Spacing of joists on L-girder benchwork?

[exodus]

I've been reading Linn Westcotts' How to Build Model Railroad Benchwork (1996 Edition) for the Nth time, in preparation for building my around-the-walls dream layout. Nowhere in his description and directions for building L-girder benchwork can I find a recommendation (unless I have missed it) as to how far to space joists on top of the girders. By checking the spacing of joists on the layout designs he gives at the end of chapter 5, I find they are spaced on average about 20" apart. However, some spacing is significantly more or less. On my middle-of-the-room 9'x10' flat-top test layout I spaced them at regular 18" intervals (how I arrived at that, I don't remember).

I realize that spacing of joists will vary depending on what they are supporting, but obviously 6' will be too distant and 4" too close. However, does anyone have a suggestion as to what might be a good average spacing (perhaps 20"?) and how much one might reasonably vary from that (say 6" or 12"?). I don't want to waste lumber by spacing at 18", if I can space them at 24." Spacing of the girders is not an issue for me, it's spacing of the joists which I'd appreciate feedback on.

Thanks, Malcolm

[SpaceMouse]

The spacing varies not only on what you are suppoting , but how long you are spanning. In other words, what is the distance between the supports on the L-girders. It also makes a differece what size lumber the l-girders are made from.

[claycts]

From the book,.:1x3L with 1x2 flange Max length 14'0" Max spred of legs 9'6" (overhange 38")

1x4L with 1x2 flange Max length 21'-8" Max Spred of leg 13'-0" (overhang 52")

Joist is based on TABLE TOP (Sub Road bed) 16" max Center to Center for 1/2" and LESS 24" for 5'8" and UP.
Hope This helps
George P.

[selector]

I agree. The whole thing has to be able to self-support.

Are you ever going to get up on this table? How long are the joists going to be? What material will you be supporting?

18" would be reasonable if you were planning on supporting 1/2" plywood over an 8' span, particularly if you are using 1X2" joists and will have to get up onto the layout at times. I used 2X4 construction in the form of 'pony' walls and placed three of them lenght-wise under my 12' long layout, spaced about 34" apart. They allow no sag for my 5/8" plywood.

If all you are supporting is plywood, or foam and plywood, and the structure and track, you will only need five or six 1X2" joists on the layout you describe.

[dehusman]

I would reccommend staying in the 16-18" range. The limiting factor is the sag of whatever you are supporting, whatever you are using as roadbed, when it is fully loaded. Even foam will sag with 10-20 lbs of load on it. You can "get by" with 20-24" spacing, but for the cost of 3 or maybe 4 extra joists in an entire room (maybe $10 extra total for the layout), I'd put them narrower.

Dave H.

[Adelie]

I pretty much have always used 16" joist centers. I'm not sure how I started that. May have been something I read somewhere, or the fact that wall and floor joists are 16" apart, and if it is good enough for them...[(-D]

I'm currently using half-inch plywood subroadbed and find 16" supports it quite well. If you are using 3/4" plywood, you could move it out to 18". I wouldn't put them much wider than that, however. Even though 3/4" plywood will support a refrigerator with joists at 18" spacing, as Dave H. said, you want to minimize the chance of any sag later on.

[claycts]

Mark which cad program did you use? Your plans look like 3rd planet, are they?

[Anonymous]

I'm using 16" center to center for my joists on a 17' long center of the room peninsula. I'm using foam for this one but also went with 16" centers when I was using 1/2" plywood and homasoate roadbed. No problems with sagging but I never had to walk on it either.

[Adelie]

I've been using CadRail for about 10 years now, George. Getting a workable bitmap image that is somewhat readable and not huge was a little bit of a chore. I copied the image into PowerPoint and used that to save it as a jpeg.

One day, I'll clean it up (take out all the extraneous turnouts and building that I copy).

[claycts]

Thanks Mark, looks like a beeter output than what I use. If I wash and dry it and run it thru Autocadd I get what I want. Looks like you get it straight from the program.
Thank You
George P.

[mounteer]

George P.,
Is your layout 20 x 22? Or did I count wrong?
Thanks,
Grant M.