behaviors (bt=tt ¼ 32。3 and 48。0)。 Mursi and Uy (2004) studied effects of column slenderness (bt=tt ¼ 22。0 ∼ 52。0)。 Sato et al。 (2009) tested stub-columns with ultra-high-strength concrete (com- pressive strength fc0 ¼ 153 ∼ 202 MPa, and bt=tt  ¼ 20。5 and 28。0)。 More recently, Khan et al。 (2013) and Aslani et al。    (2015)

tested stub-columns to investigate various combinations of concrete strength and width-to-thickness ratio (bt=tt ¼ 14。0 ∼ 40。0)。 As such, the majority of the previous high-strength RCFT specimens belong to compact or noncompact sections (AISC 2010)。 Only a few test results have been reported for RCFT columns with slender sections (Nakahara and Sakino 2003; Mursi and Uy   2004)。

As the yield strength of steel increases, the thickness of the steel plates needs to be decreased for economy。 In this case, to prevent early local buckling of the slender section, the use of vertical stiffeners can be considered。 Tao et al。 (2005) conducted axial load tests for RCFT columns strengthened with stiffeners (steel    yield

strength  =  234  MPa,  concrete  compressive  strength  =  50。1−

54。8 MPa), and proposed a design equation for the stiffener rigidity。

However,  the design equation  was developed  for mild steel   tube

columns。 Thus, the applicability of the equation should be verified to high-strength steel tube columns。

In the present study, eccentric axial loading tests were per- formed for RCFT columns with a high-strength steel slender sec- tion。 To investigate the axial-flexure capacity under high axial load, small values of eccentricity were used for the column specimens。 On the basis of the results, the applicability of the current design codes to high-strength steel slender sections was evaluated, and a design equation for vertical stiffeners was  developed。

Width-to-Thickness Ratio

In ANSI/AISC 360 (AISC 2010), the sectional slenderness for RCFT columns is  classified  into  three  categories: compact (bt=tt  < λp),    noncompact    (λp  < bt=tt  < λr),    and    slender (bt=tt > λr) sections, where bt  is  the  maximum  inner  width of the steel tube and tt is thickness of the tube plate。 The slenderness limits λp  and λr  are defined as   follows:

Fig。 1。 Cross sections of test specimens: (a) E1 and E3; (b) E2; (c) E4;

In  ANSI/AISC  360-10,  a  rectangular  section  that  has  both

where Es = elastic modulus of the steel; and fy;t = yield strength of the tube plate。 The plastic strength can be used only for the compact

section。

Stiffener Rigidity

Vertical stiffeners can be used to improve the structural perfor- mance of RCFT columns with  slender  sections  by  restraining the local buckling of the steel tube。 The second moment of inertia of the vertical stiffeners, which is required to adequately restrain the local buckling, was proposed by Tao et al。   (2005)

。w。3。5  fy;t

slender plates and compact plates is defined as a slender section。 Specimen E2 was tested to investigate the structural performance of RCFT columns that have both compact plates and slender plates

in a cross section。 For this, E2 was designed as a rectangular section (□ − 300 × 150 mm) using high-strength steel plates of tt ¼ 5 mm。 To maintain the same strength contribution ratio of the steel section

as that used for E1, the nominal compressive strength of the infill concrete was increased to 90 MPa (average cylinder strength fc0  ¼ 83。6 MPa)。