1/2 inch is plenty

With this type of plan, once everything is screwed in place you'll be fine.  3/4 inch top won't offer any advantages. 

Joe

1/2 inch

I have been using 1/2 inch ply for the last 33 yrs. No issues whether it was in Calif, Minn, or here in MO. And I have used the same type of bench work you are considering. Yes it flops around before it is screwed down but not afterwards. I don't use blocks in the corners.

Jack

Torsional rigidity for a portable layout

I understand your concern about torsional rigidity for a layout that might need to be moved. It's not too much of a concern for a layout that stays in one place, especially if one side can be attached to a wall for added stability. Instead of a thicker top, I think a more rigid framework with diagonal members is the answer, a goal being to minimize weight and construction complexity.

I'm considering ideas for a small portable layout and am pondering this idea with diagonal braces. Drawn with SCARM freeware. The framework is 30" x 60" with 1x4 lumber, the legs are 2x2's. I haven't shown the table top, or diagonal bracing for the legs. Legs should be detachable. May need additional cross members to support a continuous flat table top.

Probably better to inset the legs from the corners so they are less likely to get kicked accidentally. A similar framework might be used for a 4x8 table top, perhaps with 1/2" plywood overhanging the frame three or four inches on each side.

Doors work well for layout platforms but the largest common size is 36" x 80".

Diamond or X

Instead of a diamond shape for rigidity, would an X be better, like that used for the legs?  There would probably still need to be joists scattered within the X. 

"Instead of a diamond shape

     If you use 1/2 inch plywood and securely nail ,or screw,or glue it to the frame you don't need diagonal bracing, plywood is very strong as a shear wall( or sheer plane on a table top), it just needs to be braced to keep the center  from deflecting in a table top situation so best framing pattern is 16 inch on center across the sheet  just like floor joists. Table legs will need sway bracing unless they are very strong and rigidly fixed to the table top( which means many of the framing parts have to be over built so become needlessly heavy and expensive)......DaveB

If you build a TORSION BOX

You don't need 1/2 or 3/4 plywood!  I have a freemo 3 section module that other than the required 3/4 plywood for the end panels is made from 1/4" luan plywood, which isn't 1/4 inch but somewhat less. 

It does have cross bracing on two of the legs as it's one of three sections.  The legs fold up and the total weight is around 45-48 lbs hard to see the scale when you are holding a 7 ft long module.  It was built almost 3 years ago and has traveled over 4 thousand miles since then.  So if you want you can build a heavy module and use it to work out but I rather build a lightweight module and save my back. 

Dan

If you want to be able to

If you want to be able to jump up and down on it, use 3/4" ply.  The image provided at the top shows them using 1/4", which is enough. 

Regarding torsional rigidity for a movable layout table:

Regarding torsional rigidity for a movable layout table:

A ladder frame works well for long and relatively narrow benches or modules, not so well for a wider 4x8 table unless you use rather heavy frame members with firm multiple joints.

An X-frame made with wood needs one piece spliced where they cross, requiring some extra frame depth above or below the table center. An extra bump below the table might interfere with moving or storage. A bump above the table could be concealed with scenery. The diamond frame avoids that issue.

A continuous plywood top will of course provide excellent racking resistance, but diagonal frame members provide better torsional rigidity for a wider table - if it is wanted for strength when moving. I've learned through trial and error of actual construction with different methods.

A table top of 1/4" plywood can be adequate, with suitable framing to support it. Some may say that heavier construction dampens noise better, but model train noise has never been a problem for me.

A well-designed 1x4 framework and thinner plywood top will save a lot of weight while still maintaining strength for durability, making the layout much easier to move if that becomes necessary.

Why not use 2x4 and 1/2"

Why not use 2x4 and 1/2" plywood?  Build it in a ladder just like building a wall, screw the plywood securely around the borders and to the pieces across the center.  Should be able to build it in an hour with no complicated cuts and easy to get material.   That design has held my house up since 1962.    It would be cheap, easy, strong and simple.         K. I. S. S.       As long as the "skin is screwed off well the framing can be simple.

1/4" luan plywood with  a few

1/4" luan plywood with  a few cross braces will be plenty strong.  You can always throw a  1" foam panel over it and it still I thin enough to allow under table switch  machines to work.  More important to make  sure the leg diagonal bracing is  correct. 

