The concept of L girder bench work was developed by Linn Westcott and it is based on some simple engineering principals. The basic idea is to minimize the sag in the middle of the bench work, while using the least wood possible. If you are planning to build your first layout using L girder bench work, then I suggest that you get a copy of How to Build Model Railroad Benchwork Second Edition by Linn Westcott. I found it to be extremely helpful. In fact, I suggest that you buy this book rather than Basic Model Railroad Benchwork by Jeff Wilson as I found the detailed explanations to be more helpful than the additional pictures in Basic Model Railroad Benchwork. (That said, I'm a professional engineer so I'm always interested in the details.)
For those of you who are looking to learn about L girders, but not yet ready to buy a book, this article aims to address your questions.
Most folks first train tables are a 4 x 8 sheet of plywood on two saw horses or something very similar. This is a very quick to assemble and relatively cheap first layout bench work. At some point, many want a more permanent layout. When you start thinking about building a more permanent layout, there are two big things to consider: 1. will the surface of the layout be completely or nearly flat and 2. will the layout edges be mostly right angles, or mostly curves. If the layout is going to be mostly flat with right angles, then open grid bench work (similar to most modular layouts) is probability the best choice. The L girder concept is at its best for layout with multiple levels and curved fascias. Another big advantage of L girder bench work is the flexibility to move supports for obstructions such as under surface turnout controls.
The core concept of L girder bench work is the use of two girder to support independent joists which support the track surface. [Insert picture] The L girder is supported by legs not at its ends but at about the 20% and 80% length locations. This leave overhang at each end and better balance the load to minimize sag of the girder (Once facet of Beam Theory in Engineering).
The biggest question is how to easily make L girders. My
recommendation is to buy good quality 1 x 6s, which are actually about 3/4" by 5
3/4". Then I rip them longwise into two pieces the first is exactly 2 inches
wide and the second is slightly less than 3 3/4 inches. Next I set the 1 x
2 piece on the edge of the 3 3/4 piece. I line up the edges very carefully on
one side and have about a 1 1/4 overhang on the other. Then I clamp the two
pieces together. Using coarse dry wall screws I pre-drill and countersink holes
for the screws at 12 inch spacing, then I screw the two pieces of wood together.
Next I mark the alignment of each end of the wood on both pieces, then I back
out the screw just enough to separate the two pieces. I lay a bead of
carpenter's glue on the top edge of the 3 3/4 piece, the relay and align the 2
inch piece.
The screw should fall into their original holes, then I retighten each screw.
Now the L girder is assembled. I leave the girder to dry for 24 hours, then I
remove the screws, as they will not be accessible once the table is completed.
(This article still under development.)