Tornado Safe Structure

[Westward]

Hey, I was hoping someone can direct me to the current guide lines for designing tornado proof structures. I have looked to the FEMA references but was wondering if ASCE 7 had anything out that anyone was familiar with. Thanks in advance for any and all help.

 

[vandede427]

FEMA 361
FEMA 431

 

[dhengr]

Several feet of earth, a real thick concrete roof slab, and the rest of the building below that slab; is a fairly good design against tornados. Above ground structures can be made more tornado resistant, as in design for coastal hurricane conditions. But, tornados can be horribly erratic and really carry a punch, above and beyond most hurricanes, I believe. Look at blast design, as in Homeland Security design for susceptible infrastructure.

 

[vandede427]

make sure all your walls and roof are constructed with a rated system that meets the missile impact test.

 

[dicksewerrat]

Bucky-Fuller domes with 12 inch concrete panels and as dhengr said bury it.

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[TXStructural]

Is the purpose survivability of the occupants, or having a lack of significant repair?

Paraphrasing: All wind is local. While there will be significant wind pushing and pulling on the main wind force resisting system, much of the damage starts as components fail in a progressive manner.

For survivability, 6 inch reinforced ICF walls with a CIP concrete ceiling/roof works quite well. If you don't want a (nearly) flat roof, you can use the concrete lid as an attic floor. Windows and residential doors are sacrificial under tornadic wind, as will be the cladding.

Having some strong interior walls (4 inch ICF, grouted CMU, plywood on stud) will reduce wind and debris from entering through failed windows. Be sure the partitions are sufficiently anchored.

There is limited good research on tornadic wind fields and houses. Unless you know how well something was built, and exactly what the wind was, you cannot correlate wind to damage sufficiently to get good design advice. Hurricane wind is much more easily correlated to destruction since the damage is typically less complete and the winds are more easily determined. Additionally, the risk of a single structure being hit by a tornado in its lifespan is infinitely small.

I am a proponent of taking reasonable measures, and my next house will be ICF and concrete roof, and the plans have sacrificial cladding and windows, and interior partitions which will protect occupants in sleeping rooms and other areas of refuge.

 

[concretemasonry]

The FEMA standards (316? I beleive) are the most comprehensive. They are not structural requirements because they are concerned with the safety of the occupants and not just the strength of the structure according to some tables on a national code.

The main criteria is projection from projectiles and impact. It is based on tests at Texas A&M(?) over the years on different wall sections. The they tested numerous wall section and have only approved a very few The test procedure uses a 12' long 2x4 fired from an air cannon at about 140 mph and there must be no pentration.

The actual standard also includes roof and anchorage requirements, hardware requirements, door requirements, ventilation suggestions and direction of door swing to allow exiting after the storm.

This iinformation is based on about 20 years of development.

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

 

[JAE]

FEMA 361 is a guide. The ICC has come out with ICC 500 to codify what 361 only suggests...

Link to ICC 500

 

[TXStructural]

Texas Tech did the testing. The problem is that you never know what the exposure will be, and the testing really only compares one wall to another. Generally, CIP concrete is better than drywall on stud. In between, there are options:

http://www.depts.ttu.edu/weweb/Research/DebrisImpact/TestingLab.php

Start with FEMA's website:

http://www.fema.gov/plan/index.shtm

General info:

http://www.fema.gov/rebuild/smart_strong.shtm

FEMA 361 (

http://www.fema.gov/library/viewRecord.do?id=1657)

is good info, but you are not planning for a known event, and there is no one answer to "how strong is just strong enough?" You are looking at an extreme event, and if you are serious about surviving an F5, plan on building the proverbial brick outhouse. It is the uncertainty that requires additional strength.

 

[Westward]

Thanks A Lot Everyone!
Ok so here are a few more specifics concerning this particular project: (I should have stated earlier)
The shelter is to be constructed of (cast-in-place) reinforced concrete walls and a (pre-cast, not pretension) reinforced concrete roof. The building is roughly 31'x32'x10' with no interior columns. After reviewing mainly the FEMA 361 document I have determined that the design wind speed for this structure will be 250 mph. This is actually the same speed I get with the ICC 500 document. Anyway, I am pretty sure I am correct but would greatly appreciate it if someone can confirm that my next step is to use the ASCE7 05 approach to determine my design pressures. Also, now that you guys have a little better picture of what the structure looks like I would love if anyone had any ideas for or would like to share connection you've used between the walls and the roof. Asesthetics is always a factor however here I doubt it is a big deal, this shelter is already inside a larger pre-fabricated metal building so that i'm sure will look funny by itself. Once again thanks for the advice in advance.

 

[concretemasonry]

My apologies to my friends at and from Texas Tech for making a major error by saying Texas A&M, when I did know better. - I was just told that over the phone by a friend. That is what happens when you visit the wide open spaces to see testing methods and procedures.

Granted, the FEMA is the classic "belt and suspenders" approach, but it is for life safety and survival. - Just ask the thousands of people that have been through a tornado where the instrumentation cannot provide a number to relate to general standards. The wall sections listed are the basis to survive no matter where the structure is located or oriented. The key parts are the connections and access suggestions. All too often, the "Johnny-Come-Lately" marketing people may object since they did not have the foresight to be involved in the historic testing. - This can easily lead to "cut and paste" applications without looking at the true life safety basis of survival.

Fortunately, I has not involved in the political and tactical procedure to transfer testing and suggested standards into a politically correct and enforceable potential legislative document. The late comers typically come with a too late plea for for their individual systems that are not approved by performance and details suggested.

Dick

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

 

[vandede427]

Anybody ever seen an air cannon used to test the missile impact? I have and I can tell you it's very impressive. In school, a Mech Eng prof teamed up with a Timber Eng prof for a grad research program and built one. We watched them shoot 2x4's through plywood. Pretty cool. It was rumored that they shot a tennis ball clear across campus.

JAE is correct, ICC 500 is what you want to use as a base for structural design. But my understanding is that ICC doesn't specify a lot of stuff related to architectural considerations (minimum sf per occupant, egress, ventilation, etc). These are specifically laid out in FEMA 361. But you only have to use FEMA if the facility is receiving public money to be designated as a Community Shelter. If the shelter is private, than just stick with ICC but encourage your owner to consider the architecutral requirements of FEMA 361 as good measure.

 

[TXStructural]

Consider that the outer structure WILL come down around and on your shelter. Impact of structural steel members and other heavy items should be considered in the design.
--
I'm not sure exactly how the use of 2x4's as projectiles became the standard, except that they are ubiquitous and easy to accelerate. The test ONLY allows the comparison of one wall system to another which has undergone the same test. It is an easy, inexpensive test.

John
Texas A&M 1988
Texas Tech 2004 (structures and wind)

 

[Seversen]

For structures of this type that I have been involved in, rebar has been cast in the top of the cast in place walls and bent over the top of the precast roof. Then a topping is placed over the precast roof. This serves to tie the roof to the walls and provide additional load to resist uplift.

 

[Seversen]

I have also done a number of these inside buildings as you describe.