Okay everyone, here's my dilemma. I didn't fully think about my design before I started building it and now I've run into the problem of not actually having enough room to support my upper deck from below in a few places, so I'd like to run a couple ideas by you and see what ideas you have in turn. So my first idea, which will work on one side of the mushroom but not both, was to have 2x2" supports coming directly up from the floor to meet the bottom front of the benchwork for the upper deck and support it using the kind of plywood supports Mike Danneman is using, while the rear would be supported by 2x2s coming down from the rafters in the ceiling, like so: http://hostthenpost.org/uploads/e8c3bb3375cd1c3858b4aa0d93aba672.jpg Now, on the other side there's only space for a support at each end of the peninsula, so there would be about 10 feet between the two supports. It would be fully supported from the ceiling, like so: http://hostthenpost.org/uploads/9247aa775e61f28c651077918b8e5cba.jpg The shelf itself would be composed probably of 3/4" ply cut into 1x4s making a frame, and vary in depth between 16" and 24". Here's a screenshot of the plan for the upper deck: http://hostthenpost.org/uploads/656c85720e84b4034902e3e4ceaa742f.jpg One thing to keep in mind is that Farr Creek Bridge will cause a depression in the framework, and the left side is the one that is harder to support from the floor. Okay, ideas? Criticism?
Deck height, deck separation, deck thickness, and depth of the upper decks will all interact with each other to affect how much of the lower deck will be visible. 1. How high is the surface of each deck from the floor? (affects viewing angle) 2. How much space is there from the underside of the upper level to the surface of the lower level? (affects how much of the lower level can be seen) 3. How thick can the upper level be and still let you see to the back of the lower or middle levels without bending over? 4. How deep can the upper deck be and still let you see to the back of the shelf (or at least to where the trains will be operating)? Deck thickness will depend on construction method: a 4 inch frame+3/4inch shelf will allow a deeper upper deck without obstructing the view on a lower shelf than the 5 inch thickness of a 1x3 L-girder+1x2 stringer, but 4 inch frame construction will require supports every 7.5 feet instead of every 9.5 feet for a 1x3 L-girder or supports every 13 feet for a 1x4 L-girder. (See "How to Build Model RR Benchwork," by Linn Westcott.) For the shelf along the walls, you can brace at an angle from below but will need to be careful how far below the upper shelf the angled brace intersects the wall, so it is not easily visible within the viewable area of the lower deck. If you want the entire floor-supported peninsula covered by an upper deck that has no supports at all from the floor, it could be done by suspending two 10 foot long L-girders at the proper height between lower deck and the ceiling, and then running stringers across them. If your peninsula is 3 or 4 feet wide, then the 2 L-girders would have to be about 16 to 24 inches apart from each other and the stringers would have to be 1x2s or 1x3s (depending on how far outside the L-girders you want them to extend). The size of L-girders and stringers will also interact with how far the stringers extend away from the L-girders as they support a wider upper deck. (1x3 stringers will support a wider peninsula, but will necessitate a thicker upper deck which will reduce how far you can see into the lower peninsula before the lower edge of the upper deck's fascia boards block your view of the lower deck.) If you are able to attach the L-girders to the wall at the base of the peninsula and drop suspending 2x2s from the ceiling at 3, 6, and 8 feet away from the wall toward the end of the peninsula, then you could probably use 1x3 L-girders.
I thought out deck separation more thoroughly than I did construction. Minimum rail-to-rail clearance is 16" at the first point of overlap by the helix, and it only goes up from there. The dip for the bridge will be a bit obscuring from some angles, but it isn't in a critical spot. I've attached the 2 decks on top of each other. L-girder construction is my preferred method for the lower deck (I LOVE how solid it is!), but I have concerns for depth when it comes to the upper deck. I might use it along the walls, where there's 24+" between decks, but I don't have to worry about that for a while. I thought that a 1x4 plywood grid would be very sturdy, while not using too much vertical space. I would worry most about weight on the front putting leverage on the frame, but perhaps that could be solved by braces under the scenery, since the entire thing's on a hillside? I'd presume that the 2x2s from the ceiling would do most of the weight-bearing work.
Instead of 2x2's Coming down from the rafters, why not use threaded steel rods? I have a 3 level layout and they work beautifully!
