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.
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designing for missile impact on safe room 2
- Thread starter MikeE55
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I've never used a "slowdown distance" for a impact problem before. I have used (to figure generated forces) estimates of impact times (typically ranging from 10 ms to 0.1 ms). A even better means of figuring impact force (IMHO) is the method/formulas in Blodgett's 'Design of Welded Structures' (that takes into account stiffness). (See Section 2.8.)
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For FEMA shelters, the missile impact is by tested systems only. If you have a system that isn't tested, you have to pay for it to be tested. The list of approved systems can be found through Texas Tech's website. And yes, you would have to include a load for the high roof falling on the lower roof in addition to the 100 psf required roof live load.
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.
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.
- Thread starter
- #4
Thanks WARose. The slowdown distance may be a misnomer, since it came from a physics text where the example was a car crash test. I think it is the deflection, since the unit is in feet or inches. I'll check out Blodgett - that's a great book.
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.
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.
search around here for "impact loads". it's been talked about a lot ! the issue is how long (usually expressed in time, but distance works too .. t = s/v) the impact takes. It would be interesting to see if people have analyzed test results.
another day in paradise, or is paradise one day closer ?
another day in paradise, or is paradise one day closer ?
The force from component and cladding wind loads is going to greatly exceed the force from a 15 lb 2x4 shot at 100 mph, so the wind load controls the design of vertical reinforcing. You design for the required loads and then as long as you are grouted in every cell (I also reinforce every cell) you are good to go for impact loads.
Per what mike20793 suggests above - FEMA has records of tested wall (and door) assemblies that met various impact tests.
Here's a link to walls:
Just start here and look around:
Check out Eng-Tips Forum's Policies here:
faq731-376
Here's a link to walls:
Just start here and look around:
Check out Eng-Tips Forum's Policies here:
faq731-376
VoyageofDiscovery
Structural
Though this may be a bit of a stretch, this may be applicable.
racookpe1978
Nuclear
Be very, very careful about impact damage (tornado - in your case) but any collision or explosion.
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.
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.
VoD: I don't know if it's still standing, but about 40 years ago they constructed the West (Wet) Coast Transmission Building in Vancouver, Canada. This had a central core and the four corners were hung by cables from above the roof. This was done for seismic resistance...
Racookpe: Hehehehehehe... and passing or failing the test only occurs after the event.
Dik
Racookpe: Hehehehehehe... and passing or failing the test only occurs after the event.
Dik
- Thread starter
- #13
I checked out the FEMA website for suggestions, but their design limits the wall height to 8'. My architect insists on using a 10' high wall, so I do not know if the FEMA design will be helpful. I am also exceeding their maximum horizontal wall dimensions by a few feet. I wanted to find a way to calculate the force from the missile impact to assure myself it would not control over the wind loads. My calculations are not encouraging me that this is the case. I found a formula on a website called engineeringtoolbox.com. Their example is for a car crash, and for a human falling from a table. The formula is F = 1/2 x m x (V*2)/S, where S is the deflection or distance over which the force is absorbed. I calculate F = 1/2 x 15 lbs/32.174 x 132 x 132/S = 4061 lbs/S. To get the mass I am dividing 15 lbs F by g=32.174. To get the velocity I am using 90mph = 132 ft/sec*2. Since the result is 4061/S, you can see the answer is highly dependent on S. If S = 1", then the force of impact becomes 4061/.083 = 48.9 kips. This seems very high so maybe S should be greater.
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.
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.
racookpe1978
Nuclear
Add a outside layer as an attractive but expendable weather sheathing, an insulation layer as a "crumple zone", THEN your strength wall against penetration of the 2x4 impactor. This is the same technique used in the "lightweight" armor in the M-1 tank - and several other western tanks using the British multi-layered Chobham armor, in aircraft carrier hulls against cruise missile attacks, and will increase weatherproofing as well.
Mike
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
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
Update:
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
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
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