火災時における鋼梁の塑性変形と構造耐火性能: 架構内に配された鋼梁の熱応力減衰能力と素材歪硬化の関係

摘要

鋼構造耐火分野においては、今までに多くの研究や設計指針で銅rn部材の終局強度の定量化が進められ、それらは部材と構造全体の耐rn火性能評価に活用されている。一方、鋼部材の高温塑性変形に関しrnては、火災時の熱応力の減衰という極めて重要な力学的役割があるrnにも関わらず、高温終局強度のような直接的、且つ、分かり易い構rn造耐火性能上の評価指標では無いがために、その重要性が影に隠れrnてしまい、それゆえ鋼部材や鋼架構の塑性変形と構造耐火性能の関係に対する詳細な議論・検討はあまりされてこなかった。%It has been known that plastic deformation of steel members causes thermal stress attenuation in a steel frame at fire. Although existing design guidelines consider this thermal attenuation behavior, design equations for estimating the ultimate strength of the frame do not directly consider the thermal stresses in the members. This is introduced based on a fact that the thermal stresses do not affect the ultimate strength due to the thermal stress attenuation. It should be noted, however, that this condition is only possible when the members have large plastic deformation capacity. To have better understanding of this thermal attenuation behavior, an investigation has been conducted in this paper on the structural fire resistance of a statically indeterminate steel beam at elevated temperature. Since the plastic deformation capacity is related in part to the strain hardening of steel, a relationship between the strain hardening and thermal stress attenuation is emphasized. Based on both theoretical and numerical analyses of the steel beam subjected to the thermal stresses, this paper clarifies that the strain hardening of steel plays an important role in attenuating the thermal stress; therefore the strain hardening contributes directly to improving the structural fire resistance.