Firstly I'm not trained in this, but if I were making this my thoughts would be....
I would have to considor the weakpoints, where it can come crashing down.
I would be beefing up the squared thickness of those legs. Also I'd be doing to research on re-enforcing those corner joins. I'd add a corner bracket to ensure that those joins stay rigid and together (http://www.churchantiques.com/photos...ble_medium.jpg
Usually I look for inspiration from other similar items, such as various large tables, and load bearing joins in my house.
I'd also be ensuring it remains symetrical and adding another two evenly spaced tortionals to share the bracing load.
This software is for making a table, but it might help with the design and have some tips of building strong load bearing tables: http://www.knightlite.com/creative/
Surely there are some calculation references online to ensure certain thickness and strength of materials are used.