Earth covered home build- pre-cast hollow core or bar joist/ decking poured concrete?

[lvbutcher]

I'm nearly ready to build the home I've been planning for decades. It will be an earth home, 35'x110', under 2' of soil with the south face exposed and mainly glass. I've hired a good structural engineer to figure wall thicknesses, footings, rebar, and the structural steel for the front wall. BTW, I'm a retired union Ironworker of 30 years. I've always planned on using pre-cast slabs for the roof, and found a company within 90 miles of the build site that has figured out the specs for what I'll need. The back wall will be 12" thick, 10' high and the front wall will be structural steel square tube columns (6x6x3/8)carrying WF beams from w24x55 to w21x44, depending on the span. (They will all be concealed in the front wall). The pre-cast hollow core slab design calls for (27) 1'x4'x35' hollow core slabs, weighing approx. 13,000# each. The engineer figured this size to hold 2' of soil and various short pedestal walls for skylights in the rear rooms, plus a reasonable amount of live loads, such as a compact tractor for mowing, and also any equipment needed to install the soil (backhoe, skidsteer). There will be some rebar, a 2" concrete topping, EPDM rubber for water proofing, etc. The slabs will be free-span from front to rear walls, set flush with the inside corner of the side walls, so as not to interfere with deflection due to loads.

I'd like to know what it would take to do the same job with bar joists and bridging, with a 4" poured slab on corrugated floor decking (commercial/industrial type galvanized). The total price for the roof slabs I've gotten is about $45k, delivered on site, with openings and frames for the 3 skylights included. Since this is the only company I know of close enough to deliver reasonably, (3 slabs per truck, or 9 truck loads), I'm okay with the price and the company's helpfulness. But just thinking about 175 tons of concrete slabs and another 400 tons of soil above is a little unnerving.

How would I go about finding what size bar joists could carry the decking, 4" of concrete and necessary rebar or mesh and waterproofing, and still carry 200# per sq. ft. of soil? I've set many a building using bar joists and I've studied a few books on them, and researched the subject, but I'm not finding an answer. The trick is, if I switch to bj's, they'll have to have a maximum depth of 2' if I want to keep an 8' ceiling. I'm assuming I would have to forego the free span and run a beam down the center length of the house and garage for a carrier beam with 2 rows of joists, but I'd rather not if I don't have to. So, here's my question- what size bar joists would it take to accomplish this feat? Thanks for any and all replies. I haven't counted the use of the slabs out yet. I just would like to know what my options are. Lee

"Knowing is not enough, we must apply.
Willing is not enough, we must do."
Goethe

 

[BAretired]

Finding a bar joist to do the job is not difficult. Just look up one of the many bar joist catalogues listing allowable loads for various spans.

I would not select bar joists for that job, however, as they are subject to corrosion in the event of leakage through the slab. That is the reason they are not recommended for parking structures.

No matter what kind of roof you select, waterproofing is an important consideration. You don't want to be removing two feet of soil every time you have to repair a leak. You need good drainage and excellent waterproofing.

Another issue has come up in the past with regard to below grade housing, namely someone driving heavier than anticipated vehicles over the roof, causing excessive deflection or even structural failure. With that in mind, I would recommend some type of barrier to prevent vehicular access to the roof, other than the approved tractor.

BA

 

[kipfoot]

vulcraft joist catalog

also plank spans

I've hired a good structural engineer

Please expand the scope of work for your structural engineer to include the roof! I think you're pushing the limits of either system and would be well served to consider shortening your span with interior columns.

This would greatly reduce your deflections and reduce the depth of beams that you have at your window wall.

 

[BridgeSmith]

You should also consider prestressed concrete options, such as tri-deck or decked bulb-tees, which would likely be more efficient for that load and span than the hollow-core slabs. It's also alot easier to control the profile and avoid the sag with prestressed sections.

Btw, whatever you use for the roof support system, you'll still have the 400 tons of soil overhead, so if you're 'unnerved' by that, you're going to have to rethink your whole plan.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[bones206]

Lee -

You could do cast-in-place concrete dome roofs. There are companies that specialize in that method for earth-covered home construction. Like BAretired said, drainage and waterproofing are key. One advantage of a dome is that it drains well in all directions. It's also inherently stronger than a flat roof. I've worked on underground bunkers designed for nuclear armageddon that leaked like sieves because the roof was flat - despite having a waterproofing membrane.

