what's wrong with my OPENGL orthogonal view

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OpenGL

I drawed a house, many pillars(auxsolidbox, height is 0 to 18.6meter), and two floors(auxsolidbox, height is 6.55m, 12.6m). It's okay in glOrtho view, and I can change the camera's position using KEY_DOWN.
I am puzzled with one thing : when I look down at the Ortho space(-37,37,-20,20, 1,500) at the (0,8.5,0)position, I can't see the pillars. I thought the pillars are hide by the floor. If I change the Ortho space(-37,37,-20,20, -500,500), the pillars appears. and If I change the Ortho space(-37,37,-20,20,1,4) at the (0,5.5,0), pillars gone again.
Help me please, I am so puzzled.:-(

I thought the Ortho space would cut the objects whose part in the space, and ignore the objects outside the space. But it seems that is wrong. and I don't know why.

here are some code:


glViewport(0, 0, (GLsizei) cx, (GLsizei) cy);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
if(m_Width<=m_Height)
glOrtho(-nRange,nRange,-nRange*m_Height/m_Width,nRange*m_Height/m_Width,fnear,ffar);
else
glOrtho(-nRange*m_Width/m_Height,nRange*m_Width/m_Height,-nRange,nRange,fnear,ffar);

glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
gluLookAt(m_Camera.ex, m_Camera.ey, m_Camera.ez, m_Camera.cx, m_Camera.cy, m_Camera.cz,
m_Camera.ux, m_Camera.uy, m_Camera.uz);
DrawPillarandfloor();