Robert Falk, a structural engineer with the U.S. Forest Products Laboratory in Madison, Wisconsin, realized this while researching a deck-building manual five years ago. Falk had heard about the death of the woman in the deck collapse near Kalamazoo, and he wanted to find out the reasons for the failure. Using a database to search five years of newspaper articles from around the country, he found that nearly every collapsed deck had been attached with nails, rather than bolts, and that investigators had pinpointed the nails as the cause of collapse. In practice, it's a different story. As people gather on a deck, their weight and movement translate not just into a downward force but also into an outward force that acts as a lever prying the deck away from the house.
Nails work well to resist the downward force but are no match for the outward force.
Held in place only by the friction of bent wood fibers, nails tend to loosen when wood alternately shrinks and swells with changes in moisture content and temperature. Once nails loosen, they offer even less resistance to the prying forces of a crowd. When they pull out, they pull out. A screwed-in connector behaves differently. It gains increased frictional strength from the wedging action of wood fibers along the entire length of the shaft.