| Abstract |
Steel structures are widely used in the construction field due to their superior strength and stiffness. Hollow steel sections (HSS) combine steel benefits with the aesthetic appearance and the expanded strength of a closed shape, allowing hollow steel sections to become a significant portion of existing steel construction worldwide. Recently, many steel structures have failed to meet structural and functional requirements as a result of increased service loads, corrosion, or improper maintenance. In most cases, retrofitting an existing structure to extend its service life is significantly more cost-effective than replacing it.
Fiber reinforced polymers (FRP) are one of the most successful techniques used in the strengthening of reinforced concrete and steel structures. Fiber reinforced polymer (FRP) possesses notable advantages as a structural material, including high strength and good corrosion resistance. In addition to avoiding the risky welding process, FRPs also have easy applicability in limited access areas. Furthermore, carbon fiber reinforced polymers (CFRP) have comparable strength and stiffness to steel. When compared to welded steel plates, CFRP is a safer and more effective alternative for reinforcing steel.
Over recent decades, many studies have been carried out to investigate the response of various steel elements strengthened with carbon fiber material. Some of these studies showed a high prospect of enhancing the structural behavior of the reinforced elements. However, studies of CFRP applications on HSS are relatively few, and not sufficient to thoroughly understand the structural behavior under various conditions. Therefore, the objective of this research is to investigate the role of utilizing CFRP sheets in increasing the bending capacity and flexural stiffness of strengthened rectangular and square hollow steel sections. Experimental and numerical studies were performed for these objectives.
In the experimental study, twenty specimens were subjected to a four-point loading test. Four distinct CFRP-wrapping techniques were used to reinforce sixteen specimens made from four different rectangular and square hollow sections (RHS and SHS). Each technique has a different orientation and/or number of unidirectional CFRP sheets. Applied loads, beam deflection, and CFRP strains were monitored during the test procedure. According to experimental results, the flexural stiffness and strength of hollow steel beams were effectively improved by applying CFRP strengthening. The buckling tendency of steel sections showed a significant role in the strength improvement ratios achieved by strengthening hollow steel beams with CFRP wrapping techniques. When compared to other techniques, using double layers of CFRP in the orthogonal orientations resulted in the greatest improvement ratios in ultimate strength by 46% and deflection by 35%.
Numerical models were developed to predict the failure loads of tested beams using nonlinear finite element software as well. The CFRP laminate failure criteria with the progressive damage model were adopted. Firstly, the results of the numerical model and the experimental work were compared and found to be in good agreement. Then, a parametric study was conducted, utilizing the results of 135 beam analyses. The analyzed beams were carefully chosen to investigate the parameters that influence the strengthening of RHS and SHS using CFRP techniques. The investigated parameters included the aspect ratio and wall thickness of the steel section, as well as the orientation, number, and nominal dry thickness of the CFRP layers.
The numerical investigation revealed that reinforcing the beam with both longitudinal and transversal layers at the same time resulted in the greatest improvement in ultimate load and deflection of up to 314% and 62%, respectively. Also, longitudinal wrapping outperforms transversal wrapping in steel beam strengthening as long as web crippling is not an issue for the steel beam. Moreover, increasing the dry thickness or the number of CFRP CFRP layers has a greater influence on improving the strength and deflection of the strengthened beams. |
| Keywords |
Hollow steel section, CFRP, Retrofitting, Strengthening, Stiffness, Strength, Pucks’ failure criteria, Progressive damage, unidirectional carbon fiber, stiffness degradation |
