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Notched solid timber and glued laminated timber (glulam) beams are very common in structural engineering practice. Notches represent a weak spot in structure, and it is advisable to avoid them. However, there are various reasons for beam notching. The most common one being limitation in construction height at the supports, and others such as: stabilization of structural elements against lateral buckling, intersection of members and joint details etc. [1]. Adequate design of notched beams is necessary in these cases. 

The load carrying capacity of timber beams is considerably reduced as a result of stress concentration around the notch. Notches made on the tension side induce tensile stresses perpendicular to grain which, accompanied by shear stresses, can cause longitudinal splitting typically starting at the notch corner (Figure 1). Cracks are unattractive appearance from aesthetic point of view, but are also very dangerous from structural perspective because crack propagation as the load level increases can lead to a failure of a beam. Reinforcement of such members is a cost-effective alternative for enhancing the load carrying capacity of structures in service.

The stress state at a crack can be described by different fracture modes. Mode 1 is a tensile opening mode characterized by separating of crack surfaces in the direction that is perpendicular to them - Figure 2(a). Mode 2 represents in plane shear mode where crack surfaces slide one over the other - Figure 2(b). The combination of the previous two modes is a mixed mode fracture, as shown in Figure 2(c). Although both stresses (shear and tension perpendicular to grain) appear, crack opening is an apparent failure mechanism of a notch and it is caused by tension perpendicular to grain. Therefore, Mode 1 fracture is the most common failure mode of end-notched timber beams [2]. However, shear component usually exists and it should be also taken into consideration.

Notched ends of beams should be reinforced so as to avoid brittle failure and increase load carrying capacity of the beams. There are various types of strengthening techniques which are mainly based on preventing the expected cracks. Parameters such as ease of installation, invisibility of reinforcement, simplicity of design approach and cost are all important for determining the strengthening method. Different types of elements (rods [3], screws [4,5], plates and sheets [6]) and materials (wood-based materials, steel, advanced composite materials like carbon or glass fibre based polymers [7,8]) have been successfully used as reinforcement of notched timber beams.

In the past decades, many researchers have dealt with notched timber beams and it is obvious that notched beam strengthening and repair represents very important topic in the field of timber structural design. In the PhD thesis Jockwer [1] gave a thorough analysis of different design approaches of both unreinforced and reinforced notched beams. Franke, Franke and Harte [9]dealt with methods for repair of structural performance of timber beams, including the ones with notches. In paper Oudjene et al. [10] presented a numerical approach for modelling both unreinforced and reinforced notched beams. Dietsch [11]talked about the necessity of new design approaches of strengthened timber beams, including strengthening of notches, and implementation of these in a new section of Eurocode 5 [12], emphasizing the importance of adequate analytical design.

This paper presents experimental results of endnotched glulam beams that were reinforced with screws. Screws are economic and time-efficient solution for reinforcement and they can be easily applied [13], which is the reason why they were  hosen in this study. Five unreinforced and ten reinforced end-notched glulam beams were tested in bending to the point of failure, with two different reinforcement schemes considered. The results in terms of load-deformation relationship, failure
mode, ultimate load carrying capacity and stiffness were compared between tested beam series. The conclusions on effectiveness of screws as a reinforcement method were made.

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Marija Todorović, asistent - student doktorskih studija, Građevinski fakultet Univerziteta u Beogradu, Bulevar kralja Aleksandra 73, Beograd
Boško Stevanović, dr - edovni profesor, Građevinski fakultet Univerziteta u Beogradu, Bulevar kralja Aleksandra 73, Beograd
Ivan Glišović, dr - docent, Građevinski fakultet Univerziteta u Beogradu, Bulevar kralja Aleksandra 73, Beograd
Dijana Stevanović, asistent - tudent doktorskih studija, Građevinski fakultet Univerziteta u Beogradu, Bulevar kralja Aleksandra 73, Beograd