Thanks.
I think that works for display them seperately, but usually you use them together, what I mean is even you have left, top and bottom menu, they actually work together, when you select one item from bottom menu means you don't select any from top or left menu. So I think it is better to put them together. I created my sitemap like this:
<siteMap enableLocalization="true" >
<siteMapNode url="/root">
<siteMapNode url="/main" title="category_home">
<siteMapNode title="Home" description="Homepage" resourceid="91C9EE4D-9343-4F2D-8C8C-42D65B3F631A" roles="Admins"/>
<siteMapNode title="Courses" description="" roles="Admins" />
</siteMapNode>
<siteMapNode url="/header" title="category_header">
<siteMapNode title="Student Area" description="" roles="Admins" />
</siteMapNode>
</siteMapNode>
</siteMap>
some node don't look right, but I am using my own sitemapprovider to parse them, so I can do some processing there. I use the two nodes "/main" and "/header" to seperate the menu to two sections.