Calculate formula with & function

Hi,

I've pieces of a complete formula in several different cells.

In another cell I'm using & function to join all the pieces of the formula. The problem is that excel (2013) puts the formula in text and don't execute it. If I write all the formula manually, all Ok.

How can I join the differents pieces of the formula in one cell and at the same time that the calculation of it is executed?

Thanks,

Hugo Pinto