Structural strength of bamboo stair treads

[Daniel Gantenbein]

For years accessing the upstairs room in my studio lead over stairs too steep and short. A serious termite infestation lead to a complete reorganization of my studio. In this process I got to cut and install new longer stringers. The truly interesting and amazing little buggers had great eating for a while! I'll promise to post this story with a studio tour once further a long. I am grateful for the Saw Mill Creek. It's here that I got my first introduction to SketchUp; a big thank you to David Richards.
I came to love the intuitive 3D software as a wonderful tool, even if I still have much to learn. I make simple skp files to arrive at crucial measurments and/or work out possibities. My unfinished model for my stairs with planed drawers allowed me to cut my stringers.

(no components for treads and risers since the slight fan like movement you see from above makes each one a bit different)


The big question: Are bamboo treads with screws from the risers back strong enough? (bamboo flooring glued together into risers and likewise with a stairnose into treads).

By my guesstimation this is structurally sound yet a friend thinks a reinforcement of some sort is necessary.

Who is right? What the best solution? Thanks for any suggestions or ideas.

[Jamie Buxton]

You might get more responses if you post jpgs. Not everybody has Sketchup.

[Daniel Gantenbein]

Thanks Jamie

How to turn it into a jpg? Here is a png file of it.

The span between the stringers is about 30".


Daniel

[Steve Schoene]

After looking at the drawings I'm still confused. What is at the top of the stairs -- in other words, to what is the top tread attached? Why would it be desirable for the stringer to stop short of the top tread?

My intuitition says it absolutely needs the support of a stringer all the way to the top, under the top tread, and that the stringer needs structural support at its end.

What does the building inspector say? His is the view that counts. Stairs have lots of rules.

[Jim Becker]

More information about the bamboo flooring you're planning to use...thickness? Stair width? How many stringers? This kind of information will be helpful to folks who have the knowledge to help you.

BTW, I'm moving this to General Woodworking as it's more likely to catch the eye of our "stair experts", despite this being a design question.

Jim
SMC Moderator

[Todd Crawford]

I am a structural engineer by day, so take this for what it's worth, but I don't see how you are planning to make this work. I have run across some pretty strange stuff but never have I designed or suggested to anyone not to take the stringer all the way to your landing. I don't see any reason to leave your stringers short like that. I don't think that will pass code and I don't have the current IRC with me, but there is a lot of information about what you can and cannot do with stairs and personally, I would run my stringers all the way to the landing.

[Jamie Buxton]

...How to turn it into a jpg? ...

Your png works fine. However, for the record, in my copy of Sketchup, you can pull down the File menu to see a line about "Export". Go there, and one of the selections is "2D". Select that, and you can export in jpeg, png, and others.

[Jamie Buxton]

I'm not completely sure I understand the details of your construction. Are you planning on fastening the bamboo flooring to some construction lumber understructure? For instance, you could be making the stairs like you were going to put carpet on it. In that structure, people use plywood or OSB treads and risers. Then the carpet or bamboo would fasten to the OSB. If so, the structural strength of the bamboo is immaterial. What's holding up the weight is the OSB underneath.

[Cliff Rohrabacher]

Termites huh? Well, some where I read about using antifreeze to kill and prevent infestation by termites, carpenter ants, powder post beetles, even mold and anything else that might want to make your structure into dinner. Apparently the stuff soaks in and never breaks down. It's supposed to be the fatal bullet for all lifeforms taking mandible or spore to wood.

I haven't used it, as I've got reservations about the vapor pressure of Ethylene Glycol and atmospheric build up in a closed environment like a cellar or house. I suppose it can't be any worse than any of the bug poisons and mold inhibitors that have been used commercially for years But it's always embarrassing to die by one's own hand accidentally.

[David Keller NC]

"I haven't used it, as I've got reservations about the vapor pressure of Ethylene Glycol and atmospheric build up in a closed environment like a cellar or house."

In general, ethylene (and propylene) glycol has a very low vapor pressure - low enough to be negligible at room temperature and pressure. It's also not terribly poisonous - you have to drink it to really cause a problem (and drinking it gives you an exposure way beyond what you could reasonably get by breathing the tiny atmospheric concentration over the liquid). Were this not the case, auto mechanics would be in serious trouble.

However, what I'd be concerned about is that the glycol would never go away, and it would be contiuously slippery - yuck.

