By - frackyou
It could be a number of factors, for example, you tend to lean back or sit too far forward or tilt the chair on the two back legs or who knows what.
Unfortunately, I doubt that any amount of engineering calculations will satisfy the management company that the cost of replacing the broken chairs is their responsibility.
Maybe you can find that the manufacturer has identified an issue or has had lots of complaints. That might persuade the mgmt company.
Best case, they will take the chairs and provide you different ones.
Otherwise, if they keep breaking, you might be better off setting those chairs aside then buying & using your own chairs.
This definitely looks like some shit someone whose worked in polymers (or at least doesn't understand the fundamentals of wood) would design and slap into a factory. Crossgrain strength of wood is absolute shit, it's maybe 10% of the parallel grain strength thats typically cited for "how strong is wood", and it gets even worse in soft woods like that.
The big killer is those leg ends on smooth vinyl floors... if there is no fixed end support and no bottom tie, the ends want to slip away from each other and bending moment on that center joint will easily double. These chairs would be perfectly fine on some grippy carpet I'd bet, which is probably what they were designed and tested on.
Do the calculations that /u/bebenona said to find torque from load. You can look up some soft grain woods on MatWeb.com to find the cross grain tensile strength (in force per area, such as PSI). Estimate the mated area of the joint by finding thickness and height at its narrowest section, then we can find a rough value for max tensile stress with some other fun that I'll have to pull out my strengths of materials note cards for.
Ok, I absolutely LOVE this. I'll be home tonight and work on all that. I'm actually enjoying this because of all the things i'm learning and the enthusiasm of people like yourself. Standby /u/bebenona and /u/empirebuilder1 !
Tell them to send you different chairs. Or pay one last time and return them to mgmt and buy your own chairs.
Also they seem unsafe because of you sat in edge of chair they would tip forward. Most all chairs have the front legs leaving the front of the seat
I agree that we need to get our own chairs and will do so. I'm not paying for a 3rd chair though. This is the hill I will die on.
Have you considered they are glueing these and sending them back to you too break again?
The real flaw is the leg has that curved piece that will always break off because if the grain of the wood and no reinforcement like dowels.
You might be able to tell them you are not paying because you are using them normally and it's a defective product they should send back to mfg and to be glad you are not suing for injury created by defective chairs
Oh, excellent point there. If they're fixing them, they're just really making money. I also like the idea of stating they might be liable. Either way, I'm done using these death traps.
This is some quick trig for you: [https://imgur.com/a/njKvnLr](https://imgur.com/a/njKvnLr)
You just need to estimate your weight (F), the length of the leg (l), the angle of the leg (theta and alpha) and you should be able to get the torque value at the joint.
If you want to send me the approximate measurements, I can do it for you.
Hope this helps!
Oh, wow. That's awesome. I really want to scare them will a well-researched formula so this helps. I feel like it'll confuse them enough so that they just don't want to deal with me.
F = My BF weighs about 180lbs
l = Each leg is about 18" long
I'm up for figuring out the other pieces of this if you need them (angle, torque)
Edit: BF's weight was wrong.
I estimated the angle as 45\* and did the calculations: [https://imgur.com/a/gVlwO56](https://imgur.com/a/gVlwO56)
Bottom line, this chair is designed really poorly!
Now how much torque is too much?
And wouldn't it depend on the type of wood (density and strength of fibers), whether it's glued or screwed or both, whether the screws are into the end grain or into the face, number & length & diameter & thread pitch of screws, type of glue, dryness of wood, etc.
You’ve pretty much pointed out the failure of the chair. The tension is in such a direction that pulls the grain apart. If they won’t take that as an explanation, perhaps finding who makes the chair, and seeing if others have left such reviews could strengthen your case, or if the company themselves have admitted to making a faulty product.
So I was a woodworker \~10 years and I'm an engineering student. The one thing that stands out to me is how clean that break is at the glue line. Generally speaking, the glue will bring parts of the wood grain with it during a break.
It makes me wonder how they manufactured those chairs. That split looks as if the glue didn't actually get into the wood itself. It looks similar if you try to glue two finished pieces of wood together.
I'll look at the brand. It sounds like we'll all enjoy hating them.
I appreciate hearing that it's not us!