Tornadic Shelter Design Loads
Tornadic Shelter Design Loads
(OP)
I am currently researching design loads to withstand a tornadic event for an upcoming project. I have found the FEMA 361 document that contains a bulk of the information I have been trying to locate.
However, one issue I am having difficulty in finding information deals with impact load due to a collapsing structure. My structure is a two story building, but the first level will only be designed as the shelter in an effort to save money. This assumption would indicate that the upper level is not designed to withstand the extreme tornadic loads, thus giving the potential for collapse of the second level onto the hardened structure below.
Does anyone have any recommendations or references on how to account for the impact loading created by the collapsing structure above? I am sure this involves impulse-momentum theory to a certain extent, but was hoping to find something simple so I do not have to relearn kinetics.
However, one issue I am having difficulty in finding information deals with impact load due to a collapsing structure. My structure is a two story building, but the first level will only be designed as the shelter in an effort to save money. This assumption would indicate that the upper level is not designed to withstand the extreme tornadic loads, thus giving the potential for collapse of the second level onto the hardened structure below.
Does anyone have any recommendations or references on how to account for the impact loading created by the collapsing structure above? I am sure this involves impulse-momentum theory to a certain extent, but was hoping to find something simple so I do not have to relearn kinetics.






RE: Tornadic Shelter Design Loads
Design the second floor to blow away and you won't have to worry!! Probably what will happen anyway....
Toto - we aren't in Kansas anymore!!
RE: Tornadic Shelter Design Loads
RE: Tornadic Shelter Design Loads
RE: Tornadic Shelter Design Loads
RE: Tornadic Shelter Design Loads
RE: Tornadic Shelter Design Loads
RE: Tornadic Shelter Design Loads
RE: Tornadic Shelter Design Loads
The real problem sounds like the owner/architect has placed too many constraints on you to be able to arrive at a reasonable solution. If the owner justs wants to see cmu on the interior maybe you could use light framing with a thin adhered interior that resembles cmu.
Good luck
J
RE: Tornadic Shelter Design Loads
The impact of the collapsing upper floor and still being in the same county during a storm is a very nebulous situation.
Is this a residence or a commercial structure? If you are attempting to design the first floor and everything that can happen to the second floor, complicated by the usual overly conservative, simplying design assumptions, you can easily create a monster. - A tornado covers a very narrow path and F3 to F5 storms are not that common on a probability scale for a single location.
Mass and reinforced conrete masonry/concrete are obviously your best options.
The FEMA suggestions (life safety - not property) can easily be adapted to below ground (basement) or to first floor situations. Frequently, a large walk-in closet (8x10) can serve the purposes and an adjoining bath can easily/economically be an added safe area.
Trying to design for an F5, especially with the simplifying assumptions is just a folly, since the economics and constructability pose huge problem.
I saw a home near Long Beach/Biloxi, Mississipi that withstood the 29' direct hit of storm surge from Katrina (lesser winds, but storm surge). It looked just fine 4 months later. It was on over 1200' ($10,000/ft) of shorefront, so that owner could afford to pay for the design and constructon. - I hope you have that type of client.
RE: Tornadic Shelter Design Loads
RE: Tornadic Shelter Design Loads
RE: Tornadic Shelter Design Loads
RE: Tornadic Shelter Design Loads
RE: Tornadic Shelter Design Loads
Look up the F factors for tornadoes - but 150 mph is quite common if I remember correctly. I think an F5 goes into the range of almost 300 mph!!
RE: Tornadic Shelter Design Loads
RE: Tornadic Shelter Design Loads
The FEMA suggestions (FEMA 320) I quoted earlier are regarding life safety are are intended to provide a safe haven that is immunue to the penetration od projectiles.
Tornados are a very brief weather condition that affects a very, very small area. The general philospohy is that an EF5 tornado with 250 mph winds will almost detroy any normally usable structure and that there must be a "hardened" safety area the is close and conventient enough to save lives.
A tornado is not at all like a hurricane - much much stronger, much faster, no advanced warning and unexpected.
Falling debris is minor of little consequence if you can evacuate. The higher level of facility protection is a management decision predicated on the value and importance of the facilit, while life-safety is not a management decision. A straw can pentrate a body (if it survives) much easier than the 2x4 that it does. - You do not see wood frame structures (even with Kevlar layers) for a good reason. The FEMA tests at Texas Tech have shown the folly of the concept.
Dick
RE: Tornadic Shelter Design Loads
TTUengr51, did you have any luck finding how to estimate the impact loads on the roof? FEMA 361 gives a couple of roof sections and describes how they perform against penetration, but I don't find anything the tells you what loads you apply to the support structure of that roof from the impact load.
RE: Tornadic Shelter Design Loads
RE: Tornadic Shelter Design Loads
RE: Tornadic Shelter Design Loads
RE: Tornadic Shelter Design Loads
RE: Tornadic Shelter Design Loads
As a minimum, you should assume the 6000# unit drops on the top of the shelfter in addition to other building debris. If the shelter is below grade, the people may be lucky and have a big enough wind (EF 3 to 5) to remove everything, but there are no guarantees with life safety.
Depending on the room size, even the FEMA 320 suggestions may be adequate, so reinforced block or reinforced concrete (8" thick and steel at 8" o.c.) based on a 12'-2x4 shot at 150 mph should take care of the debris danger. The structural design will depend on the building/cell configuration and dimensions.
The doors and floor/wall/roof anchorage are critical. The doors should resist any pentration and should open into the safe area to eliminate people being trapped with debris.
RE: Tornadic Shelter Design Loads
After these structures were built and series of storms the question of hurricanes and tornadoes came up as far as the existing blast resistant design would perform.
The blast resistant designed buildings have reinforced concrete walls and roofs. I've been trying to find out what the roof design load was as in the event of an explosion there would be a lot of flying debris.
Evaluation of the current design revealed that the buildings would stand a cat 5 hurricane and an excellant chance at an F5 tornado.
Still recalling from memory, the roof on these buildings was a relatively simple design that caused no discussion in the design meetings about cost.
The point of my post is would this type design fit your second floor design not as the roof but floor?
RE: Tornadic Shelter Design Loads
I'm utilizing a composite steel framed floor (shelter roof) with an 8" concrete slab on 2" composite decking, which gives me the recommended uniform thickness of 6" as described by the FEMA 361 missile impact testing.
To design the floor/shelter roof, I am using a impact load of around 220 psf, which accounts for the roof weight (20 psf) and wall weight (8" CMU w/ 4" brick veneer ~ 90 psf) multiplied by an impact factor of 2. I determined this factor by using some impulse momentum principles. In addition to the impact load, I have additional roof live load of 100 psf as designated by FEMA 361.