Analysis Of Buckling Restrained Brace Encased In Concrete/Mortar And Steel Tube

The Buckling Restrained Braces that are currently in practical use can be categorised as those using mortar or concrete in addition to steel for a restraining part, and those using only steel. Iwata et al. carried out a literature search of the different types of buckling restrained braces reported to date and compiled search data on their performance and features. Test results indicated that the BRB using mortar-filled rectangular steel tubes as a restraining part provided stable hysteresis characteristics when the core plate was subjected to a 3.0 per cent strain. The results further revealed that compared to BRBs using mortar-filled steel tubes, those using only steel as a restraining part cannot easily provide adequate hysteresis under high strains.

Rational arrangement in a steel frame is very effective in reducing the deformation and response of a building frame during a strong wind and earthquake. When braces of high slenderness ratio are used, the deformation capability exists unless joints are broken when tensile forces work on the braces. However when these braces undergo compressive deformation, lateral deformation occurs easily and they cannot bear compressive forces hence they exhibit hysteresis of the slip type under repeated loads. It is against this background that Watanabe et al. designed a brace with a stable forth deflection characteristic that enables compressive yield strength to be considered equal to tensile yield strength. The brace designed consisted of a steel core with a rectangular section restrained at its ends by concrete encased in a steel tube. Five specimens with varying sectional dimensions of the steel tube were used (as shown in the figure 2) with the ratio of Euler load, Pe, of the steel tube to the yield load, Py, working on the core member with Pe/Py between 0.55 and 3.82. The size of the steel core was kept constant.

After the experiment, Watanabe et al. found that when this type of BRB is incorporated in a frame and the ends subjected to the effects of bending moment, buckling of the whole structural member does not occur if Euler load of the steel tube is greater than the yield strength of the core member. Furthermore, he acknowledged that the concrete in the steel tube contributes to the flexural rigidity of the steel tube. According to Watanabe et al., in making the designs, the brace member should be encased in a steel tube having Euler load about 1.5 times the yield load of the core member. With this type of BRB, it is possible to determine the initial rigidity and yield strength of the bracing member from the behaviours of the core member under compressive and tensile loads independent of the problem of buckling.

Based on the findings of Watanabe et al. (1988), Iwata et al. (2006) further developed another type of Buckling Restrained Brace that enables strict quality control as well as increased design freedom at both ends of the core plate while providing stable hysteresis characteristics even under high strain, as shown in the figure below. They believed using this approach enables visual confirmation of the initial failure of the core plate, as well as the status of the mortar filling and application of the un-bonded material, facilitating quality control. Unlike conventional braces where a core plate is inserted between a pair of steel mortar planks, sandwiching a core plate between a pair of steel mortar planks increases design freedom for the core plate end sections. Furthermore, core plates and steel mortar planks can be delivered separately for on-site erection. Three different thickness settings of 12, 16, and 22mm were adopted for the core plate. The width of the section of the core plate, or flat steel without strengthening ribs, was varied. When the buckling-restrained brace is subjected to compressive forces, a minute bending deflection occurs in the core plate plastic region. The steel mortar plank prevents progress of the local bending deflection in this particular deflection location where the core plate comes into contact with the mortar, causing the minute bending deflection to spread over the entire core plate plastic region to attain a high-order buckling mode. Seven types of core plates with different sizes of core plate plastic regions were prepared. The width-to-thickness ratios of these core plates were within the 11.0–4.0 range. The steel mortar plank dimensions were set in accordance with the PE/Py values. Iwata et al. (2006) then conducted static, cyclic axial loading tests of their designed buckling-restrained brace using steel mortar planks.

Based on their results, all specimens were found to have stable hysteresis characteristics to withstand strains of up to 1.0 per cent. Specimens excluding that with Pe/Py = 0.9 possessed stable hysteresis characteristics to withstand strains of up to 2.5 per cent.