Wat i did is, i go to 'view'--> 'element 3d view'--> 3d cutting place' draw a line trough the land, then activate '3d cut way'. I can see the height of my land is dropped in 'Generic perspective' but not in the 'elevations'. what steps am i missing?
PS: i heard that i have to go 'design'-->'story settings' to change some settings, if so, what kind of settings i have to change there?
Anyone can give me a help?
I do this all the time. It is alot easier to model the mesh when the elevation figures you enter are the actual levels, no need for doing maths in my head.. 
The way I get around this problem is follows:
1. Build the mesh as you have done using actual levels.
2. Now trace the mesh with a 100mm thick slab that is positioned about a metre below the lowest point on the mesh. Make the slab the same material and section fill as the mesh you are using.
3. Use Solid Element Operations to subtract with downwards extrusion using the slab as an operator and the mesh as the target.
This method will cut off the bottom of the mesh just like the 3D cutting plane, but will show correctly in both 3D and sections.
The way I get around this problem is follows:
1. Build the mesh as you have done using actual levels.
2. Now trace the mesh with a 100mm thick slab that is positioned about a metre below the lowest point on the mesh. Make the slab the same material and section fill as the mesh you are using.
3. Use Solid Element Operations to subtract with downwards extrusion using the slab as an operator and the mesh as the target.
This method will cut off the bottom of the mesh just like the 3D cutting plane, but will show correctly in both 3D and sections.
Matthew is quite right you can simply insert a negative number in the dialog box (see picture) this will chop off some of the bottom.
If the land mesh is placed on the same storey as your floor slab you should position it's height releative so to place the SL=0 of the mesh at 0.
In your case if your storey is set to match a floor slab at 173m. The mesh should be placed at -173000 and you could put -150000 in the box I mentioned. This would place the mesh in the correct relative position with the bottom 150m missing.
If the land mesh is placed on the same storey as your floor slab you should position it's height releative so to place the SL=0 of the mesh at 0.
In your case if your storey is set to match a floor slab at 173m. The mesh should be placed at -173000 and you could put -150000 in the box I mentioned. This would place the mesh in the correct relative position with the bottom 150m missing.
Brewer wrote:This is a neat trick also but not quite what I meant. If you set one of the two reference levels to sea level (relative to project zero - see attached) then you can simply adjust the mesh elevations relative to that reference level.
Matthew is quite right you can simply insert a negative number in the dialog box (see picture) this will chop off some of the bottom.
If the land mesh is placed on the same storey as your floor slab you should position it's height releative so to place the SL=0 of the mesh at 0.
In your case if your storey is set to match a floor slab at 173m. The mesh should be placed at -173000 and you could put -150000 in the box I mentioned. This would place the mesh in the correct relative position with the bottom 150m missing.
This saves having to do the somewhat counterintuitive negative thickness and eliminates the need for a sea level story height. In fact it separates the whole issue from story settings altogether.
Here is the mesh settings dialog showing the height to sea level option.
