Plastic Elastic Tubesheet Calculation

[innovation2]

I am designing a tubesheet that is integral with the shell and channel and is constructed of 304 SS. The shell of the unit is 304 at 250 F and the channels are 304 at 250 F. When I input the information into my design software without an expansion joint, but using the plastic calculation I get it to pass at 0.75" thick. However when I use an expansion joint the thickness increases to 2" with the plastic calculation. My question is when you enter an expansion joint does the plastic calculation still allow the shell and tubesheet to yield? In my software it looks as if it doesn't allow yielding of the shell or tubesheet but it does allow it for the channel. Is this an error in the software or is this correct for tubesheet calculations according to UHX. If anyone would like to confirm these results the design information is below.

Here are my design conditions:
shell: 70"OD 432"LG +/- 15 psi at 250 F 1/4" thick L=60"
channel: 70"OD 42"LG 100 psi -15 psi at 250 F 5/16" thick
tubing: 1" 18 ga 432"LG required qty. 2500 P=1.25"
Distance between supports L: 74"

There is one lane in the tubesheet 3" wide 70" long