# Thin-Walled CFST Columns for Enhancing Seismic Collapse Performance of High-Rise Steel Frames
## 4 Collapse Mitigation Using CFST Columns
### Collapse Capacity Enhancement  
As discussed in the previous section, the 40S-FB\({}_{\text{c}}\)-FB\({}_{\text{b}}\) building showed the most significant sensitivity to global collapse in the range of the intensity of ground motion (e.g., Art-Hachi: \(\text{PGV}=210\) m/s, \(\phi=2\)). The thin-walled CFST columns with equivalent flexural stiffness are used to substitute the HSS columns in the 40S-FB\({}_{\text{c}}\)-FB\({}_{\text{b}}\) building, aiming to improve the intensity of ground motions associated with the incipient collapse. Figure 13 shows the distribution of maximum story drift angle of the 40S-FB\({}_{\text{c}}\)-FB\({}_{\text{b}}\) and the comparative 40 CFT-high-det. (FB\({}_{\text{c}}\)-FB\({}_{\text{b}}\)) buildings when subjected to the Art-Hachi with \(\phi=2\). It is noted that sidesway collapse at the bottom two stories of the 40S-FB\({}_{\text{c}}\)-FB\({}_{\text{b}}\) building is efficiently prevented by using a thin-walled CFST column. This implies that high-deterioration significantly influences the collapse resistant capacity of high-rise SMRF buildings under high-level ground motions that have a larger probability of occurrence than the building model with a low level of deterioration effect. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Case**} & **Component** & **Height** & **Width-to-Thickness** & **‘First Mode’** & \multirow{2}{*}{_B/t Ratio_} \\  & **(Column, Beam)** & **(Story)** & **Ranks (Column, Beam)** & **Natural Period** & \\ \hline \multirow{4}{*}{SMRF 1 **} & \multirow{2}{*}{\
\begin{tabular}{c} \(B\)/\(\hbar\) Ratio \\ \end{tabular} } \\  & & & 81 m (20) & FA, FA & 2.32–2.38 s \\  & & H-shaped beams & 121 m (30) & FA, FB & 3.54–3.61 s \\  & & 161 m (40) & FB, FB & 4.53–4.66 s \\ \hline \multirow{2}{*}{CFST-MRF 1 **} & \multirow{2}{*}{
\begin{tabular}{c} CFST columns, \\ H-shaped beams \\ \end{tabular} } \\  & & 81 m (20) & FA, FA & 2.26–2.32 s \\  & & 121 m (30) & FA, FB & 3.32–3.47 s \\  & & 161 m (40) & FA, FB & 4.12–4.38 s \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Details of various high-rise building models. Figure 12: CFST columns equivalent to the HSS (hollow steel section) columns. For the collapse safety of various high-rise buildings, the SMRFs with thin-walled members face larger challenges of collapse than the equivalent CFST-MRFs. From the demonstration in Figure 14, the incipient collapse has been induced with different intensity levels, indicating that the collapse capacity of the SMRFs with thin-walled members is enhanced by the CFST-MRF buildings with the equivalent flexural stiffness. The 40-FB\({}_{\text{c}}\)-FB\({}_{\text{b}}\) building could collapse until the \(\phi\) is up to around 1.5, while the incipient collapse of the 40 CFT-S\({}_{\text{c}}\)-FB\({}_{\text{b}}\) building model did not occur until the \(\phi\) achieves around 4.0. The high-rise CFST-MRFs demonstrate greater collapse capacity than the equivalent SMRFs, but this finding is only supported when the input ground motion is Art-Hachi. As mentioned before, the Art-Hachi wave is characterized by a flat velocity spectral shape in the domain of the long-period branch. Similar waves such as the BCJ-L2, Yokohama and JSCA-Kobe are selected to perform the IDA to compare the collapse capacity of these two types of high-rises. Seismic response tends to become sensitive and collapse may occur suddenly after going beyond the limit-state of their collapse capacities. Thus, a collapse safety coefficient is proposed to quantify the seismic safety of buildings to prevent deterioration and collapse. A process that progresses from elasticity to elasto-plasticity until ultimate strength to component-deterioration and global-collapse. The PGV of earthquakes is considered to be associated with the corresponding performance demand in performance-based seismic design, e.g., the so called 'Level 2' is treated as the maximum capacity earthquake. The PGV at the Level 2 can be used as the reference to quantify the collapse margin.