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Thread: The Big U Lift Bridge

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    Default The Big U Lift Bridge

    At our last show we were lamenting how our club lift bridge didn't work so well anymore. We know it's at least 15 years old, probably closer to 20, so we really can't complain. However, it got me to thinking about a "better" design. Our current one simply clamps to the modules on each side, which means if the layout has any pressure on one side it can be a bit difficult to close. That's part of why it took some abuse.

    I drew this up as a result in SketchUp.





    The idea is that the two sides are cut from a single sheet of plywood in a big "U" shape. This provides continuous support when the bridge is open, so they layout can't move around. I've left out two boards so you can see the construction, the bottom cover (that you would step on) and the inside vertical cover on one side (identical to the one on the other side).

    Additional U shaped braces are cut and installed inside on the bottom so the thin bottom (3" tall, 1" off the ground, 4" step total) has more support. The panels that affix to other modules have slots cut in them so the clamps can be moved up and down to optimal position for maximum clamping force. What look like 45 degree cuts on the sides of the top are actually more like 42 degree, so when open the bridge goes just past 90 degrees before resting on them so it will remain open on its own. The base is 1' wide to provide more clamping area and stability, while the lift bridge itself is 7" wide. All parts are cut from 3/4" plywood, it almost all can come from a single sheet but it takes a few inches of a second sheet to make some of the U braces. Note that when closed there is a block under the hinge side, and it closes on the supports on the latch side, so it can't "sag".

    The only exception is the alignment piece on the bottom of the lift up part. That would be 3/4" hardwood of some sort so it could be sanded into a taper to help with alignment.

    N-Trak bus (12ga w/powerpoles) would run down and under. There would be three feeder drops, one on each fixed section, and one on the lift up section. Feeders on the lift up would be as close to the hinge as practical, and the wires would simply be exposed on the bottom for a very short run.

    I would probably finish as if it were furniture, stain and poly, or maybe even varnish. The tracks would get ballasted between the uprights to make it a bit more visually appealing than many bridges.

    4 pinball leg levelers would be installed. They can hold up a 400lbs pinball machine, so even if someone stood on it they should be fine to support the weight.

    This is obviously larger (to transport) and heavier (I estimate about 50-60lbs) than most of the collapsable bridges I've seen clubs use. But unlike the collapsable bridges it's weight and stiffness means the layout should move less, and it could be used next to corners or other less-stable modules without trouble.

    I welcome comments though before I go commit saw blade to plywood. I know there are plenty of folks on here who have built modules and may have thoughts. My current thinking is to build a prototype for my home layout (I finally have a loop of my own modules in the basement!) and then see if my club has any interest in a duplicate. Right now many are skeptical due to the weight and size, but I think while it will be a little more difficult to set up it will be trouble free during the show. Our current bridge has to be fidgeted with every opening, which is a lot less fun. Feel free to even say I'm crazy, if you really think it's a bad idea. I want honest feedback!
    --
    Leo Bicknell

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    I did a similar thing for my drop-leaf bridge on my home layout, except that instead of a pair of big U shapes, I built up two towers that rise from a common base of a 1x8 that sits on the floor. That's not an approach that would work for a portable layout, so I'm not recommending it here. Rather, I note that my home design did nothing to create a "moment" connection between the two towers, as the 1x8 is free to flex as much as a 1x8 can, but I just assume it's going to repose always on the same floor, and thus the towers will stay in the same alignment with each other.

    Here though, you need that "floor" connector to be very rigid, since it's got to do the job of aligning the side towers DESPITE the undulating floor conditions below it. I have my doubts about whether you have enough thickness there to do what it needs to do. I could see a bit of torque developing that might twist one tower relative to the other, and I could also see the floor beginning to sag since what you'd have would essentially be a pair of floor joists that span about 3 feet (maybe more) between where they are supported on the adjustable feet. They are tall enough that they become a trip hazard, which makes it all the more likely that folks are going to actually step on them rather than just over them.

    So, I'm wondering if it might be better to make the connector be a few inches taller, to set the top at a regular stair riser height of 6 to 7 inches, and anticipate the folks will actually step on it. A riser of that height would be able to span without sagging under the weight of foot traffic.

    Another thing one might consider is that the width of the opening might be less at the floor than it is at waist height, i.e. the tower could taper a bit. If you have 36" clear at the top, you could go down to maybe 24" or even 20" at the bottom. Though straight sides might be better if you're thinking of rolling a hand cart in over the floor, or if you picture wheelchair access being a thing (it would need ramps).

