Flexural behaviour of post-tensioned glass beams: Experimental and analytical study of three beam typologies

The concept of post-tensioned glass beams builds on the concrete analogy of the reinforced glass beams. By additionally prestressing the reinforcement in a post-tensioned system, a compressive pre-stress is applied on the glass, enhancing the initial fracture resistance in bending. This paper presents three post-tensioned beam systems tested in four-point bending. The effects of both adhesive bonding and mechanical anchoring of tendons are explored in order to define an optimised beam system which can provide a significant level of compressive pre-stress and maximise the efficient use of material. The tests demonstrate the feasibility of post-tensioning by providing a substantial level of additional load capacity and redundancy to commonly applied laminated glass beams. Several specimens have demonstrated premature failure governed by lateral-torsional buckling or by shear. Since glass beams are generally not strengthened through transversal reinforcement, as concrete elements, further investigation of the mechanism of shear failure is particularly important. Analytical models, based on the pre-stressed concrete theory, provide expressions for determination of the initial fracture resistance, initial stiffness and ultimate flexural capacity. Compared with the experimental results, the models provide close prediction values for the initial fracture loads, while the predictions of the initial stiffness and ultimate post-fracture load capacity show less consistency. Closer prediction requires a better understanding of the effect of specific anchoring, the level of composite action and the post-fracture behaviour of laminated glass (including long-term and temperature effects).

Post-tensioned glass beams

Jagoda Cupac

Over the last few decades, the application of glass in building construction has extended from small four-side supported windowpanes to areas traditionally reserved for other materials, such as roofs, floors, staircases and full building envelopes. Glass is nowadays applied as a primary structural element - a beam, column, wall or shell; however, its intrinsic brittleness calls for adequate safety concepts to account for the consequences of glass failure. Although very strong on the inter-atomic level, the flaws which arise on the glass surface during production and service life of glass elements, significantly decrease its effective tensile strength. The concept of post-tensioned glass beams continues on the concrete analogy of the reinforced glass beams, which has been validated over the past years for various loading and environmental conditions. Adding ductile reinforcement in the tensile zone of a glass beam significantly enhances the post-cracking behaviour and redundancy. By additionally prestressing the reinforcement in a post-tensioned system, a compressive pre-stress is applied on the glass beam, enhancing the initial cracking resistance by compensating for rather low tensile strength of glass. Compressive pre-stress may fully counter the tensile stresses induced by permanent loads, diminishing the negative effect of stress corrosion on long term tensile strength of glass. Furthermore, in case of glass cracking, the tendon bridges the cracks, providing ductile failure behaviour, as in the reinforced beam concept. This research aims to increase the understanding of the structural response of post-tensioned glass beams through experimental and analytical investigations and provide calculation tools which will facilitate the application of the developed beam systems in structural glass projects. The chosen beam layouts aim to maintain the visual simplicity of the common glass beams, comprising laminated glass panes and flat stainless steel tendons, incorporated in the glass section. To investigate the structural behaviour of post-tensioned glass beams, three beam systems with varying beam layout, post-tensioning method and level of pre-load are tested in four-point bending. The observed behaviour is interpreted with analytical models, which focus on the failure mechanisms at two stages: during post-tensioning procedure and during tests in bending. For the considered failure modes, which govern the load capacity of explored beam systems, allowable post-tensioning pre-load and flexural resistance can be determined based on the provided analytical expressions. Failure controlled by shear is qualitatively analysed based on the kinematics of a critical shear crack, which shows the potential importance of this failure mechanism and will serve as a basis for future research on the shear capacity of glass beam members. The proposed systems demonstrate the feasibility of post-tensioning glass beams in terms of enhanced resistance at serviceability limit states with a more efficient use of material, as well as increased safety in the event of glass failure achieved through ductile failure behaviour. The complete data of the experiments can be found in a complementary technical report EPFL-REPORT-230071.

