Firestation Design for a 250MPH Tornado Wind Load 8

[TomOlson]

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?

 

[ztengguy]

Not sure if anything here will help. Some stuff for blast design, etc.

http://www.dhs.gov/building-and-infrastructure-protection-series-tools-0

Whats the uplift on a roof for 250mph??

 

[BadgerPE]

I used this source when we were in the D-D phase of a project. We ended up using precast panels though to help offset the large (100+ psf) uplift forces. Although it is only research from a university, hopefully it gets you on the right track!

 

[TXStructural]

I find it unlikely that such a roof structure will really resist tornado loads. The economy of lightweight joist construction necessitates light weight, and I have not seen anything that verifies it capabilities under these kinds of loads. That said, if you do end up with joists, be sure you account adequately for the extreme uplift in the connection design at ends, and for bracing. Suddenly-applied loads require about twice the strength as gradually-applied (static) loads. Tornado winds come and go suddenly, and failure of slender members and connections are typically seen in lightweight construction. The structure will be partially-enclosed, and lightweight members are frequently exposed to C&C-level loads over large areas, since the low stiffness of these materials means that they are acted upon by the "small" winds modeled by C&C design loads.

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.)

 

[JedClampett]

Why does the Architect want metal deck? Appearance? Weight?
Doesn't he/she understand that resisting 250 mph wind calls for a non traditional approach?

 

[racookpe1978]

250 mph winds? Yeah. Right. Sure. OK.

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.

 

[msquared48]

Well, the doors of the fire station will be the weak link here, unless there is a circuitous route to an underground bunker for the trucks. I do not see that happening, so we may be looking at large blast doors here. That will fit into the budget nicely...

Mike McCann
MMC Engineering

 

[JAE]

racookpe - note that tornadic wind pressures at 250 mph are in the 130 psf to 160 psf range on walls and roofs.

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.

 

[JAE]

Also...I agree that a fire station door would be a tough one.

 

[racookpe1978]

I was thinkning more of the "blast" effects of debris and sideways force against the walls and lifting force up for the roof.

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.

 

[ztengguy]

racookpe1978...Its not the station (hopefully) they are trying to protect. Its the equipment (trucks) and the crew. One new ladder truck these days fully equipped can run north of 1 million. The avg fire station will have a ladder, engine and a medic unit. Close to 2 million worth of equipment. So even if the second station survives, the equipment is whats important. Most new trucks take 3-9 months, since they are usually custom built.

Plus, think of the operating costs for 2 stations.

Not the best idea.

 

[JLNJ]

FEMA has a lot of info on tornado shelter designs. They are costly, and ugly and, as mentioned, not well-suited for doors and windows. You blow the doors and windows out and then the entire inside is scoured clean by windborne debris. Yikes!

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.

 

[XR250]

@JAE;

Where are you getting earth pressures of 50 - 90 psf?

 

[JAE]

Typical at-rest pressures for clay soils run between 65 pcf to 85 pcf depending on drainage.

For a 12 ft. deep wall you get 12' x 65 = 780 psf at the bottom.

 

[a2mfk]

The codes for garage doors here in Florida have been made MUCH more strict over the years after many a garage door blow-out during a storm, which then led to failures of the roof or entire structure (IMO they are the achiles heel of most houses in wind). Many now have removable dowels that you place through the bottom track of the garage door into the slab when a hurricane is approaching, however these types of reinforcements are not so useful with the unpredictability of a tornadoes.

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.

 

[concretemasonry]

The FEMA suggestions for small structures are different than the larger group shelters.

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.

 

[GlobeTrekker]

FWIW, ...doors could be designed to have vertical and lateral
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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[RacingAZ]

I have only done 1 building in a tornado alley so my experience is limited on this topic. To save money, they designated 1 room as the tornado shelter instead of designing the whole building for it.

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.

 

[JStephen]

We've designed some equipment for data centers that was designed for higher-than-normal wind speeds due to tornadoes, but not the 250 mph. I don't know exactly what reasoning went into selecting the load level used, but you don't have to take an all-or-nothing approach to it.

 

[concretemasonry]

Racing has a good point to consider. If a large area can be broken up somewhat, the spans and heights are less and more designable than for a single large space. They would usually have smaller doors that are more practical and economic to implement and use. The main function of the planned structure may preclude this luxury.

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.