Mechanics of the spar design

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This topic contains 41 replies, has 9 voices, and was last updated by  Sundiver 7 years, 6 months ago.

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• #2063

Anonymous
#2065

portager
Participant

#2066

portager
Participant

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; Mike

#2067

thebastidge
Participant

… 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.

#2068

Participant

“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 ?

#2069

Participant

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.

#2070

portager
Participant

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; Mike

#2071

Anonymous

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.

#2072

Anonymous

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.

#2073

Anonymous

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)?

#2075

portager
Participant

“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.

#2076

thebastidge
Participant

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.

#2077

Eelco
Participant

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.

#2093

Participant

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.

#2094

Participant

What about putting the butresses inside the spar ?

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