Tornadic Shelter Design Loads 1

[TTUengr51]

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.

 

[JAE]

3 ft thick walls?

 

[civilperson]

The ends of joists welded to embed plates in top bond beam and three rows of bridging in the same axis direction of the deck can brace the top of the CMU wall against cantilever type moments, (rather than simple span). The joists with a positive connection to air handling unit will negate any impact from falling HVAC.

 

[concretemasonry]

If you are designing for a day care center, the main criteria is life safety. Tornadoes are very short duration, but much more violent that hurricanes or other storms. If you have a lightweight structure (steel frame, wood), you should expect total destruction for a EF 3 or better (worse?).

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.

 

[unclesyd]

Way out of my league but having been an observer several years ago on the hardening of some control rooms and electrical facilities by using a ASCE blast resistant design.
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?

 

[TTUengr51]

To recap my situation and research...

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.