Steel I Beam Falling Through Water

[Michael1234513423643]

So I'm going to try to keep this as brief as possible, but I have a somewhat advanced physics question that my nuclear background hasn't prepared me for and I need an argument settled at work.

Say you have a 30 ft deep pool and you need to protect the contents, which comprise the bottom ~10 ft. The risk is steel I beams falling and puncturing the pool liner (steel, say it's 1/4") or damaging the contents.

The physics for a beam falling through water aren't beyond me, and I've reached the conclusion that an I beam, falling vertically, would likely puncture the pool liner as the beam wouldn't be slowed enough by water resistance to lessen the impact to acceptable levels. My question is, and it's a bit out there, what if the pool were filled with rubber (or some similar, more-dense-than-water material) spheres? I don't know how to approach the physics of the interactions between the I beam and the water, the I beam and the spheres, the spheres and the water, and the spheres with those surrounding them.

If you are willing to look into this question, I'll provide a few basic assumptions:

Standard pressure, temperature, etc.
Say the pool is a 20' x 60' rectangle, and as said earlier, 30' deep
Balls are arranged uniformly
I'm not sure what type of material would be best, so just assume a density of 1.2 (basically just that the spheres do not float)

The argument is whether filling the pool with these spheres would reduce or prevent the impact of the beam falling as a spear (completely vertical, I assume that is worst-case) to either an acceptable level (no puncture of liner) or stop it completely (beam becomes suspended somewhere in the mass of spheres). I'm hoping someone dramatically more familiar with dynamics than I am reads this and feels intrigued enough to take the time to help out. Thanks in advance, and if you need any other assumptions feel free to either make them yourself or ask and I'll try my best to respond promptly.

 

[anorlunda]

What an intriguing question. I expect that the answer may depend on the diameter and density of balls in the pool. (Note the 1.2 density of the materials, but the number of balls per unit volume.

I can visualize very small balls in high densities acting like a solid. Think of sand on the ocean floor. So I expect that you have to further specify the range of parameters.

 

[Michael1234513423643]

Good call, thanks! The spheres I had in mind would NOT be small, likely basketball-ish size depending on various other requirements. The pools we're discussing at work have inlet-outlet ports and obviously you wouldn't want these sucked into them. With basketballs in mind, we'll go with 10" diameter for now and I'll update later if I find out this isn't large enough. Thanks again for the observation!

 

[anorlunda]

What about packing density? Can we assume that the max possible number of basket balls are jammed in?

 

[anorlunda]

I just thought. If the basket balls sink, then they layer up from the bottom. If you put max balls in the bottom layer, they can't be pushed aside easily. Again, packing density is a critical parameter.

 

[sophiecentaur]

Think of sand on the ocean floor.

When I read the OP, I thought it was a serious practical problem and SAND was the answer that came into my mind instantly. To resist an object falling through a non-solid material, the component 'particles' need to be random sizes and shapes, to prevent laminar flow past the object falling through it and to introduce a friction effect. Also, it needs to be dense so that a lot of work is required to shift it about (to displace it). A non-Newtonian fluid - a stronger, more dense version of Cornflour would form a strong barrier against initial impact but allow pumping or draining at a slow speed.
After use, sand can be pumped out when stirred up with water. It can settle out of the re-cyclable water and disposed of or stored. I would think that the sand best suited to the job would be sharp sand which has many flat edges and a vast variety of particle shapes and sizes.

 

[anorlunda]

Energy absorbing foam such as used in vehicles for crash resilience sounds like a good material for a layer of balls on the bottom of the pool.

Contact the foam manufacturers for detailed data sheets.

 

[Michael1234513423643]

So while I agree that sand would very likely do the trick, this wouldn't be an option for my current discussion. We're discussing a nuclear spent fuel pool, and sand would gunk things up too much for future processing.

As far as the packing density, I think it would be safe to assume that the "near max" amount of balls would be present. By near max I mean that in our theoretical situation, you have an emergency and basically dump a ton of these rubble balls into the pool to protect the pool and its contents. I would imagine these would distribute relatively uniformly, but not perfectly.

 

[Michael1234513423643]

Energy absorbing foam such as used in vehicles for crash resilience sounds like a good material for a layer of balls on the bottom of the pool.

Contact the foam manufacturers for detailed data sheets.

I looked into foam but believe it would be very difficult to get foam all the way to the bottom of the pool (in my mind a foam tends to be less dense than water). Another issue would then be removal of the foam afterward, but granted in my initial question I didn't mention anything about cleaning it up in the future so that's my fault.

 

[anorlunda]

Drill a hole to the center of the foam ball, and put a weight inside heavy enough to sink it.

The harder problem is the analysis. Compare the difference if the corner of the beam lands exactly in the center of a ball, with the case when the corner of the beam lands exactly in the center of the inter-ball-void.

I think it would work better with 3-4 layers of baseball size balls rather than one layer of basketball size balls.

Or just fill a steel box with foam, solid or balls or whatever, and lower it to the bottom of the pool. Lift it out with a crane when necessary.

 

[JBA]

You included the protecting the 10 ft of "contents" in the bottom of the pool. Are these contents strong and stable enough to support, without being damaged, the covering medium when it is struck by the falling beam?

