The hysteretic behavior and collapse mechanism of steel two-tiered braced frames (STBF) under rare earthquake was studied. Quasi-static test was conducted on a 1/2-scale STBF

and numerical simulation was carried out for 31 validated models of STBF. The failure mode

deformation

and internal force of STBF under cyclic load were analyzed. The influences of parameters on the collapse mechanism of STBF were investigated

including the load on the top of the column

slenderness ratio

diameter-thickness ratio

and tier-height ratio of braced frames. Results show that the bracing failure of STBF under cyclic load was primarily in one tier. The tier underwent a larger buckling deformation outside the brace plane

which might develop to fracture failure. The column experienced in-plane bending moment due to the asynchronous failure of upper and lower braces. With the increase in the tier-height ratio

the energy dissipation capacity of STBF increased. When the axial compression ratio was larger than 0.5 or the tier-height ratio of the braced frame was less than 0.5

STBF was damaged prematurely due to the instability of columns. Therefore

it is recommended that the axial compression ratio of the column should not be larger than 0.5

and the tier-height ratio should not be less than 0.5.