Thanks for your answer , here is the simplified file{F572920}. i did some test, i have seen that the problem maybe is in transparency of the leaves, because if i bake in cycles the stairs it crashes if in the leaves there is a material with transpaency, infact if i put on the leaves the basic materials it works, instead if i leave the transparent material (i need it for the shadows of the leaves on the wood) and i use Gpu , then it works.
But if i use 2.77a version of blender it works also with the CPU and never crashes, this is weird :///