Earth Sheltered Roof System with CMU 3

[HMROSS]

I am in search of the best roof system to use in conjuction with reinforced CMU bearing walls for spans up to 20 feet for the purpose of carrying earthen dead loads ranging from 140 to 340 #/sq.ft (1 to 3 feet of soil plus other materials), live loads from 60 to 100 #/sq.ft, and a possible accidental live loading from live stock (I am also searching on how other designers have dealt with the possibility of cattle breaking through fences during thunderstorms and then feeding on the grass of an earth sheltered roof--accidental live loads). Does the 100 #/sq.ft. live loading take care of such potential accidental loading? The system must be able to be incorporated with CMU construction at semi-remote sites (with in protected trees). I am currently investigation precast pre-stressed planks/slab systems but want to cover all the bases. Any suggestions out there are welcome, especially for systems which allow curve linear plan layouts (escaping the block house syndrom).

 

[jheidt2543]

The quickest and most cost effective would, IMHO, would be precast plank. Check with Flexicore, Spancrete, and Fabcon. They all have experiance with "earth sheltered" dwellings.

Your soils engineer can give you some values for the fill loading and the additional 100 psf live load would cover the infrequent, meandering livestock.

 

[HMROSS]

Thanks Jheidt2543, I have already contacted Spancrete manufacturers and Fabcon. I will also follow up on Flexicore--I don't remember seeing them as a memember on the Calif. Precast list. Are you aware of anyone providing arched precast units other than Conspan?

 

[Lutfi]

Funny you mention this because I designed similar type building over 20 years ago and it is still in great shape. I used steel bar joist, metal deck and concrete slab on top. Of course hollow core type planks will work as well.

The critical part in this type of structure is to be able to safely support the dirt/soil, and allow additional load for moisture weight.

DO NOT forget a good waterproofing system.

Good luck.

 

[epr]

Drainage of the roof is also a good idea. Install a perforated pipe drainage system in permeable fill on top of a pitched roof, overlay with a geotextile drainage fabric, and this will help prevent complete saturation of the soils. However, I would still allow for saturated weight of soils and a 100 psf live load.

Also, I am a firm believer in using clay (Volclay panels and the like) as a waterproofing membrane, because it seals itself. The worst job, is trying to find a leak in a synthetic membrane under soil after-the-fact.

 

[jheidt2543]

HMROSS,

I'm in the midwest and all three precast systems compete against each other around here. I'm not sure if Flexicore is in California, but I would assume so. I've believe I've see Conspan ads in some national publications, but I've never seen them on a job in my neck of the woods.

Lutfi and epr make good points regarding waterproofing and drainage of the deck. One nice thing about precast is it has camber to help keep water from ponding in the middle of the span (I'm assuming a clear span application here). Regardless of material selection, provide positive pitch, just like a normal roof system and also waterproof.

This method of construction was really popular in the '70's and 80's, but has become less prevelant the last ten years in this area.

 

[denoid]

I, too, designed a similar residence in rural Wisconsin back in the early 1980's. We used a precast plank roof, (I seem to recall it was Spancrete, but I could be wrong), with poured concrete walls. We sloped the plank roof sufficiently for saturated soil on the roof.

A caution, however, with using Volclay panels and joint sealant for waterproofing. We used Volclay on the roof and walls, with a "French drain" and perimeter drain tile at the foundation, but ended up in a minor lawsuit from the homeowner due to moisture intrusion early on after they moved in. The volclay products work well once they are saturated, and expand to form a sealant against moisture. We weren't on site during the laying and caulking of the panels, and somehow I don't think the panels were pre-saturated to form a good joint sealant prior to backfilling. We relied on the architect for specifying the product and some of the observations, and I'm not sure everything was done per Volclay's recommendations. Eventually, I'm told, the problem resolved itself, once the panels did reach sufficient saturation to seal up the joints, but not before we had to deal with an unhappy new homeowner.

I agree with epr that it's a good application, but, to avoid issues, maybe a "belt and suspender approach" like Volclay over a membrane isn't a bad idea to save on problems down the road. Good luck!

 

[epr]

denoid makes a good point. Whatever membrane or waterproofing system is installed, it must be installed correctly. Inspection is a must because it will (hopefully) be forever hidden.
Relative to jheidt2543's comment regarding camber though, camber with precast prestressed plank is usually gone after so much dead load (300 to 400 psf of soil)is placed. That is why the pitch of the roof is crucial.

 

[HMROSS]

Thanks for all the good information from EPR,DENOID, JHEIDT2543, and LUTFI. All of you are helping me wade through these issues.

Due to the need to preserve a large (2.5' Dia.) blue oak tree, we have reconfigured our footprint. The result is clear spans of 16 feet.

If you had to construct the roof system yourself with friends and family in a remote site, what system (no wood please) would you use to clear the 16 feet and carry the 300 lbs/ft^2 deal load and 100 lbs/ft^2 live load?

Also, Using the 97UBC what load combination would be the most appropriate given these large loads?