I’ve read quite a bit on this but have come away with a pair of questions that I’m hoping you all may have some additional answers! In all examples I’ve seen, text and pictures are geared for 4x8’. the braces that support the 8’ lengths are met in the middle of the L girder. While this makes sense for an 8’ length, what if one were building a 4x16’ table? Based on the Westcott chart, up to a 21’ span can be built with just 4 legs. Would the braces supporting the girder have to meet near the middle around 10’, or could shorter braces, say 5’ long be used. How much structural integrity would be compromised if the braces aren’t supported from leg assembly, to the direct middle of the girder?
My second question would be, the on center spacing of each joist. Does the standard home construction 16" center rule apply, or can that distance be increased to 24"? FWIW, I’m going with 1x4 joists, 1x2 flange & braces, 2x4 legs, 1/2" ply under 2" extruded foam. The railroad is a 19x32’ J shaped beast that is finally moving into the benchwork phase.
The diagonal bracing does not have to meet in the center. The diagrams you see for the typical 4X8’ layout show a situation where such braces naturally end up meeting in the middle.
Spacing is entirely up to you. One of the basic ideas behind L girder is that joists can be located wherever needed or desired.
Your description raises a question for me though. Are you doing the plywood plus foam flat surface everywhere? I so, traditional L girder may not be the optimal way to go. Its structure is optimized for a “cookie cutter” approach which allows for roadbed on risers and scenery to drop below the tracks. If you’re building what amounts to a flat plywood tabletop, something like plain open grid may be easier and sufficiently strong.
And if it is basically a flat tabletop, 24" spacing shoudl be fine. My layout is made up of (mostly) 2x8 sections so I can take it apart and move it if need be (the track and roadbed is continuous over the gaps, so I’d have to cut it), and I have 2" foam (2 layers actually) over 1/4" plywood and the cross braces are only ever 2’. No sag.
The chart for 4 legs up to 21’ would be these legs attached to the 1x4 L girder. The legs need not be placed @ the very end. They should be placed at least 1 ft inboard, could even go as much as 18". At a distance of (now 18- 19’ span) and 4 legs gives a 6 to 6’-4" span between legs.
Although the leg bracing will actually help support the L girder, the braces are more to stabilize the leg itself. I would set legs @ 4ft and only brace as needed to stabilize the leg and take the lateral movement out of the frame. Trying to run too many or too long a leg brace will make under layout access rather difficult. Try to set any bracing at an angle of no more than 45 degrees ofset from the post. Placing legs at 4ft will insure that the 1x4 used for the L girder will carry the load and not rely on any a
If I recall correctly, Lynn Westcott recommended putting legs on L-Girders at 1/5th and 4/5ths distance along the longer legs. With my 3 islands of 8 feet (96"), i’ve found that Good support is obtained by cutting 2x6 lumber into two legs, tapered by marking in 1 3/4 " from opposite sides on opposite ends and cutting on the diagonal. to give two trapizoidal legs. I then position the “Big end” under the L-girder 9 or 10 " from either end, with the taper on the outside end and fasten them with two, 1/4" carraige bolts offset 1 " in from opposite corners. If you still want additional longitudinal bracing, 20" 1x2’s cut with 30 and 60 degree angles on either end, will give you an additional support at the 2/5 and 3/5ths distances along the L-girder. (Mark 12" along girder and 16" down the leg to get a 30/60/90 degree triangle), Before mounting the leg, you may want to drill and tap in adjustable T-bolts to allow vertical allignment, if your floor is not already level. Between L-Girders, I also use 40" 1x2’s in an X to provide lateral stability for both sides. Put a scrap piece of 2" pine at the crossing of the braces, and fasten with another 1/4" carraige bolt. Hope this helps…JWH
Optimal L-girder design calls for the legs to be at the 1/5 and 4/5 points of L girder length - and there’s nothing preventing you from using a glue-lam L girder and having a 30 foot clear span on a 50 foot girder (which would be deeper than 4" and didn’t exist when Linn Westcott was making up his tables.)
The braces work best when they’re somewhere near 45 degrees to both leg and girder. Mine are short, but they still prevent wobble.
L girder joists can be spaced as needed to support whatever is above them. I noticed that I had one joist with no risers attached to it (tracks span the gap in steel stud `rain gutters’ or on solid pine 1-by planks) so I removed it and recycled it as risers elsewhere.
One necessary but seldom-mentioned brace runs on a diagonal across the riser bottoms from one L-girder to the other. Without it, the bare frame wobbles like tofu. With it - granite.
Chuck (Modeling Central Japan in September, 1964 - on steel stud `C acts like L’ girder benchwork)
This photo, below, may be one of the ones to which you’re referring. L-girder didn’t make much sense for my layout, either, so I opted for open grid: easy to construct, solid, and easy to move. It’s also totally independent of the support structure, although it is attached to it with screwed-on cleats, a couple of which are visible in the photo.
Korrection, Korrection, Korrection!! In my previous response to this, I wrote that I placed the legs at 9 or 10 " from the ends of an 8 foot l-Girder: Should have written 19 " (Nineteen Inches), which is much closer to 1/5 of the 96 " span. (I’ll blame it on reading the Lion’s posts, which can scramble a little more than the rules of gramar, evidently.) Keep up the good info flow, Lion!! JWH
