First Let me make it clear. I'm new to Simulation

I am doing a static analysis on a sheet metal part which is 2000mm x 60mm with 3mm thickness. For which im getting FOS as 0.7 using PCS material by default.

I need to stiffen the sheet metal part to increase the FOS which is my objective. But I have tried adding flanges and gussets to the sheet metal base which decreases the FOS, i couldnt able to pinpoint the cause for decrease in FOS. Can anyone tel me why is this phenomenon happening in Software, while in real time the gussets would stiffen the object. I couldnt reproduce the effect of stiffening in the sheet-metal.

Kindly suggest me a solution for this.