The Butler purlin in the 1960's was a 9.5" deep purlin with 90 degree lips. The 90 degree lips require the simple span purlins you are seeing because the lips don't permit the continuous laps at the supports like the angled lips used today.
The common rule of thumb that I have heard from the experts is if the top flange has moved approximately 1" laterally in relation to the bottom flange you are approaching collapse. I don't know that there is (or was) what would be described as a "tolerance" for this dimension. The 1" value has been determined by a number of researchers as being a point of concern. Current purlin anchorage criteria specifies d/20 as a requirement (d being depth of purlin) at the supports, but those sorts of provisions did not exist in the 60's. In a building this old, the roof panel is a through fastened roof, which is providing a lot of lateral support to the purlin and is certainly helping in regard to the capacity. It would not surprise me if the majority of the rotation is near the supports with the middle of the purlin staying fairly vertical albeit moved somewhat laterally. There is limited restraint to the purlin at midspan which allows the whole purlin to move as a unit. At the supports the bottom of the purlin is more restrained (or should be if all of the bolts are installed)which then results in the top of the purlin moving laterally from the bottom of the purlin. Your picture seems to show the purlin is fairly straight which would indicate that it has not at this point been severely overstressed. Look for kinks or other deformaties especially in the lips.
Since you are observing some of the connections with four bolts, the standard detail would have been calling for that. The simple span connection is in effect two connections, each with two bolts in a vertical pattern. I am not aware of a Butler detail that would permit a single bolt to connect a member.
Overall, keep in mind that this is an approximately 50 year old building so it has performed very well despite as you have noted large snows and increasing design loads on the structure over the years. Cold-formed design provisions have also become more stringent over the years which could, per today's codes, make this building deficient based on today's specs even for the original design requirements. Butler would be happy to provide you with whatever information we can, although admittedly that information will likely be fairly limited for a building of this age. We tend to lean on local people for a building of this age since we have no way of knowing the full physical condition of the building, such as the connection conditions you have noted.
You don't indicate where the building is located, but feel free to contact any of our regional offices with your contact information. They will either have someone that can assist or more likely they will forward your request on to our R&D folks in Kansas City.
The one downside to these forums is that it is hard to take the process off-line since they frown on posting things like email addresses (with good reason).
Al..
