F-2 Tornado Proof Structure 4

[sponcyv]

I am working on a job on a FEMA facility and they are specifically requiring it to be designed to withstand an F-2 Tornado. I have found the wind speeds for an F-2 tornado go up to 157 mph. Has anyone ever had this type of requirement on a job before? I'm not familiar with designing for a certain type of event. I have always designed from the ASCE 7 and the wind speeds are based on location and other parameters. I would assume I would just use 157 mph for my wind velocity and follow ASCE 7.

 

[TTUengr51]

I have done a couple of tornado shelter designs. To do so, I used FEMA 361 as my design reference. It prescribes wind speeds and other various factors to use in conjunction with ASCE 7. It also has some prescriptive requirements to account for missile impact (2x4's flying thru the air) on the walls, roofs, etc.

Nick Deal, P.E.
Michael Brady Inc.

http://www.michaelbradyinc.com

 

[a2mfk]

All I can say is the devil is in the details on this. No doubt you can make the structural skeleton work, but be careful when you get down to the components and cladding...

 

[H57]

While FEMA 361 provides some guidance, I would recommend ICC 500-2008. This is the standard for the design and construction of storm shelters. This is the referenced standard for storm shelter design in the 2009 IBC. In general it uses the same criteria as the FEMA 361 recommendations, but there are differences. The new edition of FEMA 361 discusses some of the differences and also references ICC 500 as the standard for the design.

 

[JAE]

H57 is correct - FEMA 361 is a handy guide but ICC500 would be the code to use.

 

[JedClampett]

I remember from my Nuclear days, the F5 tornado involved the tornado rotating winds (about 220 mph) plus the tornado speed (like 80 mph). The controlling case added together the two of them (300 mph). On the other side of the tornado, the internal speed was reversed (-140 mph)causing a torsion on the building. There was alos an internal pressure component.
Anyway, maybe there's some NRC documents on tornados with all the F scale tornados explained.
You're going to have problems with window and door qualifications. In Miami Dade County, they do testing to 146 mph and even that will limit you to just a few manufacturers.

 

[concretemasonry]

There can be a major difference between designing an over-all structure from a structural standpoint (wind loads, etc.) and a FEMA "safe cell" for protection and survival of individuals.

The classic FEMA tornado structure designed for protection of individuals does not provide any information on wind loads to be applied to a general structure. It is predicated on a smaller structure and contains some well developed plans including specific hardware, ventilation and and layout concepts. The suggested wall sections are not based on structural criteria because of the size and aspect ratios. The acceptable wall sections (reinforced concrete, reinforced concrete masonry and a sandwich of 3/4" plywood sandwiching a steel plate) have been tested for projectile penetration at Texas Tech over a period of many years. - It looked like a very entertaining job/project. After looking at the wall and roof construction requirements there is no need for detailed structural analysis since protection from projectiles over-rides any wind loads. Generally a reinforced concrete roof and concrete foundation is required.

The suggested details are quite well thought out when it comes to doors, where inward swinging door are shown to permit egress from the space if the exterior has debris around the area.

The standard for personal protection is not engineer-related but is well presented to address the needs.

In a larger facility, this could be used in an interior area, in a basement or as a separate detached facility.

Dick

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

 

[JAE]

there is no need for detailed structural analysis since protection from projectiles over-rides any wind loads

concretemasonry - I'm not sure I understand your point here. Severe storm shelters, using both FEMA 361 and ICC 500, require the engineer to design for BOTH projectile and wind load resistance. They are two different things.

Dealing with projectiles alone doesn't negate the requirement to design for the wind forces - which is what your statement above seems to imply.

Just asking for clarification. Thanks.

 

[sponcyv]

JAE,

I think concretemasonry was stating that the missile projectile design governs. It definitely does in my situation. I'm only doing a small "F-2 / EF-2 tornado proof" structure, not a community safe room. This is really just a tiny guard shack, but the concepts are still the same. The masonry wall sections that are approved and tested for missile projection in FEMA-361 show every cell grout filled with #4 bars.

The interesting piece is that the guard shack is being designed to survive an F-2 / EF-2 tornado because the old guard shack was demolished by a tornado and the guard was in the restroom at the time.

 

[concretemasonry]

JAE -

I was generally referring to FEMA 320 document. Which is intended to provide a safe area (for up to 16 people in the case of a tornado. The FEMA 361 document refers to a safe area for communities that generally must be engineered.

The following is a portion of the 320 document.

Quote - "Why is the term "safe room" being used instead of "shelter"?