EPFL2017

Comparative Study of Post-tensioned Glass Beams with Bonded Tendons

Jagoda Cupac, Pieter Christian Louter

This paper explores the system of laminated annealed glass beams post-tensioned through stainless steel tendons bonded to the lower glass edge. The tendons are first pre-tensioned and then adhesively bonded to the glass. The compressive pre-stress applied by the tendon increases the fracture strength of the beam; in case of glass fracture, the steel tendon provides ductility enhancing the post-fracture behaviour of the beam. In order to explore the optimal adhesive properties for such application, three types of structural adhesives were selected and applied in a post-tensioned glass beam system. The selected adhesives cover a range of stiffness from semi-rigid to rigid. Simple laminated glass beams were post-tensioned with a 15 kN force applied through the tendons and subsequently tested in four-point bending. The results show a structural response strongly dependent on the properties of the adhesive. While the application of pre-tension through stiffer adhesives significantly increased the initial fracture strength of the beams, an immediate loss of pre-stress was observed with the most flexible adhesive. Nevertheless, all the beams showed, to a varying extent, ductile postfracture behaviour and reached ultimate failure loads above the initial fracture load.

Glaston Finland Oy2015

Revisiting Detailing Rules for Bending Bars, Anchorage and Minimum Shear Reinforcement in Concrete Structures

Frédéric Monney

Reinforcement detailing rules describe the shape, geometrical dimensions, and amount of steel to be placed in reinforced concrete structures. They allow for simple and fast designs, account for several effects neglected in the design, ensure a satisfactory behaviour under serviceability conditions, a sufficient robustness, and an adequate behaviour in case of unexpected actions. Most detailing rules provided in codes have not been updated to correspond to current manufacturing processes, material performances and scientific knowledge. They are often based on "rules of good practice" which, while satisfactory, lack a sound scientific basis and may not be needed. This is one reason why detailing rules may differ amongst countries and design codes. Some of these rules are overly conservative, in particular when evaluating existing structures, while others may neglect significant effects. Even though these rules play a major role in the economy and safety of concrete structures, little research has been performed in this domain. Several detailing rules have been identified as needing additional investigations to verify their adequacy to current practical needs and recent technological evolutions. This thesis presents a research programme on three main detailing rules: bend detailing and required mandrel diameter, anchorage of shear reinforcement with bends and hooks and minimum amount of shear reinforcement. This research aims at developing mechanical models, simplified formulas and detailing provisions, on the basis of experimental results. These were obtained using state-of-the-art measurement techniques such as Digital Image Correlation or Fibre-Optic Measurements.Bends and hooks of reinforcing bars are usually obtained by plastic bending of the bars against mandrels. Codes specify minimum mandrel diameters to avoid splitting or spalling failures that may limit the resistance of the detail. The thesis includes a research programme on the detailing of bends and the required mandrel diameter to avoid concrete failures leading to spalling of the concrete cover. A mechanical model for the design of bent reinforcement was developed. A simplifying standard bending procedure is proposed, in which details formerly requiring various bend diameters can be obtained by using a single mandrel, allowing for faster automated manufacturing of bent reinforcement.Bends and hooks at the end of the bars are efficient solutions for the anchorage of reinforcement. However, these details are relatively sensitive to the cracking state of the surrounding concrete. For shear reinforcement, spalling failure of the concrete cover for bars close to the concrete surface can occur. The thesis includes an investigation on the mechanical response and performance of bend and hook anchorages. A mechanical model and practical considerations on the activation of shear reinforcement in beams are presented.The required minimum amount of shear reinforcement in beams and slabs has been discussed for decades. It is crucial to ensure economic and safe new structures and to accurately assess existing ones. The investigation shows that the shear behaviour is strongly dependent on the amount and the post-yield response of the shear reinforcement (ductility class). For all investigated detailing rules, the implementation of these findings into codes is discussed, highlighting the consistency of the recent changes, particularly with the new generation of Eurocode 2.