Massive door at North Point MTR exit B2

Submitted by RS on Tue, 12/12/2017 - 18:58

Hi

I recently moved to North Point and couldn't help notice that MTR exit B2 (the one leading to MacDonalds) has a massive metallic door (about ten inches thick!). That's totally odd, considering that other exits are protected with a simple metallic curtain, one of which is even installed behind the big door...

My first assumption was that there might have been a bank on the other side and this could have been a vault's door, but then I realised that the lock is apparently on the MTR side, not on the shop's side.

Does anyone know the story of that door? I have written to the MTR but they didn't answer.

Thanks!

Tags

I've seen them at other stations but don't know what they're for. They look as though they could be protection against blast or flooding. But then you'll see another entrance nearby that just has the simple metallic curtain, which seems to put an end to that theory.

Hi!

@Oldtimer

MTR just answered my email (maybe triggerred by you!). They confirmed that the door is a "a flood gate which is in place to prevent possible flooding situation".

 

@David

That was actually a good guess. Come to think of it it is logical: other entrances are under MTR's supervision and control, and are at ground level (plus the usual few steps); in case of flooding, the MTR can quickly reinforce them... but if McDonalds fails to contain the water they will get flooded up to the ceiling. The MTR needs to protect themselves from that risk, thus the door.

Wong Tai Sin Exit D2 is the same situation: the exit is underground and leads to a mall that is not an MTR mall. The door limits the MTR's risk at their doorstep literally.

 

Thanks to all!

You are very welcome, RS.  It looks like they made the door using a steel plate having its perimeter reinforced.  I cannot tell how thick the door is, however it does not need to be thick unless it is designed to hold back water at considerable depth.  Too heavy a door they need stronger hinges which appears not so in the photo.

My calculation is a bit rusty and hope the decimal is at the right place.  Assuming the door is 8 feet high and 5 feet wide, and flood level rises to and stays at the top, the horizontal load exerted by the water on the door is about 10,000 pounds they call it static load H-1 in the diagram at the web site below, other loads are considered negligible.  Regards,  Peter

About loads on dams (also suitable here) - 

http://community.dur.ac.uk/~des0www4/cal/dams/load/lf2.htm

At least at North Point, the door is one full level below the ground; it takes two flies of stairs to get there (I'd say the bottom of the door is c. 8m below the ground). Also the door there is much wider, more like 8 ft x 8 ft.

That would make more like c. 200,000 pounds of water pressure to resist, and the door is a good 8-10 inches thick.

Greetings.  My calculation is not a professional one, but this gives a reasonable estimate.   The specific weight of water is about 62.4 pounds per cubic feet.   The pressure is nil at the water surface , it increases linearly (good enough assumption here) with depth.    At 8 metres (26 feet) below ground it is 1,622 pounds per square feet at the door's bottom.  At the door's top (18 feet below ground level) it is 1,123 lbs/sq ft, and the average of the two numbers is 1,373 lbs/sq ft acting on surface area of the door 8 ft by 8 ft = 87,846 pounds pushing on the door.   No wonder the door is so thick.
It appears that designers expected the possibility of large quantities of storm runoff for extended period of time and there are no means of removing the water by pumps fast enough (on either side of the door) when the door is closed.
At this moment, my problem outside the house is heavy snow and blowing snow (Toronto area).  Time to get out with a snow shovel to deal with "stiff" water.   Regards,  Peter