Subgrade modulus of soil in RISA

What program are you using? I suspect you're using RISAFoundation. Is that correct?

If so, there are probably ways this could happen.
1) If the size of the footing changes (i.e. the weight), then this would affect sliding resistance.
2) If the FEM results change (due to a size change) this could also affect the results. Say, you have a combined footing and the relative stiffness of the components change then the force distribution between them would change.

There are probably other ways that this could happen as well. But, it's been a long time since I've used the RISA programs. Heck, they could have totally screwed up the programs in the last 5+ years for all I know. Certainly, they've lost most of their most knowledgeable staff during that time. So, I've certainly been expecting to see some big changes in quality. That's just what happens when your best employees with the highest technical knowledge leave and the main ones left with institutional knowledge are the sales / marketing folks.

Caveat:
I am the former VP of Engineering for RISA who left in late 2017 when Numchetk took over. I have some hard feeling about the way I was treated by current management during that time frame. Plus, I also saw a number of the colleagues I respected most get pushed out around the same time as well. I'm not exactly an impartial observer. So, keep that in mind when you read my opinions of the company. Also, know that I now work for their largest competitor (CSI the company behind SAP2000/ ETABS / SAFE). Therefore, I have double-bias.

Thanks! I was keeping all the parameters as it is and just changing the subgrade modulus of soil. This leaded to the change in Vr as well that's why I am getting confused.

If I remember correctly, RISA makes some assumptions about the vertical and rotational springs for a footing. So, one of the things you should do when you get weird behavior like that is hard wire the size of the footing, so that RISA doesn't get to guess about those spring stiffness values.

Instead of initially questioning the algorithms of the software, I recommend contacting support (support@risa.com).