本論文以三維非線性有限元素軟體所建立的數值模型，模擬內填充混凝土箱型鋼柱於高溫下承受軸向壓力的結構行為，數值模型採用定載升溫的方式，在內填充混凝土箱型鋼柱模型表面施予ISO-834的溫升歷程，透過順序耦合的熱傳與結構分析，進行其高溫火害的數值模擬，本研究先針對內填充混凝土箱型鋼柱的柱長、載重比、混凝土強度、柱板厚度進行參數分析，計算柱之破壞溫度並探討其破壞模式，藉以瞭解各參數對內填充混凝土箱型鋼柱高溫結構行為的影響，此外，本研究亦針對內填充混凝土箱型鋼柱的五種高溫補強策略（即：普通鋼柱板加勁肢補強、耐火鋼柱板加勁肢補強、普通鋼內十字形板補強、耐火鋼內十字形板補強、全耐火鋼柱板補強）進行數值模擬、分析與比較，尋求最佳溫度提升的補強策略與最佳經濟效益的的補強策略，數值模擬結果顯示：耐火鋼內十字形板補強策略之溫度提升效果最佳，能提升約21.3%的破壞溫度；普通鋼內十字形板補強策略有最佳的經濟效益。

The Numerical Simulations for the Fire-Resistance Enhancement Strategies of Concrete-Filled Steel Box Columns at Elevated Temperatures

Author: Kao-Chu Chien
Advisor: Professor Hsin-Yang Chung
Department of Civil Engineering
National Cheng Kung University

SUMMARY

This thesis utilized the numerical models developed by a three-dimensional nonlinear finite-element program to simulate the structural behaviors of concrete-filled box columns (CFBCs) with the constant axial compressive load at elevated temperatures. The numerical models employed the sequentially coupled thermal-stress analysis to conduct heat transfer and structural analyses for the CFBCs with the constant axial compression and elevated by the ISO-834 time-temperature curve. First, this thesis conducted parameter analyses for column length, load ratio, concrete strength and column plate thickness of CFBCs to investigate the failure temperatures and the failure modes of CFBCs, and to understand the influences of the four parameters on the high temperature structural behaviors of the CFBCs. In addition, this thesis also performed numerical simulations, analyses and comparisons for five kinds of fire-resistant enhancement strategies (including the normal steel column-plate stiffener strengthening method, the fire-resistant steel column-plate stiffener strengthening method, the interior cross-shaped normal steel stiffener strengthening method, the interior cross-shaped fire-resistant steel stiffener strengthening method and the fire-resistant steel column plate strengthening method) to find the best strategy of improving failure temperature and the most economical strategy of improving fire resistance. The numerical simulation results showed that the interior cross-shaped fire-resistant steel stiffener strengthening method demonstrated the best failure temperature improvement strategy, which could improve failure temperature by 21.3%. The interior cross-shaped normal steel stiffener strengthening method was the most economical strategy of improving fire resistance.

Key words：Concrete-Filled Steel Box Column, Fire, Nonlinear Finite-Element Method, Failure Temperatures, Enhancement Strategies.
INTRODUCTION

The tall building designs in Taiwan frequently adopt steel structures. One benefit is to decrease the self weight of building by reducing column size. The other benefit is to increase the bay length (i.e. beam length) and the usable floor area. Considering the seismic effects, structural designs in Taiwan for beam-to-column connections especially pay more attention on the ductility and toughness. The design concept of “Strong Column and Weak Beam” are always adopted in building design. As a result, steel column constructions in Taiwan usually utilize the design of concrete-filled box column (CFBC). The CFBCs are different from the steel reinforced concrete columns in which steel is covered by concrete against fire. The CFBCs are frequently utilized in the columns of the lower floors because they bear the heavy weights of the upper floors. In CFBCs, the steel box bears more load than the infilled concrete. In fire, the stiffness and strength of steel decrease when the fire temperature increases. This jeopardizes the structural safety of steel buildings. How to increase the fire-resistance of the CFBCs is very important. Our group have successfully simulated the three-dimensional nonlinear finite-element steel H-shaped frame model at elevated temperatures with a finite-element program and have successfully strengthened a three-dimensional nonlinear finite-element steel H-shaped frame model at elevated temperatures using fire-resistance steel. I will continue our research group’s works to develop the CFBC numerical models using the three-dimensional nonlinear finite-element program to investigate the structural behaviors of the CFBCs with the constant axial compressive load at elevated temperatures and to find the best strategy of improving failure temperature and the most economical strategy of improving fire resistance from the five kinds of fire-resistant enhancement strategies, including the normal steel column-plate stiffener strengthening method, the fire-resistant steel column-plate stiffener strengthening method, the interior cross-shaped normal steel stiffener strengthening method, the interior cross-shaped fire-resistant steel stiffener strengthening method and the fire-resistant steel column plate strengthening method.

