A friend of mine did a steel tubing failure research program 29 years ago. I believe the results were published in Stock Car Racing Magazine at the time. The bottom line he says unless you pre-heat every weld to at least 300 degrees you will have parent material failure when the loads are high enough. I have some frightening photos taken of a SM at Summit Point last year that has multiple tubing failures. The attached photo was my interpretation of how to build some protection into a door bar and to meet the idiotic SCCA 8 point cage attachment rule which is the 1st rule that needs to be changed to improve driver safety.