Let me preface this by saying that these are just my thoughts and I am certainly not an expert with the situation described.

1. Skidmore testing - the purpose of this test is to verify that the installation technique is installing the bolts to the proper pretension. Typically, one of the major factors that affects this is coating on the nut and/or bolt in the actual installation conditions. As you won't be using a true nut, it seems to me the most proper thing would be to fabricate a "nut" with the same plate that your bolted connection is using and use that for the skidmore testing.

2. There may well be some issues with getting the proper pretension without stripping the tapped plate as a typical nut (A563DH) is quenched and tempered (maybe not?). I would say you will probably get full thread engagement though as the plate threads will yield locally and deform to bear on the bolt threads.

Thank you dauwerda. I was thinking the same about fabricating a plate of the same material to fit the Skidmore. I just didn't know if this was standard practice for a situation like this. I was very surprised that there was no guidance in RCSC or anywhere else that I could find that addresses this.

I determined the shear area of the threads and calculated the shear stress based on the pretension load for an A490 bolt of this size, and found it to pass based on an allowable of 0.4Fy.

Bad idea IMO. Threading plate not the same as using a nut and this situation is outside the RCSC recommendations.

Can you use a one sided connector like Hollo Bolt?

http://www.lindapter.com/american/News/?view=69

As follow up, is this bolt going to be loaded in shear only? I would be more willing (comfortable) to use this type of connection in a shear condition as opposed to a tension condition.

However, there has been some research and qualification for tapped HSS utilizing a process called flowdrilling. The difference being that by flowdrilling, the material that was in the hole is displaced to create a localized thicker section of wall that is then tapped. Assuming your tapped plate is plenty thick though, I imagine it would be comparable without the flowdrilling part that is required for thin-walled HSS. Cidect Design guide 9 covers this to some extent, see exerpt below:

If connection design alternatives are allowed, maybe provide a sketch and we could advise? A490s are damn strong bolts, so you're obviously not working with toothpicks.