Replies

1. 
   BossHog | Oct 22, 2003 03:20pm | #1

   I don't believe an 8" slab will be worth squat with nothing but rebar in it.

   You might suggest he look into spancrete. Surely they would have a slab sized for that short of a span.

   Why, Sir, most schemes of political improvement are very laughable things. [Samuel Johnson]

1. 
   IronHelix | Oct 23, 2003 05:57am | #5

   Boss...

   A local "old salt" always used 1" thickness of concrete per foot of span/ with a 4" minimum, as a rule of thumb.   But this was for basement walkins/root cellars...old time stuff without big loads on top or spans past 8 feet.

   Thanks for the thought about "Span Crete"....should be something available in the St. Louis Area....I'll take a look.

   .................Iron Helix

2. 
   VaTom | Oct 22, 2003 05:01pm | #2

   How thick should the slab be?

   What rebar specs would be appropriate?

   Are there any rule of thumbs on this?

   This could be scary.  I routinely put 2' of dirt on my roofs with spans up to 40'.  I don't believe you have what you need.  When I originally got my engineering, the engineer asked what load I wanted to support.  Dirt doesn't all weigh the same.  We settled on 300 psf total load.

   Bar joists are what we use, primarily as an economy measure.  Several years ago I ran into an rc structural engineer who proceeded to engineer T beams for me.  Cost went out of sight.  He told me that's why steel is so commonly used.

   You need real engineering with specified loads.  For instance, here we use 80 psf live load.  How much snow do you get?  What's your saturated soil weigh?  Your building dept. will quote you live load but the rest is probably up to you.  Assuming you have an inspecting building dept., they may require more documentation.

   The standard load table for K-series joists lists 8K1's at 444 psf total load for 12' span.  That's an 8" tall joist which then gets decked and, usually, a thin slab (non-structural) on top.  Each joist would weigh 61.2 lbs.  Suggestion is to buy some rc engineering, or use pre-engineered steel and save a bunch of money.  

   PAHS Designer/Builder- Bury it!

1. 
   IronHelix | Oct 23, 2003 06:08am | #6

   VT....I need to profess my ignorance about the meaning of "rc" engineer! Please clue me in!

   I like your idea using the bar joist......and your loading spec are close to our wet-soil loading.    This would require a little more elevation on the existing sidewalls, but that is probably do-able.

   A little snow, every now and then, and a rare 24" in 1978......What is a building inspector?   We live in the sticks...never seen such an animal...the locals would probably shoot it!  Engineers of any sort are almost as rare as an inspector.

   More fodder for the cannon......Thanks for the imput.

   .......................Iron Helix

1. 
   VaTom | Oct 23, 2003 03:55pm | #8

   I need to profess my ignorance about the meaning of "rc" engineer! Please clue me in!

   Sorry.  Reinforced concrete.

   As the bottom foot of my roof soil is always dry by design, you'd need more load capacity.  You're wanting to support 50 tons or so.  Putting a car up there is no trick.  Dirt's heavy, especially wet dirt with a foot of snow sitting on it.  Ever get any sleet?  Much heavier in just a few inches.  We live in central Va, hardly the snow belt.

   There's a factor called "soil arch" that allows me to run my tractor on top of the soil without point overloading.  Tractor's a small one, only 4-5k lbs.  Your guy may be planning a wheel barrow.

   Price the bar joists with galvanized decking and I think you'll be pleasantly surprised.  I shoot the decking to the joists, but welding works fine.  If you have to extend your walls that 8", plan a steel plate for each bar joist to rest on and be welded onto.  I use rebar stubs down into the wall to keep the plates in place. 

   Good luck.PAHS Designer/Builder- Bury it!

1. 
   IronHelix | Oct 25, 2003 02:35pm | #9

   Great option for the problem!

   Looks as if your suggestion on bar joist has caught the eye of my client.

   We will go locally to find a supplier for correct specs on our loading.

   Thanks....................Iron Helix

1. 
   VaTom | Oct 25, 2003 03:06pm | #10

   Looks as if your suggestion on bar joist has caught the eye of my client.

   We will go locally to find a supplier for correct specs on our loading.

   Hope you can work it out.  To keep it dry, we use a couple layers of 6 mil sheet plastic, rock protected by old carpet scraps.  But our lowest foot of dirt is the heating/cooling system and has to stay dry to work.  Above that is more plastic sheeting, then the planting dirt.  Cheap and effective. 

   You might find somebody who will sell you some bentonite rolls if you are more concerned.  It's the best water-proofing I've found.

   Good luck.PAHS Designer/Builder- Bury it!

3. 
   BillHartmann | Oct 22, 2003 05:10pm | #3

   My guess is that is at least in the right area.

   I have a 24x24 garage floor over a basement that consists of a 6" slab and w #4's 6" O/C both directions. It is has a beam running across it so that the max unsupported area is 12x24. Close to your conditions.

   Don't know about the weight of 2ft of direct compared to cars.

1. 
   IronHelix | Oct 23, 2003 06:12am | #7

   Sounds reasonable for a car that averages 4000 pound, but I'm affraid that 480 cu/ft of soggy clay will top out around 50,000 pounds......

   Thanks.....................Iron Helix

4. 
   CapnMac | Oct 22, 2003 08:05pm | #4

   Actually, my concern would be the "retaining walls" on either side.  They are just presumed to take the load of a concrete slab and two feet of damp soil.

   Now, one idea that come to my mind for this is precast planks.  But, the bar joists and metal deck probably offer as elegant a design as is likely possible. 

   Occupational hazard of my occupation not being around (sorry Bubba)