Firestation Design for a 250MPH Tornado Wind Load
Firestation Design for a 250MPH Tornado Wind Load
(OP)
I'm currently designing (D-D phase) a fire station for 250 MPH tornado wind loads. The architect for the project wants to use a metal deck and bar joist roof structure. I've researched both FEMA 361 and ICC-500, and haven't found any testing information for missile impacts on the metal deck. Is there any information available on this subject?






RE: Firestation Design for a 250MPH Tornado Wind Load
http://www.dhs.gov/building-and-infrastructure-pro...
Whats the uplift on a roof for 250mph??
RE: Firestation Design for a 250MPH Tornado Wind Load
RE: Firestation Design for a 250MPH Tornado Wind Load
If I was designing such a structure (even if I wasn't in the concrete industry), I would strongly consider a structural concrete roof deck to minimize the debris and uplift considerations. Cast-in-place concrete buildings do not fail in tornadoes, mostly due to the mass and continuous nature of the structure. Properly designed structural steel performs well, but the cladding and members spanning between structural members is critical to protecting the occupancy.
That said, design for what needs to be protected, and be sure provision is made to protect equipment in the bays sufficiently as the doors are impacted. It may be sufficient to keep the roof structure and walls from collapsing, and the decking and doors may be acceptable losses. The researchers at Texas Tech tell me that loss of cladding (and subsequent shedding of wind loads) is a chief reason more structural losses aren't currently seen in tornadoes.
(For those of you in Dallas next Thursday, I will be presenting on this topic at the TSPE lunch meeting.)
RE: Firestation Design for a 250MPH Tornado Wind Load
Doesn't he/she understand that resisting 250 mph wind calls for a non traditional approach?
RE: Firestation Design for a 250MPH Tornado Wind Load
Look, it would be cheaper to build two fire stations 1-1/2 mile apart that could withstand 75 mph winds. tornadoes run from the W-SW to the northeast, so put the second south-southeast of the first. Problem solved. Crew quarters underground in a shelter UNDER THE GROUND so the winds go by overhead.
A tornado is small diameter destruction circle. At this mph you're trying to resist a nuclear bomb overhead at about 3/4 mile to 2 mile blast radius.
So, put it on a hill so you don't get flooded out.
Then dig down into the ground 1/2 wall height or more, then do the "WWII Atlantic Wall" bunker design: Thick 1-2 foot wall about 10 ft high with a sloped dirt berm outside sloped up to near roof level. Over lap the concrete pre-cast roof as shown above, but use a longer than usual overhang of the precast roof over the walls. If it moves sideways a few inches, you don't want the precast roof members sliding sideways off of the walls and themselves falling down.
RE: Firestation Design for a 250MPH Tornado Wind Load
Mike McCann
MMC Engineering
RE: Firestation Design for a 250MPH Tornado Wind Load
Earth pressures on buried walls are typically in the 50 to 90 pcf range equating to 600 to 1080 psf for a 12 ft deep wall.
I've designed a number of tornado shelters.
It is a challenge but not at all like a nuclear blast....well - I have to admit I wasn't at Hiroshima.
RE: Firestation Design for a 250MPH Tornado Wind Load
RE: Firestation Design for a 250MPH Tornado Wind Load
See, if the walls are submerged - as you point out - the debris from the storm goes "over the top" and there's little sideways problems apart fro the already-existing steady wall pressure from the dirt. But big garage doors?
Like I said, build two cheaper fire stations, then plan on losing only one station and no crew.
RE: Firestation Design for a 250MPH Tornado Wind Load
Plus, think of the operating costs for 2 stations.
Not the best idea.
RE: Firestation Design for a 250MPH Tornado Wind Load
It's up to you to bring the players to the table and explain what they are up against. It SHOULD be the architect, but they often don't seem to know what a "tornado shelter" is or means. Once you present some of the items in FEMA even an architect will see the impracticalities.
Calling something shelter with the word "Tornado" in it implies (to me, at least) that you are meeting the FEMA requirements. The public would expect and TRUST that this is the case. If all that is desired is a bit of a hardened wind shleter, then that's what you will have to call it. Decide how much you want to spend and stiffen and strengthen to suit.
We designed a cast-in-place conrete addition to a low-rise bar joist food processing plant in Arkansas. During the design, some nasty tornados tore through other parts of the midwest and the client's project manager wanted to know if our CIP addition was "tornado proof". I told him no, and if you get a direct hit on the site, the entire factory (except for our concrete tower) might well be leveled. The concrete addition would be a good place to hide out, but we could not guarantee it would be a true tornado shelter.
