How to define non-linear Stress Strain response for the elements subject to thermal loading?How do I include stress-strain relationship( data from excel) within the Material properties? 

---

Answer:
In order to consider the nonlinear stress-strain curve of material for the temperature loads, we should go for solid element model to which plastic material is assigned. Beam element cannot be used for this analysis. Once the solid element model is prepared, follow the steps below to perform the plastic analysis as follows:
1. Go to the Analysis>Analysis Control menu.
2. Select the 'Material Nonlinear' option.
3. Go to the Properties>Plastic>Plastic Material menu.
4. Define a material model using Tresca, von Mises, Mohr-Coulomb or Drucker-Prager.
5. Assign the plastic material to the corresponding solid elements.

Please note that there are some limitations in midas Civil
- Stress hardening of material can be considered but stress softening cannot be considered.
- Modulus of elasticity and thermal coefficient of concrete are changing along the temperature. This cannot be considered.

I don't know whether or not you have midas FEA. The above limitations are not there in midas FEA.
http://en.midasuser.com/product/fea_overview.asp