Syd,
I haven't a clue what all that means because i haven't actually read all the standards you refer do [certainly not in a while - 1995?].
If all these standards seem to be saying "if a structural engineer says its okay, its okay" I'm not so certain that it is. Its possible to prove something can theoretically can carry a load, but it can still fail a stability test.
Take note that I am not an engineer so take all that I say about structures with that one on board.
But in relation to 300mm cavity walls 100/100/100 I was speaking to an engineer about this only on Thursday of last week.
I had posed the question in a theoretical way stating that the ties always acted in tension as they had a vary narrow section and therefore negligible compressive strength and I was wondering how they contributed to the strength of the cavity wall.
He told me that empirical tests have been undertaken in relation to a 100mm inner leaf where this was loaded for a normal two storey house load, with and without an outer leaf bonded to it with wall ties.
The load bearing capacity was not the issue. i.e. the crushing strength of the masonry was adequate. The failure of the leaf occurred at higher loads due to the buckling of the wall due to lack of lateral restraint. The wall ties, even acting in tension alone, provided sufficient lateral restraint so that the inner leaf could take a higher load.
Now apply this - if we can - to the present case.
I suspect that widening the cavity but still with relatively thin ties may lessen the lateral restraint available. I think this might need a test to determine it fully. I think either you'll end up using something like a 200mm multi-beam purlin vertically, or your ties will end up being 6mm s/s sections with a twist for a cavity drip and splayed ends for building in, much like the old galvanised pressed metal ties.
AND WATCH THOSE CAPS!!!
ONQ.