RE: Firestation Design for a 250MPH Tornado Wind Load
Where are you getting earth pressures of 50 - 90 psf?
RE: Firestation Design for a 250MPH Tornado Wind Load
For a 12 ft. deep wall you get 12' x 65 = 780 psf at the bottom.
RE: Firestation Design for a 250MPH Tornado Wind Load
I have no idea how you will find a manufacturer with test results for doors for those types of wind pressures and wind-borne debris impact. I would start the whole design discussion with the doors. They may need to be fabricated custom steel swing doors or something similar, with a lot of detail paid to the hinges and other mechanical connection points where the loads will be concentrated on moving parts.
I agree with TX, and I am not a concrete guy, but you mention tornado and I think underground and concrete.
But I do find the push back to the very idea of making something tornado-proof by structural engineers interesting
I always get annoyed when I hear politicians (or whomever) on the news after a tornado say there is nothing you can do if it is a F4 and F5, no such thing as tornado-proof, it is in god's hands.... And my response that we have been designing bunkers for much higher pressures/loadings for 60+ years, of course we are ABLE to design and build such structures! But the building will not be pretty and who will pay for it?
It would be interesting to see the cost breakdown of this fire station versus "normal" wind loads.
Keep us posted on this one, very interesting topic.
Read an article after the big Oklahoma tornado about why such few basements in OK? Besides some areas of high water table, and no need to do so because of frost heave, it was COST. But you would think there would be a big market for pre-cast concrete bunkers, just big enough for your family and Fido. The makers of septic tanks could make them. I have seen them for sale here in FL, and a quick google saw costs starting at $2500 or so. Worth my money if I lived in tornado alley.
RE: Firestation Design for a 250MPH Tornado Wind Load
One of FEMA's concern is the projectiles that cause most of the tornado casualties (according to FEMA).
What goes up and gets blowing around comes down eventually. A car is also a projectile that I saw in many places after 3 months of Katrina damage. Ssome on the south shore of Lake Ponchetrain) were not even found. One large piece of debris (a Chevrolet as an example, would not do as much damage to a typical reinforced 8" thick concrete 8'x8' shelter, but for a larger structure, I suspect there might be a different criteria for larger structures, which could decrease the safety of the structure, but being near a emergency facility (fire station) the requirements could be relaxed somewhat.
a2mfk is correct about the Florida door concepts for the critical storm areas and they also have additional residential requirements applying to gable ends to qualify for the large insurance discounts. I do not know if that concept applies to the legal code requirements.
Engineer and international traveler interested in construction techniques, problems and proper design.
RE: Firestation Design for a 250MPH Tornado Wind Load
bars engaged in to the surrounding framing, similar to blast
proof doors, or similarly, ...like the doors on ships &
submarines with "dog ears" to resist the wind forces. Also,
not pretty, but lateral reinforcement could be placed on
the interior side of the doors, similar to what the log
cabin settlers did when they were being attacked [ i.e.
- place a thick piece of wood across the door in to brackets
on both sides of the door frame ].
Unfortunately, ...these types of doors are very expensive to
design & install. If you want them to be aesthetically
pleasing, ...that drives up the costs even more.
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RE: Firestation Design for a 250MPH Tornado Wind Load
As what's been said already, you can calc it out and you'll find that the numbers would steer you towards using concrete instead of bar joists/deck.
RE: Firestation Design for a 250MPH Tornado Wind Load
RE: Firestation Design for a 250MPH Tornado Wind Load
The smaller individual "safe cells" have a design concept based on life safety for projectiles and there is no real structural "design" because the end product looks like it was designed by an engineer that wore both a belt and suspenders even when he went to bed at night.
The first structure I designed suffered from a total roof collapse even though all the pressure analysis from the rocket engines did not give total protection and the 6 ply observation "blast proof" windows with varying angles of the panes to the concerned area of blast survived with no problems. there is always something very unique (impossible) that can happen to cause a failure. - No one was in the building, so there were no life safety compromise, but many people at NASA, Air Force and corporate got interest and recommended to consider the impossibilities, but everything was designed with every possible situation. It made a great and rude awakening to real life and design. It was the best and most valuable job I ever had and opened my eyes after a 5 year curriculum in the classroom.
Dick
Engineer and international traveler interested in construction techniques, problems and proper design.