question on trusses. contractor is building a stand-alone garage
24'x30'. 2nd floor is for storage. he's using 2x8 lumber for the floor
joists, mated in the middle with a gusset plate.
is this sufficient for bearing weight on the 2nd floor for storage?

If the 2x8 is the bottom chord of an engineered "attic truss", it's
probably sized for the intended loads. The other chords of the truss help
carry the load, the same way a 2x4 is typically used to build a standard
truss. The 2x8 would essentially be sized to carry the load between the
truss chords (maybe 10 feet for a typical attic truss?). Assuming the truss
is being built by a truss company, they should be able to supply you (or
your contractor) with a copy of the engineering diagrams and the loads it
is designed to carry. In fact, that's one of the required items for
building permits around here, so it may already be part of the plan
package.

On the other hand, if you're talking individual 2x8's for spanning 24'
you're going to have big problems. Even when sized for a ceiling joist, a
2x8 at 16" OC is only good for about 17 feet, and that's for a very light
load. You'll never get by spanning 24 feet with a 2x8.

Just for reference, I spanned 24' in my garage using 2x12's at 16" OC. Even
that is way undersized for a floor joist. It works OK for my light attic
storage (empty boxes, Christmas decorations, suitcases, etc.), but it is
far too bouncy to be considered a usable floor joist.

14" I-joists would probably work well if you have the vertical space to
install them.

"IF" you have a beam running down the middle of the garage, your span would
only be 12', in which case 2x8 joists would probably be perfect for floor
joists. You could even splice shorter joists over the beam if needed.

However, if you DO NOT have a beam, any splice in the joist is going to be
an immediate failure point (unless it's part of an engineered truss).

Anthony