Post 914: Fine-tuning the floor-to-chair aid.

Posted on December 12, 2020

Post #913 was a proof-of-concept.  It demonstrated that you can make a sturdy, portable staircase, suitable for use as a floor-to-chair aid, at home, using nothing but readily available materials and a few simple hand tools.  At modest cost.

That initial design has a lot of drawbacks.  It took a lot of time, used a lot of materials, had a lot of unwanted variation in the cut-up size of those materials, wasn’t really the right size, may or may not stand up well under the pressures generated by the pushup bars (used in lieu of grab rails.)

And so on.

So the point of this post is to fix what I can, to make this faster, cheaper, and better.

1:  I’m sticking with using identical new boxes, at least for now.

I’m sticking with the idea of building these up from packages of identical new boxes (via Amazon).  It gives them a reasonably finished look and, practically speaking, it’s the only way I can make up a set of instructions that somebody could readily follow.  Otherwise, I have no idea what the end user is working with.

2:  You can skip reinforcing the lower boxes.

Savings:  Roughly one hour of time, and six fewer cartons required for the 3-step model.

I suspected this was true from the start.  The corrugated cardboard in these cartons is rated to 32 pounds per linear inch in the “edge crush test”.  That gives a roughly 500 pound theoretical load, for these boxes, before they would fail, as long as that 500 pounds were spread evenly across the tops of the box walls.   (That calculation is based on this short Wikipedia article).

So I tested the theory.  I removed the internal supports from a box at the rear bottom of the stairs.  And when I sat on the stairs, nothing happened.  There is no perceptible difference between the side with all boxes reinforced, and the side where one box is not reinforced.

This works in part because the boxes themselves are strong enough.  But also because the first level of boxes — the part you actually sit on — serves to spread out any loads.

There is a risk here, in that the reinforced boxes are more robust to injury.  You could damage the box wall, and still have plenty of corrugated inside, holding up the box.  With unreinforced boxes, there’s just one layer of cardboard.  If it gets seriously damaged, the box could fail.

I considered adding corner reinforcements to the now-empty boxes, but commercial reinforcements are only sold on large lots.  The alternative of gluing up reinforcements, from layers of cardboard, is unappealing.

For the time being, I’m doing nothing.  I think these short boxes are robust enough on their own.  And if a sidewall gets damaged, just cut out the old box and put in a new one.

This has a lot of benefits.

This reduces the burden of cutting and carton assembly roughly in half.  And means that you save six cartons’ worth of cardboard.  It also makes for a neater overall look, as there will no longer be “overstuffed” cartons that bulge, below the top level of cartons.  It lowers the height of the steps because the tops of these boxes no longer bulge.  And it allows me to customize the height of the steps much more easily.

3:  Pay more attention to form factor/box size; customize step height if needed.

The prototype turned out with steps that are too tall.  I naively thought that a 6″ box would yield 6″ steps.  But in fact, a 6″ (interior dimension) box is actually 6.5″ tall.  Add in some “bulge” from over-cut support pieces, and the steps average just under 7″ tall.

Assuming I can eliminate the problem of bulging boxes, and noting that the hardboard only adds to the height of the first step, I now know what my options are, using stock cartons.

I can find 3″, 4″ and 5″ tall cartons whose other dimensions would work for the other dimensions of the staircase.  Each step adds a foot to the length of the staircase.  I’m assuming that 4′ long is the practical limit, but that’s just a guess.  With those constraints, to get the top step near a common wheelchair seat height (18″ to 20″), I only have three options using stock (uncustomized) boxes:

  • steps x rise, total height
  • 4 x 4.5, 18″
  • 3 x 5.5, 16.5″
  • 3 x 6.5, 19.5″

Anything other rise height or total height would require me to cut down the boxes to a custom height.

If I had to redo the current staircase from scratch, I’d just buy shorter boxes.  As it stands, rather than waste the boxes I have, I think I’m going to shorten them.  This will also be a test of whether or not I could conveniently build one to some exact specification for riser height.  It’s not hard to reduce the height of an empty cardboard box.  Tools for doing that are readily available for under $20 (such as this one, from Amazon).

The only difficult part is resizing the very first step, because that one requires resizing both the carton and the reinforcing cardboard in the carton.

Which brings up the next point.

4: Create a cheap jig for making accurate cuts in the cardboard.

The need for perfectly-cut interior supports is a real drawback.  Uneven supports weaken the load-bearing capability and they make the boxes bulge, which makes the final assembly tough and makes the results look slapdash.  In the prototype, I sorted the supports by size.  But what I really need to do is cut them the right size in the first place.

There does not appear to be a commercially-made “knife guide” to meet my needs.  Searching for “knife guide” bring up kitchen appliances and sharpening aids.

But, in a mini-quest, I think I can easily make one.

  • Take a $3.00 piece of metal corner bead, from Home Depot,
  • Hacksaw into three pieces.
  • The finished device, when viewed on end, will look like an L,  followed by a closely-spaced L and backwards L.
  • The factory cardboard edge rides within the first L.
  • The L and backwards L form a knife slot, duct-taped together, spaced by a couple of used utility knife blades.
  • The whole assembly is connected together by strips of cardboard at either end, slit to hold the knife slot at the exact right distance.

