Just want to make sure I'm on the right track.

I'm using $\displaystyle y=ax^2$ where a<0 for the bridge.

Data (arbitrary units):

Height 90

Length 500

$\displaystyle 90=a(\frac{500}{2})^2$

$\displaystyle -a=0.00144$

$\displaystyle \therefore y=-(0.00144)x^2$

Or is there a better way of doing this?