 

[Michael1234513423643]

anorlunda - The top of the pool is not clearly accessible in this scenario. Kind of a big deal and I forgot to mention it, I apologize. If we could easily access the top of the pool it would make things dramatically easier, as I'd just recommend a series of steel sheets be laid across the top and ventilation increased to allow for removal of decay heat. I definitely agree that the analysis, to me at least, seems incredibly difficult. Your insight about the various points of impact is my reasoning too - knowing the incredible amount of variables makes me tend to go with a "worst case" scenario, which I believe would be non-centered hits on the spheres but not quite void space hits either (in theory, assuming relatively even distribution, hitting one layer centered in the void space would mean a centered hit on a sphere in the layer below). I don't know if any analysis can really be done that would grant a measurable level of confidence, but in my mind this approach really seems like it would stop a pretty substantial load drop/fall.

JBA - The structure at the bottom of the pool is relatively robust, though not intended to be impacted or load bearing. The focus is more about distributing the load over a large area, which would prevent damage from a point impact with the liner/contents. The structure is a lattice configuration, comprised of 1/4" "boral" (steel laced with boron, acting as a neutron absorbing agent for the spent fuel). So while, as I said, this isn't intended to be impacted or load-bearing, it's certainly not a straw house either.

 

[anorlunda]

Is this a fuel storage pool?

As I said, the problem becomes much simpler and easier to analyze with 3-4 layers of baseball size balls. The beam directly impacts only the top layer, the forces are shared with the layers below where orientation of the balls is different than the top layer. I think it would be easy to simulate with a Monte Carlo analysis with exact placement of the balls the random variable.

Again, contact the manufacturers of the energy absorbing foam. Describe your project to one of their engineers and let them make suggestions.

 

[JBA]

What are the access options available to you for installing some type of impact absorbing item in the tank?

 

[Baluncore]

Double knit a mesh from large round section neoprene foam, then float that on the surface, a falling beam will be wrapped in mesh as it enters and sinks, progressively increasing buoyancy and drag, protecting the contents and the liner.

 

[essenmein]

Might the best solution be to design the building in such a way that I beams are prevented from being able to fall into said pool vertically? Rather than trying to protect against a rather difficult failure mode, prevent or move the failure mode else where? Maybe spec the architect so as to ensure no I beam joints above pool (just an example) so it can't fall in end first?

I would think slowing a heavy falling spike like that would be quite difficult.

 

[anorlunda]

Might the best solution be to design the building in such a way that I beams are prevented from being able to fall into said pool vertically?

Yes, that's sound advice. But if it is a spent fuel pool, it might be a case where new criteria are being applied to an existing structure. It might also be a case of not just roof beams, but also overhead cranes that could drop. Cranes are used to move fuel into and out of the pool.

 

[essenmein]

Yes, that's sound advice. But if it is a spent fuel pool, it might be a case where new criteria are being applied to an existing structure. It might also be a case of not just roof beams, but also overhead cranes that could drop. Cranes are used to move fuel into and out of the pool.

Maybe high tensile steel net attached to steel cables on a drum with some sort of brake eg magnetic brake. Basically a catch it and absorb the energy over some distance sort of thing.

 

[trurle]

So I'm going to try to keep this as brief as possible, but I have a somewhat advanced physics question that my nuclear background hasn't prepared me for and I need an argument settled at work.

Say you have a 30 ft deep pool and you need to protect the contents, which comprise the bottom ~10 ft. The risk is steel I beams falling and puncturing the pool liner (steel, say it's 1/4") or damaging the contents.

The physics for a beam falling through water aren't beyond me, and I've reached the conclusion that an I beam, falling vertically, would likely puncture the pool liner as the beam wouldn't be slowed enough by water resistance to lessen the impact to acceptable levels. My question is, and it's a bit out there, what if the pool were filled with rubber (or some similar, more-dense-than-water material) spheres? I don't know how to approach the physics of the interactions between the I beam and the water, the I beam and the spheres, the spheres and the water, and the spheres with those surrounding them.

If you are willing to look into this question, I'll provide a few basic assumptions:

Standard pressure, temperature, etc.
Say the pool is a 20' x 60' rectangle, and as said earlier, 30' deep
Balls are arranged uniformly
I'm not sure what type of material would be best, so just assume a density of 1.2 (basically just that the spheres do not float)

The argument is whether filling the pool with these spheres would reduce or prevent the impact of the beam falling as a spear (completely vertical, I assume that is worst-case) to either an acceptable level (no puncture of liner) or stop it completely (beam becomes suspended somewhere in the mass of spheres). I'm hoping someone dramatically more familiar with dynamics than I am reads this and feels intrigued enough to take the time to help out. Thanks in advance, and if you need any other assumptions feel free to either make them yourself or ask and I'll try my best to respond promptly.

Elastic balls seems to be a bad idea for following reasons:
1a) Margin of safety is not well controlled - balls do move and arrange randomly
1b) Need increased safety margin for the balls elastomer likely degradation in high-radiation environment
2) Problems replacing the balls leading to high maintenance costs - many small and short-lived parts
3) Reliance on elastic energy storage instead of plastic deformation will lead to over-weight, expensive solutions

As alternative, i would say you should put a steel crush-able grids/honeycombs/gratings over pool lining. Gratings are the common solution to protect delicate flooring against rough handling of heavy weights.
Please google for
"expanded metal grating"