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Thread: Door loads, strap tension, geometry.

  1. #1
    Farmboy's Avatar
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    Door loads, strap tension, geometry.

    Since we have an ongoing morning debate about bifold hangar doors, I thought I’d throw this out to all the engineers and door experts.

    On a typical bifold door, where the straps wind at the bottom edge from the carrying beam overhead, is the strap tension constant from bottom to top? Or does it vary due to the folding action and therefore the force of the upper door pulling away from its top hinges and bottom door pushing against the rollers on the building?

    Assuming a wind factor of zero, is there any reason to position door when open in any particular position?

    Inquiring minds...
    Pb


    Transmitted from my FlightPhone on fingers...
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  2. #2

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    When the door opens the force starts to pull the top away from the building, it does not stay vertical. A friend was renting a local hanger that had a door fail from this issue. Turns out that the building design was correct but the builder cut corners and the carry beam for the door was only toenailed into the supports below. basically the only thing holding the beam in place was the exterior siding and interior sheetrock, no tie in to any other roof structure. No one took notice of the sheet rock cracking along the first inboard truss until the door failed. This was a condo unit of 8 hangers if I remember correctly all of them had the same issue and needed correction to the tune of 15,000 each. So from a safety point I would say only open as much as needed, from a practical point I would say build it to handle a fully open door in the wind and don't worry how far you open it. Caution not an engineer.
    DENNY

  3. #3
    Farmboy's Avatar
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    Yup, I understand the load on the beam, but that’s a sidebar from the question.
    Does the load on the straps/cables change during opening?


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    wireweinie's Avatar
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    The load on the straps should be greatest in the first foot or two. At that point the straps are pulling against a near vertical load. As the door begins to fold, the strap load will decrease as they are no longer pulling against a vertical panel, they are trying to 'fold' the edges together. Just listen to the motor during a door opening. You can hear the work load decrease as the panels begin the folding action.

    Web
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    Dave Calkins's Avatar
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    This is an interesting question.

    I agree with Web. But what happens at full opening? pArtial opening?

    I would love a drawn description of the loads

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    When I was looking for a new door for my old cinder block hangar, I was fearful of the loads from a bi-fold door hanging below the door opening, so I opted for the Higher Power door which is basically self supporting. The costs seem to be about the same. I bought mine during Oshkosh so I did receive a discount, they probably all do that. This is a good question and it's important to know how your hangar was constructed before buying a door.

  7. #7
    flybynite's Avatar
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    The door on my hangar is a manual door with counterweight tied via cable from the counterweight box at the back of the hangar through two pulleys to the bottom of the bifold.

    The weight in the box does not change from closed to wide open. There is no mechanical advantage, just a straight cable run. The door will begin to open when unlatched and need a small bit of assistance from about 3 ft open to nearly fully open.

    The tension in the cables is determined by the combined weight of the door and counter weight, which do not change, therefore the change in tension is negligible.

    Wayne
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  8. #8
    courierguy's Avatar
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    Quote Originally Posted by DENNY View Post
    When the door opens the force starts to pull the top away from the building, it does not stay vertical. A friend was renting a local hanger that had a door fail from this issue. Turns out that the building design was correct but the builder cut corners and the carry beam for the door was only toenailed into the supports below. basically the only thing holding the beam in place was the exterior siding and interior sheetrock, no tie in to any other roof structure. No one took notice of the sheet rock cracking along the first inboard truss until the door failed. This was a condo unit of 8 hangers if I remember correctly all of them had the same issue and needed correction to the tune of 15,000 each. So from a safety point I would say only open as much as needed, from a practical point I would say build it to handle a fully open door in the wind and don't worry how far you open it. Caution not an engineer.
    DENNY
    On my HydroSwing door, (different I know, but similar stresses,) both the 19' wide shop and the 38' hangar door, I ended up welding 1/4" by 2" flat bar to the door frame, in the center, and then ran the flat bar up tight against the ceiling clear back to the end wall, so that now the door would have to take the entire building with it to go anywhere.

    Another HydroSwing related item: the short little hydraulic control lever is touchy, and it's real easy to put a bigger than neccessary shock load on the door mounts that first couple inches of opening, where the loads are highest. I always feather mine, just ease it gradually that first little bit, still a bit touchy to do real smooth though. But recently I had the bright idea of slipping a 12" length of scrap tubing over the little lever, in effect making it less touchy. I got rid of my remote control for the shop door long ago, as it was all or nothing, and what alerted me to the need to do some end wall re-enforcing.

  9. #9
    Dave Calkins's Avatar
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    From a crane guy/rigger perspective, how do you see the strap tensions on the bifold.

    Waynes observation is hard to argue.

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    Farmboy's Avatar
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    Door loads, strap tension, geometry.

    Deleted

  11. #11
    Gordon Misch's Avatar
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    Yup, if the counterweights balance the door in all positions, there's no need for paper analysis of strap tension. However the in-out (horizontal) loads on the building do indeed vary with position. Essentially zero with the door closed, and maximum with the door fully open.
    Gordon

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    moneyburner's Avatar
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    Quote Originally Posted by wireweinie View Post
    The load on the straps should be greatest in the first foot or two. At that point the straps are pulling against a near vertical load. As the door begins to fold, the strap load will decrease as they are no longer pulling against a vertical panel, they are trying to 'fold' the edges together. Just listen to the motor during a door opening. You can hear the work load decrease as the panels begin the folding action.

    Web
    You can also measure the current on one of the motor phases and graph it. I've got one of those, and the next time I open it, I'll do that, but I have a few stacks of insulation and so forth and don't want any wind getting in there and creating havoc.
    Quidquid Latine dictum sit, altum videtur
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    cubdriver2's Avatar
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    Quote Originally Posted by moneyburner View Post
    You can also measure the current on one of the motor phases and graph it. I've got one of those, and the next time I open it, I'll do that, but I have a few stacks of insulation and so forth and don't want any wind getting in there and creating havoc.
    Don't think that will work? As the strap rolls up on the rim it get bigger in diameter and the gear ratio is changing and increasing with ever rotation so the load on the motor would be increasing also?

    Glenn
    "Optimism is going after Moby Dick in a rowboat and taking the tartar sauce with you!"

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    Is Isaac Newton on this site?
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    moneyburner's Avatar
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    Quote Originally Posted by cubdriver2 View Post
    Don't think that will work? As the strap rolls up on the rim it get bigger in diameter and the gear ratio is changing and increasing with ever rotation so the load on the motor would be increasing also?

    Glenn
    Good point; I hadn’t thought of that.
    Quidquid Latine dictum sit, altum videtur

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    wireweinie's Avatar
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    If that is a factor, it will show as an increase in current draw as the door nears the top of it's travel.

    Web
    Life's tough . . . wear a cup.

  17. #17
    cubdriver2's Avatar
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    Quote Originally Posted by wireweinie View Post
    If that is a factor, it will show as an increase in current draw as the door nears the top of it's travel.

    Web
    Unless the door bottom gets lighter as more of the weight is transferred to the top hinge?

    Glenm
    "Optimism is going after Moby Dick in a rowboat and taking the tartar sauce with you!"

  18. #18
    wireweinie's Avatar
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    If that's the case, I guess it really won't matter.

    Web
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