Steel members strengthened with carbon fibre reinforced polymers - A study of local and global structural behaviour

Investigations have been conducted by several research groups to study the effect of using carbon fibre reinforced polymers (CFRP) as reinforcement on steel beams and a brief summary of this work is given in this thesis. Laboratory studies of reinforced steel beams demonstrate that the bending stiffness and the ultimate load-carrying capacity can be significantly increased. Furthermore, field applications of this reinforcement method have been used on bridge elements made of steel or wrought iron, with the aim of either increasing the load-carrying capacity of the beams or restoring the origin capacity of beams that have deteriorated. In addition, field applications have also been made to increase the fatigue life of bridge members. The results of these field applications reveal that the main aims of the reinforcement application could be realised and it was also revealed that this reinforcement method could be cost efficient in comparison with traditional reinforcement techniques. Investigations have been conducted by the author to investigate the global behaviour of double-symmetrical steel beams that were reinforced with CFRP laminate on the tension flange. This took the form of a parametric study in which laminates and adhesives with different geometric and material properties were used. The study was carried out using laboratory tests and analytical and FEM analyses. There was good agreement between the results produced by the different methods in terms of global behaviour. It was revealed from this study that the maximum increase in the load-carrying capacity of the beam may be limited by the resistance of the steel sections to supporting compressive stresses. However, studies of the shear stress distribution in the adhesive layer at the end of the bond line revealed that it is difficult to define the interfacial stress distribution from laboratory tests using strain gauges applied to the laminates.The shear and peeling stress distribution in the adhesive layer was found to be similar in reinforced steel beams with high sections and steel plates reinforced with laminates on both sides and loaded in tension. A laboratory investigation was therefore conducted on double-sided reinforced steel plates where the shear and peeling strain distribution in the adhesive layer was defined with an optical measurement method. The laboratory investigation revealed that, in the case of non-linear adhesives with high ductility, a substantial redistribution of the stresses in the adhesive layer took place. The redistribution of the stresses resulted in a stress concentration close to the adhesive-steel interface, followed by the successive degradation of the adhesive joint until debonding failure occurred. Due to the redistribution of the stresses in the adhesive layer, predicting the stress distribution could be very complex. The laboratory tests also showed that using a linear-elastic model for predicting the joint capacity may result in a very conservative design, particularly for non-linear adhesives with high ductility. This thesis also presents a suggestion for a new design method, which considers the force in the laminate as the only governing parameter. This method inherently takes the redistribution of stresses in the adhesive layer into consideration

Strengthening steel beams with adhesively bonded composite laminates

Strengthening steel structures with carbon-fibre-reinforced-polymer (CFRP) laminates bonded to the steel substrate has become an interesting alternative to the conventional strengthening methods. The current research comprises analyses of the global and local behaviour of double-symmetrical steel beams strengthened with CFRP laminate bonded with adhesive to the tension flange. The beams were subjected to loads applied in four-point bending. In this research, the global behaviour is the load-strain or load-deflection behaviour of a strengthened beam, while the local behaviour comprises the interfacial stresses obtained in the bond line. These behaviours were studied in both the serviceability and the ultimate limit state. Methods for conducting these analyses were based on analytical models, laboratory tests and FE (Finite Element) models. Furthermore, in this research, different configurations of the strengthening system were studied, where CFRP laminates and adhesives with different material and geometrical properties were used. Another aim of this research was to study the failure modes and obtain practical experience from the preparation work on the strengthened beams. The results produced by each method have been analysed and compared reciprocally. The results reveal that the load-carrying capacity could be increased by about 20% for a double-symmetrical steel beam strengthened with bonded CFRP laminate. For the analysed beams, the increase in stiffness in the elastic phase of the steel beam was negligible. However, the analytical solutions revealed that the behaviour of the strengthened beams in the elastic phase can be affected by selecting another configuration for the strengthening system, if CFRP laminate with stiffer properties could be selected. This will then affect the magnitude of the interfacial stresses, which could be critical for the strengthening system. The analyses of the interfacial stresses reveal that both the shear and peeling stresses in the adhesive exhibit large-scale variations over the width of the bond line, with their maximum values at the location closest to the web plate. In addition, the shear stress that developed as a function of the magnitude of the applied load, after the steel beam had reached yielding, was almost linear and was therefore not affected by the yielding in the area at the mid-span of the steel beam. However, the interfacial shear stresses near the mid-span of the strengthened beam were strongly affected by the yielding in the steel beam and the magnitude of the shear stress in the vicinity of this area might well exceed the shear capacity of the adhesive

