Damping a trampoline on the 2nd floor

[Rocksolid]

My son has autism. He is 15 and weighs 235 pounds. He is also 6 feet tall. He loves to jump on his trampoline in our living room, yeah I have a trampoline in my living room. We live on the 2nd floor of an apartment. I noticed the ceiling in my downstairs neighbor's home bending a little every time my son jumps on the trampoline. I'm worried about the ceiling so we moved the trampoline to another part of the room. The trampoline has 6 metal legs and we put those foam pads under each leg to absorb the shock. Does anybody have a better idea on how to keep the shock down so it will not damage the ceiling below? Thanks.

 

[Dale]

Oh, wow! Autism or not, that is a big kid! And I know that kids with autism can have a bit of a lack of motor control so you probably cannot have him reduce the amplitude of the jumping much.

To reduce the shock you will want to increase the amount of time that he is in contact with the trampoline, basically dampen the jumps. The foam pads are a good idea, but you would want to make them fairly thick pads.

Another possibility is to remove a couple of the springs. This will make the trampoline "stretchier", but does run a couple of risks. One is that he could easily "bottom out" particularly given how large he is. The other is just that the remaining springs will wear out faster.

You could also put a foam pillow under the trampoline to limit the amplitude somewhat, when he makes a large enough jump to hit the pillow then some of the energy will be sapped out. However, that may startle him or make him off balance a bit, so I would be pretty cautious about that (although if you are anything like my sister with her autistic child "pretty cautious" is the default anyway).

 

[Rocksolid]

Thanks Dale. My son is like a football player and still growing. Good ideas. The trampoline uses bungee cords but I don't think removing any would be good since it does support his weight. I was thinking maybe some kind of a shock absorber that I could attach to the legs like cars have with tires.

 

[Dale]

Yes, that would work. You would probably be better off with the shock absorbers for bicycles than the ones for cars.

 

[Rocksolid]

I don't know how I would put them on or where they should be installed. Maybe other people will chime in.

 

[Greg Bernhardt]

Does anybody have a better idea on how to keep the shock down so it will not damage the ceiling below?

Put it outside?

 

[Rocksolid]

Yeah, I can't put it outside. It needs to be in my home. Is there a hollow cup with a strong spring at the bottom that the legs can sit in? Would that work?

 

[Greg Bernhardt]

I'm no structural engineer (no engineer at all!), but whatever else you do, maybe find the strong points in the room. Find out either where a load bearing beam is or put the trampoline in an outside corner wall where you know structural support is strong.

 

[Rocksolid]

Maybe the legs can sit on something like this?
https://www.lowes.com/pd/Danco-SEATS-and-SPRINGS-FOR-DELEX/1082005

 

[Greg Bernhardt]

Maybe the trampoline legs can sit in something like this.

Maybe the legs can sit on something like this?

Given the weight of your son you'll need something much more hefty

 

[Rocksolid]

Ok, so the spring would have to be thicker and more tightly wound.

 

[JBA]

Rocksolid, post: 6156319, member: 619611"Maybe the legs can sit on something like this?"

Springs on each leg is not really a good solution because if/when he lands off center then the compression of the springs on the legs on that side of the trampoline could cause it to tilt and throw him off balance.

 

[Rocksolid]

I was thinking a metal cylinder about 6 inches long. I would put a tight spring that fit snug into it for about 3 inches and then I would put the leg of the trampoline into the remaining 3 inches of the cylinder. Then I would put the leg cap on the bottom of the leg which is there now. I would do that for all 6 legs of the trampoline. I would have to make sure they were all the same size. He usually just bounces in the middle since the trampoline is not that large.

 

[Nik_2213]

You may need a hefty sub-frame spanning the room, with eg foam cushioning above and beneath. Think 'sprung dance floor'...

Really, you need to make the sub-frame a fair fraction of your son's plus the trampoline's mass, the better to soak up energy transfer.

Think 'base isolator' stuff as used to mount machinery to skids and tote-frames...

