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Tidskriftsartikel (refereegranskat)abstract
- Material properties of wood under compression perpendicular to the grain and rolling shear are important for the engineering design of timber structures. This regards the short-term stiffness and strength, their dependence on the moisture content of wood, as well as the time-dependent behavior. Norway spruce clear wood properties in the transverse plane of wood were studied inan experimental campaign exploiting an earlier developed biaxial test setup. The moisture dependence of the stiffness and strength and the short-term time-dependent creep deformations under compression in the radial direction and under rolling shear were characterized. Loading and unloading stiffness, as well as the strength, were determined in quasi-static tests at five different moisture contents from 4% to 29%. The elastic and viscous stiffnesses were identified in creep tests at three compressive stress levels of 0.50, 0.75, and 1.00 N/mm2, and at two rolling shear stress levels of 0.33 and 0.50 N/mm2. The test data complements the existing experimental database, especially with novel data regarding the moisture dependence of the rolling shear strength, which showed less moisture dependence than the compressive strength perpendicular to the grain. The results of the creep tests revealed different material properties for the different loading and material directions of wood.
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| 8. |
- Akter, Shaheda T., et al.
(författare)
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Numerical modelling of wood under combined loading of compressionperpendicular to the grain and rolling shear
- 2021
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Ingår i: Engineering structures. - : Elsevier. - 0141-0296 .- 1873-7323. ; 244
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Tidskriftsartikel (refereegranskat)abstract
- Numerical modeling is an efficient tool for experimental validation and for gaining a deeper understanding of complex material phenomena, especially when causal relationships are overlaid by material variability. Wood is such a highly orthotropic and complex material, which in engineering problems however is considered as macro- homogeneous. The aim of this study is to numerically investigate stress and strain states of wood in the radial- tangential plane and the influence of the orthotropic material behavior on the structural response. Model vali-dation is based on experiments performed on clear wood of Norway spruce (Picea abies) by using a biaxial test setup. Three material models were used, namely Hill’s plasticity model, the Hoffman criterion and a novel quadratic multi-surface (QMS) criterion. After validation on the local material scale, the models were applied to the engineering problem of compression perpendicular to the grain for studying the effect of the unloaded length. As a novel part, the influence of the annual ring structure on the local material behavior and the global elasto- plastic force–displacement behavior of wood under compression perpendicular to the grain were numerically investigated. Hill’s failure criterion was found to be the least suitable at both length scales, local material behavior and global structural response. The Hoffman and the QMS criteria showed quite good agreement with the biaxial experiments in terms of force–displacement relations and strain distributions for different loading situations, especially for combinations with radial compression, while there was less agreement with experiments for the behavior of combinations with tangential compression. Application of these material models to compression perpendicular to the grain for studying the unloaded length effect yielded similar trends as observed in structural tests. A reasonable and similar force–displacement response by Hoffman and QMS criteria was observed, while Hill’s model yielded significantly overestimated force carrying capacity. Differences in force-–displacement response for different loading situations were well in line with literature findings and the infl-ence of the annual ring curvature on the overall force–displacement behavior could be quantified.
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| 9. |
- Akter, Shaheda T., et al.
(författare)
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Stiffness of cross-laminated timber (CLT) wall-to-floor-to-wall connections in platform-type structures
- 2021
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Ingår i: World Conference on Timber Engineering 2021, WCTE 2021. - Santiago, Chile : World Conference on Timber Engineering, WCTE.
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Konferensbidrag (refereegranskat)abstract
- Wall-to-floor-to-wall connections are important for the performance of multi-storey, platform-type cross-laminated timber (CLT) structures. Their stiffness properties are studied by means of a numerical model, which was previously validated with experimental data from material testing and CLT connections loaded perpendicular to the grain. In this work, the stiffness of CLT wall-to-floor-to-wall connections is derived and its dependence on the compressive loading in the CLT walls and on wall and floor thicknesses were investigated. The compatibility of the local model with the connection size in structural design models, was investigated by studying the effect of the floor length and the wall height in the numerical model. The results showed that both rotational elastic stiffness and moment capacity of the floor connection increase with increasing compressive force on the CLT wall. However, a moderate decrease in stiffness, but a stronger rotation hardening was found for higher wall pressures, while lower wall pressures yielded an ideal plastic behaviour. The wall thickness showed a higher influence on the connection stiffness and moment capacity than the floor thickness. The influence of the support condition on the deflection of a CLT floor was exemplified. This study includes novel stiffness data for the design of CLT floors in platform type constructions.
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| 10. |
- Muszyński, Lech, et al.
(författare)
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On the need for reliable rolling shear characteristics in CLT lamellas and for efficient related test methods
- 2019
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Ingår i: Bool of abstract: CompWood 2019, International Conference on Computational Methods in Wood Mechanics – from Material Properties to Timber Structures. June 17-19, 2019, Växjö, Sweden.. - : Lnu Press. - 9789188898647
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Konferensbidrag (refereegranskat)abstract
- Effective modeling of structural behavior of cross-laminated timber (CLT) elements requires reliable input on the mechanical properties of its laminations. The cross-lamination of layers provides for dimensional stability of CLT elements. In this arrangement, however, all laminations in shear walls and the layers of floor elements oriented perpendicular to the major strength axis transfer shear stress in the radial–tangential plane, often referred to as rolling shear. It is among the least documented characteristics of wood, since it had been of marginal interest for structural lumber and engineered wood composites until the emergence of CLT. While the numerical models may easily account for the contribution of rolling shear in the immediate and long-term deformations of laminated panels, simulations are charged with wide margins of uncertainty because of shortage of reliable experimental data. Rolling shear is not the easiest property to measure, and it received only limited coverage in the literature [1-7]. What has been documented was that the rolling shear strength and stiffness in the cross-layers in CLT floor panels is related to the species, density, growth ring orientation, and manufacturing parameters, but there is no evidence for a meaningful correlation with the grade of lumber, whether established by visual or machine grading. In the presentation, we will discuss the pressing need for reliable data on rolling shear characteristics in clear wood and in structural lumber, their statistical distributions in species important for CLT industry, as well as for efficient test methods to allow generation of relevant data in timely manner. Prototype methods and preliminary data will be presented.
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