Membrane element only has the in-plane stiffness. If the periods are very large that means some membrane type slab are not supported by beams or columns which have bending stiffness in the out-of-plane direction of the slab.
Shell element has both in-plane and out-plane stiffness. So the period value becomes reasonable.
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只看楼主 我来说两句我估计是因为：
1.楼主做的膜楼板模型，没有定义diaphragm.就是常说的：无限刚性楼板假定
2.如果用壳来模拟楼板，由于存在面外刚度，相当于有存在一定的数值的刚度的楼板，所以周期会改变。
建议在做周期以及位移比验算是，作无限刚性假定。在计算楼板局部开大洞时再取消。
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Shell element has both in-plane and out-plane stiffness. So the period value becomes reasonable.
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