International Journal of Advanced Structural Engineering
This paper investigates the mechanical performance of longitudinally cracked glulam columns under eccentric compression loads. Experimental investigation was conducted to explore the influence of initial cracks on the failure modes and load bearing capacity of glulam columns. Two different crack patterns named DC and IC, and two column lengths (i.e. 600 and 1100 mm) were considered in the experiments. It was indicated that these two crack patterns reduced the capacity of slender glulam columns and the difference of failure modes was observed between glulam columns with and without initial cracks. Further, a numerical model was developed and validated by the test results. With the application of cohesive zone material model, the propagation of initial cracks could be considered in the numerical modeling. A parametric study was carried out by the verified model and the influence of crack lengths and crack locations was further investigated. From the numerical analysis, it was found that through cracks reduced the capacity of glulam columns significantly. Also, crack location impacts the capacity of glulam columns and the extent of impact relates to the slenderness ratio of the columns, while cracks with different lengths have similar influence on the capacity of columns.
This paper deals with assessment of glulam twinned columns to beam circular bolted connection. This kind of connection is used for embedded assembly. Because of the moisture content variations, cracks often occur in the direction parallel to the grain. The aim of the study is to understand the mechanism responsible of the cracks happening. In the same time, another aim of this study is to evaluate the residual resistance of a damaged assembly. The assembly has been designed according to Eurocode 5. Two different initial conditions have been tested. For the first assembly, the columns and the beam have been dried before machining and tested dry. For the second assembly, the beam was wet and the columns were dry before machining, then the assembly was tested dry. The difference of moisture content implies a huge tensile strain in the direction perpendicular to the grain of the columns before loading. In order to qualify the assembly behavior, strain gauges techniques have been used. This analysis allows a better understanding of the phenomenon of cracks initiation and propagation due to the coupled effect of shrinkage/swelling and loading.
Nowadays, the impact of knots on the failure behaviour of glued laminated timber (GLT) beams is considered by subjecting the single lamellas to a strength grading process, where, i.a., tracheid effect-based laser scanning is used to obtain information about knot properties. This approach single-handedly defines the beam’s final strength properties according to current standards. At the same time, advanced production processes of such beams would allow an easy tracking of a scanned board’s location, but, at this point, previously obtained detailed information is already disregarded. Therefore, the scanning data is used to virtually reconstruct knot geometries and group them into sections within GLT beams. For this study, a sample of 50 GLT beams of five different configuration types was produced and tested under static four-point-bending until failure. As for each assembled lamella the orientation and position within the corresponding GLT beam is known, several parameters derived from the reconstructed knots can be correlated to effective GLT properties. Furthermore, the crack patterns of the tested beams are manually recorded and used to obtain measures of cracks. A detailed analysis of the generated data and their statistical evaluation show that, in the future, dedicated mechanical models for such timber elements must be developed to realistically predict their strength properties. A potential approach, using fluctuating section-wise effective material properties, is proposed.
Moisture may significantly influence the dimensions and behavior of wooden elements and, thus, it is important to consider within both serviceability as well as ultimate limit state designs. Dimensional changes, also called swelling (during wetting) and shrinkage (during drying), are non-uniform due to the direction-dependent expansion coefficients of wood and usually lead to eigenstresses. If these exceed certain strength values, cracking may occur, which reduces the resistance to external loads, especially to shear stresses. The current standard Eurocode 5 takes these circumstances very simplified into account, by so-called service classes, defined based on the surrounding climate and average moisture levels over the course of a year. Accordingly, reduction factors for strength values and cross section widths are assigned.
For a better understanding of the climate-induced changes in wooden beams, we exposed 18 different beams with varying cross sections to a representative climate of Linz, Austria, within the framework of a finite element simulation and investigated the resulting moisture fields and crack patterns. For this purpose, expansions and linear-elastic stresses were simulated by using the thermal and moisture fields obtained in the first simulation step and expansion coefficients. Using a multisurface failure criterion, two critical points in time were determined for each cross section, at which advanced crack simulations were carried out using the extended finite element method. The resulting crack lengths showed that the Eurocode 5 assumption of a linear relationship between crack-free and total width could be verified for both drying and wetting cases.
In future, the obtained crack patterns might also be used to investigate the actual reduction of load-bearing capacities of such cross sections, since the position of a crack and, for example, the maximum shear stress may not coincide. For the first time in this work, a consistent concept is presented to estimate the resulting crack formation in a wooden element from any moisture load based on a mechanical well-founded simulation concept. For this reason, this work is intended to lay a basis for a more accurate consideration of climate-related loads on wooden elements up to timber constructions.