    I suppose you could also deal with potential sag by adding adjustable feet to the middle of the span, and thereby supporting it right where traffic loads would get applied. But I still don't see how you could make that step be any less than about 4 inches in total, and so I think it's better to go ahead and just round that up to 6 inches for a normal riser height.

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    I could also see this being weak in terms of racking plan-wise; imagine the modules to the left of the bridge tugging inward while the ones to the right want to thrust outward. On an ordinary N-Trak module, its rectangular plan shape resists such racking, but the two independent towers here have little to help them in this regard. What they will rely on will be the base, and in this case, the tread that was omitted for clarity will play an important role. I think I would cut that tread so that it is a full 4' long, with its ends inserting between the two vertical walls of each tower.

    I was going to suggest that the crotches where the tower's vertical wall becomes the floor beam could be cut with a 2" or more radius, rather than a 90-degree corner, to ease the stresses that will get applied here. But if you insert the tread as stated above and have it firmly anchored all along its length, then it will serve as reinforcement for that corner and accomplish much the same thing.

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    You're both touching on the same concern I have, the twisting resistance of the base. The sad truth is, without some sophisticated modeling software I won't know until it's built. I attacked the problem 4 ways:

    1) The reason the ends go top to bottom is that makes the towers very twist resistant. And there's a similar piece on the inside, not quite all the way up so it's possible to reach clamps and wires, but most of the way.

    2) For the bottom, I am trying to keep it low so that it's easier to get a cart over. I assume people will step on it, in fact I figured the tread I would put the non-slip stuff on. That's the reason for the additional U shaped beams inside the bottom. There are 3 on each side, making the "board" a full 3" thick. They would be fully glued (entire face), and then the tread would be glued down to the top of them as well. They also get glued to the inside and outside ends. I have no doubt this would hold a person standing on it, but would it resist the twisting enough?

    3) This module will be heavy. A sheet of 3/4" quality plywood is about 65 lbs. I figure this module will be 55-60 lbs finished. It's going to want to stay put.

    4) The module is a full 1' deep. Most of our lift bridges are around 7" deep, which gives them less "lever" to resist the twist.

    Now, going to your suggestions. The taper is a very interesting suggestion. The opening is 36" at the top, but could be 30" (standard door) at the bottom, providing more strength and less beam length. That one has me thinking, it causes some additional plywood waste but may be a good choice.

    I don't think I need additional feet in the middle. I'm pretty sure the plywood will hold a person without significantly bending as drawn. They don't help with the twisting problem either, I don't think.

    The radius to reduce stress was my original idea, but I realized it was very hard to deal with on several levels. Primarily I'd need something very flexible to bend around it, and most of those materials wouldn't have much strength if someone accidentally kicked them. It just seemed to complicate construction more than it added.

    I also considered making the U shaped braces I described above come up higher. Right now they are drawn as 6" tall (3 inches along the bottom, coming up an additional 3 inches on both sides into the towers). I could make them say, 9" tall, so they come up further into the towers.

    One last idea, I could put in some perpendicular braces in the bottom between the front and back U shaped rails. I'd need to drill some small holes so the wires could go through them, but that might add some additional connections to help resist twisting.
    --
    Leo Bicknell

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    Googling around I found the "Sagulator" for bookshelves! http://www.woodbin.com/calcs/sagulator/

    Now, consider just the tread, omitted from the diagram. If I put in:

    Plywood, Fir
    200 lbs load (a generic person)
    36" wide
    Fixed (attached only at the sides)
    Center Load
    12" deep
    0.75" thick

    I get 0.1" deflection in the center, which it lists as borderline for a bookshelf.

    Keep in mind this is just the flat piece! 3/4" ply will hold a lot of weight!

    So even though those 3" high sections don't look too much, all they are doing is providing enough support so we don't have the 0.1" deflection if the board was just supported at each end.

    I think this also provides some idea of the twist resistance. Now obviously the towers act as long levers, but I actually suspect the bottom in this design could provide several hundred pounds of twist resistance. Given most of our layouts move maybe 1/4-1/2" when the current bridges are opened, and are easily moved back into proper alignment with a single hand, I think this is enough.

    I think being the operative words.
    --
    Leo Bicknell

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    I am not sure if the continuous U of plywood is any stronger than 3 separate members connected by multiple bolts. In other words, make the bottom connecting support a separate piece and connect each of the side pieces to it with 3 or 4 bolts.

