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Someone help me figure this out – PLEASE!

Home Forums Research Engineering Someone help me figure this out – PLEASE!

This topic contains 33 replies, has 8 voices, and was last updated by Profile photo of shredder7753 shredder7753 3 years, 3 months ago.

Viewing 15 posts - 1 through 15 (of 34 total)
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  • #1618
    Profile photo of shredder7753
    shredder7753
    Participant

    I need to know how to calculate the strength of flat concrete steel reinforced walls with (X) wall thickness. According to my spreadsheet. I can use up to .5m thick walls and still have a floating Bergstead. I know with 100% confidence it wont take .5m thick walls. Would a box with .35m thick walls/bottom/top and 3m between floor and ceiling be able to withstand pressure at 25m depth? I dont want to know “well, based on my experience…” I need quantitative data! what is the exact breaking point in theory. I suspect we might have 5x as much strength as needed in a static water tank. If that’s the case than this design will work.

    Where can we find this formula?

    I do know that the pressure at 25 m is 3.5 times atmospheric pressure at sea level. 1 atm = 14.7 PSI. 3.5 atm = 51.1 PSI. thats not a huge amount.

    Wait A Minute! I just realized. if concrete has strength of 3000psi compression, and we only need 51psi (or 90psi at 50m), then there is absolutely no reason why you cant have flat walls!

    #15265

    shred, how big the hydrostatic load on a piece can be depends mostly on the load case. This is why submarines have round hulls that cook down to a ” compression arch load case” – the failure depth depends most on the “expected failure mode” especially if we are talking about failure by buckling (elastic / non elastic) or by compression of the material itself (for thick walled round structures).

    For the case of a box structure that does absolutly NOT work as a compression arch at all, you need to calculate the walls as beam supported on the ends – what in this case is the failure mode, is the tension in the rebar. How tension forces in rebar come up depend mostly on how in the concrete you colocate the rebar. Best would be just below the inner surface of the wall.

    Even for the most experienced and highest paid engineers on earth – the exact calculation of flat walls under hydrostatic load and prediction of their failure can be tricky – the most expensive engineering desaster in civil engineering history , the failure of a flat walled connector piece in sleipner A at a not expectd depth – shows the magnitude of the problem.

    So in your case – you are best advised to “just test it out” build a model – submerge it – see what you get. I would expect 3-5m rather than 25m destruction depth for a cube shape.

    flat walled “tricell” piece failure in sleipner A – the implosion of the structure caused a magnitude 3 earthquake…

    Wil

    concretesubmarine.com

    #15266
    Profile photo of shredder7753
    shredder7753
    Participant

    its hard for me to understand (little exp working around water) how a Berg could possibly fail at just 3-5m. the pressure there is only 50% more than the atmospheric pressure!

    also, remember – the interior air pressure has to be controlled. there will likely be some air tanks to pump in the excess air i think.

    wow, sleipner was a disaster. hope that dont happen to me :-)

    AND you just made my point!!!! you clearly argued that a flat walled part of the sleipner hull FAILED at a depth of 99m! thats twice the max depth i want! wahhoooooooo!

    ____________

    My Work II

    “Leadership and do-ership are not the same thing”

    #15268
    Profile photo of shredder7753
    shredder7753
    Participant

    also i understand why the walls become “beams supported on both ends” – BUT they are also supported along their length by the edges of the floor and ceiling.

    ____________

    My Work II

    “Leadership and do-ership are not the same thing”

    #15267

    exactly, as you see from above example the “hard to understand” for flat walled structures under hydrostatic load even applies to the best engineers and their sophisticated FE computer models…so the best way to get a better understanding quick is to test it out in test series that compare directly .

    To start with you need to understand that the difference between round structures and square structures is GIANT not just a bit. It is because they fail completly different and due to completly different load cases.

    If you think atmospheric pressure is “just a little bit” check the internet videos what 1bar does to a structure when you suck the air out and let 1bar work

    #15269

    test it – you will see…

    also keep in mind – things like the tricel and the marmaray tunnel segment (foto below) are square and submerge to considerable depth – but check their floating line (hardly float) which indicates the big DIFFERENCE in wall thickness versus diameter compared to your “berg”. Also check their rebar distribution which is key for their load bearing performance…

    Wil

    concretesubmarine.com

    #15270
    Profile photo of shredder7753
    shredder7753
    Participant

    u made ur point – but still there has got to be a FORMULA out there that can help me get somewhere in the ballpark.

    ____________

    My Work II

    “Leadership and do-ership are not the same thing”

    #15271
    Profile photo of georgeberz
    georgeberz
    Participant

    I think you should try to find bridge engineering specifications or parking garage specifications.

