Suggestions

While it's great to have something be as light as possible, there's a practical side that must be considered.  Personally, I don't see how the design above resists diagonal or horizontal warping. What the design above lacks are horizontal frame pieces.  You want an L or a T shape running lengthwise.  Not only does this strengthen the structure, but it provides a wide platform to glue foam down.

However, tweaking the design above should yield great results and you can certainly get below 70lbs per module.

Just be careful not to try and shave off every ounce because mass contributes to both strength and stability.  The ideal module is one which is incredibly portable and yet is as stable as a kitchen countertop.

Hello James and thank you for

Hello James and thank you for your informative reply.

What do you mean by "L or a T shape" ?

My other two modules are made of hollow core doors framed with 3/4 inch ply that have stood the test of time and have even been dropped from a truck's ramps. Very sturdy - haven't warped or bowed, but a headache to move.

https://model-railroad-hobbyist.com/node/39987?page=1

Cheers, Deem

My layout: https://model-railroad-hobbyist.com/node/31151?page=30

Options

Mr Willmus raises a good point about the risk of the unit racking. The inset foam and track plywood will provide some racking resistance but the foam could crush with enough force applied.  Simple triangular plywood gussets at the corners would probably be adequate.  
Deemiorgos At the module size you are building it’s still going to have some decent weight of wood framed.  Since weight reduction is desired, alternatively - and especially with today’s very high lumber prices -you might consider steel stud construction.   https://forum.mrhmag.com/post/the-shelfless-shelf-layout-12207253
jeff

L / T-shape

Deem,

Look at the below picture of my module under construction. The middle flat piece is the top of the T on which the cork roadbed will go. This ensures the section under the track will not bow over time. It also helps (somewhat) against deformation of the entire frame.

Side view:

As you can see one part doesn't have plywood supporting roadbed, which is where my bridge will go.

Bessemer Bob, You mean like

Bessemer Bob,

You mean like steel that is used for metal stud framing?

Thanks for the demo,

Thanks for the demo, Yaron.

Near to the middle of the module I want to make, I want to create a bridge scene like this. I may need at least 3 inches deep of foam.

https://model-railroad-hobbyist.com/node/40762

Deem Yes these are the steel

Deem

Yes these are the steel tracks and studs used in place of 2 x 4 or other dimensional lumber in framing construction. I build rectangles out of them, top with a sheet of 1/8 inch masonite. Gorilla glued to the rectangle,and then whatever thickness of foam needed on top of that. So far I have built 10 modules this way and been very happy with them. I don’t have a scale but I will try to borrow one and weigh a few to give you an idea of potential weight savings. My older blog postings have a lot of info and pictures on this method.

Also Jaron‘s pictures show the triangular corner gussets I previously mentioned.

Thanks

Jeff (not Bessemer Bob-I just like his signature and I leave credit to him)

Should you prefer

to stay with wood or plywood, Deemiorgos, you might consider cutouts on the main structural members.  A beam need not be solid to to be strong.  Think of the 'opera window' cutouts in a center beam flatcar.  If you want to save weight on a piece of 1/2" ply that is, for example, 3 1/2" wide x 72" long, simply cut 2" x 5" windows in the piece along it's length, with 1" solid sections between them.  That'll cut the weight of the beam quite a bit, without bothering it's strength much at all.  If I were doing this, though, I might use strips of ply, or lumber, and laminate up the beam.

There are many ways to skin a cat.  If it was me, I'd seriously look at the steel stud approach; a few minutes with snips, a drill, and a handful of pop rivets or sheet metal screws really ought to produce an excellent frame; then you need to 'skin it', then adhere the foam and roadbed to it.  Your center 'chasm' should be planned into the metal frame, of course.  Oh, one thing on the studs - mind the sharp edges.  You might want a bit of duct tape to cover over any sharp points.

Blair

L/T shape

Here's what I mean by "L" or "T" shape pieces:

That image is from: Building Your Model Railroad: Benchwork

That article explains it better than I could.

In it's simplest form, a wooden module consists of endplates and girders (I call them "runners"). The endplates have two jobs, (1) provide a square, flat surface to connect modules to each other and (2) properly space the girders for module width/depth.

The girders are designed to keep the structure rigid, which means avoiding any twisting, bending, cupping, or warping.  All types of wood, even plywood, can warp and twist depending on humidity, temperature, and gravity.  Plywood just happens to be more stable since it's a lamination of thin sheets of wood.  The grain in the sheets is alternated, which gives the structure incredible strength for its weight.

As Yaron shows, he has braces in each corner that keep the whole module square.  And as Blair said, you can cut holes in the girders to reduce weight without reducing strength. I would just avoid cutting holes in the endplate since a flat, smooth surface is what you want to aim for.

Waffles

Waffles.  Waffles waffles waffles.   Waffles waffles. 

Strength vs. lightness

A torsion box is a great way to get something strong that’s also pretty light.  A hollow core door is an example of a torsion box.  The waffle construction mentioned above is partly a torsion box (although often there’s so much of the bottom panel left out that I don’t think it gets the full torsion box strength).