[Daniel Gantenbein]

Thank you all

Sorry I wasn't clearer. The new stringers do run all the way to the top. The previous shorter stringers butted up against a crossbeam which supports the floor joists midway of the 25' long upstairs floor. The only way I could lengthen the stairs was by supporting the beam with a new column and then cut that section out and move the landing further back in the upstairs floor. I beefed up of the existing joists where necessary and strongly braced between them.
I realize a picture is worth a thousand words.

:( Naturally I had to move an existing airline... And to make things really exiting I replaced the column supporting the beams other end, which is in the middle of my downstairs studio by a steel beam with a jib crane. This required cutting the existing concrete floor for the necessary footer but that allowed me to run 6" dust exhaust piping and electrical to my Table saw island. I am far from done but will attach some progress report pictures shortly.

The structural integrity of the new stringers with temporary treads have proven themselves during construction... What a great relieve to walk on a sound upstairs floor, so far only the 3/4" T&G ply underlayment still minus the Bamboo flooring! No more falling off the joists and putting my legs trough the sheet rock ceiling; my dog either out of sympathy or because of some mice up between the joist tried that too!

My original question remains: are 5/8" thick glued up bamboo stairs of 8.5" depth between a span averaging 30" strong enough?

Thanks Daniel

[Jamie Buxton]

I reread your post. Now I understand you're thinking of just edge-gluing flooring into panels to use as treads, without any structure underneath it. If so, that's not a good idea, for two reasons. First, the edges milled into flooring is not intended for this. Generally the sides of the flooring are not exactly at 90 degrees to the top. They tilt slightly to ensure that the top corner meets tightly, and the bottom corner may not. If you glue them up, you'll get a cupped panel. Second, you'll have a tread which is only 3/4" thick. Stair treads are thicker than this; 1" or 1 1/8" is common in oak.

Do you know you can buy bamboo in other forms? You can buy it in 4'x8' panels like plywood, and you can buy it as stair tread stock, complete with the bullnose.

[Daniel Gantenbein]

Thanks again Jamie

This is what i did... without realizing the drop down where I now see jpg . Grateful and often overwhelmed for technology:eek:

[Todd Crawford]

5/8" is too thin especially spanning 30". My stairs are 38" wide with the stringers spaced at 12" and my stair treads are 1" thick.

[Daniel Gantenbein]

Todd and Jamie

Thanks. I rethink my structural confidence into unsupported bamboo treads. I do believe strength wise alone Bamboo is very strong but there is its brittleness.

[Cliff Rohrabacher]

In general, ethylene (and propylene) glycol has a very low vapor pressure - low enough to be negligible at room temperature and pressure.

How very interesting.


It's also not terribly poisonous - you have to drink it to really cause a problem (and drinking it gives you an exposure way beyond what you could reasonably get by breathing the tiny atmospheric concentration over the liquid).

The source for my statement was a web site run by a galoot. He wanted to see whether antifreeze would prevent mold fungus and insect contamination. One of the things he did was treat various pieces of wood and leave them out with other pieces of lumber that were not treated. His result was that the treated survived ground contact unharmed over a long period while the non treated was moldy bug infested and useless.


However, what I'd be concerned about is that the glycol would never go away, and it would be contiuously slippery - yuck.

Well The places I'd put any bug / mold treatment wouldn't be used for foot traffic or hand contact. Beams, joists etc are whereI'd be spraying it. I have powder post beetles in my 200 year old oak beams.

When I was considering using ethelyne glycol I chose to try Borax instead. It was cheaper and non toxic to mammils. I made a super saturated Borax solution and hosed everything down with that and it seems to have dramatically reduced the incidence of the little flying moths from the grubs. I haven't seen byut three or four in the 2 years since I did the borax treatment. I was unsure about the efficacy of the stuff but maybe I was wrong. Before the treatment the little moths were a constant & annoying reminder that I had a serious infestation.

[David Keller NC]

"Well The places I'd put any bug / mold treatment wouldn't be used for foot traffic or hand contact. Beams, joists etc are whereI'd be spraying it. I have powder post beetles in my 200 year old oak beams."

Cliff - you don't say where those beams are. If they're already in the structure of your house or shop, you've a really serious, and potentially life-threatening, issue. I'd seriously consider hiring a professional to assess the damage and recommend a treatment (preferably a modern insecticide based on organo-phosphates). Powder post beetles, like termites, can cause a tremendous amount of damage inside a piece of lumber without it being obvious on the outside.

