building metal frame shelving (due to wood $$$)

Hi, i'm aware this is "Woodworking" but if you've seen the price of wood this summer you'll understand why i'm posting it.

Anytime in the past I needed to build something i'd stick it together with wood - I had a tolerable guesstimate of how strong something was by eyeballing it and that's fine.  Yet I have zero experience with metal and don't even know what questions to ask for instance.  I was hoping someone here who happens to know both could enlighten me.

Quite simply I want to build a metal framed shelf with 8-10+ foot wide span.  (whether I use wood panels for the surface or yet another type of metal for the shelf panels is also open to discussion)  I say 8 feet because there can be more than one vertical post if I carefully place it.  I've built plenty out of wood just using 2x4's at the corners for posts/vertical load and a pair of 2x4's under a 2x8 foot plywood sheet for the shelves and it holds all the weight I care to put on it. (i'm not storing engines or something like that)  Though I planned this one to be alot deeper than before because I need to make it a shelf OVER a car/pickup parking space inside a super tall RV garage - so its going to be more like a 8 foot wide by 20 foot long top surface. (out of wood or metal)  So maybe its a 20 foot span if I just do four corners but thats not essential - I can probably put at least one post about in the middle, but a car DOES need to be able to park under it and still open/close a door is all.

Can someone give me some rules of thumb or safe guidelines along with what would make a fairly easy construction (maybe what size/grade bolts and such? using pre-holed mild steel?) AND deconstruction since it will eventually come down when moving out in 6-12 months?  Ideally i'd learn alot more about proper engineering for long term use and such but this is more of a time emergency - i'm being forced to move out of a house due to medical bills and shove all my stuff into an RV height storage garage.  :-/

Just a thought -- wholesale lumber prices have dropped way down. It will just take a while for the overpriced stuff to work its way out of the retail system.

Meanwhile, metal may look more attractive. If I were designing a temporary structure, I would think about possbile ways to repurpose it right from the start.

Brian, have you looked into used/surplus warehouse pallet racking? You might find some on Craigslist, or at an industrial surplus dealer, at fairly inexpensive prices... since generally, it is too tall for most home use.
It generally has 8' wide spans (for 2 pallets) and various depths (also generally related to pallets, 42" or 48") It assembles with a mallet, clips or bolts for safety, and wire grid decks are available, or just use pallets. It'll hold an enormous amount of weight.
You could set up three (or more) units each in front of the next, and park underneath. If you left a gap for your car door, you could maybe bridge that gap with pallets and a sheet of plywood resting on the adjacent units.

For the car, there are also wheel dollies, in case you want to roll your car sideways around your garage or maybe under a shelf...

I know it will come down eventually, but I wasnt sure whether it was actually better to still make things out of wood is all, I was trying to be sure I wasn't suffering from 'wood myopia' thinking its the solution to everything just because it's what i'm familiar with.  I wanted to price out comparable shelving in metal, that I could custom make for my exact use case, and get comfortable making simple things (bolt together not welding yet) since in the future I had other things I thought i'd want to make also. (DIY exercise equipment for instance)

I can look into used pallet racking also, if I find something available i'll maybe just jump on it - but i'm still mildly curious about future building in metal so i'll always have an idea of what a DIY solution looks like and costs.  

Here's two metal framing systems, the first is a bit more approachable, and often used for shelving and workbenches; the second is often found in commercial/municipal/industrial construction for supporting mechanical systems (electric/plumbing)

https://en.wikipedia.org/wiki/Slotted_angle
https://en.wikipedia.org/wiki/Strut_channel

And a couple more... The 80/20 company is the original, but there's knock-offs/clones of it as well. McMaster-Carr is both a source for stuff, and a great design resource since their catalog is very well done, easy to search.

McMaster-Carr (railing fittings and t-slotted framing)
80/20 Inc.

If you want to just get your own steel, and drill holes yourself, go to a steel supplier... (not the hardware store or "home improvement" stores, $$$$)
They'll likely sell in 20' lengths for bars and angles, 22'-24' for tubes, and may charge a fee for cutting, or a fee for delivery. Since you have a big space to build out, getting long pieces delivered might save you some time and effort joining.
If you do get cuts made, you could just get a few cuts just to make the pieces fit into your truck, and then cut smaller parts yourself. (a simplified example: 8' + 12', or 8' + 8' + 4')
And if you need a LOT of nuts and bolts, check McMaster-Carr, Bolt Depot, or Fastenal before the "home improvement" stores.

Thank you for the suggestions and ideas...  i'm looking at the McMaster stuff now...  i'm just not sure how strong is needed for instance.

I'm pretty sure i'll be limited to 10-12 feet of length for anything because that's what can fit in my longbed pickup with a red flag on the end - the storage garage isn't in town where i'd be getting stuff.  So i'm thinking six posts so the whole thing is just 20-24 feet at most.  (a little extra room for the center post means I can move the vehicle under a bit forward or back so I can still open the door of different vehicles)

Eyeing the slotted angle stuff i'm wondering if I should just use L-channels like that?  Vs C channels or fully squared off...  I need strength not flimsy, nothing would suck more than the whole thing skewing sideways and collapsing.

Images like this stimulate my thinking, just putting the bottom 'shelf' at floor level to drive over and scaled up to 8 feet wide...

https://static.grainger.com/rp/s/is/image/Grainger/3W151_AS01

Does anyone think that wood would be simpler and cheaper and less failure-risk still over L-channel steel for a use like this?

What grade bolts should I be considering, Grade 8?  Wondering if there's a shear force chart for different thickness of different grades to give me an idea what I can risk putting on top of such a shelf...