Mechanics of the spar design
May 18, 2008 at 5:51 pm #2063
Something like this: http://img363.imageshack.us/my.php?image=buttressesbc8.png ?May 18, 2008 at 6:52 pm #2065
“You assume no inertia from drifting, but don’t the currents in some places get significant speeds ?” I assume by inertia you mean lateral loading. Inertia would apply if the structure were accelerating and decelerating due to lateral loads. Actually I assumed that lateral loads would be minimal because the currents would be small since the Spar Buoy will be anchored in deep water. In my analysis I found that a the loading from 5 knot current would be small provided you used boat anchoring techniques and the anchor was attached near the center of loading. The anchors should have a minimum of a 7:1 scope, which means that the anchor rode is 7 times the water depth and at least 1/7th of the rode should be chain. The chain is critical in damping the snub loads. As the tension in the line increases it lifts chain off the bottom, which gradually takes up the tension. This makes acceleration and deceleration minimal so the inertia loads are insignificant and people won’t randomly fall on their faces or other parts. If you assume that the current is constant with depth, then the center of loading would be at the mid-draft. This minimized the tipping of the Spar Buoy due to anchor loads. “You say that buckling is the main concern when building a spar out of steel, which seems was the reason TSI had chosen ferrocement instead, …” Ferro cement is good in compression but not in tension. Buckling is what happens when you stand on an aluminum beer can. If you are very careful you can stand on it, but as soon as the loading becomes asymmetrical it buckles. This occurs because the wall of the cylinder is too thin for its tensile strength. To make Ferro cement resist buckling you will need to reinforce it with rings that have tensile strength. “and I remember one point my brother rised was that, around the water line the compression would be awful from the buoyancy below and the structure’s weight above – if I understand that right we can expect double the Archimedes’ force at that place, right ?” So, when I stand on the bathroom scale, the floor is pushes up with a force equal to my weight. Therefore, the scale is measuring twice my weight, right? The buoyancy of the hull has to match the weight above water, these forces are equal and opposite to the cancel each other out, they do not add. “My main concern is dynamic changes of the loads: wind changes combined with ballast trying to rebalance the structure from tilting, which may put enormous bending efforts on the spar ; current direction change with a significant speed, as well ; and above all vertical tear from rapid changes in waterline.” OK, by “rebalance” I think you mean “right” or return to vertical. I haven’t got a clue what you mean by vertical tear. Lateral loads due to wind and current produce shear and bending. Lateral loads will also cause the Spar to tilt slightly (~1 degree in a major storm) which will produce additional bending. Waves will cause the waterline to increase or decrease which will increase or decrease the buoyancy and increase or decrease compressive load. The key design feature of the Spar design is that the area at the water plain is very small relative to the total displacement. The change in displacement due to a change in depth is the water plane area times the change in depth times the density of water. If a large flat bottom vessel like a barge sees a change in draft equal to it’s normal draft it will experience a 1 g vertical acceleration. My Spar Buoy House design has a displacement of 364,000 lbs and a 10 foot diameter at the waterline. The change in buoyancy is 5026 lbs per foot or 0.01389 g/foot. Therefore, a 10 foot instantenous wave will generate 0.1389 g vertical acceleration a 20 foot wave would produce 0.276 g. You might think that a 40 foot wave would doule the acceleration again, but a wave that large would have a wave length of 280 feet so the buoy starts to ride over waves that large and the change in displacement will be less than the wave height. “I’d really like your opinion on this: it seems that the oscillation period of the structure when in waves would have to be higher than the highest wave period observed (wouldn’t that be something like 7 seconds ?), in order to damp vertical oscillations instead of resonating in them, but that means waves with enough amplitudes would cause enormous transient loads, both in excessive buoyancy (doubled at the water line, I think, because of the whole structure’s inertia) while the spar accelerates up, and in excessive tear while the spar accelerates down.” Maybe tear translates to tension? Initially I shared your view on the oscillation period or natural frequency, but for very large waves you need the structure to be responsive enough to ride over them, otherwise it would need to be even higher off the water. It is OK for the structure to be excited at its natural frequency provided there is enough damping to bleed off the excess energy. Fortunately water is viscous and therefore a much better damper than air and the Spar design has a lot of surface area. For example take a pendulum and see how long it goes underwater versus in air. Inertial does increase and decrease the compressive load by 1+/- the vertical acceleration, so in the case of my design in 20 foot seas that would be 1+/- 0.276 = 1.276 to .724.May 18, 2008 at 7:21 pm #2066
