The mystery of the train seat uncovered

Will the train seat withstand 120Kg? was I seeing things? Maybe the weight was more since the person had a backpack. But the load was extreme.

Seat deformation
Seat deformation

I calculated 5 mm deformation on the cushion. Could I have seen it? Well maybe. Stresses were far away from the plasticity limit, however the structure feels loaded.

From my analysis the structure could take even more load and wobble even more. Maybe the squeaking sound over exaggerated my notion of wobbling.

Undeformed to undeformed seat
Undeformed to deformed seat comparison

The light blue image is the deformed state,  we can see the deformation direction, which moves in a expected direction.

Close up of seat
Close up of seat

In this close up we can see a slight displacement. But very small.

So my conclusions:

The heavy train traveller was safe, the seat could stand a few extra Kg’s of weight with some more camping equipment.

The design seems fairly robust, however it actually wobbles.

Train seat analysing

Finally I got time to mesh and solve my imaginary problem. I will not dive into meshing since it is a chapter on its own. I used a tetrahedron mesh as standard as it gets. The work flow is simple; get a CAD model, get material data, mesh it, add supports and loads and finally get some results, either by stresses or displacements.

Train seat mesh & Result
Train seat mesh & Result

My results for a small load of 1.2Kg shows that the T tube element will suffer most when we sit on it. I would say that if it breaks it will be there. Ok this case is more then evident since the only thing holding my weight is this connection.

But will it wobble with 120Kg weight?

Free Body Diagram

In order to build up my model some boundary conditions need to be defined. For this a free body diagram is helpful. With it it’s possible to visualise what is interacting with my target body.

Free Body Diagram Train Seat
Free Body Diagram Train Seat

With this in mind we have two fixed supports that constrain (X, Y, Z and fixation null torque in all directions Mx, My and Mz).

I will continue more in-depth since im am curious regarding tubular meshing and to actually know if the seat can take a 120Kg weight on top.

Will it hold?

My today’s train trip is being quite interesting. As someone sat on a seat it wobbled. It made me think “how much would it hold”. Well, I enjoyed the seat design with it’s tubular shape and table attached to it.

Train seat
Train seat

This made me think also. How would I model this?

I have a few inputs/assumptions I already can imagine:

  • Assuming a 120Kg (more or less 250 pounds) person
  • Isotropic behaviour structural steel
  • Perfect welding connections (I know I know but it’s just a rough analysis)

Maybe these boundary conditions, hold the seat in place. I will also assume only one seat is present instead of the hole bundle.

Boundary condition of seat
Boundary condition of seat

On my next post I will follow up on the model building.