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Comment by InvisibleCities | Hacker News Reader

>This is why emergency exit doors must open outwards. People have died in fires in theaters trying to push on doors that said “Pull”. The crash bar on an emergency exit door is what’s known in design as an affordance.

Is this true? I always assumed it was because it's pretty much impossible to pull open a door when you are being crushed by people pressing against one another trying to escape.

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Yes and recent.

>An inward-swinging door - three times cited as a code violation by West Warwick inspectors and three times replaced by club managers - was blocking the exit closest to the stage.

https://www.bostonglobe.com/metro/2013/02/15/series-errors-s...

For some context in case anyone is curious, two sentences later the article reports that "Bouncers opened it immediately after the fire broke out".

The door in question's main problem appears to have been that bouncers wouldn't let people use it, not its orientation.

AFAIK you're correct, a vertical handle is an affordance to that indicates a door should be pulled, but the reason is mechanistic. A crush of people will open the door with a push-release but won't with a pull handle - whilst the handle affords pushing some numpty will try and pull it but the people behind them will soon ensure that they push the door, with their broken ribs if necessary.

Norman doors https://99percentinvisible.org/article/norman-doors-dont-kno...

I think it's true to some degree, I've seen people push on doors that say "pull" even when there is no fire.

But I think your assumption is probably more correct, past a certain point they have no option to pull anymore.

The push/pull "mistake" is usually caused by poorly designed doors. Like putting a vertical grab handle on a door that pushes open.

It's the same sort of myth as "why are manhole covers round?" and there are lots of attributions for why that ignore history for a just-so story.

I don't understand the reference. Why are manhole covers round? I always thought it was to prevent the cover from falling into the hole. Is that not the case?

That is the argument but actually any lipped shape can give this guarantee. Even historically manhole and utility covers had to be somewhat flush to the ground and so they've always had to be lipped for this.

You can see lots of utility covers that are hinged, square, or other options, but all are lipped for safety. While a circular profile makes the lipping easier, it doesn't seem to influence many utility covers.

You can easily put a square or a rectangular utility cover into the hole, because the length of the side is less than the diagonal. This isn't possible with a circular cover because the diameter is uniform.

The parent’s point is that a lip prevents this for every shape, by making the hole smaller than the side-length.

Only with a sufficiently large lip. The diagonal of the hole has to be less than the shortest side of the lid. As I picture it in my mind this is an excessively large lip.

If you have a very rectangular shape then the lip would have to be large.

Stop for a moment next time you walk in an urban environment. Look for circular utility covers you see in your urban environment as opposed to squaed ones. Look at the features of the circular ones vs the non-circular ones.

One of the reasons this myth irritates me so much is that everyone is so certain that they know exactly the answer but their actual daily experience doesn't line up with the results at all.

I've heard several people offer explanations from a geometric safety option (which is attractive for free-standing covers) to the simplicity of manufacturing (e.g., that it's very easy to make circular molds and get even density compared to square molds) to simply what the contractor suggested. I've also heard people suggest that metal cylindrical templates were something very common to manufacture for a variety of industrial uses.

I'm not certain, that's why I asked the question in the first place.

I'm not sure what features I would look for because the lip on a rectangular cover would be under the lid. How do I know that the rectangular covers have sufficiently large lips to prevent the lid from falling in? I'm willing to accept that in some cases the lids are rectangular and there is a risk of them falling in.

If only this were true for certain utility holes in my neighborhood with rectangular lids. I I've sprained my ankle a few times. I mean, they won't fall in accidentally because of the lip. But you can lift one out and put it in the hole, due to negligence or a taste for vandalism.

> You can easily put a square or a rectangular utility cover into the hole

Then why do rectangular (including square) and triangular manhole covers exist? Rectangular ones are quite common, triangular less so.

Grandparent is correct, lips are what, in practice, prevents manhole covers from falling in, not being circular (which many are not.)

Well maybe in some cases the risk of the lid falling in is acceptable. Or maybe there are other mitigations in place.

I don't know why rectangular utility holes exist, I assume cost.

FYI, I've personally dropped a rectangular manhole cover into a properly lipped hole. They do fit all too well.

They are only round when they cover a round hole. I've seen a number of covers that are other shapes, often square.

There are engineering reasons to prefer a round hole, but there are sometimes other considerations which push a different shape.

Sewer grates are frequently square. For what it's worth.

(Not always, but quite frequently.)

They have the same gravitational-gradient dynamic as manholes.

> Why are manhole covers round?

Manhole covers exist in several shapes, as illustrated on the Wikipedia page.

https://en.m.wikipedia.org/wiki/Manhole_cover

What's the myth? There are good reasons for round manhole covers.

Also fun fact: That used to be a Microsoft interview question.

That is not where the myth originated, but it is where I heard it first.

Thanks - you correctly caught an error and I’ve edited the post to address it. The danger is in the confusing affordances, and the point was about that and not the direction of the door swing (which as you’ve correctly pointed out has other more primary considerations.)

Don’t worry about that. Just retweet it as a fact, pronto.

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