الفهرس 
INTRODUCTIONOnly 14 pages are availabe for public view
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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