火灾下钢-混凝土组合梁内力变化的试验研究

实际工程中，钢-混凝土组合梁大多具有轴向约束，从而使得组合梁在大变形时产生悬链线效应来继续承担外部荷载。为研究火灾下钢-混凝土组合梁的悬链线效应产生机制，开展了2块足尺钢-混凝土组合梁的火灾试验，测试组合梁截面内力随受火时间的变化过程。介绍了试件设计、加载方案和测量内容，描述了相关试验现象及破坏特征，得出以下主要结论:升温时，组合梁截面温度梯度分布变化较大且易产生附加弯矩，对组合梁极限承载能力产生不利影响；火灾下组合梁的破坏模式主要表现为梁端负弯矩处形成明显的塑性铰，同时板底钢梁的受拉下翼缘出现整体侧向失稳，这与常见单个钢梁的受压翼缘屈曲而整体失稳的现象相悖；荷载比是影响组合梁抗火性能重要的参数之一；通过实测得到火灾发生时组合梁内力的变化过程，分析出在同等条件下的相同时间内，荷载值越大组合梁悬链线效应越明显。

EXPERIMENTAL STUDY ON INTERNAL FORCE VARIATION OF STEEL-CONCRETE COMPOSITE BEAM UNDER FIRE

In practical engineering, most steel-concrete composite beams have axial constraints, which cause the composite beam to produce catenary action to continue bearing more external loads at large deformations. To study the mechanism of catenary action of steel-concrete composite beam under fire, two full-scale tests of steel-concrete composite beams were carried out to obtain the internal force variations with the time. The test designation, loading scheme and measurement contents are presented in this paper. The experimental phenomena and failure characteristics are also described. The test results show that the temperature gradient distribution of the composite beam section changes greatly under fire, and so it is easy to generate additional bending moment, which has an adverse effect on the ultimate bearing capacity of the composite beam. The failure mode of the composite beam is mainly the formation of obvious plastic hinge at the negative bending moment area at the beam ends, and the overall lateral instability occurs on the tensioned lower flange of the steel beam at the bottom of the beam, which is contrary to the common phenomenon that overall unstability of a steel beam is usually attributed to the compression flange buckling of a steel beam. The load ratio is one of the important parameters affecting the fire resistance of the composite beam. Through the actual measurement, the changing process of the internal force of the composite beam in fire is obtained. It is found that a larger load value leads to more obvious catenary action of the composite beam at the same time under the same conditions.