Bookcase Shelf Thickness vs. Span
Whether you construct or purchase a bookcase, understanding the relationship between shelf thickness and span helps you determine how much weight each shelf can support. This, in turn, gives valuable information on the bookcase dimensions necessary to hold all your favorite books without fear that any of the shelves might break under the weight of your library.
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Thickness
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Thickness refers to the thickness of each individual plank used as a shelf. As the thickness increases, the shelf can support more weight since the weight of each book distributes over a greater amount of mass within the shelf. Consider using shelves with considerable thickness if you have larger, hard cover books such as dictionaries, encyclopedias and other tomes. Similarly, smaller paperback books require less thickness to support their weight.
Span
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Span refers to the length of each bookcase shelf and directly relates to how much weight each shelf can support. Pegs or some other holding mechanism anchor the ends of each shelf. This holding mechanism gives each shelf additional support to hold weight, relieving pressure applied directly onto the shelf. A shelf with a short span holds less books due to the limited space, and thus requires both the holding mechanism and the shelf to distribute less weight. A shelf with a long span, on the other hand, holds more weight, which puts additional pressure onto the shelf and holding mechanism.
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Testing and Materials
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Test your bookcase shelf thickness and span before purchasing or constructing the shelf. Place several of your heaviest books on the shelf. If constructing the shelf yourself, place the shelf between two tables or chairs that hold the shelf at both ends. Test different shelf materials as well. For example, some woods have a denser composition than others, which allows you to place more weight on the shelf without fear of breaking. Other materials include metal and plastic.
Deflection
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Measure deflection when placing your books on the shelf. Deflection is the amount of bending that occurs on each shelf after you apply weight. Use a tape measure or ruler to determine the amount of deflection while placing books on the shelf. Stop when reaching a deflection of 1/8 inch. Anything more than this indicates significant pressure on the shelf, which may result in a collapse. Alter the books by removing heavy ones and replacing with lighter books.
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References
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