Robert Jockwer

State-of-the-Art Review of Approaches for the Design of Timber Beams with Notches

AbstractNotches are details that significantly reduce the load-carrying capacity of beams made of timber. The amount of reduction was the subject of investigation in numerous studies from an experimental or theoretical point of view. The singular stress distribution around the notch corner can be described by equations based on linear elastic fracture mechanics, fracture energy, or complex solutions. From experimental tests, simplified equations can be proposed. Standards, codes, and handbooks for the design of timber structures are based on such equations. When comparing the dimensions of notched beams tested in experiments with those that are relevant in practical applications, specimens of small dimensions were primarily tested. The application of design approaches verified by tests on specimens of small dimensions and the use of material properties determined from small clear specimens do not account for size effects that are important when working with beams of large dimensions. The influence of dura...

Fully Threaded Self-tapping Screws Subjected to Combined Axial and Lateral Loading with Different Load to Grain Angles

In order to benefit from the advantages of fully threaded self-tapping screws as reinforcing elements, it is essential to have detailed knowledge about the strength and stiffness of screws with different shank to grain angles subjected to combined axial and lateral loading. In this paper a design model is proposed for the calculation of the load-carrying capacity and stiffness of screws with different shank to grain angles subjected to loads perpendicular to the grain by accounting for the effective embedment of the screw in the timber. The proposed model is based on commonly used material properties and other established design models and fits well the influence of the angle between shank and grain direction on the joint’s capacity and stiffness. However, detailed knowledge about the specific input parameters in the design approaches is necessary in order to achieve a reliable prediction of the load-carrying capacity and stiffness of the individual screw.

Impact of growth characteristics on the fracture perpendicular to the grain of timber

The natural material wood is commonly graded with regard to the parallel to the grain strength and stiffness properties and taking into account different growth characteristics such as knots and grain deviations. In this paper the impact of knots and grain deviations on the fracture perpendicular to the grain of timber is analysed by means of numerical models. The results are used for the calibration of an analytical model. With this model it is possible to evaluate the impact of growth characteristics on the perpendicular to the grain fracture and compare the results with test data from literature. The evaluation shows that certain growth characteristics increase the strength perpendicular to the grain. This is in contrast with current grading procedures, where such growth characteristics are considered as being strength decreasing. The results are compared with a model for the description of the effects of growth characteristics on the distribution characteristics of the strength perpendicular to the grain. This strongest link model can be used to describe phenomena with a parallel system of failure events.

Quasi-brittle fracture and size effect of glued laminated timber beams

The load bearing capacity of glued laminated timber beams is size dependent. A material based size effect occurs due to the natural variability of the timber properties and can be explained with the Weibull weakest link theory. The Weibull type size effect has received much attention in the timber research community in the past as it is the dominating size effect regarding the load bearing capacity of timber beams. In this paper, it is shown that a further deterministic size effect, which originates from the timber fracture behavior, influences the load bearing capacity of glued laminated timber beams. It is the size effect associated with quasi-brittle behavior. The paper covers a short theoretical review on fracture related size effects and a study comparing experimentally tested beams of several sizes with detailed numerical simulations considering the natural growth characteristics of timber. Further, an analytical model illustrating the features of the fracture related size effect on the load bearing capacity of glued laminated timber beams is presented. It is demonstrated that the simulated load bearing capacity of the tested glued laminated timber beams distinctively improves if the quasi-brittleness is taken into account by using a softening constitutive law.

Load-carrying capacity and failure modes of glulam beams with reinforced notches

The structural behaviour and failure mechanisms of unreinforced and reinforced notched glulam beams were studied in a test series. It could be shown that shear failure of the reinforced notched beam is an important failure mechanism and that adequate reinforcement in tension perpendicular to grain and shear is required in order to regain the load-carrying capacity of the reduced cross-section of the glulam beam.

Impact of knots on the fracture propagating along grain in timber beams

Fracture propagating along the grain of timber is one of the most relevant failure mechanisms due to the wood’s low strength and its brittle failure behaviour when subjected to excessive shear and tensile stresses perpendicular to the grain. According to the current procedures, the fracture energy of wood is determined on small clear specimens. However, for the prediction of the structural behaviour of full-scale structural timber members, the influence of knots has to be accounted for. The discrepancy between the fracture behaviour of small specimens and members of structural size has already been observed in several studies. The aim of this paper is to provide a review on a selection of these studies and to discuss their relevancy for the evaluation of the fracture properties and the structural behaviour of full-scale structural timber.