基于梁铰机制的柱端弯矩增强措施研究

Study on the moment amplification measures of column ends based on the beam sidesway mechanism

汶川地震中钢筋混凝土框架结构塑性铰普遍出现在柱端,该震害现象促使我国抗震规范在2010版提高了"强柱弱梁"措施。新的柱端弯矩增大系数取值对一、二抗震等级而言与已有研究结果相近,但低烈度区、三级抗震的取值仍缺乏依据。该文按现行中国规范设计5个位于Ⅱ类场地的不同设防烈度分区、不同抗震等级的规则钢筋混凝土平面框架,讨论现浇板钢筋、梁下部纵筋等对"强柱弱梁"措施的影响规律。以强震下除底层柱下端外其他柱截面纵筋不屈服的梁铰机制为原则建立非线性计算模型,对每个框架分别输入30条符合要求的地震波,在OpenSees平台上对5个框架进行罕遇地震下的非线性反应分析,并对柱端弯矩增大系数需求进行统计分析。结果表明,对于7度区三级抗震框架的柱端弯矩增大系数,抗震规范取1.3明显不足,建议提高其取值,并同时附加将中间节点处绝对值较小的梁端负弯矩取为零的计算原则;8度区二级框架、9度区一级框架的柱端弯矩增大系数需求的统计结果较规范取值偏大得相对较少。基于梁端实际配筋和材料强度标准值的柱端弯矩增大系数需求的统计特征值离散性更小、沿楼层变化不大,同时其已包含板钢筋的贡献,是相对更好的"强柱弱梁"措施形式,此时抗震规范的一、二抗震等级的弯矩增大系数仍有待提高。

Due to the damage mechanism of reinforced concrete moment-resisting frames that plastic hinges developed mainly at columns ends in 5 ·12 Wenchuan Earthquake, the strong column and weak beam measures are strengthened in 2010 Chinese seismic design code. The new values of MAFC, namely the moment amplification factor of column ends, are similar to the suggested values in former researches for the 1st and the 2nd anti-seismic grades. However, the values of MAFC for the 3rd anti-seismic grade of lower earthquake intensity region are insufficient. In this paper, five regular reinforced concrete plane frames located at Ⅱ-type site with different earthquake intensity regions and different anti- seismic grades were designed according to Chinese code. The influence of several factors on strong column and weak beam measures was analyzed, such as the reinforcing bars in the site-casted slab, bottom longitudinal bars of beam, et al. Nonlinear analysis model is established with the principle of beam sidesway mechanism, which implies that stress of longitudinal bars of columns end sections is below yield stress under strong earthquake excitation with the exception of lower end section of the first floor. For every frame, 30 earthquake waves are selected, nonlinear dynamic responses under rare earthquake excitation are analyzed based on the OpenSees framework, and statistical analysis of the MAFC demand are carried out. The results show that MAFC of 1.3 for 7 intensity region 3rd anti-seismic grade in Chinese seismic design code is obviously small. The value should be increased and additional calculation principle of taking negative bending moment of beam ends with smaller absolute value to be zero at interior joints is suggested. The statistical values of the demand of MAFC for 8 intensity region 2nd anti-seismic grade and 9 intensity region 1 st anti- seismic grade are a little bigger than the corresponding values in Chinese seismic design code. With respect to the demand of MAFC based on practical longitudinal bars of beam and nominal strength, the discreteness and the variety of statistical characteristic value along the height are comparatively small. The contribution of the reinforcing bar of slab is considered, so it is a relatively better expression of strong column and weak beam measures. For these cases, MAFC for the 1 st and the 2nd anti-seismic grades in Chinese seismic design code needs to be increased.