How to reduce the stiffness of force versus deflection linear curve at elastic phase analysed with ABAQUS?

Dear Abaqus Users,

I have modelled and analysed a simply supported RC Slab strengthened with FRP using ABAQUS 6.8-2. Plastic analysis with Concrete Damaged Plasticity has been chosen to simulate the behavior of the RC Slab loaded at the centre. After running the analysis, the results show a very stiff linear graph at elastic phase. Is there any way that could reduce this stiffness graph? I have heard about putting material damping  may reduce this stiffness. anybody knows what is the reasonable value of this damping factors such as Rayleigh, Alpha or Beta, composite or strucutural factor? To which material shall I put this damping factors?

Your kind assistance and guidance is very much appreciated

ELMI