Animation with a liquid layout

Hey people.

I’m doing this site (swf is attached) and as you can see the header, footer and the menus are liquid, the width changes depending of the size of the window.

My problem is: How can I animate that 3 components if they are liquid ? After the loading of the website I’d like to animate the bars growing up from the right to the left, how can I do that if I don’t know the widths ?

Tks in advance

PS: Sorry for my bad english.