If you can, align frame with joists, bridging the room centre. Do you have access to vertical pillars, such as you'd attach a bracket for punch-bag or such ??

Plan_B could be persuading your young man to take up swimming...

 

[Rocksolid]

Thanks Nik. That all sounds pretty fancy. Not sure if I could do all that...and to be honest I don't really know what all of that means. He likes to swim but he needs to do something in the house that he will find calming and soothing. I just might have to move to the first floor, lol. Keep the ideas coming

 

[geoh777]

Your candidates for solutions to your problem are mass (as Nik has specified), or shock absorption *as you have already mentioned). Or a combination of the two.

Available shock absorbers (e.g. screen door closers, SUV rear window and car trunk lid supporters, motorcycle shocks) would have to be somehow adapted to your use. That difficulty, the trial-and-error approach necessary with the risks inherent thereto, plus expense might rule this out.

Next would be mass. If you check your local building codes, you can find the allowable floor load, both dead and live, in pounds per square foot in your area. (Just as an image of what that might consist of, think of several four-drawer file cabinets [each of which when loaded can be very heavy], arranged in a circle like petals on a flower. It is not at all unlikely that these file cabinets could violate the allowable residential floor loadings, especially for a load in the middle of the floor.)

So, I would think that the best and lowest cost solution for you would be to get six 94 lb. sacks of Portland cement, build a wooden case or box for each (3/4 inch plywood bottom and top, 2x6 sides, screwed together, with flat pieces of plywood taking up all of the space between the top of the sack and the bottom of the lid, line the boxes with thick visqueen to keep cement contamination into the room to a minimum), and place one of these under each of your six legs. Look out for your rough boxes poking holes in the visqueen, snagging rugs, and tripping people moving around the trampoline.

That would be 235 lb. working against around 600 lb. You might stand a chance. (You could try adding six more boxes stacked on the first six for more mass.)

Going back to shock absorption, maybe building the boxes like above and placing one or more layers of foam material in each box (arriving at the number of layers by trial and error), with a plywood "piston" on top of the layers, and each leg pushing down on the piston, maybe you could get enough damping that way. Maybe the piston would be best is doubled (two pieces of plywood to total 1 1/2 inches thick glued and screwed), and a candidate for the foam might be those cheap, interlocking foam panels at Harbor Freight or one of those big lots stores.

Good luck solving your problem.

 

[willem2]

You do NOT want a shock absorber. The trampoline itself is already a better shock absorber than any amount of foam/springs/cement/anything else that has been suggested. A trampoline is already a giant spring that spreads the force from the inpact from a through as long long a period of time as possible.
Damping material will only help against quick, shocks, like jumping on the floor without a trampoline.
The problem is that the force when the trampoline is at its lowest point can be many times your body weight.
(depending on jump height)
This force will somewhat bend the ceiling, which is not good if your downstairs neighbors have plastered it.
The only thing to do is to spread out the force over a large area, by putting the feet on stiff planking, or even a metal frame. It should be stiffer than your floorboards. (I thing you have a wooden floor?)

Another thing, how high is your ceiling? Mine is 8 feet (2.45m). A 6 feet person on a trampoline won't be able to jump at all.

 

[Ketch22]

Rocksolid, What is your physics background? Willem2's input is very good. The concept is impact impulse. With each bounce there is a force that needs to be dissipated (and returned in the case of a trampoline) which is what is bending the ceiling. Anything that you can due to divide the forces over a longer period of time will be beneficial.
Also to be considered is the loading on the joists of the intermediate floor. The shear strength of the framing is greater than the bending force. If you can define the true load bearing structures and transfer the forces to where they are as close to shear as possible this will reduce the deflection. The structure here should be as stiff as possible. This will transfer the load from the longer lever (impulse applied to supporting wall) to the shortest possible lever.
In summary, reducing the impact impulse will reduce the applied force. (Keep in mind this will also cause the trampoline to feel "softer"). Shifting the forces to the supporting structure will reduce the bending. This will require a relatively stiff structure and possibly prestressed structures that will involve direct engineering.
The trade off here is that by moving the forces from flexural (bending and somewhat repeatable) to shear (stronger but closer to a failure mode) one moves from a nuisance to the neighbor closer to a catastrophic failure affecting the neighbor.

