I solved a implicit dynamic problem for a solid with length = 2m, width =2 m depth =1m, density = 0.5, Young’s Modulus = 1 E+7, poission ratio = 0 and apply the triangle impulse loading(time,loading) {(0,0);(0.01,100);(0.02,0);(100,0)}.Fix time_step =0.0001s to running. Using the same boundary condition for one element Q4 linear in 2D model(CPS4) and for one element H8 linear 3D(C3D8) (just apply the forces in xy plane), set up the poission ratio = 0. But compare the stresses in 2D model and 3D model, the results are very difference. The result in 3D model look like have a poision ratio affect (although the poission ratio = 0), so it still has stress components in z direction. Why have difference between 2D and 3D model like this? Do the results run in 3D model in this case not enough exactly? How can i neglect the Reaction forces in z direction in 3D model and make the result in 3D the same in 2D model?