Torsion Box Parameters?

Re: Torsion Box Parameters?

Dick Coers

I'd use 5/16 - 3/8" thick (resawn 4/4 in half) poplar, 3" tall, on something like an 8" to 10" grid.

Re: Torsion Box Parameters?

Barry Irby

I agree with Dick.

The thicker the box is the stiffer it will be, within reason.

My first shot would be poplar ribs 2 to 2 /12 inches tall and 5/16 to 3/8" thick and I would use four. I would put ribs crosswise about every ten inches but would not notch them into the lengthwise ribs at all. Notching them would weaken them. I would use the thinner plywood for the skins. And, paint the top surface with sand paint of some nonskid surface.

Try making a mock up and see if it fits your hands. A 2" one may be a lot easier to handle.

Look up "Stressed Skin Panels" to see if you can find more information.

Re: Torsion Box Parameters?

John McGaw

Maybe you are overthinking it. Have you looked into aluminum loading ramps? The cheapest Chinese ones are plenty sturdy for your use by a factor of five. Those which have no solid surface, having a rung-like construction, could be adapted for doggie use with a piece of 1/4" ply. Some of the folding ones are available for < $75 from places like Harbor Freight.

Re: Torsion Box Parameters?

Mark

William, as the fellow owner of an overweight Retriever ( 90-95 lbs) , my father recently made a ramp to assist them get the dog into their SUV when looking after her.

She will jump out , but needs the white glove service to get in.

It was a length of 3/4" VC plywood about 14-15 inches wide and 6 feet long..period! He put his foot on the bottom to keep it in place and up she went.

Worked great and no signs the plywood couldnt handle the weight. They just threw it in the back of the vehicle after use with no need to strap it to a roof rack.

You might find a simple length of plywood works fine before engineering a higher maintenance solution.

Re: Torsion Box Parameters?

Stuart Johnson

I'm with the others thinking you are over building. A strip of 1/2" plywood with a 1x1x(a couple inches less than the ramp) glued and nailed to the middle of the bottom would be more than enough. The 1/2" plywood would hold the weight and the 1x1 would prevent sagging.

However made, put carpeting on top

Kneale Brownson

Friend with similar issues found the dog liked going up the ramp much more after a carpeted surface was added.

Re: Torsion Box Parameters?

Keith Newton

I tried posting something similar to this a while ago on my phone, but see it didn't come through.

I was thinking of 7/16 OSB, or CDX with a 1x2 along the edges.

I'm not sure your mutt is going to like the tapping sound of his nails on the thin deck. Sometimes they can wig out over the least things.

Life is hard enough without going overboard on something this simple. ha

Re: However made, put carpeting on top

Pete in Holland MI

How bout a section of a hollow-core bi-fold door from a Habitat Restore ? Might need a thicker top skin, and some carpet for traction.

I like the aluminum ramps from Harbor Freight. Rain proof if you leave them on your roof rack.

Re: Torsion Box Parameters?

William Duffield

Thanks for the "Stressed Skin Panels" search parameters. This document from Georgia Pacific looks like it will be very useful:

https://www.buildgp.com/DocumentViewer.aspx?repository=bp&elementid=3815&usg=AFQjCNHn2xSseG_fOfjtapTHMhWzOVYqvQ

Lots of math will be required for an optimum design. This paper points out a design feature I had not considered: vent holes in the ribs are required so every compartment has pressure relief. I will probably also put a couple hand holds in each side.

A non-skid surface is an important design parameter. I'm going to tack lateral hardwood ribs on the upper surface, probably every 6". If I can find some old carpeting, I'll probably use that too.

Re: Additional Parameters

William Duffield

The ramp must be as light as possible and still support the dog. It needs to be secured to the roof racks, because the length of the dog compartment is only 4'10", its width is even less, and the ramp needs to be stowed after the dogs, dog beds and other gear have been loaded. There are currently two large dogs in the family, and I expect to add one more. The roof rack needs to share space with either kayak racks or bicycle racks. When traveling, it needs to go up and come down rapidly and easily for pit stops.

I want to use a wooden torsion box design, because I already have all the materials and tools on hand, so there will be no out-of-pocket expense, and I won't need to travel many miles to purchase what I don't have. I want to learn and practice the techniques before I use the methodology to build anything important. I know it will be lighter, and can be designed to fit my needs more precisely than either CDX plywood, OSB, MDF, or a manufactured aluminum ramp. OTOH, I know there are other materials, e.g., honeycomb in plastic, carbon fiber or aluminum, fiberglass, epoxy, etc., that could be used to make it much lighter, but I can't justify the additional expense.

Right now, I'm sure 3/8" thick ribs will be strong enough, but if I could reduce them to 1/4" or even 3/16" it would work better. If the ribs could be ripped from 2x (1-5/8" thick) pine, that would simplify things even further.

My Golden mix was hesitant

Dick Coers

She was pretty nervous during the first week or so of using the ramp. I don't think I would use the horizontal hardwood strips. If the stride distance is off, the paws will be off and on the strips. Plus you get the visual look of a barrier with the strips blocking the path. Carpet or texture paint is the way to go. For mine, I just sifted out some dirt and mixed that into some old exterior latex paint.

like a birch skin door?

