designing for missile impact on safe room
designing for missile impact on safe room
(OP)
I am designing a safe room for tornado loads and wondered how other engineers are calculating the force due to 2x4 missile impact. I think the appropriate formula is F = 1/2 x m x V x V / S, where S is the slowdown distance. The appropriate mass seems to be 15 lbs / 32.174 = 0.47 slugs, and the velocity is 90 mph in a 200 mph zone. What would you use for the slowdown distance for a missile impacting an 8" concrete masonry wall? Any advice would be appreciated. Part 2 of the question would be the safe room is located in a metal building, and there is a possibility of approximately 3000 lbs of roof falling about 10 feet to land on the roof of the safe room. Any thoughts on the slowdown distance involved for this type of collapse? The construction of the safe room would be an undetermined thickness of concrete slab supported on concrete masonry walls.






RE: designing for missile impact on safe room
RE: designing for missile impact on safe room
Edit: I should note that I have had peer reviewers claim both ways: the 100 psf live load includes the high roof and that it's in addition to the 100 psf live load. For a FEMA Shelter, I would much rather be on the conservative side.
RE: designing for missile impact on safe room
Mike20793, thanks for the Texas Tech reference. I plan to use the 8" CMU with every cell filled and reinforced, so their test results include the wall I want to use. My wall height is taller than the FEMA prescriptive height, so I was thinking it was necessary to calculate the force involved. Maybe the taller wall height would need to be tested. I agree with you on the roof design - I don't think the 100 psf is enough to include a falling load. I still would like to hear what someone else has used for a formula, or for that "S" distance.
RE: designing for missile impact on safe room
another day in paradise, or is paradise one day closer ?
RE: designing for missile impact on safe room
RE: designing for missile impact on safe room
Even if untested (unless it is requirement), I would prefer a safe room that was designed as opposed to one 'just constructed'.
Dik
RE: designing for missile impact on safe room
Here's a link to walls: https://www.fema.gov/wall-sections-passed-previous...
Just start here and look around: https://www.fema.gov/fema-p-361-safe-rooms-tornado...
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RE: designing for missile impact on safe room
RE: designing for missile impact on safe room
http://www.wsdot.wa.gov/news/2016/11/14/wsdots-new...
RE: designing for missile impact on safe room
Sure, you are "designing to specification", but the tornado IS NOT "going to follow" Washington's "rule of the law" for maximum size, weight, and length of objects thrown by high speed winds. (Well, nobody in Washington is following the rule of law anymore, so why should a tornado or terrorist, he asked rhetorically.)
Sure, it is a weighted estimate of objects, and that estimate is (supposedly) made after judgement and investigation of previous tornado impacts. But they remain only estimates of the weight and direction of impact (end-on at assumed maximum wind speed, no debris trapped on the 2x4, no other roof structure attached, etc.) Make your best estimate of the worst case impact and its result, but recognize it is all based on estimates of estimates of guesses. Like bird impact damage studies on planes, testing would be the final proof, but would still only show that you met "design case" not real world tornado. Which we all hope the structure is never hit by - whether worst case or minimal case.
RE: designing for missile impact on safe room
Racookpe: Hehehehehehe... and passing or failing the test only occurs after the event.
Dik
RE: designing for missile impact on safe room
In their example for a body falling from a 4' high table, they calculate the energy as 200 lb x 4' = 800 ft-lbs. They calculate the force by assuming a 3/4" deflection in the body, so the force becomes 800/.0625 = 12,800 lbs. Wow! If I use a similar calculation for my 3000 lb steel beam falling on the roof from a height of 10 feet, I get 30,000 ft-lbs. Assume a deflection of 1", this is 30,000/.083 = 360,000 lbs. The size of the load gets crazy.
I can see why cars are designed to have a crumple zone to absorb the shock. It doesn't work very well with a rigid building.
RE: designing for missile impact on safe room
RE: designing for missile impact on safe room
We design protective tornado barriers at nuclear power plants. They must withstand a host of tornado missiles
-8lb 1" reinforcing rod going 135 mph
-6" diameter pipe going 135 mph
-12" telephone pole
-railroad tie going 200mph
We use a two step design method:
1) Design for perforation: We use BC-TOP-9A to determine the required steel thickness to prevent perforation. Its an empirical formulation based on test data.
2) Design for displacement: We use ductile plastic design to dissipate the missile's kinetic energy. We use Single Degree of Freedom charts (Biggs) to establish the peak plastic displacement following impact
Hope this helps
Jeff
Jeff
Pipe Stress Analysis Engineer
www.xceed-eng.com
RE: designing for missile impact on safe room
Not sure this post is still active but we recently uploaded two blog posts on designing for missile impact loads that may help. Posts are focused on both global response (ductility) and perforation using empirical methods
Tornado Missile Evaluation: Global Response
Tornado Missile Penetration Calculators
Any feedback is always appreciated
Have a great weekend!
Jeff
Pipe Stress Analysis
Finite Element Analysis
www.xceed-eng.com