Claudio,

I want to add a second floor to my garage, which is made of cinder
block 21 x 22 feet. I am very new at woodwork and my English is pretty
poor. So please be patient with me. I need some input about my design
for the second floor, which will be used as storage, not a living
space. The garage is by itself, completely separated from the house.
I am planning to have an engineered center beam that span 21 feet and
will support all the 2x8" 11' joints 16" oc. The second floor
will be 8' at the center and 6' on each side with 2x6" 12'
rafters 16"oc. These rafters will be supported at the center by a
4x8 solid ridge that spans 21 feet. The framing will be made of 2x4
16"oc. One of my concerns is about lateral support for the side
frame.

Your design sounds unnecessarily complicated to me.

I would forget about all the beams, and simply span the 21 feet with 2x12's
at 16" OC, and use 2x6 rafters at 16" OC. Nail the rafters to a 2x8 ridge
board at the peak.

This arrangement would give you a clear span inside the garage, without the
point loads you would have with a large beam. And, the floor joists will
also serve as collar ties to keep the rafters from spreading the supporting
walls outward.

We used this exact structure on our garage which is 24'x28', and it works
very well, despite a free-span of 23'. It's too great of a span for a
living space, but it works great for light storage.

You would actually have better results with a 21' garage (should result in
a 20' span or less?). If you needed more support, you could switch to 12"
OC spacing, or use engineered wood I-Joists.

It's also much easier to lift 21 foot 2x12's into place than it would be to
lift a 21' engineered beam.

Good luck with your project!

Anthony