Cracks in timber members influence the stiffness and load-carrying behaviour but only rudimentary rules are given to evaluate cracked members. Therefore, an investigation to gather information about the most frequent characteristics of cracked timber structures has been carried out. This investigation provides the main characteristics of both the timber elements and the crack distributions encountered. These main characteristics have then been used to define a numerical model in order to investigate the impact of cracks on the stiffness and load-carrying capacity of timber beams. Based on these results, the existing rules considering cracks in timber beams can be evaluated and new rules can be developed.
The increasing number of wood structure amongst large and potentially public buildings gave a new impulse to the assessment of timber structures. For assessing the state of timber elements, cracks are a key indicator. Therefore, experimental and numerical investigations on not cracked and partly cracked timber members were carried out and analysed. The results show no influence on the stiffness and modulus of elasticity for partly cracked beams. The experimental results were used for the development of analytical and validation the numerical solutions for the assessment of the residual load carrying capacity of cracked timber members. Several models predicting the residual load carrying capacity depending on the crack situation are presented.
Cross-laminated timber (or CLT) must be recognized as a “precracked” wood composite material where the non-bonded edges within each layer act as cracks in the structure. Furthermore, differential shrinkage between the layers of installed CLT panels subjected to variations in moisture and temperature will result in additional cracks forming parallel to the initial precracks. Fortunately, there is a large literature on the effect of such cracks in cross-laminated composites used in aerospace composites. This paper applies prior literature (when available), and extends it (when needed) to derive all mechanical, thermal expansion, and moisture expansion properties of CLT as s function of the number of cracks in each layer. These results can be used to better design CLT structures. Furthermore, because CLT will form additional cracks when in service, these equations should be a key component of any durability analysis.
The evaluation of damages in large-span timber structures indicates that the predominantly observed damage pattern is pronounced cracking in the lamellas of glued-laminated timber elements. A significant proportion of these cracks is attributed to the seasonal and use-related variations of the internal climate within large buildings and the associated inhomogeneous shrinkage and swelling processes in the timber elements. To evaluate the significance of these phenomena, long-term measurements of climatic conditions and timber moisture content were taken within large-span timber structures in buildings of typical construction type and use. These measurements were then used to draw conclusions on the magnitude and time necessary for adjustment of the moisture distribution to changing climatic conditions. A comparison of the results for different types of building use confirms the expected large range of possible climatic conditions in buildings with timber structures. Ranges of equilibrium moisture content representative of the type and use of building were obtained. These ranges can be used in design to condition the timber to the right value of moisture content, in this way reducing the crack formation due to moisture variations. The results of this research also support the development of suitable monitoring systems which could be applied in form of early warning systems on the basis of climate measurements. Based on the results obtained, proposals for the practical implementation of the results are given.
Under varying climate conditions, cracks are commonly observed in bolted joints, owing to the shrinkage of wood and confinement from slotted-in steel plates and bolts. A three-dimensional finite element model was developed to investigate the mechanical behavior of bolted glulam joints with initial cracks. Wood foundation was prescribed in the model to simulate the local crushing behavior of wood surrounding the bolts. The behavior of wood in compression and the foundation were defined as transversely isotropic plastic in the software package ANSYS. Cohesive zone model was applied in the numerical analysis to consider the propagation of initial cracks and brittle failure of wood in the bolted joints under tension load. The numerical model was validated by the experiments conducted on full-scale specimens and it is indicated that the numerical model has good ability in predicting the failure modes and capacity of tension joints with local cracks. To further investigate the influence of crack number, length and locations, a parametric study was conducted with the verified model. Moreover, to study the effects of cracks on the behavior of bolted joints with different failure modes, another bolted joint including bolts with different strength grades and diameters was designed and analyzed in the parametric study, which was expected to have bolt yielding failure mode. It was found that the initial cracks can decrease the capacity and initial stiffness of tension joints by up to 16.5 and 34.8%, respectively.
A reduction of the shear resistance was introduced with the crack factor kcr in Eurocode 5. The factor 0.67 corresponds to cracks that have a depth of 1/3 of the beam width. The aim of this project was to learn more about different types of cracks and their importance for the shear strength of glulam beams. The project started with tests of five types of glulam beams, with or without cracks. The cracks had different depths and locations, three beam types had cracks made by sawing and one type had cracks from moisturing and drying. The beam dimensions were 115 mm x 315 mm x 2600 mm. Five beams of each type with cracks were tested and ten beams without cracks. The beams were Swedish standard beams made of Spruce and taken from the normal production. Three-point bending method was used for the shear tests. The beams of type 1 without cracks got mostly bending failures; the characteristic shear strength was at least 3.5 MPa. Beams with sawn grooves got lower characteristic shear values and this means a reduced cross section should be used for beams with cut grooves along the beams. Beams with drying cracks got more shear failures, but the characteristic shear strength of the beams was about the same as for beams without cracks.