The strength comes from the box having all six sides (4 sides plus top and bottom).  To see why, take any shipping box you get in the mail, with the top opened.  You can easily twist it.  Now tape the top shut again.  Becomes very difficult if not impossible to twist.  For larger areas spacers across the middle to keep the top and bottom sheets spaced properly adds strength.  You don’t need to build it from 3/4 ply, much lighter should be good enough.  If you’re going to put a layer of foam insulation over the top anyway, you might be able to build one from as light as 1/8” ply.

I should make it clear that I haven’t actually built modules this way yet, I’m in the planning phases.  But am I inspired by the super light construction in aerobatic r/c planes - my extreme flight 60” edge (airplane) has a very rigid and strong airframe whose core is made with thin (1/16”?) ply and carbon fiber.  It’s got a 1 horsepower electric motor on the front and does exciting things at 60+ mph in the air, and has given the ground a few cringeworthy whacks with no damage.  If something built that light can take that abuse, I don’t think I need my model rr benchwork to be built from 3/4” ply.

2” foam

Also the 2” foam itself is quite rigid and provides some strength to the assembly if well adhered to the structural pieces. 
jeff

Weights and measures

Top module is 10.5 x 42” lifttout. 7.4 lbs. 

Bottom module is 61 x 12” five track yard. 12.0 lbs. 

both have some 1/2” plywood parts (endplates) and aluminum French cleats. The track is laid directly on the foam board. The bottom module has a Masonite fascia.    

If my math is right they are both about 2.4 lbs/sq ft. surface area.  

That may not be a perfect metric for your module but at that factor yours would be 46.2 lbs. 

Also mine have 1” foam not 2” but that’s not a big weight driver. 

Cheers

Jeff

Lighten up

Hi,

the wish to lighten up is just as understandable as the wish for more stability

Yet there is another way to lighten up - build two modules instead of a single one. That would allow you to use rather stable construction and still have modules that are easy to move

There seems to be a very interesting trend in bicycles - to make them lighter they do not add padding to the seat. But that's uncomfortable, so the padding goes into the pants. Yipieh, the bike weighs less (the biker's weight doesn't count)... So yes, I think there is such a thing as "over-optimization."

I'm having good luck with some rather puny dimensional lumber and 6mm plywood (~1/4") which has been stable for years. It's more about being awkward to handle than heavy in my case.

Have fun!

  "While it's great to have

I agree, you might move it once every few years or less but you work on it and run trains often so I'd think about how important shedding a few pounds really is. There's not too much difference between moving a 70 pound module and a 40 pound module, just 15 pounds per end if two people are carrying it but the 70 pounder might feel a lot more stable when set up. I'd also consider the sharp edges and all the wiring that's done under there if you think steel studs are a good idea.....DaveB

A friend of mine made

4 x 8 waffle style modules for his N scale layout out of 1/8 " Luan Plywood.  He constructed eight cavities unlike the one in the picture above that has only four.  He glued quarter round trim into every corner.  I saw him stand on it.  I couldn't believe how light it was.  Think about the ribs in an airplane wing which weigh nothing and support the weight of the whole aircraft.  This idea about making benchwork out of 3/4 " 5 ply plywood, to ME, is just plain overkill.   We are only supporting plastic models and yet you could put a car engine on most modules I've seen.  There are other alternatives to the "old school" ways of building benchwork.

While I'm on my soapbox, I want to tell you of a layout I saw that was built on sheetrock.  It was all hand laid track and the sheetrock held the track nails really well.  No worries of warping there at all,  and a whole lot cheaper than buying plywood.  It's OK to think outside the box.  There are a lot of great new ideas to be found out there.

Fabricated piece

I built a staging yard, meant to be removeable, using 1/4" luan plywood and then cut pieces of 1/2" with pine.  the sides and ends were 1/4" plywood strips about 4 inches wide.  I glued the pine to to top and bottom of the Luan, to make a de facto 1x4  that was "C" shaped.  I also used 3/4 square pieces as glue blocks for the cross braces and 1/4" plywood as a top surface.  I have had it for about 10 years and its still going strong.

Hello Jeff, I found your blog

Hello Jeff,

I found your blog and I'm in the midst of reading it.

What tool do you use to cut the steel? I will need to make a list of tools to buy. I will be going to Home Depot this weekend.

https://model-railroad-hobbyist.com/node/38428

Jeff, Very nice, tidy and

Jeff, Very nice, tidy and mobile! Thank you for sharing.

Blair, steel stud framing

Blair, steel stud framing looks very promising, as I have no room or place cut wood not to mention enough tools.

Dave, sounds very durable and

Dave, sounds very durable and will last for decades. I'm going to look up what "luan" is.

James, thanks for the image,

James, thanks for the image, explanation and overall informative reply.

For now I can only dream of having the tools required to work with wood for such a project.

rjthomas909 They look great!

rjthomas909

They look great! Somewhat remind me of the construction of the wings of wooden planes.

I knew a model railroader that had his entire layout done in a similar manner. I did not know they were called waffles. For some reason I'm now craving maple syrup.

Jeff, good to know about the

Jeff, good to know about the 2" foam. I think I may need up to 3" in foam to accomplish the tide out look.