中華民國內政部建築研究所中華民國內政部建築研究所

一、研究緣起
高強度鋼材的研發與應用已大量增多，高強度鋼材逐漸應用於橋梁與中高層建築物。相較於目前常用的鋼板，高強度鋼材具有高降伏強度與高抗拉強度的特性，因而高強度鋼材的應用於高層建築物有其優勢，特別是承受大載重的低層樓柱構件。國內外對於鋼柱構件於火害中之行為，已有相當程度之研究；然而過去研究大多集中於一般鋼材之火害實驗。反觀現今高強度鋼材的中高層建築物已日漸增多，但採用高強度鋼材的鋼柱於火害下之受壓強度與行為之研究甚少。高強度鋼材在高溫下，其降伏強度和彈性模數的衰減幅度皆大於一般鋼材，故本計畫將進行高強度鋼柱火害實驗，探討其高溫下的受壓強度與行為。
二、研究方法與過程
本研究方法採火害實驗與建立受壓強度計算模式。首先為蒐集與彙整國內外相關文獻與規範，了解影響鋼柱耐火性能之參數與各規範計算之方法。規劃四組SM570MC鋼材之實尺寸箱型鋼柱進行火害實驗，探討高強度鋼柱於火害下之行為、受壓強度、破壞模式。進而參考各國規範進行受壓強度計算模式的建立，計算試體的受壓強度並與實驗結果進行比對，建立高強度鋼材箱型鋼柱於高溫下受壓強度之計算模式。
三、重要發現
由實驗結果得知相同溫度下細長比愈大，受壓強度越低；而當鋼柱溫度愈高，鋼柱受壓強度亦愈低。當溫度高於600˚C後溫度對受壓強度的影響較細長比對強度影響大。箱型斷面採全滲透銲接，實驗後觀察銲道並無破壞跡象。試體設計時箱型斷面皆採用塑性設計斷面，但實驗結果顯示四支試體中寬厚比較高之兩支試體於高溫下亦發生局部挫屈，常溫下符合塑性設計斷面之試體於高溫下仍會發生局部挫屈之現象。由試體受壓強度之計算結果發現以AISC (2016)之鋼材高溫機械性質折減係數折減降伏強度與彈性模數後，代入國內鋼結構極限設計法之銲接箱型受壓構材公式計算所得之受壓強度最接近實驗值。
四、主要建議事項
1.建議一
高強度內灌混凝土箱型鋼柱於高溫下抗壓強度之研究：立即可行建議
主辦機關：內政部建築研究所
協辦機關：中華民國鋼結構協會
國內實務上使用之高強度箱型鋼柱，一般亦內灌高強度混凝土，以提高其受壓強度。此種柱構件受火害高溫作用時，鋼材的導熱性高，鋼材強度與勁度將逐漸衰減；然而內灌的混凝土為熱的不良導體，兩者將相互影響柱構件的受壓強度。後續的研究可探討高強度內灌混凝土箱型鋼柱於高溫下的抗壓強度。
2.建議二
高強度銲接箱型鋼柱於高溫下受壓強度之研究：中長期建議
主辦機關：內政部建築研究所
協辦機關：中華民國鋼結構協會
國內外對於高強度銲接箱型鋼柱高溫下受壓強度之研究甚少，本研究使用四支箱型鋼柱進行火害試驗，由此建立計算模式之應用範圍有限，後續之研究可進行不同之溫度與細長比的定溫加載實驗，以建立完整之受壓強度計算模式。

The development and application of high-strength steels have been increased significantly. The high-strength steels are widely used in bridges and medium- and high-rise buildings. The high-strength steel has higher yield strength and tensile strength than mild steel. Therefore, the application of the high-strength steel on high-rise buildings has advantages, especially for columns subjected to heavy load. The behavior of steel columns in fire has been studied considerably. However, most of the study focused on the fire test of the mild steel. The use of the high-strength steel has been increasing, but the research of the compressive strength and behavior of steel columns using the high-strength steel under fire is lack. At elevated temperatures, the yield strength and elastic modulus of the high-strength steel decrease more significant than those of the mild steel. Therefore, this study carries out an experiment of the high-strength steel columns in fire to investigate the compressive strength and behavior under high temperature.
The fire test and compressive strength calculation were carried out. Relevant literature and codes were first collected to understand the parameters affecting the fire resistance and various codes’ methodology. Four large-scale steel box columns using SM570MC high-strength steel were tested in fire to explore their behavior, compressive strength, and failure mode. Calculation model was studied to calculate the compressive strength of the specimens and to compare with test results. The calculation model for the compressive strength of the box column using high-strength steel at high temperatures was established.
The test results showed that the higher the slenderness ratio, the lower the compressive strength and the higher the temperature, the lower the compressive strength. When the temperature is higher than 600˚C, the influence of the temperature on the compressive is more significant rather than slenderness ratio. The complete joint penetration weld used to build-up the box section resulted in no damage on the weld. Although the cross section of the specimen is a plastic design section, local buckling occurred at high temperatures for two specimens with greater width-to-thickness ratio. Based on the calculation of the compressive strength of the specimens, it is found that the most accurate compressive strength at elevated temperature of the box column is calculated by domestic codes using relevant reduction factors of the mechanical properties at elevated temperatures.
For immediate strategy:
The high-strength box columns used in engineering practice are generally filled with high-strength concrete to enhance their compressive strength. The strength and stiffness of the steel are gradually attenuated when the columns are at high temperature. However, the internally filled concrete is a poor conductor of heat. The steel and concrete will affect the strength of the column. Further study should be conducted to explore the compressive strength of the high-strength box column filled with high-strength concrete. 
For long-term strategy:
There is little research on the compressive strength of high-strength steel box-columns at elevated temperature. In this study, only four specimens are used for fire test. The application of the calculation model based on this test result will be limited. Further study is needed to include other temperatures and slenderness ratios and to establish a complete calculation model of the compressive strength used for high-strength box column at elevated temperature.