Old, old Building, New Purpose:

How old? 1896! Yes as in Eighteen Ninety Six.
Planning floor layout details.
As real work in building gets going, one issue is maximizing usable area. Rent, lease, or buy; cost is considered primarily in square feet. When building, the height becomes another factor. But within considerations mentioned, the available CUBIC feet are kind of free. I moved here for that exact reason. What I bought equaled 18 months rent in Southern California. Replacing the entire roof works out 46 months total. While eventual goals aren't disclosed, they'll unfold as it goes along. What is in my favor, while 3-4 places are similar within 100 mile radius, they're promoted in very old school ways.

So, in July the punch presses moved in, where a mezzanine is next. That added 560 sq ft will clear shop floor with machine tools already in place. This will be self built. A big impasse of interior height 15' 8" is thickness of combined deck material. Past few days I fleshed ideas that have been in mind awhile. The pic expands with notations. Seems 'Lift Pocket' only feature that won't make the cut.
Once the floor grating thickness is known [lbs. per square foot], the supporting structure materials will be factored. Frank S likes doubled purlin "I-beams" as economical, easy to handle joist material. They'll center 28.5.
More vertical savings would be how to use pallet rack beams; being indented with a step. They don't seem available in sufficient [desired] length. Is moving posts inboard under the splice practical?
The stair and landings work with a short 'dead' wall space, without blocking adjacent drive through door. The open corner fits around head of large press, it's 10' 6" tall.

Certainly, will consider and rework accordingly with every suggestion from entire HMT community.
Attachment 32993
Just tested diagram, it opens quite large, text readable by scrolling a bit.

The list of design considerations not listed for this mezzanine are many, including lateral stability, deflection, vibration, anchorage, and potential impact to supporting columns from materials or moving vehicles such as a forklift. The best advice for you is to engage a licensed design professional from your jurisdiction to ensure safety and adequacy of the structure for its intended use.

road-warrior's list points out legitimate concerns. Exactly how any project should be handled.

I'm not one to step into an engineer's office and say "I want something for so and so". Like attorney's, most bill by the hour.
The right way, set the parameters for HIM to work within; not other way round, "This is what's needed, within this space...".
Then, so armed with road-warrior's questions and others, "Where are the impact guards,..." [etc].

One good thing; bulk of lateral stability is ledger at west wall, 18' high cinder block.
Right now, search is on for load bearing capacity of initially proposed floor joists of doubled purlin. Floor grating appears to be 195 lbs over 36" by 4'6" openings, where my material is only 2'4" wide.

Not unlike the roof replacement, nearly flat mopped asphalt. Met with all kinds of roofers who made a big show standing atop a 24' ladder. Peeking over te pediment, waving arms about, pitching what mounted to a maintenance contract [without cost controls on permits/ material/ labor], patch this hot mop that. On other side of ridge, one corner was collapsed.
One, he'd raise common rafters OVER existing roof, enclosing it. OMG
Not one even asked inside to adjudge condition of the 15 supporting posts. 6x6's in 3 rows -5 each- 6x6's, most termite eaten to half original size. 2 so far gone, they'd scabbed on 2x8 splints.

One didn't bring a ladder, asked inside FIRST. "We can't save this. The walls [brick] are sound. I'll cut old roof out, swing 65' trusses over, set on lumber plate 2"x12"s. It'll be 65' free span, plywood decking, 30lb paper, 50 year shingles and no posts at all. "65' BTW is longest [and highest at 4:12, ~10'9"] they can trailer through town."
Guess who got the job?
A few joke-roofers came by later. "I would have done this way also..." To each, the same response.

I'm a machinist. It's not my job to know roofing, even construction. You're used to homeowners, clients who have maybe no capabilities, just a sagging, leaky roof. You did nothing appraising entire situation, yet provided a quote. This is 50 year guarantee roof, for less than your 3.33 of re-dos. [15 yr warranties].