Nope, not the child theme directory...look in here...
../wp-content/themes/thematic/library/layouts/
In there you'll see some stylesheets with your different layout options. You can include any of these stylesheets into your child theme. In your child theme stylesheet.css file, at the very top, you will see this:
/* Reset browser defaults */
@import url('../thematic/library/styles/reset.css');
/* Apply basic typography styles */
@import url('../thematic/library/styles/typography.css');
/* Apply a basic layout */
@import url('../thematic/library/layouts/2c-r-fixed.css');
/* Apply basic image styles */
@import url('../thematic/library/styles/images.css');
/* Apply default theme styles and colors */
/* It's better to actually copy over default.css into this file (or link to a copy in your child theme) if you're going to do anything outrageous */
@import url('../thematic/library/styles/default.css');
/* Prepare theme for plugins */
@import url('../thematic/library/styles/plugins.css');
See the call up there that looks like this...?
/* Apply a basic layout */
@import url('../thematic/library/layouts/2c-r-fixed.css');
You can change this to call a different layout stylesheet. Just change
2c-r-fixed.css
to reference a different sheet.