Concrete Ceiling - Is it safe during a tornado? 2

[keharrisuiuc]

I'm not 100% sure this is the right place to post this question but I have to start somewhere. I recently bought an early 1900's, 2 story farmhouse. There is a small room (8'X 8') on the east end of the basement. The ceiling in this room is a concrete slab. The room sits directly below an old enclosed porch and the ceiling is about 10 inches lower than the rest of the house. My question is, first of all, what would been the purpose of a concrete ceiling? And two, would this be a safe place to take shelter in a tornado? It would seem like it to me but my concern is that it could potentially backfire and the ceiling could collapse in the event of a severe tornado. I'm attaching some photos. I would appreciate any advice I can get. Thanks!

 

[concretemasonry]

It was probably built for the cool storage of vegetables and root crops, so they were available for long periods. It was very common since help maintain a fairly uniform temperature and relative humidity in cooler climates.

Similar construction is the FEMA requirements for "safe cells". Usually, the design recommended is to use a reinforced 8" concrete slab supported by either 8" reinforced concrete walls or 8" reinforced concrete block walls. The criteria is to provide protection from debris and projectiles with no penetration since it is a life safety issue. The recommendations are based on many years of actual testing at a Texas University. These recommendations apply for "safe cells" within a structure (below or above grade) or for remote structures that are buried or partially buried. Obviously, it is not practical to design individual cells from from a structural standpoint, so the engineers used a performance-based criteria for individual elements. AS far as debris, whatever goes up, comes down, either vertically or horizontally.

Your 10" slab should be adequate, but the amount of reinforcement is unknown, but the 8' spans are short. The question is what are the materials for walls? It is not a typical structural design. since no codes really apply and proven performance is the key for a once in a lifetime event.

Because the FEMA standards are based on life-safety, they contain very specific details that are somewhat unusual. One key item is that the steel door should open into the room to allow egress in the case of a collapse of the adjoining structure.

Search for "safe cell" on the FEMA site for some very interesting study.

Dick

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

 

[JAE]

"Is it safe" for a tornado is a relative phrase...there are degrees of safety and compared with other areas of your residence, this might be the "safest" place. That doesn't mean a tornado wouldn't collapse the space, or that projectiles wouldn't penetrate inside (through the window in your photo).

To know for certain you'd need to hire a structural engineer to come in and evaluate the pros and cons of your space, explain the relative risks of the space and even perhaps determine the load capacity of the ceiling/walls. They could also offer recommendations for things you can do to add to the safety such as blocking-in the window.

 

[concretemasonry]

The window is obviously a weak point when it comes to life safety, but it is an easy and obvious item to correct. Upgrading is an opportunity to add some ventilation and FEMA does have that information, but the needs are minimal for a short stay in a shelter.

Good luck finding a structural engineer with a knowledge of the practical effects of a tornado shelter since he would be in uncharted territory since there are no standards or codes to use as a guide for F3 to F5 tornadoes. Finding and engineer to evaluate the strength of the slab and connections, but for a cellar the very short term wind has little effect on the walls if the roof of the cell is intact.

The lack of practical engineering knowledge is the reason that the "safe cell" concept (semi-prescriptive) is based on the properties of the acceptable elements and gives a guide to survival. The suggestions (not a code or requirement) are definitely over the top, but provide a practical guide to increase the ability to provide life safety. That is the reason it is used for safe cells within a new frame structure that often are also used as a large closet or a bathroom.

Dick

Dick

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

 

[JAE]

there are no standards or codes to use as a guide for F3 to F5 tornadoes.

That isn't true - check out ICC 500. It is a "code" and provides explicit design requirements for wind forces from tornadoes and hurricanes.
All of FEMA 361 and ICC 500 are based on designing for tornadic activity and provide guidance for tornado wind/pressure forces (250 mph wind) as well as guidance for projectile resistance.

With a window above the ground level (per the photo) the wind loads DO have an effect on the brick walls. It would be stupidity to just ignore that.

Earlier you stated, "Obviously, it is not practical to design individual cells from from a structural standpoint," - I disagree with you - I've designed numerous tornado shelters based on applied wind forces and pressures both under FEMA 361 and ICC 500. It is practical and done all the time.

To keharrisuiuc: Many engineers know these documents as well as other literature out there - so getting a structural engineer to review you situation is a good step.

 

[concretemasonry]

The FEMA documents are not written in code language, so enforcement is near impossible but are great references and many locations reference them.

I erred when I used the word "practical" when I should have been stated that it is not practical because very few engineers would be able to justify the billable hours for a detailed analysis for a small hole in a 100 year old farmhouse wall that for the most part is subjected to much greater loads from the soil since the exception/hole in the wall has little to do with the ability of the "ceiling" to withstand very short term loads and impact. The opening is still an obvious point to consider when it comes to vertical loads and the strength of the members forming the opening if the slab is bearing. When it comes to safety, filling the opening with more masonry is compatible when you add some ventilation, if needed.

The analysis of windows in basement walls for long term loads is possible and has been beat to death for years even without considering the actual fixity. It is a fun, classic problem to analyze when there is nothing else to do since feel people could justify paying the cost in the real world. I first did it 40 years ago because there was nothing else to do and was encouraged to do so because it was similar to the problems we had designing concrete instrumentation building downstream from rocket test stands, but my time was paid by corporate, NASA and the Air Force. - One of the perks you get when employed by multiple sources that want the time used in the best way.

Dick

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

 

[JAE]

Dick - per my above post - ICC 500 is a CODE that encompasses FEMA 361.

Please go review it on the icc website: ICC 500 link

The rest of your post makes no sense to me. It seems to suggest that engineers simply can't design a storm shelter.
That is ridiculous.

 

[msquared48]

All I know is that I don't want to be anywhere around an F1, let alone an F5.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[rowingengineer]

I agree with JAE, get a local engineer involved, he should be able to provide some level of confidence as to the soundness of the structure.

http://www.nceng.com.au/

"Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning."

 

[shobroco]

The house might be early 1900s but the slab doesn't look like it. The board forms were covered with something to make the concrete smooth & prevent leaks, likely plastic, so that will give you an earliest possible date of 1950s & more likely 60s. The excellent condition of the concrete supports this date. You say an enclosed porch, but was it an open porch when it was poured, & enclosed later? Was the whole thing added to the house (are the walls different than the basement)? There are many more questions than just your OP & only a physical investigation will answer them, if even then.