Fixing spotted gum deck

[airmech953]

Hi folks, I fixed a spotted gum deck (80x19 boards) direct to a steel frame using galvanised 10ga decking screws, 2 per joist. My problem is that the lateral expansion of the timber shears one of the screws at each joist. I have sourced some 12ga climagard screws but I am concerned that they will break too.
What I am now considering is sourcing rectangular speed nuts and fixing one screw at each joist with a speednut through an oversize hole in the joist, allowing a bit of lateral float as the board expands and contracts.
Has anyone else had a similar problem and what solutions did you come up with?

[TermiMonster]

It may be too late for you, but I would fix a batten to the steel first, then screw the decking to that.
TM

[airmech953]

Yes, it's too late. The decking is all fixed to the steel. Even so wouldn't the movement of the decking make the screws loose in a timber batten given that spotted gum expands about 7% across the grain?