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.
Timber-concrete composite structures were originally developed for upgrading existing timber oors, but during last decades, they have new applications in multistorey buildings. Most of the research performed on these structures has focused on systems in which wet concrete is cast on top of timber beams with mounted connectors. Recently investigations on composite systems were performed at Luleå University of Technology in Sweden, in which the concrete slab is prefabricated off-site with the connectors already embedded and then connected on-site to the timber joists. Similar studies have been carried out also on timber-concrete composite structures with prefabricated FRC slabs at Lund University in Sweden. Two kinds of shear connectors were incorporated in the prefabricated FRC concrete slabs. These last systems can be considered globally as partially prefabricated structures because only the slabs were cast off-site with already inserted shear connectors and then the connection with the timber beams is done on the building site. An innovative composite system for floor applications is presented in this thesis. The entire structure is prefabricated off-side, transported and direct mounted to the building on site, that can be seen as full prefabricated structures. Noticeable benefits of a full prefabricated structure are that the moving work from the building site to the workshop reduces construction costs, is more simple and fast of manufacture and erect, and of sure, has better quality, that means more durability. Self-tapping full-threaded screws to connect concrete slabs to timber beam were used. Dimensions of the composite beams and the spacing between the screws has been chosen by discussing different FE model in order to reach the optimal solution. The experimental campaign included:
(i) two short-time bending tests carried out on two dierent full-scale specimens,
(ii) dynamic tests conducted on one full-scale specimen,
(iii) long-time bending test carried out on one full-scale specimen,
(iv) compression tests on three cubes of concrete,
(v) nine withdrawal tests of the screws with different depth in the concrete.
The results of the experimental tests show that the composite beams have a very high level of resistance and stiffness and also allow to reach a high degree of efficiency. Last, comparisons between FE results, analytical calculations and experimental values have been performed and from them it can be concluded that FE model and theoretical calculations well interpret the behavior of the composite structure and provide reliable results.
A timber-concrete composite structure (TCC) is economically and environmentally friendly. One of the key design points of this kind of structure is to ensure the reliability of the shear connectors. The objective of this paper is to study the mechanical property of stud-groove-type connectors and to provide shear capacity equations for stud-groove connectors in timber-concrete composite structures. Based on the Johansen Yield Theory (European Yield Model), some mechanical models and capacity equations for stud-groove-type connectors in timber-concrete structures were studied. Push-out specimens with different parameters (stud diameter, stud length, groove width, and groove depth) were tested to obtain the shear capacity and slip modulus. The experimental strengths were used to validate equations given in the paper. The shear capacity and slip modulus of stud-groove-type connectors was in direct proportion to the diameter of studs and the dimension of the groove. Comparison between the theoretical and the experimental shear strength results showed reasonable agreement. The highlight of this study on shear capacity equations could significantly reduce the push-out tests before investigating the other properties of TCC.
This report concerns element strength and stiffness of cross laminated timber (CLT) at in-plane beam loading and includes presentation of experimental investigations and a review of some analytical models for structural analysis. A total of 20 individual tests were carried out, divided into five different test series which each comprise four nominally equal tests. The test series include prismatic beams (two test series), beams with a hole (two test series) and beams with an end-notch (one test series). All CLT elements were composed of 5 layers of laminations, with three layers of longitudinal laminations of width 40 mm and two layers of transversal laminations of width 20 mm.
Test results relating to beam strength are presented in terms of maximum applied load and also in terms of stress components as calculated by analytical models. The review of models for stress analysis reveals significant influence of the element layup and laminations width on the predicted stresses. Test results relating to beam stiffness are present as element shear stiffness and element local and global modulus of elasticity, evaluated based on the European test standard EN 408.
This thesis deals with experimental tests and methods for strength analysis of glulam beams with holes. Test results and methods for strength analysis available in literature are compiled and discussed. The methods considered comprise both code strength design methods and more general methods for strength analysis.
New strength tests of beams with quadratic holes with rounded corners are presented. The test programme included investigations of four important design para\-meters: material strength class, bending moment to shear force ratio, beam size and hole placement with respect to beam height. One important finding from these tests is the strong beam size influence on the strength. This finding is in line with previous test results found in literature but the beam size effect is however not accounted for in all European timber engineering codes.
A probabilistic fracture mechanics method for strength analysis is presented. The method is based on a combination of Weibull weakest link theory and the mean stress method which is a generalization of linear elastic fracture mechanics. Combining these two methods means that the fracture energy and the stochastic nature of the material properties are taken into account. The probabilistic fracture mechanics method is consistent with Weibull weakest link theory in the sense that the same strength predictions are given by these two methods for an ideally brittle material. The probabilistic fracture mechanics method is also consistent with the mean stress method in the sense that the same strength predictions are given by these two methods for a material with deterministic material properties.
A parameter study of the influence of bending moment to shear force ratio, beam size, hole placement with respect to beam height and relative hole size with respect to beam height is presented for the probabilistic fracture mechanics method.
Strength predictions according to the probabilistic fracture mechanics method is also compared to the present and previous test results found in literature and also to other methods for strength analysis including code design methods. The probabilistic fracture mechanics method shows a good ability to predict strength, with the exception of very small beams.
The aim of this paper is to investigate the performance of CLT beams with and without holes loaded in-plane under various loading conditions with special emphasis on shear loading and the in-plane shear behaviour considering the complex internal structure. CLT beams present a much better solution for beams with holes or notches as compared to glued laminated timber beams thanks to its lay-up where tensile forces perpendicular to the beam axis can be transferred by the transversal layers. In order to have in-depth understanding of the local mechanical behaviour in shear stress transfer between laminations, numerical analyses based on 3D-FE models are used.
Environmental and urbanization challenges during the last few decades encouraged steady growth of mass timber construction where attention is drawn to cross laminated timber (CLT) as one of the most interesting products in terms of mechanical properties, versatility, efficient prefabrication and sustainability. Standardisation and codification regarding testing and design of CLT elements are hence pointed out as one of the main issues within the ongoing revision procedure of Eurocode 5. A consistent and unified design approach for CLT at pure in-plane shear loading conditions (shear walls) and at in-plane beam loading conditions is however still missing. This paper deals with analytical models for the determination of stress components related to predictions of load bearing capacity of CLT with respect to shear failure mode III – shear failure in the crossing areas constituted by the flatwise bonded areas between laminations of adjacent layers. This failure mode is relevant for both pure in-plane shear loading and in-plane beam loading conditions. The paper presents a review of previously proposed models for the prediction of shear stresses in crossing areas of CLT, for both loading conditions. Comparisons between FE-results and model predictions are reviewed indicating significant differences between them concerning the predicted influence of the CLT element lay-up and values of maximum shear stresses. Based on simplifications of models previously presented, a unified design proposal that is based on a rational and consistent mechanical background for both loading situations and that shows overall good agreement with FE-results is presented.