You could also consider replacing some of the earth cover with geofoam to lighten to the load.

I also feel bar joists are not well-suited for this type of construction.

 

[phamENG]

Lee - you should have restarted your thread from last summer.

thread507-471350

In that thread you said you're a retired iron worker. That's a tough job and I respect your skill in being able to do it. But keep in mind that just as I'd never be able to erect this house on my own, you can't design it on your own. Are there load tables for bar joists? Yes. Are there rules of thumb for slabs, etc.? Yes. But those are, generally speaking, for preliminary design. This is a fairly unique building with a lot of possible considerations. You need to have an engineer design this for you. A group of engineers on the internet can't put it together for you either.

 

[dik]

YOur building envelope is critical as well as drainage. Good luck... steel joists, with care, can be economical as well as HC slabs (35' HC slabs, unless 12" thick may be problematic for that span and load). Steel joists for parking garages (one in Winnipeg was a disaster) due to salt de-icing are really bad.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[lvbutcher]

Thanks for the replies, everyone. I was waiting to get email notification, but none came, so sorry it took me so long to reply myself. I spoke with an old friend who is a long time commercial builder, and he told me he would estimate 24"-28" depth of joists, and suggested a center carry beam in order to reduce those depths. He calculated that it would be cheaper, but more labor would be required. From our talk, I pretty much decided to stay with the pre cast.

Per Kipfoot's comment on getting the structural engineer's input on the slabs, he works pretty closely with STL Pre-stress and knows the company's engineer who figured my job. Their businesses are located within a short distance of each other, and the structural engineer actually referred me to the company, so they were working together on the loads. Nathan needed Blake's input on slab weights in order to figure the footings and walls that would hold them. But thanks for mentioning that, since it would be a disaster for the structural engineer to try to figure the specs without knowing what was going to be sitting on top.

I also informed the pre-cast designer that I would want to be able to drive out compact JD 2032R tractor on top to mow. I'm sure he factored in snow, saturated soil, etc. He actually was the one to reduce our initial plan to put 3' of soil to 2' due to his estimates, which we've revised.

Also, in regards to the waterproofing and drainage, I plan on using bitumen AND rubber, (belt and suspenders logic) along a 1/4" per foot slope minimum on the roof from front to back for better drainage. There will be mid-wall and footing drains around the back and sides, and a berm/swale around the back end of the house where the 'roof' meets the ground, along with another under slab drain behind the front wall footing, which is figured to be 2.5'x4' wide. The engineer said this bigger footing will prevent creep from possible hydrostatic forces from behind. The side and back wall footings are 1'x3'. I wish I could put a pic of my prints or hand drawn plan on here, but being a geezer (62), I don't know how to get a 2'x3' plan copied with my normal size scanner. What I do have is a model that I've made of our plan, so I'll stick a couple of views of it. FYI on those, I've since moved the parapet and wing walls back to the front wall line, knowing there is no need for soil over or around the canopy on the front porch area. Thanks again for all the comments. FWIW, my biggest source of good information is a book by engineer Herb Wade titled Building Underground- The Design and Construction for Earth-Sheltered Houses. It's old (1983), but it's the most complete source and well written information I've found. If anyone has anything to add, please do! Lee Butcher, southeast Missouri

"Knowing is not enough, we must apply.
Willing is not enough, we must do."
Goethe

 

[bones206]

Lee - If it were my house, I'd want to go steeper than 1/4" per foot for drainage. Those bunkers I mentioned had a 1/4" per ft slope with cast-in-place concrete roofs and they leaked badly. Also keep in mind that the hollowcore planks will have inherent camber that will change the effective slope at any given point.

I'm not sure you want to use a traditional roofing membrane for this either. I would lean more towards a waterproofing membrane meant for geotechnical applications. Something like Cetco Voltex. These types of membranes are designed for resisting damage during construction/backfill and for long term durability in contact with earth.