Fixing spotted gum deck

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?

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

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?