The terms "safe room" and "shelter" have been used, for the most part, interchangeably in past publications. Typically the difference in usage was limited to differentiating between residential applications called "safe rooms" and larger projects called "community shelters." The release of the ICC-500 standard, as well as other national, state, and local protection initiatives, identified a need to distinguish shelters that meet the FEMA criteria for near-absolute protection and those that do not. Although both the FEMA and ICC criteria are designed to provide life-safety protection for safe rooms and shelters that meet these criteria, only the FEMA criteria provides near-absolute protection from extreme wind events. To help clarify the difference between safe rooms design to FEMA 320 and 361 guidance, the term "safe room" applies to all shelters, buildings, or spaces designed to the FEMA criteria (whether for individuals, residences, small businesses, schools, or communities). This allows for the buildings, shelters, or spaces designed to the ICC-500 standard to be called shelters. All safe room criteria in the FEMA publications meet or exceed the shelter requirements of the ICC-500. Download FEMA 361.

A residential safe room is designed to protect families or small groups of people (up to 16) while a community safe room is defined as a shelter designed and constructed to protect a larger group of people from a natural hazard event. This publication will refer to all shelters constructed to meet the FEMA criteria (whether for individuals, residences, small businesses, schools, or communities) as safe rooms.

The ICC-500 provides the minimum design and construction requirements for extreme-wind storm shelters and is expected to be incorporated (by reference) into the 2009 International Building Code (IBC) and International Residential Code (IRC). It is important that those involved in the design, construction, and maintenance of storm shelters be knowledgeable of both FEMA guidance and ICC standards that pertain to sheltering from extreme winds." - Closed quote.

The 320 structures do not generally require engineering and the standards are similar to a presciptive code with approved wall section because history has shown that the most common injury or cause of death in a tornado is due to projectiles. That is the reason for the extensive test at Texas Tech to evaluate the real resistance to pentration. Generally these type of applications are for a very robust room. The designs are for a "room" that can be in a basement, inside a slab on grade structure, a free standing building or buried (partially or completely). The general structures are usually 8' or 10' maximum wall lengths,. The walls (8" thick) are reinforced concrete masonry or reinforced concrete with reinforcement generally at 8" o.c. The foundation is a concrete slab with dowels and reinforcement and the roof is 8" reinforced concrete. Special doors and hardware are also detailed.

The reason for the prescriptive type of plans is because these structures are not always in code controlled areas and there had to be a safe standard for people to follow. Currently many are a specialized part of a new structure. For slab on grade structures they are commonly also used as walk-in closets or bathroom. They may also be built in a basement and used as a bathroom or a storage room. The exterior applications (free standing or buried) follow the traditional storm shelters, but built to a higher standard. - Just visualize a 8' cube with 8" thick solid reinforced concrete walls and roof.

Obviously, the larger safe rooms (classified as community shelters) are a part of a bigger project and are usually controlled by codes and standards with specific wind categories that should be engineered because of the dimensions and number of people.

I just naturally gravitate to the FENA standards because I am more familiar with them, have observed the testing and been involved in some construction. Sorry about any confusion.

Dick




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

 

[JAE]

Thank you concretemasonry.

 

[cvg]

see the attached for a photo of a typical structure taken in south central kansas. This one appears to be buried about half way and is solid reinforced concrete approximately 8' x 8' square.

 

[a2mfk]

Not much help for this situation, but I have seen pre-cast concrete "bunkers" for storm rooms advertised for sale down here in Florida, similar to the one in the picture that could probably be dropped off of a tractor trailer... But with our rain fall and water tables, you would not want to bury them in most locations. I have always liked the master bedroom walk-in closet idea of a "safe room" for hurricanes/tornadoes. I am randomly throwing around my terms here...

 

[structuresguy]

I have designed several EOC's, emergency shelters in community centers, and EHPA's in schools, as well as a couple tornado shelters, here in Florida. The biggest impedement to proper design is not the wind pressures, not the impact, not the ability to engineer a safe structure, it is the architect. For some reason, they insist on covering these "bunkers" with windows, storefront, glass doors, you name it.

If you keep the architects as far away from your building as possible, you'll be ok. Concrete and masonry can easily be designed for virtually any wind and impact loads.

 

[a2mfk]

Structuresguy- But architects believe all of our buildings would be bunkers without their input

I suppose with some of the buildings you were dealing with, the structure had multiple uses outside of being a storm shelter. I remember having to deal with this on gymnasiums for public schools. But like you said, if it is a straight CMU or concrete safe room/ bunker without fancy openings, it should be no problem, since the military and others have been doing this to resist nuclear blasts for 65+ years now...

 

[structuresguy]

Yep, mixed use facilities, like community centers which double as shelters, are the most challenging because they have to look decent, while still being functional.

My personal favorite is when an EOC/shelter is only in part of the building. I did one once that was about half of the ground floor of a 3 story building. So what do you design the upper floors to? what if they collapse on top of your shelter? What about once the exterior envelope above gets compromised, is the envelope of your shelter still intact? And lets hope your ground floor doesn't get flooded out.

No matter what recommendations you make to an owner/architect, sometimes they just insist on making the wrong choices.