Steel hollow sections are one of the most versatile and efficient forms for construction and mechanical applications. Many of the strongest and most impressive structures in the world today would not have been possible without hollow sections. High strength per unit weight is considered to be one of the most important characteristics of the hollow steel sections.
Buckling is the common failure of the hollow steel sections, whether local buckling such as elephant’s foot buckling mode or overall buckling. The elephant’s foot buckling mode is a typical local buckling mode of circular hollow steel tubes which appears as an outward bulge near the ends of the tube. It can occur due to outward imperfection in addition to axial compression alone, in combination with cyclic lateral loading or in combination with internal pressure. The same mode of buckling may occur in square hollow sections under axial load and it is called roof mechanism]. It commonly occurs at the mid height of the specimens. The overall buckling mode is also a common mode of buckling for the hollow steel sections due to axial load in addition to small out-of-straightness imperfection in the specimen. These previous imperfections are categorized as geometric imperfection. It means that either the steel section itself has slightly geometric deviation from the ideal shape. This imperfection arises during the manufacturing of the steel element. Also, material imperfections exist in steel in form of point, line, planer, and volume defects.
Strengthening of steel structures is commonly carried out by steel plates. Because steel plates weigh heavily and can be corroded, they have been replaced with fibers reinforced polymers composite (FRP). In addition to superior thermo-mechanical properties, FRP composites have many advantages over conventional materials such as high strength, long service life, impact resistance and excellent corrosion resistance. Therefore, FRP may be used to strengthen hollow steel sections to improve the structural buckling behavior of the steel elements.
Research work on strengthening of tubular steel compression members is very limited. Batikha et al. presented numerically a new method of strengthening tubular sections to resist elephant’s foot buckling. This method involves the use of a small amount of FRP in the critical location of the tube and this method had proven to be effective in eliminating the problem and increase the buckling strength.
Haedir and Zhao presented an experimental study to evaluate the performance of externally bonded carbon fiber (CFRP) in strengthening circular steel tubular short columns under axial load to resist the elephant’s foot buckling. The results confirmed that enhancement of the axial section capacity would be possible by strengthening the tubular column with CFRP.
Shaat and Fam provided an experimental study to investigate the behavior of short square hollow steel sections strengthened with CFRP under axial load. The predominated modes of buckling for the short columns were elephant’s foot buckling. The results showed that the maximum strength gain of 18% was achieved with two transverse CFRP layers. There are other researches that study the same modes of buckling, but the loads were not limited to the axial load.
Nishino and Furukawa conducted cyclic loading tests on a circular tube beam-column strengthened with CFRP sheet using a loading system that causes double curvature bending. They found that CFRP can increase the deformation capacity of circular tube beam-columns and the carbon fiber prevented local ring-type buckling (elephant’s foot buckling mode).
For overall buckling, Shaat and Fam provided an experimental study to investigate the behavior of long square hollow steel sections strengthened with CFRP under axial load. The predominated modes of buckling for the long columns were overall buckling. The maximum strength gain was 23% with three longitudinal CFRP layers applied on four sides.
Gao et al. conducted an experimental study to investigate the effect of strengthening tubular steel sections with carbon fiber reinforced polymers (CFRP) on the overall buckling behavior. They concluded that externally bonded longitudinal CFRP sheets were effective in increasing the axial strength and stiffness of slender braces.