 

[sophiecentaur]

I'm no structural engineer (no engineer at all!), but whatever else you do, maybe find the strong points in the room. Find out either where a load bearing beam is or put the trampoline in an outside corner wall where you know structural support is strong.

I think the approach has to be along those lines. The reason that the floor is deflecting is that it is not strong enough to deal with the forces on it. (Greatest deflection in the middle) The floor needs help in the form of extra bracing between the strong points that @Greg Bernhardt has mentioned. Problem - the headroom will probably be compromised by a thicker floor (if you use beams). An additional frame with triangular sections around the outside of the basic tramp frame would take most of the load on feet nearer to the walls and further from the centre of the floor, reducing the deflection. That (in fact any major mod) would mean the room would be less useful for other use.
PS I don't think it would be 'valid' to alter the dynamics of the trampoline itself; it will have been designed (spring k etc) to operate on a relatively unyielding floor.

 

[Tom.G]

Expanding on the idea by @Ketch22 :

Putting a few sheets of thick plywood on the floor under the trampoline would spread the forces over a larger area, and effectively stiffen the floor. You probably can not find a plywood sheet large enough to support all the legs at the same time so you will have to use at least two layers. Make sure the layers are staggered such that (for two layers) the upper layer spans 1/4 of four sheets below it. i.e. make sure the sheet edges do not line up between layers.

This has the effect of spreading the existing force over a larger area and reducing the pounds per square foot loading. As it is, those six trampoline legs are point loads!

As also mentioned, the floor has highst load capability near the walls. That is why installation instructions and building codes specify a maximum distance from a wall when installing a waterbed. Although putting a trampoline against a wall may make it not very usable. (oh, and keep clear of windows, that could make for a hard landing. )

Cheers,
Tom

 

[sophiecentaur]

Keep the ideas coming

A rowing machine is good exercise and low shock for the poor old floor. It could be linked to a TV display to give VR experience - if that's the sort of thing your son likes.

 

[Bianco]

Okay No Physic here; but a Cheap solution could be ultra strong and Thick Cardboard. It is Tuff and Sound Dampening. I had a Ex who didnt respect nor understand the noise from his three and half foot speakers. They would reverabrate through the floor to the neighbors ceiling and down their walls. So, I had large Cardboard blocks used for packing where I worked at the time that was being thrown out. I placed two under each speaker and shazam no more noise and better yet no more complaints. They were corrugated blocks super duper heavy duty type

 

[Joe591]

You can reduce the shock all you want, but 235 pounds is 235 pounds and the trampoline is exerting a decelerating force on your kid every time he comes down and the trampoline stops him. The force that the trampoline is seeing is much more than 235 pounds. No amount of shock absorption is going to take the dead weight away. Move the trampoline closer to a wall so that you reduce the bending moment on the floor. Try to figure out where the support columns or walls are in the apartment below and see if you can position the trampoline over those if possible.

 

[Rive]

Putting a few sheets of thick plywood on the floor under the trampoline would spread the forces over a larger area

I think it's not enough: the plywood spreads the forces through bending, and if it's laid on the floor, it'll bend together with the floor => no effect.

I think a kind of second or 'false' floor will be needed, with a few inch distance to the actual floor and the only support points near the walls. The false floor would do (and: hide) all the bending, and the forces would be transferred to the walls.

 

[sophiecentaur]

Okay No Physic here; but a Cheap solution could be ultra strong and Thick Cardboard.

Unfortunately, the frequency ranges of music and jumping are different and call for different solutions. Also, the peak power of the vibrations would probably be a lot higher for the jumping up and down ( 1kW or more). The Physics can't be ignored, I'm afraid.