How can I define different load factor for self weight for steel and concrete?

In MIDAS we can only specify one value for the load factor in the Self-weight function. However, to consider different factor for the different materials, you don't need to calculate the load on each beam and apply it manually, you can modify the weight density of the material by multiplying with the appropriate factor.

If you go to the Material Data definition if you need a standard material you can choose the material and the change the Standard back to "None". This will unlock the material properties to be modified and you can change the Weight Density: