局部火灾下基于网架热力效应解耦方法的杆件约束刚度求解

Calculation of restrained stiffness of chords for double-layers grid structures subjected to localized fire based on decoupling of thermal & mechanical interaction

局部火灾下,网架结构整体升温呈非均匀状态,由于各杆件轴向的温度膨胀变形不同,导致了杆件间的轴向约束效应,这种轴向约束使网架结构杆件受火屈曲后仍具有一定的承载力.定量确定网架结构杆件间的相互约束是正确评价杆件抗火承载力的重要条件.分别将上、下弦杆层连续化,参考拟夹层板法,基于力平衡方程和位移协调方程,分别得到非均匀温度场中考虑温度膨胀效应和静力荷载效应的网架上、下弦杆层的面内位移及竖向挠度解析解.同时采用数值分析方法,分别得到非均匀温度场中,在温度、杆件间相互约束和静力荷载耦合作用下的网架上、下弦杆层面内及竖向位移数值解以及杆件间相互约束效应产生的附加温度内力.上述位移解析解与数值解之差值即为网架各杆件在约束效应下所产生的轴向变形,该轴向变形量与附加温度内力之比,即为杆件的轴向约束刚度.这种轴向约束刚度计算方法有效地将杆件由于温度膨胀受到约束产生的轴向变形量从耦合效应中剥离出来,从而较准确地得出升温历程中杆件间的瞬态约束刚度.

The temperature of members in double-layer grid structures under localized fire is non-uniform.The reason that causes the constraint effect of the members is the difference of axial temperature expansion of each member.Steel structure members after buckling under fire still has certain bearing capacity because of axial constraint.Determining the mutual restraints of members is an important initial condition to evaluate the fire resistance of members.Based on the sandwich plate analogy method,the upper and lower chord layer is studied in this paper to get in-plane displacement and vertical deflection analytical solution in the non-uniform field,which considers temperature effect and static load effect.Numerical analysis method can get in-plane displacement and vertical deflection analytical solution which considers temperature effect,static load effect and confinement effect,and can also get additional temperature internal force caused by the confinement effect.The difference between numerical solution and analytical solution is the axial deformation caused by the confinement effect.The ratio of the additional temperature internal force to the axial deformation makes axial restraint stiffness of members.This calculation method is effective inremoving the axial deformation caused by the constraint from coupling effect,and thus can accurately obtain the transient constraint stiffness of the member in the heating process.