Bracing for 2nd floor laundry?

[Jim Franklin <jamesf@groupwbench.org>]

I have a 13'6 span of 2x8 16" o.c. floor, covered with one layer of  
5/8 Advantek, onto which I've placed the W&D, and being right in the  
center of this marginal span they shake the bejeezus out of the  
entire house. Has anyone overcome this by using extra subflooring,  
sprung pedestals, etc?

thanks,
jim