Storm shelter design - how to apply wind loads and impact loads?

[basims]

Hi,

I'm designing a small above-ground storm shelter constructed of 24" wide 10-ga steel panels that are seamed and bolted together.
Structure height is 7', width is 6', length is 8'.

Design criteria are FEMA 320 and ICC 500...a 15-lb 2x4 at 100 mph.

I have gotten this far:

Momentum at impact = 68 ft-sec
Energy at impact = 5,017 ft-lb

My question is what to do with this info now?

I know that the equivalent static force depends on the stiffness/deflection of the steel panels and the duration of impact. How would I figure those?

If I 'assume' a 0.1 sec duration which sounds about right, and a 2.0 impact factor, I come up with a static force of 1.36k...which seems reasonable. But it's based on an arbitrary 0.1 sec duration.

In summary, how can I come up with a justifiable impact duration to help me get an accurate static force?

Thanks,
Brad

 

[JAE]

I could be wrong but does ICC 500 allow you to do calculations to verify wall projectile resistance? I thought the wall types, doors, etc. all had to be tested in a lab.

FEMA 361 has many of these wall types and I'm pretty sure that they don't have gage metal wall systems provided so maybe that is why you are forced to calculate it out.

Impact analysis is pretty complex and you are correct that it depends on the decelleration time.

 

[basims]

Thanks JAE.
Yes, this is a new design I'm looking at for a client so it has not been tested.

How would I come up with a reasonable estimation of the deceleration period?

I've searched the forums on this site and others, and no one seems to have a clear answer.

 

[concretemasonry]

The FEMA 361 standards are generally based on life safety and prevention for no debris penetration. The suggestions are based on years of physical testing of many different wall assemblies. It has taken some system containing steel to be found to be acceptable only is sandwiched between 2 layers of 3/4" plywood of 2x6 studs and have a penetration resistant ceiling/roof.

Generally, the 361 suggestions produce a small structure that is so robust it is not worth the time to do a detail structural analysis unless the owner/developer has deep pockets and is will to wait on the test results. If it is for larger group or community structure, FEMA also has suggestions, but they do not address the question of life safety from projectiles, which is the single most common reason for fatalities.

The testing consists of firing a 12' long 2x4 at 145 mph at a wall section with no penetration as the criteria.

I use the word "suggestions" because FEMA has no power unless and accepted by a legal authority or professional. If the real life safety issue is not included, it is just a purely dynamic structural design situation with wind pressure and suction complicated by the structural effects of debris.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[JAE]

You might check with ICC to see who wrote the ICC 500 document and how the systems are "approved" for use against tornadic projectiles.

Again, I'm not sure, short of testing in a lab, how you could calculate that resistance. However, I do think there are military (department of defence) documents dealing with blast loads and these might have some bearing on projectiles - but they deal with impulse pressures vs. object impact.

 

[IRstuff]

Note that 12ft/100mph = 82.5 ms, so 0.1-s impact time is probably too long. One might posit that the full-width, half-max, impact time shouldn't be more than 1/2 of the 82.5 ms, so more like 41.3 ms might be in order.

TTFN

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