To compare the effect of steel yield strength, Specimen E3 was designed with mild steel of Grade 400 (actual tensile strength

fu;t ¼ 466 MPa and yield strength fy;t ¼ 301 MPa)。 The   other

Is;s ¼ Is;re ¼ 3。1 × 10−4 t4

parameters of E3, such as the plate thickness, sectional dimensions,

tt 280 t

Where Is;s (= tsb3 =12) = second-moment of inertia of the stiffener [Fig。 1(c)]; ts and bs = thickness and depth of the stiffener plate, respectively; Is;re = required second-moment of inertia of the stiff- ener; and w = width of a subpanel pided by the stiffener, which is defined as half the full width (B=2) when one stiffener is used per tube plate。 Eq。 (2) was empirically defined from the existing test results of mild steel tube columns  [for  the  tube  plates,  fy;t  ¼ 234 MPa (tt ¼ 2。5 mm) and for  the  stiffeners,  fy;s  ¼ 234 MPa (ts  ¼ 2。5 mm) and 311 MPa (ts  ¼ 2 mm)]。

Specimens and Test Parameters

Table 1 and Fig。 1 show the test parameters and sectional dimen- sions of the test specimens, respectively。 Five columns were pre- pared for the eccentric axial loading test。 For high-strength steel, grade 800 steel (actual tensile strength fu;t ¼ 835 MPa and yield strength fy;t ¼ 746 MPa) was used。 The control specimen, E1, was

designed as a square section (□ − 300 × 300 mm) with high- strength steel plates of tt ¼ 5 mm。 Since the tube width-to-thick- ness ratio bt=tt ¼ 58。0 exceeds λr ¼ 49。7, the tube section was classified as a slender section。 For the infill concrete, 70 MPa con- crete (average cylinder strength fc0  ¼ 70。5 MPa) was   used。

and concrete strength, were the same as those of E1。 By using the mild steel, the steel tube of E3 was classified as a compact sec- tion (bt=tt  ¼ 58。0 < λp  ¼ 59。0)。

The properties of Specimens E4 and E5 were the same as those

of E1 and E2, respectively, except for the use of vertical stiffeners in the former。 In E4, a total of four stiffeners were welded to the tube plates。 In E5, which had a rectangular section, two stiffeners were welded to the slender plates。 As a stiffener was located at the center of the tube plate, the width-to-thickness ratio of the subpa- nels was decreased to w=tt ¼ 29。0, which is smaller than λp ¼

37。5   (fy;t  ¼ 746 MPa)   for   the   compact   section。   Mild steel

(ts ¼ 5 mm and fy;s ¼ 301 MPa), which is the same as that used for E3, was used for the vertical stiffeners。 The width of the stiff- eners was bs ¼ 60 mm and their design method is discussed in de- tail in the section “Design of  Stiffeners。”

To investigate the axial-flexure capacity under high axial load (about 80% of concentric axial load capacity), a small value of eccentricity ratio (i。e。, ratio of eccentricity to  the  dimension of the section) was used for all specimens: e=H ¼ 0。133。 For the square-section  specimens,  the  eccentricity  was   e ¼ 40  mm (H ¼ 300 mm), and for the rectangular-section specimens, the eccentricity with respect to the  weak  axis  was  e ¼ 20 mm (H ¼ 150 mm)。

E1 300 300 5 58。0 746 37。5