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Thread: Hurricane-proof homes - the science & engineering of surviving 155 mph winds

  1. #31
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    Jun 2007
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    FOUND THIS LINK
    for wind loads calculations

    https://www.buildingsguide.com/calcu...ural/ASCE705W/
    as I was given 1/2 ss 7x19 cable with 22k lbs working load
    to use as the cherry on top to be extra sure the roof stays in place

    I am trying to understand the numbers for lift and how strong the cable end fittings and ground ties really need to be
    I am guessing the SS cable is over strength but got 5/8 steel turnbuckles that are only rated 5700lbs working load
    and need to buy connectors and bits to tie to the floor slab

    if I plug in my numbers at a 200mph wind speed
    I get

    one number is -93 labeled (w/ +GCpi) windward roof for end zone [lift?]

    for the next column I get -65 (w/ -GCpi) leeward roof for end zone.

    p= pounds per square foot no idea what i is w= 1/2 width

    so I get 15/-93 times ‘‘i’’ what ever the ‘‘i’’ is
    then figure the roof area in the code box and multiply that by the 15/-93 times ''i'' I think

    can any one check the link on how to use the numbers it produces ?

  2. #32
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    looking at the bahama housing destruction

    the metal roofing held up fairly well to the winds mostly
    direct water waves and flooding currents were the worst home destroyers
    as were the flying stuff that did hole a few metal roofs

    I saw a house on tv that had all the windows [impact rated] done in by water and stuff in the waters
    with a metal roof intact

    as I am in a projected to be dry area zone X that did not flood in ANDREW
    I am hoping the 1/2 SS cables with keep my roof in place

  3. #33
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    Norfolk UK and some of me is in Northern France
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    Start with the basics. 1/2 x air density x velocity squared. That is the maximum pressure. Multiply by area, either of roof or side elevation. That is a force. The lift coefficient can get over one in some shapes. Work out forces per roof panel to see how well they should be secured, then for whole area to see what forces are expected. If the wind gets inside, you get the conversion to static pressure equal to that max pressure, it can blow the roof off. Local features like other buildings can push up that velocity too. And gusting has to seen as a momentum / time event, ie an impact. Personally I would use a factor of safety of 6 multiplied by the max velocity induced force when calculating tie down ties. Density of air, use .0024 slugs per cubic foot. Velocity ft/s gives pressure in lbf/ ft2.
    sicut vis videre esto
    When we realize that patterns don't exist in the universe, they are a template that we hold to the universe to make sense of it, it all makes a lot more sense.
    Originally Posted by Ken G

  4. #34
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    Hurricane clips can make all the difference.

  5. #35
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    yes used the standard dade co approved ones doubled on sister-ed replaced beams to tie to the concrete wall caps

    and like everything else on my roof used screws not nails for more holding power
    but the clips are ONLY holding the beams in place
    in the Bahamas I saw lots of roof beams in place with little to NO roof left so they are NO sure thing

    the 1/2'' SS cables may not be needed but are cheap insurance for the increase in cat 5 storms
    and do hold the steel roofing and the tripled plywood down in addition to the underlying beams

  6. #36
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    The weak link in screwed down steel is the small head of the screw. It’s a smaller force to pull it through the steel than to rip the screw out. You can try washers under the heads. Or bolts at the edges to prevent a domino effect
    sicut vis videre esto
    When we realize that patterns don't exist in the universe, they are a template that we hold to the universe to make sense of it, it all makes a lot more sense.
    Originally Posted by Ken G

  7. #37
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    Quote Originally Posted by profloater View Post
    The weak link in screwed down steel is the small head of the screw. It’s a smaller force to pull it through the steel than to rip the screw out. You can try washers under the heads. Or bolts at the edges to prevent a domino effect
    the steel roofing is held in place with screws and washers
    and all the steel is every other sheet doubled so most of the screws are thru 3 layers of steel roofing minimum
    in the peak area the steel is lapped so most are thru 4 layers of steel roofing or more

  8. #38
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    Quote Originally Posted by nota View Post
    the steel roofing is held in place with screws and washers
    and all the steel is every other sheet doubled so most of the screws are thru 3 layers of steel roofing minimum
    in the peak area the steel is lapped so most are thru 4 layers of steel roofing or more
    So using 300 ft/s I make the max possible force 108 lbf/ sq.ft. Does that sound right? But there is the gust momentum too.
    sicut vis videre esto
    When we realize that patterns don't exist in the universe, they are a template that we hold to the universe to make sense of it, it all makes a lot more sense.
    Originally Posted by Ken G

  9. #39
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    the calculator in the top of page post gave a -93 value at 200mph
    so not too far off 108lbs f/sq at 300ft/s

    but is that value lift or force pressing on the buildings roof ?
    I think the value -93 is lift not pressure as it is negative ?
    and lift is the killer for a roof in a cat 5

    my steel is rated to 180mph in single layer use [the screws thru two layers at the joints/edges
    as I used minimum 3 layers of steel and longer screws 2 1/2 vs 1 1/2 inch long rated and tryed to get the edge screws in to the beams not just the plywood deck

  10. #40
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    Quote Originally Posted by nota View Post
    the calculator in the top of page post gave a -93 value at 200mph
    so not too far off 108lbs f/sq at 300ft/s

    but is that value lift or force pressing on the buildings roof ?
    I think the value -93 is lift not pressure as it is negative ?
    and lift is the killer for a roof in a cat 5

    my steel is rated to 180mph in single layer use [the screws thru two layers at the joints/edges
    as I used minimum 3 layers of steel and longer screws 2 1/2 vs 1 1/2 inch long rated and tryed to get the edge screws in to the beams not just the plywood deck
    My calculation for 300 ft/s approx 200 mph, is the static pressure if you stop the wind eg by letting it get inside. Its always a lift force. In addition the momentum of a gust can exceed that force. When you multiply by the roof area, thats going to be a big lift force. The shape of the roof plus the building will usually give a lift force as the calculation is basically a negative from one atmosphere. Bernoulli says p + dynamic + g force is constant, and here the one bar is being converted to wind speed. Which in turn converts back to pressure if it is stopped or diverted. You can get positive pressure on the sides as the air comes to halt. Once the air starts swirling you get gusts.
    sicut vis videre esto
    When we realize that patterns don't exist in the universe, they are a template that we hold to the universe to make sense of it, it all makes a lot more sense.
    Originally Posted by Ken G

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