Earth Homes and Tornados

[msquared48]

Anyone hears or seen any information on the success or failure of an earth home design to resist a direct hit by a tornado of varying intensities?

Just wondering - no job involved.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[JAE]

Well, the cow standing on top of the roof will most likely get blown off.

 

[msquared48]

You obviously have too much time on your hands JAE.

You are hereby designated to chase the cows.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[JAE]

Gulp. Sorry. I've been chastened.

I don't have an answer to your question directly. Based on FEMA 361 and the ICC 500 code, an earth home would have to resist both the wind forces, and the projectile impact.

For an earth home, the raw dead weight would probably be enough for the walls, but wonder about the roof - if it is 2 feet of earth (180 psf, then technically you might not meet the 0.6D+W requirement against uplift assuming 90 pcf dirt).

I'd have to calculate the wind force uplift. Per ICC 500 I think you actually take a percentage of the wind (0.6D + x% of W) since a tornado force wind pressure is considered an "ultimate" condition much like seismic.

I would guess that a couple of feet of earth would be as good as an 8" fully grouted masonry wall but perhaps Texas Tech would have to test it with their cannon.

 

[Subie31]

I've never seen testing or papers on this. Everything I've seen has been conventional construction.

 

[SLTA]

Do you mean earth homes like above ground, rammed-earth types? I've always wondered about connection of roof to walls on those. Or do you mean underground, like in the side of a hill? Seems to me those would be perfect.

Man it's windy here today.

 

[concretemasonry]

There are two major types of "earth homes" (no technical definition now).

One is an "earth sheltered" that was very "green" and built into a slope and usually has concrete or block walls and and a precast concrete roof with soil, grass and goats or cows on the roof. They usually faced toward the south with a broad exposure and they used very little energy. Living space was in the front (south) and bedrooms and baths were in the back. The one problem was emergency egress, so many had a stairway to access the surface and allow the occupants to go up up and groom the cows and goats or view the wild flowers without walking around and up the slope. Great use of thermal inertia and storing heat from the sun. - 1970's, 1980's style, with a costly and leakage prone roof system because of the high moisture content of the garden soil

The other is an "earth bermed" where the walls were concrete or concrete block, but a trussed roof was use for economy and eliminate the problems of a roof with "goats and grass" on it. One problem was that he buried walls had to be cantilever walls with no lateral support at the top, but the access to the exterior was easier and cheaper and many times the main accessed was down through a stairway into a foyer. - This followed the earlier earth shelter versions. Unfortunately, this provided little protection from projectiles from a tornado and there was a problem with anchorage or the roof system since it was essentially an airfoil in many cases when the wind blows over an open field and encounters a sloped change in elevation.

Bottom line is that FEMA recommendations (NOT STANDARDS unless adopted by a code body with authority) are well based and very good for general design and construction details, but it is up to the engineer to make the "belt and suspenders" requirements before signing off since projectiles from tornadoes and not really defined in any code. FEMA, backed up but years of testing does have some wall and roof sections that are acceptable for projectiles. There is no question that a small shelter buried is obviously adequate for tornadoes without a signed plan, but who would pay an engineer to design it?.

Rammed earth is just a method of using available materials (earth of some kind and possibly with cement)of unknown properties to build a traditional home.

There is also an movement by insurance companies to offer major (30-40%) discounts for homes built according to their standards that exceed codes. The major changes are the garage doors, gable end roof design and roof system anchorage.

Just a bit of history. -

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[msquared48]

I am not talking about rammed earth here.

Obbviously, the sidehill scenario is best, but I also noted some with imported fill around the sides and on the top.

My concern is that these homes should be considered more for rebuiding rural farmhouses that have been damaged by tornadoes. You think the banks, insurance companies, or government would try to create a program to try to mitigate future damage, at least helping to rebuilding some homes in this manner.

An additional concern, contrary to the facing of the home to the south, would be to face the front face of the home away from the most likely direction of approach of these storms. Metal roll up doors could be provided on this side that could be lowered in the event of a storm. Other exits and skylights could be provided, as well as broad spectrum lighting inside.

This should also work well in an earthquake, but Tornados, even hurricanes, are my immediate concern.

You'd think some of the fire stations and police stations that were destroyed could be rebuilt in this manner. Better than wood or stick framing - we have seen the result of that many times.

Probably a pipe dream, but, hey, what the heck!

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[concretemasonry]

Mike -

You are talking about rebuilding existing structures after they are no longer functional for one reason or another. The orientation of the structure cannot be changed and a structural engineer rarely has the power or authority to re-orient a new project based on the probability of a tornado, let alone the direction of the storm. A tornado is just a small, concentrated weather system with high winds and suction that are really not directional.

The FAA and similar organizations have similar planning problems and they provide wind maps for different locations showing the likelihood or probability when it comes to orient a landing strip or runway. Tornadoes are a different matter and generally come from the SW with a following suction winds if one ever hit the site. I can remember being on my deck cooking and watching a tornado pass to the NW as it traveled to the NE and some children were killed. Fortunately the doppler radar on TV gave an accurate path of the storm system.

Shutters are not a structural item and meaningless when it comes to projectile projection as evidenced by the various codes for opening protection.

Hurricanes are much easier to design for, especially if you have a few feet of elevation.

I have been through tornadoes and hurricanes and there is a big difference because the "advertised" hurricane wind velocity is reduced dramatically from the landfall velocities by ground friction, while individual tornadoes are relatively short-lived, and other tornadoes are frequently spawned to create a "tornado path". After looking at the Katrina damage for months, most of the damage was from the storm surge that was dissipated within 3/4 mile and then the political interference with engineering caused the inland flooding.

