Roger,
We have barns with truss-like wall configurations in some cases, but these are not PA barns.
It all gets complicated very fast with the different settlement patterns, the barns being built reflecting who settled that area, etc.
The area where we find PA style barns was settled by Dunkards and Amish in the 1840's and 1850's. These came largely out of Holmes County Ohio. The areas such as Hamilton County where we find the square barns were settled from Ohio, by a mixture of German Lutheran and Scotch-Irish settlers. The Germans tended to settle in one area and the Scotch-Irish in another, and this is reflected in the barns that still stand. Scotch Irish barns are of the English style, square barns typically not very big with centered side entry doors. German barns are of course of a German style, featuring the long braces extending from sill to plate. They also often have a threshing floor that is more off center, showing a floor plan more reminiscent of a south German or Swiss barn (arranged horse stall, threshing floor, cow stall, with hay stored in a loft above the stalls being thrown up from wagons in the threshing floor aisle)
The PA style barns tends to be the mark of Anabaptist settlement in this region, from my experience all of the barns of this style locally were built either by Amish or Dunkards.
The German barn styles fell out of use by 1900, replaced by the much larger incarnation of the English barn, the more typical Midwestern barn. The layout here is essentially the same as the smaller English barns, just bigger.
Like you mentioned, a lot of barns are obscured by additions or changes. Most barns around here have totally replaced roof structures. Often, the old purlin posts framed an 8 or 10/12 roof. These were removed and replaced by canted posts framing a 12/12 roof. At other times, I have found where short stub posts were added on top of the old purlins to support a new purlin, increasing the roof slope. This is a bad practice, these posts are unstable.
In the early part of the 20th century, very many barns had their original roof totally removed and replaced with gambrel style roofs. Barns built from this period through the 1950's are almost always gambrel roof structures.
At some point, many PA barns had the forebay enclosed, or even were extended out on the forebay side so as to completely obscure this feature. Add to this the fact that there are often a number of barns with ramps to the upper level that are not PA style barns and never were.
