[Mr V]

The first question that comes to my mind is what kind of rivets are you using? There are different grades and quality rivets. If you are buying them at your local hardware store in retail friendly packages chances are they are low strength. McMaster is one place that does sell high quality, high strength rivets also check for a local fastener supplier, Hi-Strength Bolt has locations in many areas.

I prefer rivets and/or nuts and bolts for the reason of quicker and easier repairs and changes.

Next time you are in traffic sitting next to a large truck take a look at the frame rail. You'll see that the cross-members and brackets are held on by rivets or nuts and bolts. That is so that those pieces can be replaced in the field in an expedient manner and without worry as to maintaining proper alignment or the skill of the welder and subsequent quality.