    I think at 50psi that is 7200 lbs from just 1 sq foot

    Each time you move 1 foot from the wall towards the center add that weight again as all the loads will be sent to the corners and the walls IF you can come up with the right amount of rebar schedule, you really are looking at a massive span concrete is brittle as you found out in the 1st wall failure you had.

    concrete has almost 0 tension ability, have you given thought to what even 3-4′ wave energy will need to be displaced along a flat wall?

    this is why you do not see many flat walls on boats or subs, as even a slight curve increases strength immensely

    Im sorry I dont have actual links or specifics to send you to.

    Also most countries I believe now require commercial boats to have foam ballast equal to do displace the weight of the boat if it is to be sold or rented. If it is for your own personal use, no renting out then anything really goes.

    just check on some of the rules before you get too deep into this whole thing.

    #15272
    Profile photo of shredder7753
    shredder7753
    Participant

    are u gonna send me on a wild goose chase or maybe post 1 link so i can have the slightest idea what ur talkin about?

    i googled “foam ballast” and all the links are about model railroading.

    ____________

    My Work II

    “Leadership and do-ership are not the same thing”

    #15274
    Profile photo of georgeberz
    georgeberz
    Participant
    #15275
    Profile photo of shredder7753
    shredder7753
    Participant

    georgeberz wrote:

    Im sorry its flotation requirements

    http://www.uscgboating.org/regulations/boatbuilder_s_handbook/flotation_part1_f.aspx

    great link! but im wondering, those guidelines/rules cant possibly apply for an oil tanker. so where do they draw the line. my calculations are showing a weight of 2,000 tons for the hull of a 20m x 20m Berg (yes, thats 4 million pounds). surely they dont need me to put foam and pixie dust all up in it.

    the 20 x 20 has an enclosed garage that can fit a 40′ offshore powerboat.

    ____________

    My Work II

    “Leadership and do-ership are not the same thing”

    #15276
    Profile photo of shredder7753
    shredder7753
    Participant

    u would simply not believe how hard it is to get this thing to submerge. yep even with a solid mass of concrete below it, and 4m deep of ballast on the inside, the water line is still 1m from the top.

    ____________

    My Work II

    “Leadership and do-ership are not the same thing”

    #15281
    Profile photo of shredder7753
    shredder7753
    Participant

    i think i have the dimensions i need now – at least to float AND be able to submerge. i still dont know the exact tolerance of the walls. BUT they are 0.35m (14 inches) thick. that should do the trick. this next one will be in 1:35 scale instead of 1:24. the last one was only 15m x 15m and the scale model weighed over a hundred pounds. this new one is 20m x 20m and is expected to weigh 80lbs. plus the pyramid weight which will be 60lbs on its own.

    ____________

    My Work II

    “Leadership and do-ership are not the same thing”

    #15283
    Profile photo of georgeberz
    georgeberz
    Participant

    As far as I know all new manufacture has to comply with the floatation issue, I do believe even subs now have to comply and be positively bouyant and can only sink when underway by the winglets, so if the power fails the sub rises on its own. its just what Ive heard. I dont know if it is a fact or not in the USA.

    I sould worry about the structure and the design loading of the walls / floor / roof first, you can always reengineer floatation later if need be.

    #15285
    Profile photo of shredder7753
    shredder7753
    Participant

    when it comes to submarines (i’ve been reading about rc subs for their ballast systems), there are static and dynamic subs. static is when they sink using ballast, dynamic is when they just dive using their control surfaces. its hard for me to picture an american nuclear submarine that has no way to dive without being in motion. dynamic diving subs have to be in motion all the time or they cannot stay below the surface.

    anyway… the berg pictured above can ONLY submerge if the pyramid is attached AND the ballast tanks are almost completely FULL. The way many ballast systems work is that the water is actually sucked in when an AIR PUMP compresses the air from inside the tank to a separate compressed air storage tank. i previously thought that a water pump was used to pump the water into and out of the ballast tanks, but usually thats not the case.

    ALSO – they are designed in such a way – and i dont completely understand this yet – that when a power failure occurs, the ballast water is automatically BLOWN out and the vessel will surface.

    from reading about how this works, i have come to realize thats its without question well enough ahead of my engineering skill level. but thats why i already contacted a reputable r/c submarine ballast system expert. he emailed me back today and said he was on vacation in France but he is interested in finding out more about this project.

    AND – when it comes to the required wall thickness to use my beloved flat walls, the only thing i’ll need is a single number to tell me how thich the exterior surfaces will have to be. i set up a spreadsheet that, once i plug in that number, it will tell me EVERYthing ELSE; “Total Volume of Concrete”, “Weight of the Structure”, “Height of the Water Line”, “Volume of Water Displaced”, and so forth..

    (control key and scroll up may help u zoom on this)

    ____________

    My Work II

    “Leadership and do-ership are not the same thing”

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