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    Quote Originally Posted by Mac View Post
    I am not sure if the continuous U of plywood is any stronger than 3 separate members connected by multiple bolts. In other words, make the bottom connecting support a separate piece and connect each of the side pieces to it with 3 or 4 bolts.
    i don’t have any idea what it would do to the strength issues, but certainly making it 3 pieces (2 uprights and a bottom) that are bolted together securely would make it MUCH easier to break down and transport.
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    I am sure there is a bolt-and-3-piece design that is as strong as the U design. That said, I'm not interested in bolting and unbolting for transport. In my experience anything that requires bolting and unbolting really slows things down. All of my modules have attached, swing up legs for instance. I am fortunate to have a trailer for transport, so I don't need to pack them quite as tightly as someone fitting them in say a station wagon. At the last show I took 6 modules and had them all broken down, in their racks, and in my trailer in ~40 minutes all by myself. In that time I watch people pack up one or two modules with removable legs, skyboards attached with thumb screws, and so on. Bolts simply take too long, add-on bits take too long. (For me.)

    In case it's not obvious, I'm arguing with myself here as much as you guys. I appreciate the critical feedback and suggestions, because it is making me think. In many cases they are good suggestions, but perhaps just not a direction I want to go.

    After thinking about it more, I do believe the tread (where you would step, not shown in my drawing) is a KEY component of the twist resistance. I say this both from thinking about that shelf site for computing deflection, but also my module experience. I build my modules with a 3/4" plywood frame (typically cut to about 4.5" tall), and then use a top of 1/2 plywood. I also use fairly premium (hardwood, more ply) plywood for the structural strength. I do this because I do not like cross braces under the module, they get in the way of electrical and switch machines and so on. When I build them the 3/4" frame by itself will twist like crazy. But that 1/2" top glued and screwed makes the twisting nonexistent. And they are strong enough I can stand on them, and I'm not a small guy. So it's the top offering much of the resistance.

    That's good news here. If there was too much twist across the U, it would be trivial to double up that tread. Simply add another on top, glue the whole face, and screw it down. Now it's a 1.5" plywood sandwich resisting twist. Best part, I don't have to add it if it is not needed, and if it is needed adding it only adds a few minutes to the build time. The step being an extra 3/4" tall should be no big deal. That may be my plan to deal with the problem, if it occurs.
    --
    Leo Bicknell

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    I like the original U plan. I think the U shaped ply will be more stable than bolted construction over time. Bolting and unbolting would wear.

    The only difficulty I see is some clumsy person (like me) accidentally kicking the bottom while passing through the gate.
    Cheers!
    Gordon
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    Well, if breaking down for storage/shipping isn't an issue, then the solid U construction will definitely be the way to go. it will be at least as strong as bolted (maybe stronger) and much less hassle on setup/teardown.
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    Quote Originally Posted by el Gato Gordo View Post
    The only difficulty I see is some clumsy person (like me) accidentally kicking the bottom while passing through the gate.
    The only solution I've seen is a much bigger undertaking.

    The part that may or may not be obvious, wiring has to pass from one side to the other. Most of our collapsable bridges have a bit that sits on the floor to cover the wire, and generally I find them to be a trip hazard because if you kick them they move! Some clubs simply gaffer tape down the wires to the floor.

    The "solution" was one club who had a member in a wheelchair. They built an arch, 6' tall, that the bridge lifted straight up into. The wires went up and over the top, leaving nothing on the floor. It works, it's nice, but then it's _really_ big.

    So I'm hoping that just by the fact that it won't move (much) when kicked/stepped on is a win.
    --
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    I have built something similar to this for a Commercial Bar. We ran tap lines and an Ethernet cable underneath. Due to the extremely high amount of traffic and rigorous cleaning we built the step subframe out of 1/8” steel L girders and the top of it was trapezoidal. The piece did not need to be portable and was probably quite a bit heavier, but it was bullet proof. If the bottom is kept to a minimum depth and height most people will instinctively step over it. Paint it with bright yellow caution stripes. You could also try building it as you have drawn and then add a wide based floating “step” over it so any stress placed on the step was transferred directly to the floor and not attached to the bridge unit.

    just my $.02 for what it’s worth

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    Quote Originally Posted by Hucklebury View Post
    we built the step subframe out of 1/8” steel L girders
    Maybe rather than using the additional pieces of plywood laminated inside the base I would be better off using a piece of steel.