MATERIALS AND METHODS

Research methods were mainly divided into three parts. The first part was that three-dimensional nonlinear finite-element CFBCs numerical models were developed and were verified by the experiment to confirm each step of the model establishments correct. The second part was employed the sequential heat transfer and stress analysis to simulate the surface of CFBCs numerical models heated by ISO-834 time-temperature curve in constant load. Parameter analyses were divided into column length, load ratio, concrete strength and column plate thickness of CFBCs to investigate the numerical results and to realize the effects of the CFBCs by each parameter model at elevated temperature. The third part was employed the sequential heat transfer and stress analysis to simulate the surface of CFBCs numerical strengthening models heated by ISO-834 time-temperature curve in constant load. Enhancement strategies were divided into the steel column-plate stiffener strengthening method, the interior cross-shaped steel stiffener strengthening method, the fire-resistant steel column plate strengthening method and etc. to research for the best economical strategy of improving fire resistance.

RESULTS AND DISCUSSION

The results of column length parameter analyses in the same load ratio were showed that failure temperature of the long length column was higher than failure temperature of the short length column. The normal compression strength of the long column is lower than the normal compression strength of the short column at room temperature, so the long column bears the lower compression than the short column in the same load ratio. The normal compression strength decreased in high temperatures, so the failure temperature of the long column bearing the lower compression was higher than the failure temperature of the short column bearing the higher compression as Figure 5-87.
The results of column length parameter analyses in the same load (13970 kN) were showed that failure temperature of the long length column was lower than failure temperature of the short length column. The normal compression strength decreased in high temperatures, so the failure temperature of the short column was higher than the failure temperature of the long column as Figure 5-88.
The results of load ratio parameter analyses were showed that failure temperature of the high ratio column was lower than failure temperature of the low ratio column in the same section size as Figure 5-89. According to my research, it was illustrated that the failure temperature was decreased in 60˚C per increasing load ratio by every 10%.
The results of concrete strength parameter analyses in the same load ratio were showed that failure temperature of the high strength concrete column was higher than failure temperature of the low strength concrete column in the same size section as Figure 5-90. Concrete temperature was low at elevated temperature due to the poor concrete conductivity, so concrete could bear the part axial force. The failure temperature of CFBCs was improved by increasing the concrete compression strength.
The results of column plate thickness of CFBCs parameter analyses in the same load ratio were showed that failure temperature of the thin plate column was higher than failure temperature of the thick plate column. The normal compression strength of the thick plate column is higher than the normal compression strength of the thin plate column at room temperature, so the thick plate column bears the higher compression than the thin plate column in the same load ratio. The failure temperature of the thick plate column bearing the higher compression was lower than the failure temperature of the thin plate column bearing the lower compression but the 25 mm plate thickness had the local maximum failure temperature and the failure temperature was increased in plate thickness 22 mm to 25 mm as Figure 5-69.
The results of column plate thickness of CFBCs parameter analyses in the same load (13970 kN) were showed that failure temperature of the thick plate column was higher than failure temperature of the thin plate column. The normal compression strength of the thick plate column is higher than the normal compression strength of the thin plate column at room temperature. The normal compression strength was decreased in high temperatures, so the failure temperature of the thick plate column was higher than the failure temperature of the thin plate column in the constant load as Figure 5-91.
The results of the CFBCs strengthening models were showed that the fire-resistance steel was better the normal steel to improve failure temperature of the CFBCs as the interior cross-shaped fire-resistant steel stiffener strengthening method (20mm×120mm) which could improve failure temperature by 21.3%. The interior cross-shaped steel stiffener strengthening method was better than steel column-plate stiffener strengthening method to improve failure temperature due to the interior cross-shaped steel stiffener strengthening method covering by concrete against fire to keep the stiffness and strength.

CONCLUSION

The results of the concrete-filled steel box column exposed to standard fire were showed as follows：
(1) The failure temperature of the long length column was higher than failure temperature of the short length column in the same load ratio.
(2) The failure temperature of the long length column was lower than failure temperature of the short length column in the constant load.
(3) The failure temperature of the high ratio column was lower than failure temperature of the low ratio column in the same size section.
(4) The failure temperature of the high strength concrete column was higher than failure temperature of the low strength concrete column in the same size section.
(5) The failure temperature of the thick plate column was higher than failure temperature of the thin plate column in the constant load.
(6) The interior cross-shaped fire-resistant steel stiffener strengthening method (20mm×120mm) improved failure temperatures by 21.3%.
(7) The interior cross-shaped normal steel stiffener strengthening method was the most economical strategy of improving fire resistance.

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