I have a "dead" rivet collection from the
"El" in Chicago sitting on my shelf. It's now over a hundred years
old and the rivets I've found have all been purposefully removed in the
course of maintenance. But I digress....
A few thoughts about your questions:
I'll hazard a guess that the rivets that are missing are
near deep pockets of corrosion between plies. Long-term corrosion can easily
pop rivets out if unaddressed. If it's not corrosion, you'll need to
determine why the rivets are missing.
I'm not sure why you doubt the rivets that are still in
place. Exception: pay attention to those that are near pockets of corrosion
between plies for the reasons stated above. To rehab this structure, you'll
have to get all that rust pack outta there and that will probably mean rivet
removal and replacement (they use A325 bolts in maintaining the
Welding to this old steel is probably possible, but may not
be practical, depending upon the chemistry of what you've got. If you want
to weld to it, you can investigate that further. A good reference for this
is "Field Welding to Existing Structures" by David T. Ricker. This
was published in the AISC Engineering Journal.
As far as the pony truss stability assessment, look at Joe
Yura and Todd Helwig's AISC/SSRC bracing lecture notes. That specific
example is discussed at length because it is such a notable example of
torsional bracing. If you need to get a copy, call Bob Lorenz at AISC