Wenches broke my benches

[SteamyTea]

This is, on the face of it, quite comical, but is actually serious.

We bought 3 new, recycled plastic, bench and seat combos. These are not cheap ~£700 each. We got them from the same company as the last ones that have been excellent.

 

So, a 'large' family visited, ordered food, went outside, found a bench and 3 of the women sat on one side.

The seat supports snapped, sending them to the floor.

Luckily no injuries, and we were quickly tending to them.

We removed the broken bench, offered to put two table benches together so all 6 of them could sit, but they said it was OK (2 of the men and one woman, the very heaviest ones stood and ate).

 

Come the next morning, I have a look at the damage, all the new seats seem to be failing, the old ones are fine.

The difference seems to be that the old seats had 3 bolts on each support, and they were in a smaller triangle in the arm. The new ones had two bolts, but closer to the edges, leaving a thin strip to take the majority of the tension.

Hopefully the pictures below will make it clearer.

So one for @Gus Potter, @saveasteading and anyone else interested in materials and how they can fail in usage.

 

The broken arm/seat support, with no 'meat' at the top.

 

The detached seat.

 

Close up of the failure, which shows a lot of aeration in the extrusion.

 

 

 

An old bench with the 3 bolts in a small triangle. This has lasted a few summers.

 

 

Edited August 14 by SteamyTea

[Dave Jones]

claims direct, whiplash etc

 

hope you got plenty insurance.

[SteamyTea]

9 minutes ago, Dave Jones said:

hope you got plenty insurance

A large, multi national always has plenty of insurance, this is not a two bit company like yours.

[Adrian Walker]

18 minutes ago, SteamyTea said:

This is, on the face of it, quite comical, but is actually serious.

We bought 3 new, recycled plastic, bench and seat combos. These are not cheap ~£700 each. We got them from the same company as the last ones that have been excellent.

 

So, a 'large' family visited, ordered food, went outside, found a bench and 3 of the women sat on one side.

The seat supports snapped, sending them to the floor.

Luckily no injuries, and we were quickly tending to them.

We removed the broken bench, offered to put two table benches together so all 6 of them could sit, but they said it was OK (2 of the men and one woman, the very heaviest ones stood and ate).

 

Come the next morning, I have a look at the damage, all the new seats seem to be failing, the old ones are fine.

The difference seems to be that the old seats had 3 bolts on each support, and they were in a smaller triangle in the arm. The new ones had two bolts, but closer to the edges, leaving a thin strip to take the majority of the tension.

Hopefully the pictures below will make it clearer.

So one for @Gus Potter, @saveasteading and anyone else interested in materials and how they can fail in usage.

 

The broken arm/seat support, with no 'meat' at the top.

 

The detached seat.

 

Close up of the failure, which shows a lot of aeration in the extrusion.

 

 

 

An old bench with the 3 bolts in a small triangle. This has lasted a few summers.

 

 

 

That recycled plastic doesn't look very good in x section.

[SteamyTea]

4 minutes ago, Adrian Walker said:

 

That recycled plastic doesn't look very good in x section.

No.

Aeration is a common problem when extruding 'fractional melt' plastics.

I think the main problems are that the top cord was too small and with only two bolts per arm, 'rocking' soon becomes excessive.

Triangulation is simple, cheap and sturdy.

Edited August 14 by SteamyTea

[ProDave]

You need to take this up with the bench supplier.  The reason for the failure is not your concern, though you may want to enter dialogue with them to help them improve their product and understand why it failed.

 

And if they were sold for domestic use they may reject any claim.  Where they sold knowing they were for commercial use?

[joe90]

5 minutes ago, ProDave said:

And if they were sold for domestic use they may reject any claim.  Where they sold knowing they were for commercial use?

+1, I was told years ago that things bought should be ”fit for purpose” which these are obviously not.

[MikeSharp01]

I am no structural engineer but my guess is the three bolt scheme is much better because the extra bolt is taking a chunk of the load and preventing a straight fracture across the two straight up and down bolts - the only thing I really know about structures is that 'triangles' are everything.

[SteamyTea]

1 hour ago, ProDave said:

You need to take this up with the bench supplier.

It is being taken up with them.  We are not a small company.

But I am more interested in the physics, not the legality.  I wanted to take the new ones away, seems to me an accident waiting to happen See lower down.

1 hour ago, MikeSharp01 said:

I am no structural engineer but my guess is the three bolt scheme is much better because the extra bolt is taking a chunk of the load

I think 3 bolts, stop some of the 'rocking' and as they are nearer the centre line, there is more material in tension.  A greater cross sectional area for the forces.

 

Below is a picture of one that is failing in my opinion.  The bean is already bowing and you can see how little distance there is between the bolts and the top edge of the beam.

 

 

 

[ProDave]

Are these new ones the same in all respects to the old ones apart from 2 vs 3 bolts?

 

Or are the new ones also smaller section material in places?

[SteamyTea]

Just now, ProDave said:

Or are the new ones also smaller section material in places

Not measured them, but they look to be the same extrusion sizes.

[joe90]

3 hours ago, SteamyTea said:

Close up of the failure, which shows a lot of aeration in the extrusion.

Without cutting the old ones you won’t know if the aeration is the same or not. I bought similar stuff for the soul plate of my new wooden garage (to act as a DPC) and cutting that virtiualy no “bubbles” seen. 

[SteamyTea]

2 minutes ago, joe90 said:

Without cutting the old ones you won’t know if the aeration is the same or not

When machining plastics, woodworking machines are often used, these cause micro fusion at the cut edges, which makes them look as though they are solid.

So hard to tell from a machined part.

 

[joe90]

11 minutes ago, SteamyTea said:

When machining plastics, woodworking machines are often used, these cause micro fusion at the cut edges, which makes them look as though they are solid.

So hard to tell from a machined part.

 

However I cut mine with a hand saw so no “melting” should have occurred..

[Temp]

The aerated core probably turns it into a box section. 

 

The bolt holes on the new/failed units appear to be further apart vertically so the top bolt hole appears to be through part of the solid top skin rather than the core like the older units.

[joe90]

In the end the benches are not “fit for purpose” (unless max weight limit signs are visible 🤷‍♂️) so the suppliers are liable IMO.

[Onoff]

Edge distance issue that imo. 

[saveasteading]

The weight gets applied to the bench and all goes to the pair of bolts. The likelihood is that it all sits on one bolt until it nestles down to be shared by both.

but before that happens, the load acting downwards causes the spar to rotate with the bottom bolt as pivot point. You now have lots of load down on the top bolt and also pulling it apart at the top surface.

that tiny bit of plastic above the top bolt fails.

the spar is already much reduced by 2 holes in a line.

bending continues and so does the crack,  especially as this is now dynamic load. it is moving.

all in an instant.

 

If it was wood then there would be fibres longitudinally, holding it all together.

But the aero bar is all one material and therefore   . ....someone  who understands plastic can take over....

 

With three bolts, the rotation is about one bolt but resisted by the other 2.

one extra bolt, and at least double the strength.

 

They are all a bit close to the edge.  Bolts that bite into the plastic would be best to stop initial movement.

 

I had one of them at our office for 15 years and it was fine. Then it was stolen so I can't say what the bolt arrangement was.

[Onoff]

Yep, top bolt will fail first I'll bet. Ask the next chubster family that rocks up to try a "good" bench with the offer of free doughnuts. Maybe set up a wildlife camera to look at the bolts...