问题描述:
基于对象组定义截面切割并选择【内力类型】为设计内力时,【内力输出位置】中的顶面和底面、右端和左端应该如何选择呢?
解答:
首先,无论选择分析内力还是设计内力,用于定义截面切割的对象组必须同时包含单元和节点。单元用于确定隔离体,节点用于确定切割面,更多内容详见《基于对象组的截面切割》。在此基础上,截面内力的参考坐标系与内力类型相关,具体如下:
对于分析内力,默认的参考坐标系即用户在【坐标系】下拉列表中选择的 GLOBAL 或其它自定义坐标系。当然,用户也可以根据需要,利用三个转角或高级局部轴定义非默认的参考坐标系。
对于设计内力,墙的局部 1 轴平行于 Z 轴,局部 2-3 平面位于 XY 平面内;连梁和板的局部 2 轴平行于 Z 轴,局部 1-3 平面位于 XY 平面内。上述定义便于用户以构件内力的方式输出截面内力,例如:沿 1 轴的截面内力为轴力 P,绕 3 轴的截面合力矩为弯矩 M3。当然,如果墙、连梁或板的布置方向与全局 X 轴存在一定的夹角,用户也可以通过局部 1 轴或局部 2 轴的转角进行调整。
在确定了设计内力的参考坐标系之后,【内力输出位置】中的顶面(墙)或右端(连梁和板)对应局部 1 轴的正方向,底面和左端则对应局部 1 轴的负方向。换言之,如果用户选择顶面或右端,程序将基于上述参考坐标系输出内力;如果选择底面或左端,程序将基于上述坐标系的 反 方 向 输出内力(数值等大异号)。
以下图所示 XZ 平面内的连梁为例,基于当前对象组(即虚线和“X”标识的单元和节点)定义截面切割并选择“连梁”设计内力。其中,局部 1 轴平行于 X 轴,局部 2 轴平行于 Z 轴,局部 3 轴指向平面外(即 Y 轴负方向,请读者根据右手螺旋法则自行判断)。
针对以上模型分别定义左端(ElemL)右端(ElemR)两个截面切割,内力结果如下表所示。可以看到,ElemL 和 ElemR 中的三个非零内力值(P,V2,M3) 等 大 异 号。如果针对墙肢定义顶部和底部的截面切割,内力值同样存在“等大异号”的特点,此处不再赘述。
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)
综上,内力输出位置的改变仅仅影响结果的表达方式(严格来讲是改变数值的正负号)。为了便于用户理解或应用截面内力,建议根据对象组中节点与单元的相对位置选择“左右”或“上下”。具体来讲,以参考坐标系为基准,如果节点位于单元的 1 轴正方向,请选择顶部或右端;反之,请选择底部或左端。
同样以上述连梁为例,由于对象组中的节点位于单元的 1 轴负方向,故左端(ElemL)输出的内力值符号与材料力学的规定完全一致,即:轴力-拉为正压为负;弯矩-下部受拉为正,上部受拉为负;剪力-顺时针错动为正,逆时针错动为负。