G'Day Steve.
You're going to get all sorts of advice with a question like that... so lets start.
Without knowing your soil properties and profiles, I'll talk in general principles and explain what I did.
Generally, bigger and deeper is better, but my advice is to keep the top of the footing small (where the pier connects to the footing). My reasoning for this is that just as you don't want to be swinging off the pier itself while you are imaging, you don't want to be changing load on the footing either. If it's big and deep enough it shouldn't matter, but if its not... the disappointment will be palpable!
Isolate the pier footing from the slab or floor of the obs for the same reason.
If you have clay soils the size and depth matters more due to movement. In fact in some cases you may never be able to stop the footing moving (deep clay) but a large footing will at least be stable in the short term, and you should only have to re-polar align the mount periodically.
Here's what I did. I have clay soil. So slippery when wet that clutch starting a car or bike is not possible, and fence strainer posts have spun in their holes when the fence has been impacted or re-tensioned. Having said that we don't get a lot of movement - no cracks in the house, jamming doors etc. I found the reason for this is the ground is shallow.
I planned to make my footing 350 x 350 and 1 metre deep. I didn't get there. At about 750mm down I hit rock. I keyed into the rock about 50mm (that's all I was prepared to go!
) and poured the footing. Perfect!
I have a 100mm concrete slab around the footing with 10mm foam concrete expansion joining isolating the slab from the footing.
I hope this helps your decisions.
Oh, the top plate of the pier is where you mount the mount to. The bolts are so you can level the mount. Just a tip: the bolts are (usually) the greatest source of flexibility in the footing, pier mount system so keep them adjusted as short as reasonably practical.
Al.
