Hi,
The Amplitude of your mains voltage is not 220V but 220Vsqrt(2) which is approx. 311V. Your multimeter however measures the rms value and therefore 220V. Same thing is true for the voltage on the secodary side of the transformer which doesn't have an amplitude of 12V but 12sqrt(2) = 16,97V. On the other side of your full-bridge you then have a DC voltage of roughly that value which is in your case 16,8V. Guess you will need some additional components to get your 12V.
Cheers,
Dominik