Observations and results
Were the eggshells able to support the hardcover book and several magazines?
The eggshells likely supported a surprisingly heavy load. You may have seen them hold up a small hardcover book and a stack of more than a dozen magazines before all of the eggs cracked and were crushed under the mass. (If any eggshell had a hairline crack or a big chip, however, the eggshells may have only held the book and a couple of magazines before the weakened eggshell cracked and then the others were crushed as well.) In a building the shape of the arch distributes the forces to weight-bearing piers that support the arch, and this is similar for the arch of the eggshells as well. This in turn eliminates some tension stress in the structure. Overall, the arch's shape allows it to support a relatively large amount of mass being placed on top of it.
When you are done with your tests, thoroughly clean any surface the raw eggs (including the shells and your hands) touched with soap and water because they can carry salmonella.
More to explore
The Arch Bridge, from The Science Museum and Peter Symonds College
Egg and Embryo Development, from EnchantedLearning.com
Fun, Science Activities for You and Your Family, from Science Buddies
Fallen Arches: The Surprising Strength of Eggshells, from Science Buddies