Buttresses could reduce the compressive and bending loads in the vertical tube, which would reduce the weight of the tube. The question is would the savings in the tube weight exceed the weight of the buttresses. Experience shows that the sea if very good at removing appendages from marine structures. I believe that the buttresses would be very substantial to remain attached. In addition the buttresses increase the lateral surface area so the lateral loads and acceleration due to waves would be increased. I think the buttresses should be located above the maximum wave height, such as inside the lowest level of the house. Regards; MikeMay 18, 2008 at 7:30 pm #2067
… that’s more of a “flying buttress” or a brace. I was thinking that the buttress is integral to the structure. They run the length of the vertical spar. It would grealty increase drag when trying to move the spar while vertically oriented. It also greatly increases the complexity of the shape, which is an issue as well, that might well negate any benefits people are imagining from ferrocement. I’ll try to put together a diagram and upload it.May 18, 2008 at 7:42 pm #2068
“Actually I assumed that lateral loads would be minimal because the currents would be small since the Spar Buoy will be anchored in deep water.”
OK, I didn’t realise you would have it anchored, I was talking about the inertia of the structure drifting in the current.
“Buckling is what happens when you stand on an aluminum beer can.”
Ah, I thought it was just rupture under its own weight, I was missing the bending element. Wouldn’t a ferrocement structure be more resistant to this by having much higher thickness ? For example we’d have almost 1′ of concrete, instead of 1/2″ of steel.
“So, when I stand on the bathroom scale, the floor is pushed up with a force equal to my weight. Therefore, the scale is measuring twice my weight, right?”
Oops, you’re right, that was stupid of me. I’ll have to ask my brother what he really meant.
“Lateral loads will also cause the Spar to tilt slightly (~1 degree in a major storm) which will produce additional bending.”
Yes, that’s what I mean. If we picture a tilt of 30 degrees the induced bending becomes very obvious. The lateral loads and the ballast which is needed in a spar design combine to make this problematic.
“It is OK for the structure to be excited at its natural frequency provided there is enough damping to bleed off the excess energy.”
Thanks for the advice I’m guessing that choosing a specific natural frequency for the structure is an important design step. Wouldn’t it determine the section of the spar relative to its displacement ? So in the end it would dictate the ratio between diameter and length ? Also, there must be some formula to minimize the vertical acceleration delta by choosing a good frequency that does not resonate too fast but resonates the taller lower-frequency waves ?May 18, 2008 at 7:58 pm #2069
I think it would make more sense to have multiple spars instead of a single spar with butresses. No more need for ballast (less bending forces, better “mileage” as the structure is lighter and cheaper). And you can put shorter butresses between the spars directly, if needed.May 18, 2008 at 8:45 pm #2070
I suggest you look at some of the manned spar buoy laboratories such as FLIP before you decide it is impossible. http://www-mpl.ucsd.edu/resources/flip.intro.html FLIP is a FLoating Instrument Platform that floats horizontal for transit and tips vertical to provide a stable manned research platform. FLIP is 355 feet long when horizontal and has a 300 foot draft when vertical. The cross section at the waterline is 4 meters and she supports 11 researchers and a crew of 5. Here is a link to the requirements for a new Manned Spar Buoy Lab to replace FLIP. http://www.unols.org/committees/fic/smr/buoy.pdf Regards; MikeMay 18, 2008 at 9:56 pm #2071
If the platform is to be the same height above the water to avoid waves it would probably have to be pretty huge in order to be stable without counterweights. And the forces on individual spars might even increase due to different wave action between them at any given point in time. Also, one of the strong points of the single spar buoy is the possibility to connect with other similar buoys and form larger communities and still have the possibility to move even from a position when it is completely surrounded by other platforms (by lowering and sailing out below the others). Buoys with several spars will have trouble doing this. Granted, this is pretty advanced stuff so it won´t happen anytime soon. Still for large buoys multi-spar configurations might work great.May 18, 2008 at 10:11 pm #2072
Ok, but that doesn´t seem like very efficient material use to me. Also wouldn´t simply thickening the walls accomplish the same thing without the drag penalty for instance? Buttresses that hug the wall might be efficient on a church or a house when you want to keep the garden free of braces and beams and stuff, and don´t have weight limits for the structure. I´m not sure about on a seastead though.May 18, 2008 at 10:17 pm #2073
Wasn´t one of the big selling points of the buttresses that they would resist buckling at the most stressed part of the spar (somewhere close to the water line if I´m not mistaken)?May 19, 2008 at 4:43 am #2075
“OK, I didn’t realise you would have it anchored, I was talking about the inertia of the structure drifting in the current.”