In addition to making the cutting more accurate, I think this will also speed it up considerably.

5  Modify the reinforcement in the top boxes, the ones you actually sit on.

I decided against this.  I’m keeping this the same as it is in the prototype.

The top boxes have to distribute fairly intense point loads from the pushup bars.  And they take high transient dynamic loads as the user moves up and down the stairs.  So this is the part of the staircase that needs reinforcement.

I’m not thrilled with my current method of little cardboard “V”s.  But I’m not seeing anything that looks like it’ll work better, for the same or less effort.

I ended up with the “V”s because I tried to do the standard rectangular interlocking grid reinforcement, but that took vastly too much time.   (As a means of reinforcing a box, a grid is more efficient than “V” because the cardboard supports are absolutely evenly distributed.  But it just takes too much time to produce all that grid from raw cardboard.)

I considered non-cardboard materials for the support, but didn’t find any that met all my criteria (cheap, strong, fast, recyclable).

Spray foam would be expensive and of unknown durability.  It would take roughly two cans of Great Stuff spray foam to fill one box.  (Calculated from this source — each can produces about .31 cubic feet of foam). And that would also mean that the boxes could not be recycled.

I may yet try insulating foam board.  That’s lightweight, cheap, and durable, but very tough to cut well with just a utility knife, and even tougher to cut to an exact size with a knife.  That would also require removal before recycling the cardboard.

All things considered, other than cutting the pieces to an accurate 6″ height, I’m sticking with what I have.

6)  Stiffen up the top surface.

The hardboard panels on the prototype serve three purposes.

First, they stop high point loads from puncturing the cardboard.  E.g., think how easy it is to drive a ball-point pen into a cardboard box.  So they prevent accidents like that from trashing the sitting surface.

Second, they provide a slick and wear-resistant surface.  This both helps the user to maneuver, and prevents repeated use from rapidly wearing through the cardboard.  (The tape, on the other hand, is a different matter.)

Third, they help to spread the loads over a larger area.  But they do this poorly, because the hardboard panels are themselves quite flexible.  For example, if you try to walk on this staircase, you get an unacceptable level of deflection of the step surface under your feet.  Your weight is simply not spread out enough for the cardboard to take the load without deforming.

To spread the load, ideally, you’d have a very stiff top surface, one that wouldn’t readily bend.  The stiffer it is, the better it will spread the load.  And that matters greatly, because the entire strength of the steps comes from spreading the load over a large amount of corrugated cardboard.

I used 1/8″ hardboard because I knew I could cut that by scoring it with a utility knife.  I didn’t go with something stiffer (plywood, say) because it’s too hard to cut (or cut well) with simple (non-power) hand tools.  (And it makes a mess when you cut it.)

I’ve looked at options for a stiffer step (1/4″ MDF, gluing multiple sheets of hardboard together, creating foam-core hybrid materials using insulating foam and hardboard, using thin plywood) and they all have drawbacks (that I won’t bother to detail).

Instead, I’m going to take the scraps of leftover hardboard and add a second layer of hardboard on the outside 10″ of each step.  These will be the “landing pads” for the pushup bars.  Those bars will exert considerable point force, with the user’s entire weight resting on the eight small feet of the pushup bars.  With a 200 pound user, I can plausibly expect a static load of 20 PSI and a dynamic load of twice that.  Whereas the user’s rump will generate no such high point loads.  And so, any reinforcement of the surface needs to go on the ends.

7  Try duct tape.

’nuff said.

8 Try a different assembly routine.

It would be nice if all the tape joints were visible from the outside.  That way, if a box got smashed, you could easily cut out just that one box and tape a new one in its place.  It also took quite a long time to assemble all the boxes, and I have to believe there’s a quicker way to do that.

9  Conclusion

I think that’s it.  Those are all the changes that are believe are feasible for me to do.  So the revised instructions would look like this:

  1. Choose your box size/form factor.
  2. Order your pack of boxes from Amazon.
  3. While you wait, make up your knife guide for accurate cuts.
  4. Buy the rest of the materials locally.
  5. For the three-step models:
    1. Assemble 6 empty boxes, two with handles.
    2. Cut up 6 boxes into strips, or cut scrap boxes into strips.
    3. Assemble and pack 6 boxes with internal reinforcement.
    4. Cut up 2 hardboard sheets (several additional small cuts now required).
    5. Assemble the staircase.
  6. For the four-step models (after buying one more sheet of hardboard).
    1. Assemble 12 …
    2. Cut up 12 …
    3. Assemble and pack 8 …
    4. Cut up 3 hardboard sheets …
    5. Assemble the staircase.

I am guessing that with the reduced amount of cutting of supports, the jig to speed the cutting, and so on, that this will now take under three hours.  And require less effort.

I’ll rebuild the existing staircase tomorrow.  And I’ll build the 4-step 4.5″ riser model when the boxes get her via Amazon.