Opening a frameset with different contents

I'm currently creating a website with 5 main sections. The main design of the site is 3 frames, a menu at the top, a border in the middle and a contents frame underneath.

The opening splash page isn't part of the frameset, and has 5 buttons linking to the overviews of the 5 sections. I need the splash screen to load the 5 overviews of the sections into the content frame.

I've created a function to load the specific frames into the frameset but I do not know how to load the function into the frameset as the buttons target the frameset as opposed to the function.