Fixed fluid

It doesn't really work the same way, but you do have a lot more control over the layout than before. On the theme configuration page, go into the Layout and General Settings section, under Standard Layout. Under Set the Page Width, you can make it either fixed or fluid by setting it to a fixed pixel width or setting it as a percentage. There are also the settings/media queries for tablets and smartphones so you'll need to handle your layout appropriately for those too.

I'd recommend looking at some of the AdaptiveTheme help documentation since that is all handled by the base theme.