We just built a prototype dual axis tracker out of 50 x 50 x 6mm aluminium. Its been up and running for about a week, producing about 2.5 kW per day here (NE Victoria, Australia), from 2 Sunpower 210W panels, through an Apollo Solar mppt.
The same setup produced about 1.6 kW when the panels were fixed. (Some late afternoon shading problems, so performance would be improved in full sun all day)
Some strong wind gusts recently caused the frame to flex, but not excessively. A tracker of this design is like a sail, really. Even though it's built as solid as is feasable, there will be a wind strong enough to blow it over one day, I'm sure.
Our design specs suggest that increasing the area supported by the tracker will result in the need for disproportionately stronger frames, bearings and mountings, and that there is a point where increasing system output is best achieved by increasing the number of trackers.
In answer to your question about engineers certificates, I think such a certificate would be dependant upon the maximum expected windspeed in your area. The amount of force applied to various parts of the assembly can be calculated using Betz Limits.
If anybody is interested, I can post some pics and a diagram. Our tracker frame accepts three panels, however we are intending to use the center section to mount 100 x 100 mm mirrors, for a heat exchange system.
A closing comment... solar panels are very expensive, even with the right running gear. A tracker/mppt combination can reduce your panel requirement considerably, but this must be weighed against the risk of panel damage caused by big winds, same as with wind turbines.
"Make it run, make it right, make it fast." - Donald Knuth