Strengthening steel beams with adhesively bonded composite laminates

Strengthening steel structures with carbon-fibre-reinforced-polymer (CFRP) laminates bonded to the steel substrate has become an interesting alternative to the conventional strengthening methods. The current research comprises analyses of the global and local behaviour of double-symmetrical steel beams strengthened with CFRP laminate bonded with adhesive to the tension flange. The beams were subjected to loads applied in four-point bending. In this research, the global behaviour is the load-strain or load-deflection behaviour of a strengthened beam, while the local behaviour comprises the interfacial stresses obtained in the bond line. These behaviours were studied in both the serviceability and the ultimate limit state. Methods for conducting these analyses were based on analytical models, laboratory tests and FE (Finite Element) models. Furthermore, in this research, different configurations of the strengthening system were studied, where CFRP laminates and adhesives with different material and geometrical properties were used. Another aim of this research was to study the failure modes and obtain practical experience from the preparation work on the strengthened beams. The results produced by each method have been analysed and compared reciprocally. The results reveal that the load-carrying capacity could be increased by about 20% for a double-symmetrical steel beam strengthened with bonded CFRP laminate. For the analysed beams, the increase in stiffness in the elastic phase of the steel beam was negligible. However, the analytical solutions revealed that the behaviour of the strengthened beams in the elastic phase can be affected by selecting another configuration for the strengthening system, if CFRP laminate with stiffer properties could be selected. This will then affect the magnitude of the interfacial stresses, which could be critical for the strengthening system. The analyses of the interfacial stresses reveal that both the shear and peeling stresses in the adhesive exhibit large-scale variations over the width of the bond line, with their maximum values at the location closest to the web plate. In addition, the shear stress that developed as a function of the magnitude of the applied load, after the steel beam had reached yielding, was almost linear and was therefore not affected by the yielding in the area at the mid-span of the steel beam. However, the interfacial shear stresses near the mid-span of the strengthened beam were strongly affected by the yielding in the steel beam and the magnitude of the shear stress in the vicinity of this area might well exceed the shear capacity of the adhesive

Steel members strengthened with carbon fibre reinforced polymers - A study of local and global structural behaviour

Investigations have been conducted by several research groups to study the effect of using carbon fibre reinforced polymers (CFRP) as reinforcement on steel beams and a brief summary of this work is given in this thesis. Laboratory studies of reinforced steel beams demonstrate that the bending stiffness and the ultimate load-carrying capacity can be significantly increased. Furthermore, field applications of this reinforcement method have been used on bridge elements made of steel or wrought iron, with the aim of either increasing the load-carrying capacity of the beams or restoring the origin capacity of beams that have deteriorated. In addition, field applications have also been made to increase the fatigue life of bridge members. The results of these field applications reveal that the main aims of the reinforcement application could be realised and it was also revealed that this reinforcement method could be cost efficient in comparison with traditional reinforcement techniques. Investigations have been conducted by the author to investigate the global behaviour of double-symmetrical steel beams that were reinforced with CFRP laminate on the tension flange. This took the form of a parametric study in which laminates and adhesives with different geometric and material properties were used. The study was carried out using laboratory tests and analytical and FEM analyses. There was good agreement between the results produced by the different methods in terms of global behaviour. It was revealed from this study that the maximum increase in the load-carrying capacity of the beam may be limited by the resistance of the steel sections to supporting compressive stresses. However, studies of the shear stress distribution in the adhesive layer at the end of the bond line revealed that it is difficult to define the interfacial stress distribution from laboratory tests using strain gauges applied to the laminates.The shear and peeling stress distribution in the adhesive layer was found to be similar in reinforced steel beams with high sections and steel plates reinforced with laminates on both sides and loaded in tension. A laboratory investigation was therefore conducted on double-sided reinforced steel plates where the shear and peeling strain distribution in the adhesive layer was defined with an optical measurement method. The laboratory investigation revealed that, in the case of non-linear adhesives with high ductility, a substantial redistribution of the stresses in the adhesive layer took place. The redistribution of the stresses resulted in a stress concentration close to the adhesive-steel interface, followed by the successive degradation of the adhesive joint until debonding failure occurred. Due to the redistribution of the stresses in the adhesive layer, predicting the stress distribution could be very complex. The laboratory tests also showed that using a linear-elastic model for predicting the joint capacity may result in a very conservative design, particularly for non-linear adhesives with high ductility. This thesis also presents a suggestion for a new design method, which considers the force in the laminate as the only governing parameter. This method inherently takes the redistribution of stresses in the adhesive layer into consideration

Upgrading of Steel Beams using Composite Materials

Research work has been conducted to study the effect of upgrading steel I-beams using carbon-fibre-reinforced-polymer (CFRP) plates bonded to the tension flange. The investigation comprised two sections; numerical analysis using Finite Element (FE) method and laboratory tests. The investigation was carried out as a parametric study where CFRP plates with different material and geometrical properties were used along with two different types of adhesive. The aim of the investigation was to study how various material properties of the strengthening material affect the behaviour of the strengthened steel I-beams. Additionally, the magnitude and distribution of the interfacial shear and peeling stresses in the strengthened beams were analysed. The results show that the moment capacity of the strengthened steel beam can be increased up to about 18%. It was also shown that yielding of the steel beam in the area of maximum moment does not affect the interfacial stresses near the end of the bond line, until excessive yielding and formation of plastic hing