Tornadoes are a dramatic, random natural occurrence and designing for zero damage is not economical, so the best path is to provide for life safety close the inhabitants. I always think the concepts of schools/gymnasiums/auditoriums are a problem because of the type of structure and number of people concentrated, but good for newscasters after the fact.


Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[msquared48]

Dick:

I can understand completely that reasoning regarding the economics here, but it is a situation not unlike raising the foundation of destroyed homes to be rebuilt in a flood zone. You don't want them to rebuild there, but, short of rezoning, which is possible, it does mitigate some future damage.

As for orienting the structure, some can be reoriented, but not all, true. And I do understand the cyclonic action opening the structure up to projectile attack from all sides. That being said, with three of the four sides, plus the roof, protected, your odds of damage go down dramatically. And, after the storm passes,the structure is still there, maybe not the front wall, but that can be replaced far more easily than the entire structure.

A small safe room could easily be bult in the rear of the structure, used for a storeroom/root celler.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[TXStructural]

An earth-bermed structure should withstand a tornado quite well. The soil mass acts to redirect the wind, absorb energy, and block debris (projectiles). That said, by the time you build a bunker sufficient to hold back the weight and moisture in the soil, you probably could have built it above ground and avoided he moisture issues. Obviously, underground, you won't have windows to worry about, but sleeping rooms would still have to have code-required egress directly to the exterior.

The earth cover would need to be on all sides, since the direction of travel is less important than wind direction, which could change 360 degrees as a storm passes.

For example, a home built of 6" thick ICF exterior walls and a concrete attic floor or roof deck, with normal reinforcement, would be safe for most tornadoes, provided it was appropriately designed (avoiding crazy architectural or homeowner ideas, like window walls, in a tornado-safe house, and unusual proportions.
--
For some time now, I have been working with the Concrete Joint Sustainabilty Initiative (representing the various concrete industry associations), FEMA, and the insurance Institute for Business and Home Safety, on a series of workshops emphasizing resilient construction to reduce losses in earthquakes, windstorms, wildfires, and floods. These kick off in April and May. (The only mention of concrete construction is a short segment late in the day, so it's not a day-long sales pitch. Google for more info.)
Also, Google IBHS for info on their FORTIFIED program that outlines some disaster resistance strategies.

 

[JAE]

by the time you build a bunker sufficient to hold back the weight and moisture in the soil, you probably could have built it above ground and avoided he moisture issues

.
This is true.

 

[concretemasonry]

It all depends on what the owner wants as far as a concept for the home/structure, Some just pay for a structural design and not more and try to find a way to get it built. - It was common in the hay-days of the earth shelter concept in the 1970's. The good structural engineers did the design and eliminated themselves from the construction and finishing and only assumed how many goats were on the roof. One of my classmates jokingly put the assumed number of goats(mobile concentrated loads)in the calculations.

There were many successful homes built in that 1970's period but a lot of DIY goofs that had a better idea and skimped. The best roof systems were the smaller, narrow (24") precast plank that could be set easily and quickly with minimum equipment and 3 man crew with a small truck crane.- Followed by a pumped 2" topping before waterproofing and grass and goats.

I never hear of a wind problem even though there were deaths in conventional homes and basement within 1/4 mile or less

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[Teguci]

Always had a hankering to see how concrete domes faired under high winds. Maybe a combination dome/earthen berm could be a strategy. Definitely above grade construction though. No need to go all hobbit on us.

 

[msquared48]

Depending on the water taboe and local drainage patterns (if any), the home could be sunk into the ground, say three to four feet, using the excavated material to partly offset the need for imported fill for the berm. However, I do agree that if the drainage conditins do not warrant this, put it above ground.

Good idea about the dome Teguci. Maybe a concrete Yurt style?

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[TXStructural]

IBC does require perimeter drains for most cases where soil surrounds the building, in flat places, daylighting a drain is bothersome, and maintenance of mechanical drains (pumps) is not going to be maintained.
The other thing I see in this is the volume of non-native backfill required, since much of tornado alley is expansive clay.

 

[denoid]

I did the structural engineering for an earth sheltered home in the late 1970's while working for one of my former employers. Unfortunately, the Architect used a DIY skimp method, (as concretemasonry mentions above), for waterproofing the walls and roof. It think he was sold a bill of goods by a product rep for an installation that was never really tested.

As I recall, it was a powdered bentonite clay product sandwiched between cardboard sheets that were fastened to the concrete walls/precast roof, using some additional bentonite caulking for the taped joints. The theory was, (I guess), that the cardboard would decay, and the bentonite would get wet, gel, expand, and seal the buried portions of the earth sheltered structure.

End result was that when the owners moved in, it practically rained in the house for about the first three months. The problem was eventually discovered that the waterproofing system needed time in the soil to actually work the way it was advertised. So, until the bentonite became thoroughly soaked and allowed to expand, the system leaked horribly.

Good news was that, structurally, the building worked like a charm; bad news was that, as part of the entire design team sued for the fiasco, we had to "contribute" to stay out of court. The whole situation left a bad taste in my mouth about earth sheltered anything, but, I'm told that once the owners hired someone to properly re-waterproof the roof in a conventional fashion, (partially with my ex-firm's money), the system worked like a charm.

 

[msquared48]

The only earth home I did was a small vacation structure on Henry Island in the San Juans about 15 years or so ago. I used ferrous cement over metal lath grid, and it was formed, applied and constructed the same way concrete boats were made. Used the same mix design too. Fit into the hillside great and the local cows loved it. Hard to see from the water and blends into the landscape very well architecturally. JAE would be proud.

Always wondered how that would fare in a major storm though.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com