    Perhaps something like this: http://www.speedymetals.com/pc-3993-...led-steel.aspx

    It's probably not much heavier than several layers of wood, but I bet has quite a bit more twist resistance. I could even get my buddy to weld a vertical part that came up the sides ~2 feet or so to have better lever action. That's impractical for more plywood U's (too much panel waste), but in steel wouldn't be that hard.
    --
    Leo Bicknell

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    I wonder how you are going to hinge the bridge? Whenever I contemplate building something like this, the geometry of hinging is always a concern. Obviously, many people succeed, I was just wondering how you plan to accomplish this task.
    Cheers!
    Gordon
    Rheinland Bayern Bahn
    https://www.nscale.net/forums/showthr...4-x-9-5-layout

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    I would suggest, once built, start with a hole saw and start adding lightening holes. Think aircraft framing. You won't lose that much strength, but could drop quite a few excess pounds.
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    Quote Originally Posted by el Gato Gordo View Post
    I wonder how you are going to hinge the bridge? Whenever I contemplate building something like this, the geometry of hinging is always a concern. Obviously, many people succeed, I was just wondering how you plan to accomplish this task.
    The axis of the hinge needs to be slightly above the rail head, to above pinching it when raised, so it seems that the hinge may need to be mounted on the angled faces of the side rails. That means two separate hinges, though, rather than one long continuous piano hinge, which would seem to be better. An option might be to have the track extend over a hinge such that where it it meets the stationary lead track, it drops away; this would require the stationary track to be undercut at a slight angle.

    I think it was @TwinDad who experimented with some cabinet hinges that allow for some clever geometry, so maybe we can get some input from him...

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    Quote Originally Posted by el Gato Gordo View Post
    I wonder how you are going to hinge the bridge?
    The trick is that the center of the hinge has to be above the rail head. (Think about when it's open 90 degrees, the rail that lifted up has to be above the rail that stayed fixed.)

    Most bridges accomplish this easily. The track is affixed directly to the plywood, and then hinges are used on the surface. I was thinking of using these:

    https://www.go2marine.com/product/20...der-hinge.html

    Two would be placed flat on top of the bridge near the sides. I think the hinge part is just high enough to be a few fractions above the top of the rail.

    Our existing bridge uses hinges like these:

    https://www.homedepot.com/p/Everbilt...5404/202034071

    Although I'm pretty sure smaller than that, but mounted just as a describe on top of the board.

    Some fancier designs put them inches above the rail by bringing the sides up higher and putting a pivot point through both sides, much like a real lift bridge.

    - - - Updated - - -

    Quote Originally Posted by WP&P View Post
    hinge may need to be mounted on the angled faces of the side rails
    I suspect that would work, but I actually planned to mount mine flat to the board and leave the angled faces just for holding it at about 95 degrees when open.
    --
    Leo Bicknell

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    Bit of unsolicited problem reformulation (and completely naive brainstorming) -- instead of trying to more rigidly connect the two separate sections of layout can you instead incorporate some float / movement into both or either lift bridge mounts (and lead-up track)? I guess it would depend on your modeling on either side but it sounds like your issue was minor misalignment that eventually led to wear-and-tear on the bridge / bridge connections. Might be within the range that could be handled with some unsecured flex track and a floating mount on one or both sides.

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    Quote Originally Posted by WP&P View Post
    I think it was @TwinDad who experimented with some cabinet hinges that allow for some clever geometry, so maybe we can get some input from him...
    Indeed it was me. European style cabinet hinges, and they worked quite well. I’m having some issues with the other end of the bridge, but the hinges have been fantastic. The nice thing is the entire hinge is hidden underneath the bridge, but the pivot point is above the rail, where it needs to be. And they are very adjustable (great for weather changes) and they snap in and out, so you can remove the bridge entirely while setting up then click it back in place.

    The downside (if there is one) is the thickness of the bridge where the hinges are is very thin. I couldn’t even have cork under the track for the first couple of inches (where the hinge mounts are). Doesn’t look like you’re too worried about scenery though.

    The hinges I used were the Blum Clip-Top Blumotion standard 110˚ hinges. They have a 95˚ version for thick doors that might work as well and (may or may not) give more clearance for roadbed, etc.

    If you’re interested, I can dig up some of the calculations I did.
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    I've found this type of build to be extremely monolithic and relatively light weight.
    U.jpg
    A buildup of 2x4s screwed and glued together and the angles strengthened under the layout side of the step thru with plywood gussets on each side. It's simple to make.

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