3.3　Concrete-Filled Steel Tube

3.3.1　The following combinations of steel tube and concrete may be used:

1　Q235 steel matched with concrete of Grade C30-C40；

2　Q345 steel matched with concrete of Grade C40-C60；

3　Q390 steel matched with concrete of Grade C60 or above.

3.3.2　The wall thickness of steel tubes shall not be less than 8 mm.The ratio of the outer diameter D to the wall thickness t of steel tubes should be in the range of 35×（235/fy）-100×（235/fy）.The characteristic strength value of steel fy shall comply with the provisions in Table 3.1.3.

3.3.3　The design confinement coefficient of CFST ξ0 should not be less than 0.60.The sectional steel ratio ρc should be in the range of 0.04-0.20.The parameters of ξ0 and ρc shall be calculated by using the following formulae:

Where，ξ0——Design confinement coefficient of CFST；

ρc——Sectional steel ratio of CFST；

As——Sectional area of steel tube；

Ac——Sectional area of concrete core；

fs——Design tensile，compressive and flexural strength values of steel；

fcd——Design compressive strength value of concrete.