My brother has had a couple semi's break their frames over the years. The first one was repaired with patches on both frame rails to keep the frame the same. That seemed to hold up fine.
The second truck that did it he decided to just get a complete new frame rail. That was a fun fix. Every bolt was taken out and the frame rail replaced and all new bolts went back in. We did that in front of our shed in a couple long days.
Both were Peterbilts with around 500-600,000 miles on them at the time.