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
近年來隨著鋼構造建築物高度及跨度的增加，於強度需求更高的情況下多採用高強度鋼材。相較於一般鋼板，高強度鋼板具有高降伏強度及高抗拉強度等優點，更適用於高層鋼構造建築。國內常用之SM570M系列高強度鋼板之降伏強度為420~540 MPa，抗拉強度為570~720 MPa，降伏比低於0.85，具優良塑性變形能力及銲接性。鋼構件常用的螺栓接合之施工技術要求低且品質穩定。高強度螺栓能施加較大之螺栓預拉力，使接合強度更高。鋼構造建築受火後之鋼板以及螺栓會因高溫作用及冷卻方式而導致其殘餘機械性質改變，進而影響建築物火害後之強度。國內常使用之高強度鋼板及螺栓於火害後殘餘機械性質之研究甚為匱乏，故本研究進行高強度鋼板及螺栓火害後之殘餘機械性質之探討。
二、研究方法與過程
研究方法為以實驗將鋼材與螺栓加熱至200℃~1000℃，再以空氣冷卻或水中冷卻後，進行鋼材於火害後之硬度試驗、拉伸試驗、衝擊試驗、金相試驗及螺栓之拉伸試驗。最後建立高強度鋼材及螺栓各項殘餘機械性質相對於常溫之折減係數。
三、重要發現
SM570M系列高強度鋼板之殘餘機械性質於受火溫度為600℃前，皆不因受火溫度及冷卻方式之改變而發生太大變化。受火溫度為800℃以上時，受火溫度及冷卻方式將對SM570M系列高強度鋼板之殘餘機械性質影響甚鉅。以空氣冷卻試體之降伏強度、抗拉強度、里氏硬度皆明顯低於常溫試體，伸長率及韌性明顯提升，彈性模數則不受影響。反之，置於水中冷卻試體之降伏強度、抗拉強度、里氏硬度皆顯著上升，伸長率、韌性、衝擊值及彈性模數則顯著降低。衝擊試驗結果亦顯示SM570M-CHW之衝擊韌性良好，適用於高入熱銲接。SM570M系列鋼板在不同溫度下火害後空氣冷卻的里氏硬度與抗拉強度折減係數幾乎一致，顯示火害後空氣冷卻之SM570M系列鋼板之抗拉強度可以里氏硬度預估。
F10T系列螺栓之降伏強度與抗拉強度在受火溫度500℃前，皆不因受火溫度以及冷卻方式產生明顯變化。在受火溫度500℃至700℃間，無論冷卻方式，F10T系列螺栓之降伏強度與抗拉強度皆明顯低於常溫試體。受火溫度700℃以上時，受火溫度及冷卻方式將對F10T系列高強度螺栓之伏強度與抗拉強度影響甚鉅。以空氣冷卻之試體之降伏強度與抗拉強度明顯低於常溫試體。置於水中冷卻之試體之降伏強度與抗拉強度明顯高於常溫試體。置於水中冷卻之F10T系列螺栓，彈性模數大致與常溫試體相同，並不隨受火溫度變化出現明顯變化趨勢。以空氣冷卻之F10T系列螺栓，在受火溫度700℃前，彈性模數大致與常溫試體相同；但是在受火溫度700℃至900℃之間。
四、主要建議事項
鋼結構建築物火害後非破壞性檢測之研究：短期性建議
主辦機關：內政部建築研究所
鋼構造建築火害後之損傷評估為建築之修復或重建為一大關鍵。傳統之目視檢測法對於建築物之損害程度可能有所誤判，而破壞性檢測法為在火害後之構件上取樣並藉由試驗判斷其損傷程度，雖對於建築物損傷程度之評估較為準確，但將對建築物造成不可逆之破壞並需花費時間及資源。一套有科學根據並可快速檢測建築物損害程度之非破壞性檢測法有其研究之重要性，而目前對於高強度鋼板火害後損傷評估之非破壞性檢測法仍不盡完整，因此須做進一步之探討。

With the increase of the height and span of steel structures in recent years, high-strength steels are more often used when the strength demand is high. Compared with mild steel, the high yield strength and tensile strength of the high-strength steel are more suitable for high-rise buildings. With good plasticity and weldability, SM570M series steel of high-strength steel widely used in our country has yield strength of 420 ~540 MPa and tensile strength of 570~720 MPa, resulting in a yield ratio of lower than 0.85. Bolted connections commonly used in steel members have lower requirement of construction technique and stable quality. The higher pre-tension exerted from the high-strength bolt results in a higher connection strength. The steel plates of steel structures after fire will change their residual mechanical properties due to the effects of high temperature and cooling method, which further affects the strength of steel structures. The research of the residual mechanical properties of the high-strength steel and bolt commonly used in our country is lack; therefore, a series of experiment was conducted to investigate the residual mechanical properties of the high-strength steel and bolt.
This research was conducted experimentally. High-strength steel coupons and bolts were heated to temperatures varying from 200oC to 1000oC in the increments of 100oC and then cooled in air or in water. Further, Leeb hardness test, tension test, Charpy impact test and metallographic test were carried out for high-strength steel coupons and tension test was performed for high-strength bolt. Finally, the reduction factors, relative to room temperature, of the residual mechanical properties of the high-strength steel and bolt were established.
The residual mechanical properties of SM570M series high-strength steel heated under 600oC were not changed significantly due to the temperature and cooling method. When the steel coupons were heated above 800oC, temperature and cooling method would make a significant influence on the residual mechanical properties. For steel coupons cooled in air, the yield strength, tensile strength and Leeb hardness dropped significantly, but elongation and toughness increased apparently, the elastic modulus did not vary. In contrast, for steel coupons cooled in water, the yield strength, tensile strength and Leeb hardness rose significantly, but elongation, toughness, impact energy and elastic modulus decreased. The impact energy of SM570M-CHW steel plates was good and suitable for high-heat input welding. For SM570M series high-strength steel heated and then cooled in air at various temperatures, the reduction factors of Leeb hardness were quite the same as those of corresponding tensile strength. It is revealed that the tensile strength of SM570M series high-strength steel heated and then cooled in air can be estimated by its Leeb hardness.
The yield and tensile strengths of the F10T series high-strength bolts heated under 500oC were not changed significantly due to the temperature and cooling method. When these bolts heated between 500oC and 700oC, their yield and tensile strengths dropped significantly regardless of the cooling method. When these bolts heated above 700oC, temperature and cooling method would make a significant influence on yield and tensile strengths. For bolts cooled in air, the yield and tensile strengths dropped significantly. In contrast, for bolts cooled in water, the yield and tensile strengths rose significantly. The elastic modulus for these bolts cooled in water weren’t changed significantly regardless of variation in temperature. When these bolt heated under 700oC and cooled in air, the elastic modulus were not changed significantly. However, the elastic modulus dropped significantly when these bolts heated between 700oC and 900oC and then cooled in air.
For short-term strategy: Research on non-destructive inspection of steel structural building after fire
The damage assessment of steel structures after fire is a key to repair or reconstruct the buildings. The traditional visual inspection method may misjudge the damage degree of the building, while the destructive inspection method is to take samples from the members after fire and judge the damage degree through tests. Although the destructive inspection is more accurate to evaluate the damage degree of the building; however, it will cause irreversible damage to the building and require time and resources. A set of scientifically based non-destructive testing methods that can quickly detect damage to buildings is of great importance. However, the current non-destructive testing methods for assessing fire damage to high-strength steel plates are still incomplete, and further investigatioin is needed.