>>9What are you gonna do, use a medium size dog?
A real shelf must handle being filled fully
http://www.ehow.com/list_7607477_requirements-weight-limits-shelves.html A standard shelf measurement of 3-foot by-1-inch-by-12-inch should bear the following weights: 313 pounds for oak shelving, 200 pounds for pine, 129 pounds for plywood, and 87 pounds for MDF, This Old House advises. Normal sagging is 1/4 inch, which increases by another 1/8 inch over time.
A 4-by-6-foot plywood shelf mounted to ceiling framing supports about 160 pounds