Need bar joist load information for an underground earth home roof I'm building

[lvbutcher]

I'm in the planning process of building an earth home, which will have the two side walls, back wall and roof completely covered with 2' of soil. The structure will be 35' x 110', and if possible, I'd like to make it clear span, with no center beam running down the center of the house.
We've investigated pre-stress hollow core panels, but the building site is in the middle of a rural 43 acre property we have, and the weight of the panels, around 13,000# each, plus the primitive easement road into the property and a creek bed crossing that resembles a roller coaster ride at this point, are making me really rethink that option. Not to mention the $45k price tag.
I am a retired structural ironworker and have worked with bar joists, bridging and metal decking for most of my working life, but I have no idea how to figure what the load capacity necessary to carry the decking, concrete, rebar/mesh, soil and any live loads this will require. And with live loads, that will include bobcats or tractors loading and finishing the dirt placed on top of the roof (2' deep), and later driving a lawn mower or small tractor on it to mow.
The rear concrete wall will be 1', double faced rebar, as will the side walls. The front wall, which will be exposed to the south, will be steel columns and beams for strength. We will be using a lot of glass for winter sun, and a canopy to stop summer sun infiltration. Is there any source available that would show this information? As I said, I've worked on and with bar joists most of my life, but now that I've gotten to the point of needing to know the specifics of what I need for our house, it drives me crazy realizing that I don't know the math behind figuring what I need. Thanks for any and all replies. Lee Butcher

 

[r13]

The required engineering calculation is more than you can image, albeit you have the real work experience. Hire, or consult with a licensed engineer by your locale, whether this project is for adding living space, or a wine cellar.

 

[BAretired]

That's a lot of load on your roof. Clay weighs about 100 pcf when dry, 110 pcf when wet, so two feet of soil will be about 200 psf. The additional dead load of the roof structure depends on how much concrete you are using. At a guess, I would say about 50 psf dead load. Live load should likely be taken to be 100 psf. So total load on roof = 200 + 50 + 100 = 350 psf.

Using concrete slab on steel deck on open web steel joists, you will need to provide a very good waterproofing treatment. You may want to camber the joists and use a layer of drain rock to prevent ponding.

Retired13 is absolutely correct. You will need to retain a licensed engineer for a design. I looked at a Canam joist catalog and found a 28" deep joist 28K10 weighing 14.3 #/' with a capacity of 501 psf on a span of 35 feet (see below). That would require a spacing of 1'-4" to carry 350 psf. You may find another joist fabricator with a lighter joist to carry the same load. Alternatively, you may decide to use a little less soil on the roof.

BA

 

[dhengr]

Lvbutcher:
You do not want any mid-size tractor on that roof. Min. soil cover for its insulating and cover value, a couple sheep or goats and a small riding mower is much easier to design for, and even then, some fencing or signage for no larger loading. Is this bldg. dug into a sidehill or is soil mounded up around it? Along with your roof waterproofing system, which requires some considerable study and knowledge, you might consider a bar jst. system with a top slope to one side, the low and best drained side of the lot and bldg., so as to take pounding and deflection out of the drainage picture. A constant depth stl. jst. can have two different brg. elevs., but then you have a bunch of extra interior framing to level the ceilings. Maybe a mono-slope stl. jst., bottom bearing or with different height upper bearings would be a practical solution. The stl. jst. are lighter, but at 35’+ you’ll still have to snake 4 or 5 at a time down a curvy tot road, on a farm wagon. You really should be able to get a large truck into the bldg. site, with proper turn around space. You’ve got a bunch of const. mat’l. to supply for a 35’x110’ bldg. The three side walls will be no small system at 8 to 11’ high with full height (plus) soil pressure. And, you will want exterior abutments or returns on the walls at the open end of the bldg. You really do need local Structural Engineering help.

 

[WeezerMike1]

I agree with the previous posts. This really does warrant a professional engineer getting involved. Based on your description of the build-site maybe permitting and legality aren't really concerns, but where I'm at it's illegal for someone without an engineering license to design something like this.

 

[oldestguy]

Hey snow loads also. This is not just the usual engineering job. There likely are unknowns that may happen. That estimate of soil loading is way off, more like 150 pcf. As to tractor loads, you can't control that in design, just add more steel, etc and apply a decent safety factor. I would expect the engineer wants up front money also. By the time this is all figured, likely its way too costly for the owner.

 

[BAretired]

That estimate of soil loading is way off, more like 150 pcf.

Well, I suppose it can vary a little depending on the type of soil, but I usually consider it to be somewhere in the order of 100 or 110 pcf. Taking water at 62.5 pcf, the maximum value in the chart below is 1.6*62.5 = 100 pcf. It may be a good idea to bump that up a bit to account for the possibility of it being wet. The eor will make that determination for himself when he performs the design as well as offering a little advice about the type of equipment to permit on the roof. Might be a good idea to fence the property to prevent armored tanks and heavy trucks from driving over the roof.

A live load of 100 psf should take snow loads into account but again, it would be the job of the EOR to make that determination.

BA

 

[BAretired]

Here's another chart of soil weights. If lawn is planned over the roof, I would expect a loam to be used. If you plan to grow giant redwoods, you might want to give their future weight some careful consideration.

BA

 

[JLNJ]

In many jurisdictions a homeowner can do just about whatever they want on their own property. The more rural the fewer the requirements.

If you don’t want to hire an engineer who “knows the math” and can be responsible for your family’s safety and well-being, then you can work by trial and error like they did in the Middle Ages.

 

[WeezerMike1]

JLNJ,

Well that's a terrifying concept.

 

[CrabbyT]

For clarification, are you planning to dig a big hole and put a building in it? Or, do you plan to build a structure above grade and encase it in soil?

I imagine it would be critical to check the roof's deflection to ensure that there aren't areas where wet soil or water could accumulate. It would probably be a good idea to add an allowance for piles of soil that could be created if the roof needed to be dug up, and also the weight of any potential machinery that could be used on top of the structure.

Whatever you use as a roof deck, you should try to figure out how much corrosion you could expect and size the deck to account for it over the lifespan of the structure.

Beyond sizing the beams, I think it would be wise to pay very close attention to damp proofing, drainage, radon mitigation, egress, and ventilation (to name a few). If you plan to have trees nearby, you might want to try and account for a potential falling tree load.