Nitrogarage has some solutions, but lets look at this in some detail.
First a disclaimer. I was never an Architect or Engineer. Just a draftsman and designer. Not registered. So there are some finer aspects of structural design that you should go to those folks for.
What I offer here on GJ is a look into the choices you make to design a structure, so people who don't know anything, can begin to realize that structures need to be engineered, and not to take someone's comment that "We used a couple of 2x12's and it held up fine" as a solution for their own project.
The first thing to consider is the live load that will be placed on this mezzanine. The OP mentioned some engine blocks and a large compressor. I suspect he will also fill up any additional available space. What it will be filled with, I don't know, so we'll make some assumptions. I looked up engine and trans weights on the net. Let's go with 700 pounds for an engine and 300 pounds for a trans. Air compressors of that size weigh in at around 700 pounds. So figure 4 engines and 4 transmissions the compressor and say another 2000 pounds for misc. So a total of 6000 pounds. Loft area is 256SF, so 5000/256 is 24.5#/SF.
A typical residential floor is designed for 40#/SF.
Now these are uniformly distributed loads, and the engines, transmissions and compressor can be more concentrated and point loads. The OP says the engine parts will be on pallets. That helps spread the load. Don't know if the compressor is vertical or horizontal. You can spread the load with an extra layer of floor sheathing that spans more joists. You can also space joists closer together under these loads.
But let's also up the live load from the 40# to say 50#/SF. This gives you a 12,800# total load capacity.
So lets size the joists.
Here's the calculator I use for joists:
http://www.awc.org/codes-standards/calculators-software/spancalc
I use Hem-Fir #2 and better as the species as it's cheap and available. I set the spacing at 16" to begin with and L/360. Dead load at 15#/SF. Let's try 2x12's.
Calculator says 15' 5" span. Actual span will be 15' 7" so close enough. You can also use different species and spacing.
The beam will probably need to be LVL's, so I use this info. Note the 1.9E. That strength is readily available at places like Menards.
parr.com/PDFs/LP LVL 1.9E.pdf
The contributing area for each LF of beam is 8SF times 50#/SF LL. So the beam must support 400#/LF. But it must also support 520#/LF total load. Divide that in half for a double LVL, so 200#LL and 260# total load/LF.
Looking at the charts for floor beams, a 14" deep LVL will support 274#/LF with L/360 and 378#/LF total load, so a double 1 3/4 x 14" deep LVL beam will work.
The other end of the joists need supporting, so a beam needs to do that. It can be a ledger beam, and the span between supports can be less than 16' since it is against a wall. I like to place a support under my ledger beams, in addition to lag bolting them into the studs, so 3 2x4 support posts give an 8' span. You could probably use dimensional lumber or an LVL for this ledger beam. Since this will be a single ply beam, it will need to support a 400#LL/LF and a 520#/LF total load. A 9 1/4" deep LVL will support 624# and 704# respectively. But since it is less deep than a 2x12 joist, it would need to be placed under the joists. Use a rim joist lagged into the studs.
For dimensional lumber use this:
http://www.awc.org/pdf/codes-standards/publications/wsdd/AWC-WSDD1986-ViewOnly-0301.pdf
Use an Fb of 1300 and look for w in the chart under 8' span. A 2x14 will support 594#/LF so you can use it to support the joists with joist hangers. Don't forget the 3 2x4' posts lagged to the wall and used as supports.
A 6x6 corner post will give enough beef to support the double LVL beam let into the side of it. Pay attention to the connection by through bolting with at least 2 bolts through the remaining 2" of the 6x6 in addition to resting the double LVL beam on the post. And also secure the column at the floor.
3/4" T&G ply decking glued and screwed with solid blocking at joints. The other end of the 2ply LVL beam can be supported by 2 or 3 2x4's or 2x6's in the wall. Check out the requirements in the charts.
Understand?
The point of engineering is that you know how much a structure will support along with how much it will deflect under that load. Otherwise you have no idea.
Bill
PS, This is probably over designed for what the OP will use it for. But with a 16' span for the joists, and some possible point loads, it will keep it stiffer. I would probably design mine for a standard 40#/SF LL.