You could postentially have a situation that might cause a collapse with a significant snow load or a windstorm.

[Daniel Gantenbein]

Yes termites certainly have impacted my live :eek: over the years. Before I topped of the cinder blocks with concrete I made sure any thirsty insects had enough to drink.

Years earlier our house also need a new sill... but i now love in floor heating :)
"Only" the bathroom still awaits modernisation and taking the joists of the temporary cinderblock supports.

And the studio once fully functional again ( :( As in where is that tool among all the piles i have moved for the umpteen time :mad: ) will be sweet :) .

To support the upstairs joists I built a new bearing wall in front of the sorry sill you see in the picture. The jib crane works and lifts two tons, a bit more headroom would have been niece but the big one is in the uninsulated studio.

Hope the pictures do the talking.

[Jeff Duncan]

I'm no engineer, but just from having a basic knowledge of wood I agree that 30" is much too far a span for any 5/8" thick wood material. Of course bamboo isn't really wood, but that's another discussion. You'll need at the minimum a structural tread or another stringer to use 5/8" finished treads.

And just as an FYI, it's very difficult to get any real data on the strength of Bamboo b/c there are so many species of it. And since most of the stuff on the market isn't graded into any strength or species category, well I wouldn't want to trust it for your application anyway. You can get 2 different species one very hard and strong and another not so much. With no system of labeling or regulation, you won't know what your getting

good luck,
JeffD

[Daniel Gantenbein]

A 5/8" piece of wood or grass is not strong enough to span much as a heavy foot comes down hard. But all the riser support the treads very well over the width of that span, right? And with 5 longish screws trough the riser in the back the actual distance between "support points" narrows to around 6".

Am I too much of a minimalist?

[Todd Crawford]

Yes you are.... It won't work without extra support. :rolleyes:

[Daniel Gantenbein]

Thanks to you Todd and the collective wisdom

I better come up with something sensible and overcome my minimalist tendencies. Not being a straightforward purist has me thinking about metal bracing.

All the stuff previously stored under those stairs asks for a rather flat profile.

Hopefully before next years ends I get to post pictures of the finished stairs;)

[Brad Shipton]

I guess the question I would ask is if you want code legal or not? If not, load test a sample tread with it supported one edge and approx 1" from the other. 300lbs is where I would start. If you are thinking about any dados for the risers make sure to build them into your load test. I am betting with the tread fully supported by the risers, it may work in practice, but if I sit down with my calculator and some assumed parameters (conservative) it will have no hope of working just as Todd suggests. If you want a code legal stair and you cannot find proper technical specs in the applicable building codes for bamboo, this will not pass regardless of what tests you conduct. You would need a structural eng.

If you do go forward with this I suggest you carefully consider the glue, and clamping pressure. If you use the easiest glue up method with the individual pieces being 30" long, the glue joint will be critical for these support conditions.

Out of interest I checked a couple of stair part manufacturer sites, and a couple show laminated bamboo tread options where the core appears to be plywood. Personally, I doubt many use bamboo as a structural material in NA due to the lack of code tabulated parameters.

I am a bit confused why you are trying to make the finished stair parts the structural elements in this case. That is more common for open tread designs. Here you could whip up a set of conventional stairs out of dimensional lumber and cover them up with bamboo if you like and nobody will ever be able to tell (unless visible from the underside. If you want an exposed timber stringer, make them out of whatever you like as it appears from your pic that both treads are supported by walls.

Good luck.

[Chris Friesen]

I better come up with something sensible and overcome my minimalist tendencies. Not being a straightforward purist has me thinking about metal bracing.

All the stuff previously stored under those stairs asks for a rather flat profile.

What does a flat profile have to do with anything? All we're suggesting is to put a thick plywood or solid wood tread down, and then cover it with the bamboo flooring. The flooring becomes decorative, not structural.

[Daniel Gantenbein]

Since my stringers are cut and installed I don't feel as free.
This stairs should rise evenly for the first time over the whole run. I prefer to do penance for my haste rather then to have the first and last step out of rhythm. Otherwise I definitely like syncopes.

I could cut the temporary treads made from the temporary 2"x6" down to fit between the stringers and attach them with pocket screws or go with an angle iron support to be structurally sound.