Well, I've done some digging too, and I stand by my initial position; smooth shank nails driven into green wood LOOSE their holding power as the wood dries, everything else being equal.
Here's a web page from your US Department of Agriculture, Forestry Service, Forestry Products Laboratory:
Download chapter 8 of their "Wood Handbook" and on page 3 of Chapter 8 it says:
Effect of Seasoning
With practically all species, nails driven into green wood and pulled before any seasoning takes place offer about the same withdrawal resistance as nails driven into seasoned wood and pulled soon after driving. However, if common smooth-shank nails are driven into green wood that is allowed to season, or into seasoned wood that is subject to cycles of wetting and drying before the nails are pulled, they lose a major part of their initial withdrawal resistance. The withdrawal resistance for nails that are driven into wood that is subjected to changes in moisture content may be as low as 25% of the value for nails tested soon after driving. On the other hand, if the wood fibers deteriorate or the nail corrodes under some conditions of moisture variation and time, withdrawal resistance is eratic; resistance may be regained or increased over the immediate withdrawal resistance. However, such sustained performance should not be relied on in the design of a nailed joint.
In addition, look at this report from the Virginia Polytechnic Institute dated July, 1964. It claims that the style of nail used has an important influence on withdrawal resistance:
On page 3, it says:
It is a well known fact that plain-shank nails lose as much as four-fifths of their initial holding power during seasoning of the nailed lumber, while properly threaded nails retain or slightly increase their initial greater holding power under identical conditions.
This report purports to show that, unlike smooth shank nails, threaded nails driven into wet wood increase their withdrawal resistance as the wood dries. However, if you look at "Table 1" on the last page, the highest withdrawal resistance recorded is for a threaded nail driven through oak that had a 33.6 percent moisture content and was allowed to dry for 5 weeks. In that case, the anomolous 907 pound withdrawal force (which was nearly twice it's initial withdrawal resistance) was attributed to rusting of the nail shank inside the wet wood.
I certainly can't explain why threaded nails would behave differently than smooth shank nails in wet wood, but it's clear from the test results that rusting of the nail shank is a critical factor in determining the force required to pull the nail out once the wood has dried.
J. Kirk: Please don't read my posts.
I don't read yours.
Ordjen: You said:You'd think that ceiling pops would be less prevalent if gravity is helping to prevent them. Where am I screwing up in my thinking here?Ceiling pops are more prevalent because of the downward weight of the drywall.