受火约束钢梁在升温段和降温段行为的理论分析(Ⅰ)

真实火灾中的调查和试验结果表明,约束钢梁比一般的无约束钢梁有更好的抗火承载性能。在梁端约束作用下,火灾中的约束钢梁能够产生很大的变形而且不会在瞬间坍塌。在梁中产生的轴力对梁在火灾中的行为有很大影响。影响约束钢梁行为的因素包括:荷载类型、轴向约束刚度、转动约束刚度、梁截面温度分布等。根据梁端约束情况和荷载类型,对梁的变形曲线进行了假设,据此可得出梁的轴力以及梁截面内应变与应力的分布。通过对应力的积分,可得出截面轴力及抵抗弯矩,然后结合梁的平衡方程,就可求出梁在火灾下的变形。为了进行火灾后的安全评估及修复工作,还需要了解钢梁在降温段的反应。当温度降低时,钢材的强度会得到恢复,这对恢复梁的承载能力是有利的,但是同时由于梁的收缩,梁内将进一步产生拉力,这对梁及两端的节点是很不利的。本文对约束钢梁在降温段的行为进行了比较深入的理论研究,并推荐了一种简单的计算方法。

Analysis of Restrained Steel Beams Subjected to Temperature Increasing and Descending Part 1:Theory

The facts demonstrated in the experiments and the real fire hazards showed that the restrained steel beams have better fire-resistant capability than the simply-supported beams. Under the effect of the restraints, beams in fire condition can undergo very large deflection and the sudden crash can be avoided. In addition, the axial force will be induced and play an important role in the behaviour of the restrained beams. The factors influncing the behavior of restrained beams subjected to fire include the loads on the beam, the stiffness of axial and rotational restraints and the temperature distribution in the cross-section of beam, etc. Based on the assumptions made regarding the deflection curve of beam according to the restraints and the load type, the axial force and the strain and stress distributions in the beam can be determined. By integrating the stresses, the combined moment and force in the cross-section of the beam can be obtained. Then, through subtituting the moment and axial force into the equilibrium equation, the behavior of the restrained beams in fire can be worked out. In addition, for the safety estimation and repairing after a fire, the behaviour of restrained beams subjected to temperature descending needs studying. For the restrained beams experiencing very high temperatures, the strength of steel will recover when temperature descends, but the contract force will aggravate the stress in the beam, especially at the end. In this paper, the behaviour of the restrained beam in cooling phase is analyzed, and the effect of the contract force is discussed.