Axial Compression Performance of Concrete-Filled Steel Tubular Columns with Different D/t Ratios

Table 1

CCD model Parameters level.

Parameters	Coding	Parameters level
		−1	0	+1
Response 1
D/t ratio	X1	21.22	25.46	31.83
In-filled concrete strength	X2	40 N/mm^2	50 N/mm^2	60 N/mm^2