Support conditions for uplift only

Hi all, 

I'm new to structural analysis and STAAD, and am trying to model a temporary structure which doesn't have fixed supports, ie. It's freely sat on the ground so can overturn under enough load.

I'm trying to figure how best to approach the support conditions, for overturning I need supports which allow vertical uplift but can't release vertical movement because then the model has no support at all. Ideally, I would release vertical moment upwards (ie. Away from ground) but not downwards, as the ground provides resistance.

If anyone could offer any advice on how best to go about this, it would be much appreciated, thanks in advance