Bill Tindall, E.Tn.

As I understand these things a torsion box is a grid of strips on edge and a skin on each side, kind of like a wing of an airplane (old timely kind). I suppose a birch "skin" door would be an example. A birch skin door is 1/8"plywood and it is laid up over a 1 1/8" frame with widely spaced members. I have used birch doors for years as temporary desks, and benches. I have no doubt such a door would hold several hundred pounds. I think a dog ramp could be made from two luan door skins, glued to a frame of 1/2" tall and it hold a dog readily.

Since the load is along the length the internal members should run primarily the length of the ramp. A piece of 1 x 12 shelving would hold a dog.

Re: My Golden mix was hesitant

William Duffield

You're probably right about the strips. I think I have enough carpeting to cover it. It might be worth an experiment. I could teach her to climb the ramp without sticks, and then temporarily tack them on and see if she is more hesitant, or if they improve her grip. I could do the same with indoor/outdoor carpeting. I've already gotten her to use a couple of 8 ft. 2x6s, so training her to use the ramp before I've decided on the surface won't be a problem. Somewhere, I think I still have some sanded paint that I used for my front porch steps. The only potential problem I see with carpeting is the additional wet weight and how long it will take to dry after a trip in the rain.

Re: like a birch skin door?

Barry Irby

The hollow core door is a good idea except they are absolutely not waterproof. Any of them would fail quickly in the weather. But, I know from long experience they are plenty strong to support the dog.

If you want to have some fun and save an ounce or two, cut your ribs, making them straight on the top side and curved on the bottom. Tapering from nothing at each end to maybe an 1 1/2 or 2" at the center. If you can find a clean, knot free 2x4 it would work fine. OTOH, At 80 miles per hour it might create some lift.

Re: Additional Parameters

Don Stephan

The first torsion boxes I made decades ago, the instructions suggested stapling the skin to each of the ribs. Even 5/16" would make too hard a target to hit if you go that route.

I think the strength of the torsion box relies on the glued area between skins and ribs - if so, the wider the rib the stiffer the box.

Re: Torsion Box Parameters?

AZ in Colorado Springs

Don't know what kind of car you have. I have a Silverado pick up truck. We faced a similar situation 15 years ago with our golden retriever. I used a sheet of 3/4" construction grade plywood covered on top with indoor- outdoor carpet. I put a handle on the plywood to pull it out and lift. We'd pull it out and lay one end on the ground. Up went the dog. Then lift the plywood and push it in. For the last couple of feet the dog hopped onto the plywood.

Sadly, the dog has been gone for a while now, but the vehicle and board are still there. Makes loading and unloading items in the pick up truck a lot easier. It's easy to sweep or blow dirt or other debris off the carpet.

Re: Torsion Box Parameters?

William Duffield

I have a rolling microwave oven, a Scion xB. It has plenty of room for two or three large dogs, but the cargo space is short, so the ramp has to be secured to the luggage rack after the dogs are loaded. There is plenty of headroom for one or more greyhounds, which have no problem just stepping in. The tailgate is 24" off the ground. I also have a bad back and sciatica, so lifting heavy stuff multiple times makes me dysfunctional and grumpy for the rest of the day, at least.

Re: like a birch skin door?

William Duffield

A hollow core door! Of course! Its structure and strength provide valuable insight into what I need. I'm not sure I would be comfortable with 1/2" high ribs, since the stiffness increases with the cube of the height, but a total thickness of 1-1/4" should surely be enough.

Re: like a birch skin door?

Bill Tindall, E.Tn.

1 1/4" will be enough. A birch door is only 1 3/8".

Re: Torsion Box Parameters?

Alan in Little Washington (NC)

I've been out of the country so a bit late to the parade.

A narrow (small closet or 1/2 of a set) hollow core door properly sealed and non-slip strips or carpet on the surface is the way to go. Most of them have a very thin, heavy-duty paper or cardboard web inside and thin skins, so overall are extremely light but plenty strong. Use one with birch ply rather than hardboard (Masonite) skins for better durability and resistance to weather.

I wrote an article for American Woodworker (Dec/Jan 2010) quite a few years ago now, about how to build a torsion box assembly table. My assembly table was much heavier duty, but in the article I also show an example of a torsion box I made entirely (skins & web) from thin, 3/16" hard board (and some leftover pegboard) held together with only yellow glue (I used halved joints on the web strips). It was 8' long, 16" wide, 2-1/2"(?) thick, but could hold 350 lbs of bricks center span with only 1/2" of sag! I used it as a scaffold plank for awhile until I got it really wet while washing my porch ceiling (the glue joints failed). Here is the link to the article and below it two photos of the partially completed and loaded hardboard torsion box.

https://www.popularwoodworking.com/projects/aw-extra-7413-how-to-build-a-torsion-box

Glue applied to the edges of the web and ready for second skin:

That is 350 lbs. of bricks! (two rows deep)

Re: Torsion Box Parameters?

William Duffield

That's a great article. Thanks for sharing. If on couldn't build one successfully from your information, one shouldn't be woodworking.

My guess is it 2-7/8" thick plus the thickness of the skins.