Been There, Done That

I used this exact design on my last layout using 1/2" plywood and once assembled it was very strong but light.  Were I to build another one, I would use something stiffer/more rigid than wood molding for the four side X braces between the legs and once the table is set up in place I'd run a small screw through the each leg below the bolts that hold the end cross braces just to help with the torsional stability.  Other than those two tweaks, it's a good, simple and easy to build design.  It disassembles easily and survived three moves including one from Sacramento to Phoenix, with no problems.  As I recall the plan first appeared in Model Railroader in the late 1990s or early 2000s as part of one of the project layouts.  Using 3/4" plywood is overkill for this design.

Lou

Why not use 2x4 and 1/2"

Answer: Because 2x4's are overkill for a layout. It will also cost approximately double the price of 1x4's and add weight that is not needed. 

Yes, we build houses with them but we don't have to build layouts with them.

Torsion Box

A true torsion box consist of a center honey comb sandwiched between two layers (top and bottom in this case).  Without the bottom layer it is not a true torsion box and loses a lot of its strength. Visit the sipping and switching society web site at http://s-ss1.home.mindspring.com for an example. 

Art

Torsion Box

Art if you look carefully behind the legs is a 2" strip that is glued to the sides,the ends and the ribs making it a torsion box.  I can lift the module up from one corner and it stays rigid, maybe not as well as a full covered bottom but more than enough for my purposes.  An interior door is made the same way two very thin door skins (less than 1/8") and a full frame around the outside.  The "secret" is to glue all part together of the frame so that it doesn't move.

Dan

The tale of a table

If you can cut a true right angle and 45 bevel, you can build a very strong, lightweight table following a design that is centuries old:

side view                                              bottom view

The key to this is the 45 degree blocks in the corners. (You can buy these premade out of steel.) A screw through these to the legs puts the legs and frame under tension that will resist any deforming force, except that prevented by screwing the frame to the table top. You can make the legs removable by using a lag bolt and wing nut to hold them on.  You probably have furniture built like this in your house and put hundreds of pounds on it without giving it a thought. Cutting holes in the top does not affect the strength as long as the perimeter is continuous.

If you are a black belt woodworker, you can cut the rails at 91 degrees to give the legs a bit of stance. That will be hardly visible, but makes the table tip proof. 1x3 or 1x4 rails will support more weight (way more than any layout needs) but are a little harder to attach to the top so the screws don't show. (But that hardly matters for a layout table, does it?) If you don't need removable legs, you can add short diagonal bracing from the legs to the rails for extra security.

I'm sitting at a 6' x 2' table I built this way in 1986.

It has shown absolutely no sag over the years, even though the top is particle board. It originally held tape recorders (big ones) so I added braces to the legs and some extra screws. (I knocked some off when I started using it as a desk, but that has not affected anything.) Since the 2x2s are spruce (on sale at the local lumber store), it is very light. I can carry it upstairs by myself, even now.

pqe

Tale of Grandma's Table

Good points.  My grandparent's formal dining room table (which I now have) is built exactly like that, even down to the angle blocks with lag bolts. 

Sorry

Dan. I did not see the 2 inch pieces. It is more of a torsion box than I thought. 

Art

No problem one of these days

I am going to take better pictures to show the underside as so many people at train shows ask how does all go together.

Dan

Sides and Top

We build modules for portability for our club layout, and use 1/4 inch hardboard for the base topped 1/2 rigid foam for the scenery base, hardwood 2x2 ends and sides with a notch for the hardboard. Some of those sagged a bit so extra cross braces were added. I build mine with 3/4 inch Baltic Birch plywood sides 3 inches thick with a groove for the 1/4 inch hardboard base and hardwood ends, like German beech or oak. The Baltic birch is extremely strong with 13 plys in 3/4 inch thickness, yet still lightweight, it will hold screws in the ends because it is so dense.

Sides and Top

We build modules for portability for our club layout, and use 1/4 inch hardboard for the base topped 1/2 rigid foam for the scenery base, hardwood 2x2 ends and sides with a notch for the hardboard. Some of those sagged a bit so extra cross braces were added. I build mine with 3/4 inch Baltic Birch plywood sides 3 inches thick with a groove for the 1/4 inch hardboard base and hardwood ends, like German beech or oak. The Baltic birch is extremely strong with 13 plys in 3/4 inch thickness, yet still lightweight, it will hold screws in the ends because it is so dense. The 1/2 inch Baltic birch can be cut into strips for the roadbed under the track.

I have tried

I have tried lightweight/smaller materials and heavier / overbuilt. 

The last one, I overbuilt to the point that I could walk on it. 

Allot will depend on the materials that are EASILY bought in your local area, and of those, what are dimensionally consistent.

My last one, I found that 2x4s were the same price as good 1x4s at the local big box store. 

Plywood, found that the only dimensionally consistent (for layout use) was the higher grade 3/4" stuff.