Designing for F5 Tornado 1

[DaveAtkins]

I am working with a client who is wondering what would be involved in creating a "tornado shelter" within a new building. The client thinks the locker rooms within the building could be designed as a tornado shelter.

We originally proposed that the locker room CMU walls be designed as exterior walls per ASCE 7 (even though they are interior walls), and the roof deck above the locker rooms have additional puddle welds, but no other special design be done. But the client wants to know what it will take to really design for a tornado.

I must admit, I have never done this kind of design before. I found an Eng-Tips thread from 2006 which discussed tornado wind speed:

Category F5: Incredible Damage (261- 318 mph)

Do I use this wind speed, and plug it into the ASCE 7 formulas? Seems a bit excessive to me.

DaveAtkins

 

[msquared48]

If the building has a basement, I would put it in a corner next to an RC wall, and make it RC totally enclosed. I would also provide a separate external exit with doors at both ends.

JAE could give more insight here as he iswell in the F5 zone of probability I believe.


Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[H57]

FEMA 361 is very handy for a high wind event design guide. This outlines the wind speed requirements for different areas of the country in addition to the debris impact requirements. From this you can see what wall, roof, door and windows meet the impact requirements.

From this the ICC-500 standard for shelter design was established. This outlines the structural criteria (along with civil, arch., mech., and elec.) for the design of storm shelters. This is the referenced standard for storm shelter design in section 423 of IBC 2009.

 

[H57]

With regards to your original construction:

8" cmu that is reinforced with #4 bars at ea. cell meet the impact requirements.

The only roof deck listed states that 4" reinforced concrete meets the impact requirements. I have done more internet research on this and found a table by the state of florida that outlines various roof assemblies and which ones pass the requirments for various missle impacts. At this time I don't think you'll find many metal roof decks meeting the requirement.

For wind speed it is using ASCE 7 equations to calculate wind pressures (and especially uplift for the roof) and designing the structure accordingly.

 

[Resso]

Here's a useful link to the National Storm Shelter Association:

http://www.nssa.cc/

The FEMA 361 design criterial will point you in the right direction. We have recently tested a composite metal deck/insulation/standing seam roof system to withstand 535 psf of wind uplift and the FEMA 361 missile impact criteria. Other than cast-in place reinforced concrete, I don't know of any other roof structures that will meet the FEMA 361 hurricane shelter criteria.

 

[JAE]

H57 got it - ICC 500 is the document you need. It is the "codified" version of FEMA 361. 361 is helpful (and free from FEMA) in that it includes various wall types, etc. that meet the projectile criteria.

Contact ICC (or go to their website -

http://www.iccsafe.org)

and order ICC 500.

 

[vandede427]

ICC 550 has a specific section for shelters located within a larger building.

 

[structuresguy]

Watch out for the roof over the room. THe missile impact on the roof (missile travelling vertically impacted a horizontal surface) criteria in FEMA361 and ICC-500 is very high for tornado shelters. Much higher than I think any metal deck alone could withstand.

The FLorida Building Code (FBC) has a section for south florida called High Velocity Hurricane Zone, where they list approved construction for missile impact. But beware, this is missile impact per FBC criteria, which is a small fraction of the impact momentum required in FEMA and ICC.