O, while drifting lateral loads would be virtually non-existent. The highest load would be under tow, but if you tow with a long line the elasticity of the line will minimize the spikes.
“Wouldn’t a ferrocement structure be more resistant to this by having much higher thickness ?”
It is primarily the tensile strength that prevents the sides from bulging out. Steel has much higher tensile strength than Ferro Cement. Actually, you usually assume that cement has no tensile capacity and the reinforcement carries all the tension.
“Yes, that’s what I mean. If we picture a tilt of 30 degrees the induced bending becomes very obvious. The lateral loads and the ballast which is needed in a spar design combine to make this problematic.”
Actually, one of the reasons a Spar design is used so often is because it is very stable which keeps the structure very close to vertical so the bending loads are minimal. I calculated that the worst case tilt angle would be 2 degrees so I analyzed it at 5 degrees. The thickness required to resist buckling was twice what was required for bending and compression, so it should survive a 10 degree tilt. By the way it would take a 300 mph wind to produce a 10 degree tilt.
The ballast does not increase the compressive loads as you have indicated either. The ballast is a negative load below the buoyancy, so it caused tension between the ballast and the buoyancy section. The compression load at the waterline is equal to the buoyancy minus the ballast which equals the weight above water, so the compression at the waterline is independent of the ballast weight. The amount of ballast only affects the amount of buoyancy required.
“Thanks for the advice I’m guessing that choosing a specific natural frequency for the structure is an important design step. Wouldn’t it determine the section of the spar relative to its displacement ? So in the end it would dictate the ratio between diameter and length ? Also, there must be some formula to minimize the vertical acceleration delta by choosing a good frequency that does not resonate too fast but resonates the taller lower-frequency waves ?”
Exactly. I sent a lot of time analyzing different form factors until I found a design that provided the best response. I started out with a much stouter spar. but it would have been a harsh miserable ride in a heavy sea. I ended up with a 16:1 spar length to diameter with 3/4 of the spar length below the waterline. I do not think there is a formula for the ideal natural frequency. I did it by trial and error and several iterations.May 19, 2008 at 8:36 am #2076
I’d caution against trying to get every possible generic function into the first design. More important is a practical application that actually works. Unless you’re actually going to bolt the things together, small seasteads joing a larger community probably means spread out over several square kilometers, not a few hundred meters. You don’t want to foul mooring lines or crash into each other.May 19, 2008 at 8:44 am #2077
I am also a mechanical engineer, and I do not share the concern over wind or wave loads.
The low tensile strength of concrete is indeed something to keep an eye on when designing, but it is not particularly problematic for the spar concept. Firstly, the entire column would be prestressed by the weight of the structure, so bending loads wouldnt so quickly lead to tensile stresses. And secondly, the forces due to waves and wind, while big if compared against what you can do with your hands, are small compared to the static load from the structure itself.
Stability with regard to currents and wind is an interesting subject. Currents are relatively constant in time, while wind can fluctuate strongly. If we create the attachment point of the mooring line(s) in the equivalent center of the sum of wind loads, there will be no tipping due to wind loads. This leaves a tipping force due to currents, but these could be cancelled out to a high degree by variable ballast tanks.May 19, 2008 at 4:09 pm #2093
The FLIP is made of steel, not ferrocement. In any case, my only point throughout the whole thread here is that a big spar made of ferrocement will be difficult to make. I think we should look at other designs only if we can find one that is easier to build.May 19, 2008 at 4:11 pm #2094
What about putting the butresses inside the spar ?
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