## 2 Experimental Program  
### Test Specimens  
Table 1 and Figure 2 show the section details and material properties of the HCS specimens. In this study, two HCS specimens with cross-sectional heights of 200 and 265 mm were fabricated, and the specimen names are shown in Figure 2a. The total length and clear span of the specimen were 7.0 and 6.0 m, respectively. Considering the furnace condition, 2.5 HCS units with a width of 1.2 m were continuously placed for each specimen to achieve a width (\(b\)) of 3 m. As shown in Figure 2b, the D200-T7 specimen was manufactured by pouring topping concrete with a thickness of 50 mm on the upper part of the HCS unit with a height of 200 mm. Two 9.5-mm-diameter prestressing strands were placed on the top flange of the HCS unit, whereas two 9.5-mm-diameter and five 12.7-mm-diameter strands were placed at the bottom flange. In addition, a live load of 1.5 kN/m\({}^{2}\) was applied to the upper part of the slab during the fire test. As shown in Figure 2c, the D265-T8 specimen was fabricated by pouring topping concrete with a thickness of 50 mm on the upper part of the HCS unit with a height of 265 mm. In the D265-T8 specimen, two 9.5-mm-diameter prestressing strands were placed on the top flange of the HCS unit, whereas two 9.5-mm-diameter and six 12.7-mm-diameter strands were placed at the bottom flange. The live load of the D265-T8 specimen was set to 3.0 kN/m\({}^{2}\).  
\begin{table}
\begin{tabular}{c c c c c c c c c c c} \hline \multirow{2}{*}{**Specimen**} & \(A\) & \(h\) & \(h_{t}\) & \(b\) & \(L\) & \(f_{pu,9.5}\) & \(f_{pu,~{}12.7}\) & \(f^{\prime}_{c,PC}\) & \(f^{\prime}_{c,t}\) & \(f^{\prime}_{c,jo}\) \\  & **(mm\({}^{2}\))** & **(mm)** & **(mm)** & **(mm)** & **(mm)** & **(MPa)** & **(MPa)** & **(MPa)** & **(MPa)** & **(MPa)** \\ \hline D200-T7 & 299,000 & 200 & 50 & 3000 & 7000 & 2003.2 & 1923.7 & 56.9 & 61.5 & 14.1 \\ \hline D265-T8 & 406,250 & 265 & 50 & 3000 & 7000 & 2003.2 & 1923.7 & 59.1 & 61.5 & 14.1 \\ \hline \end{tabular}
\end{table}
Table 1: Dimensions and material properties of test specimens.  
Figure 2: _Cont_.  
The nominal tensile strength (\(f_{pu}\)) of the prestressing strands was 1860 MPa, and the effective prestress of the strands was 0.65 \(f_{pu}\). The HCS unit was manufactured using an extrusion method [33]. Figure 3 shows the manufacturing process of the test specimens. The prestressing strands were first tensioned on a long-line prestress bed and then anchored at both ends of the bed. Subsequently, a machine compacted concrete simultaneously with extruding concrete with a zero slump. The slab was subjected to steam curing for 24 h and then cut to produce an HCS unit member. As described above, 2.5 HCS units with a width of 1.2 m were placed continuously (see Figure 3f), and a hole was drilled in the upper surface of the slab. Thermocouples were then installed to measure the temperature distribution in the cross-section. The specimen manufacturing process was completed by casting topping concrete. The specimens were air-cured at room temperature for 28 days, and the concrete compressive strength (\(f^{\prime}_{c,PC}\)) of the HCS unit measured prior to the experiment was 58 MPa.